APPLICATION OF MODIFIED WALNUT SHELL ADSORBENTS IN OILY
WASTEWATER TREATMENT
by

Jibrael Kingsford Odoom
B.A., University of Ghana, 2015
M.Sc., Jiangsu University, 2020

THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
IN
ENGINEERING

UNIVERSITY OF NORTHERN BRITISH COLUMBIA
January 2025

© Jibrael Kingsford Odoom, 2025

i

Abstract
Oil spills are of great concern because oily wastewater disrupts the aquatic ecosystem, causes
animal mutations, contaminates surface water resources, and causes diseases such as human
cancer. Current efforts are geared towards recovering spilled oil from marine environments and
ensuring the effective separation of oil and water in the collected emulsion. After oil separation
from the emulsion, a polishing step is required to treat the residual oil in the water before
discharging the effluent into the aquatic environment. Adsorption using natural materials as
adsorbents is seen as an attractive option due to its simplicity, cost-effectiveness, accessibility, and
eco-friendliness. Recent adsorption studies have focused on modifying eco-friendly materials to
enhance their adsorption potential. This study investigated a two-step modification of walnut shells
(WS) to treat oily wastewater. Firstly, WS was modified with sodium hydroxide (NaOH) via
the wet impregnation method to attain WS/NaOH. Then, tetrabutylammonium bromide (TBAB)
was introduced as a co-adsorbent to enhance the oil removal potential of WS/NaOH. The surface
morphologies and chemical composition of the WS and WS/NaOH were characterized by scanning
electron microscopy and Fourier transform infrared spectroscopy, respectively. A one-factor-at-atime experimental design was employed in this study. Findings from this research project indicated
that raw WS and WS/NaOH are excellent adsorbents for the treatment of oily wastewater, with a
removal efficiency of 81 % and 88 %, respectively. They generated adsorption capacities of 201
mg/g and 220 mg/g, respectively. However, with the introduction of TBAB as a co-adsorbent, an
efficiency and capacity of 91 % and 226 mg/g, respectively, were achieved, proving the
effectiveness of TBAB. Adsorption isotherm and kinetic studies were conducted to understand the
adsorption mechanism of the proposed adsorbent. It was concluded that the Freundlich and Pseudo
second-order model best represents the experimental results, as they show higher R² values of
ii

0.9695 and 0.9998, respectively. This research project presents an eco-friendly and highly efficient
adsorbent from modified WS to treat oily wastewater, offering a sustainable, scalable alternative
to traditional methods by utilizing agricultural waste to remove oil pollutants in petrochemical
wastewater treatment efficiently.

iii

Table of Contents
Abstract ......................................................................................................................................................... ii
Table of Contents ......................................................................................................................................... iv
List of Tables .............................................................................................................................................. vii
List of Figures ............................................................................................................................................ viii
Acronyms and abbreviations ........................................................................................................................ xi
Acknowledgments...................................................................................................................................... xiii
Chapter 1 Introduction .................................................................................................................................. 1

1.1.

Background ..................................................................................................................... 1

1.2.

Objectives and significance of the study ........................................................................ 2

1.3.

Organization of thesis ..................................................................................................... 4

Chapter 2 Literature Review ......................................................................................................................... 6

2.1. Characteristics of petroleum wastewater ............................................................................. 6
2.2.

Human and environmental health impacts of petroleum wastewater ............................. 8

2.3.

Treatment technologies ................................................................................................... 9

2.3.1.

Physical treatment methods ........................................................................................ 9

2.3.2.

Chemical treatment methods..................................................................................... 11

2.3.3.

Biological treatment .................................................................................................. 14

2.4. Adsorption.......................................................................................................................... 17
2.4.1. Wastewater treatment by adsorption ............................................................................... 19
2.4.2. Factors affecting adsorption ............................................................................................ 21
2.4.2.1. Effect of pH.................................................................................................................. 21
2.4.2.2. Effect of initial oil concentration ................................................................................. 22
2.4.2.3. Effect of adsorbent dosage ........................................................................................... 23
iv

2.4.2.4. Effect of temperature ................................................................................................... 23
2.5.

Adsorbents for wastewater treatment ............................................................................ 24

2.5.1.

Natural adsorbents .................................................................................................... 25

2.5.2.

Agricultural waste ..................................................................................................... 27

2.5.3.

Industrial waste ......................................................................................................... 29

2.5.4.

Nano-adsorbents ....................................................................................................... 30

2.5.5.

Bio-sorbent ................................................................................................................ 33

2.6.

Oily wastewater treatment using adsorption. ................................................................ 35

Chapter 3 Materials and Methods ............................................................................................................... 43

3.1. Materials ............................................................................................................................ 43
3.1.1. Walnut shells (WS) ......................................................................................................... 43
3.2.

Synthesis of WS/NaOH ................................................................................................ 45

3.3.

Oil emulsion preparation............................................................................................... 46

3.4.

Experimental design...................................................................................................... 47

3.5.

Material characterization and adsorption studies .......................................................... 48

Chapter 4 Results and Discussion ............................................................................................................... 51

4.1.

Characterization of WS/NaOH ..................................................................................... 51

4.2.

Batch Adsorption Studies ............................................................................................. 54

4.3.

Effect of experimental conditions on oil adsorption ..................................................... 56

4.3.1.

Effect of adsorbent dosage ........................................................................................ 56

4.3.2.

Effect of reaction time .............................................................................................. 57

4.3.3.

Effect of initial oil concentration .............................................................................. 58

4.3.4.

Effect of oil/water salinity concentration .................................................................. 59

4.4.

Adsorption isotherm models ......................................................................................... 60
v

4.5.

Adsorption kinetic studies............................................................................................. 62

Chapter 5. Conclusion and Recommendations ........................................................................................... 66

5.1.

Conclusion .................................................................................................................... 66

5.2.

Limitations of the study ................................................................................................ 67

5.3.

Recommendations ......................................................................................................... 67

References ................................................................................................................................................... 69
Appendices................................................................................................................................................ 111

vi

List of Tables
Table 2.1: Characteristics of petroleum wastewater ....................................................................... 6
Table 2.2: Oily wastewater treatment technologies ...................................................................... 16
Table 2.3: Adsorption performances of different adsorbents for wastewater treatment............... 33
Table 2.4: Adsorption performances of various low-cost adsorbents for treating oily wastewater
....................................................................................................................................................... 40
Table 3.1: Main materials ............................................................................................................. 44
Table 3.2: Main Apparatus ........................................................................................................... 45
Table 3.3: Experimental design details ......................................................................................... 48
Table 4.1: Adsorption isotherms constants for oil removal from oil-water emulsion .................. 61
Table 4.2: Kinetic parameters for oil removal from oil-water emulsion ...................................... 64
Table 4.5: Comparison of oil removal efficiency of the proposed WS/NaOH/TBAB with other
natural adsorbents ......................................................................................................................... 65

vii

List of Figures
Figure 3.1: Images of (A) Walnut shells 24 h after the addition of NaOH via wet impregnation,
and (B) Walnut shells/NaOH after carbonization for 5 h ............................................................. 46
Figure 3.2: Images of (A) prepared oil-water emulsion and (B) oil extraction step ..................... 47
Figure 3.3: Preparation steps of raw walnut shells, Walnut shells/NaOH, batch adsorption studies,
and sample analysis....................................................................................................................... 50
Figure 4.2: FT-IR spectra of RWS ................................................................................................ 52
Figure 4.6: FT-IR spectra of RWS ................................................................................................ 52
Figure 4.7: FT-IR spectra of WS/NaOHFigure 4.8: FT-IR spectra of RWS ................................ 52
Figure 4.9: FT-IR spectra of WS/NaOH ....................................................................................... 52
Figure 4.10: Preparation steps of RWS, WS/NaOH, batch adsorption studies and sample
analysisFigure 4.11: FT-IR spectra of WS/NaOHFigure 4.12: FT-IR spectra of RWS ............... 52
Figure 4.16: FT-IR spectra of RWS .............................................................................................. 52
Figure 4.20: FT-IR spectra of RWS .............................................................................................. 52
Figure 4.21: FT-IR spectra of WS/NaOHFigure 4.22: FT-IR spectra of RWS ............................ 52
Figure 4.23: FT-IR spectra of WS/NaOH ..................................................................................... 52
Figure 4.24: Preparation steps of RWS, WS/NaOH, batch adsorption studies and sample
analysisFigure 4.25: FT-IR spectra of WS/NaOHFigure 4.26: FT-IR spectra of RWS ............... 52
Figure 4.27: FT-IR spectra of WS/NaOHFigure 4.28: FT-IR spectra of RWS ............................ 52
Figure 4.29: FT-IR spectra of WS/NaOH ..................................................................................... 52
Figure 4.30: Preparation steps of RWS, WS/NaOH, batch adsorption studies and sample
analysisFigure 4.31: FT-IR spectra of WS/NaOH ........................................................................ 52
Figure 4.32: Preparation steps of RWS, WS/NaOH, batch adsorption studies and sample analysis
....................................................................................................................................................... 52
Figure 4.33: Oil removal efficiencies (R) and adsorption capacities (qe) of RWS, WS/NaOH, and
RWS/NaOH/TBABFigure 4.34: Preparation steps of RWS, WS/NaOH, batch adsorption studies
and sample analysisFigure 4.35: FT-IR spectra of WS/NaOH ..................................................... 52
viii

Figure 4.36: Preparation steps of RWS, WS/NaOH, batch adsorption studies and sample
analysisFigure 4.37: FT-IR spectra of WS/NaOHFigure 4.38: FT-IR spectra of RWS ............... 52
Figure 4.39: FT-IR spectra of WS/NaOHFigure 4.40: FT-IR spectra of RWS ............................ 52
Figure 4.41: FT-IR spectra of WS/NaOH ..................................................................................... 52
Figure 4.42: Preparation steps of RWS, WS/NaOH, batch adsorption studies and sample
analysisFigure 4.43: FT-IR spectra of WS/NaOHFigure 4.44: FT-IR spectra of RWS ............... 52
Figure 4.48: FT-IR spectra of RWS .............................................................................................. 52
Figure 4.52: FT-IR spectra of RWS .............................................................................................. 52
Figure 4.53: FT-IR spectra of WS/NaOHFigure 4.54: FT-IR spectra of RWS ............................ 52
Figure 4.55: FT-IR spectra of WS/NaOH ..................................................................................... 52
Figure 4.56: Preparation steps of RWS, WS/NaOH, batch adsorption studies and sample
analysisFigure 4.57: FT-IR spectra of WS/NaOHFigure 4.58: FT-IR spectra of RWS ............... 52
Figure 4.62: FT-IR spectra of RWS .............................................................................................. 52
Figure 4.1: SEM images of raw walnut shells (A) and walnut shells/NaOH (B) ......................... 52
Figure 4.2: FT-IR spectra of raw walnut shells ............................................................................ 53
Figure 4.3: FT-IR spectra of Walnut shells/NaOH ....................................................................... 54
Figure 4.4: Oil removal efficiencies and adsorption capacities of raw walnut shells (RWS), walnut
shells/NaOH, and raw walnut shells/NaOH/TBAB ...................................................................... 55
Figure 4.5: Effect of adsorbent dosage on the adsorption of oil from oil-water emulsion using the
raw walnut shells/NaOH/TBAB ................................................................................................... 56
Figure 4.6: Effect of reaction time on the adsorption of oil from oil-water emulsion using the
WS/NaOH/TBAB ......................................................................................................................... 57
Figure 4.7: Effect of initial oil concentration on the adsorption of oil from oil-water emulsion using
the Walnut shells/NaOH/TBAB ................................................................................................... 58
Figure 4.8: Effect of salinity concentration on the adsorption of oil from oil-water emulsion using
the Walnut shells/NaOH/TBAB ................................................................................................... 59
Figure 4.9: Langmuir isotherm adsorption of oil from oil-water emulsion .................................. 61
Figure 4.10: Freundlich isotherm adsorption of oil from oil-water emulsion .............................. 62
ix

Figure 4.11: Pseudo first-order kinetic model for adsorption of oils in oil-water emulsion ........ 63
Figure 4.12: Pseudo second order kinetic model for adsorption of oil in oil-water emulsion ...... 64

x

Acronyms and abbreviations
AC

activated carbon

AD

adsorbent dosage

ANOVA

analysis of variance

ASTM

American Society for Testing and Materials

BOD

biochemical oxygen demand

CNTs

carbon nanotubes

COD

chemical oxygen demand

CPC

chloride monohydrate

CT

contact time

DAF

dissolved air flotation

FTIR

Fourier transform infrared spectroscopy

GO

graphene oxide

IC

initial oil concentration

PAHs

polycyclic aromatic hydrocarbons

PC

principal components

PCA

principal component analysis

pH

potential of hydrogen

RPM

revolution per minute

xi

RT

reaction time

SEM

scanning electron microscopy

TBAB

tetrabutylammonium bromide

SS

suspended solids

TDS

total dissolved solids

TSS

total suspended solids

WS

walnut shells

WS/NaOH

walnut shells modified with sodium hydroxide

WS/NaOH/TB

walnut shells modified with sodium hydroxide and tetrabutylammonium

bromide

xii

Acknowledgments
I would like to express my heartfelt and profound gratitude to my supervisor, Dr. Oliver Iorhemen,
and my co-supervisor, Dr. Jianbing Li, for their invaluable financial support, insightful guidance,
and the wealth of knowledge they have shared with me. Their dedication and expertise have
provided me with a comprehensive understanding of this field, making this academic journey both
enlightening and inspiring. I also acknowledge the invaluable effort of the members of my
supervisory committee, Dr. Theresa Adesanya and Dr. Natalie Linklater, as well as the external
examiner for my defense, Dr. Pranesh Kumar. Their comments and recommendations have been
instrumental in shaping my thesis to a higher standard.
I am also deeply grateful to my friends and lab mates at the School of Engineering, and I have a
special appreciation for Mehdi Mohammadpour and Manveer Kaur for their unwavering support
and collaboration, throughout my research work. Their contributions have greatly enhanced my
academic and personal growth.
Furthermore, I extend my sincere appreciation to the School of Engineering, its dedicated staff
members, and the University of Northern British Columbia for providing an exceptional platform
that has enabled me to pursue and achieve this academic milestone. The supportive environment
and the university's resources have been instrumental in my academic endeavors.
Finally, I owe a debt of gratitude to my wonderful family, whose prayers, encouragement, and
financial support have been a constant source of strength. Their unwavering belief in me has been
the foundation upon which I have built my success throughout my time at the University of
Northern British Columbia.

xiii

Chapter 1 Introduction
1.1.

Background
Freshwater streams are increasingly contaminated because of rising industrialization and

urbanization. Significant contributors to the rise in water contamination include sewage treatment
facilities, industrial discharges, agriculture, and other anthropogenic activities. These industries
discharge both organic and inorganic contaminants into water bodies [1], including pesticides,
plasticizers, fertilizers, hydrocarbons, biphenyls, phenols, detergents, oils, greases, medications,
plant and animal tissues, etc [2]. Even in small concentrations, these pollutants are hazardous.
One of today's most crucial strategic resources and a critical factor in the growth of modern
economies is oil [3, 4]. Oil demand has increased due to the rapid expansion of the world economy,
which has also increased the amount of oil trade. This has led to the extraordinary growth of the
marine oil exploitation business. Crude oil is extracted, transported, and stored during the upstream
process and refined during the downstream phase [5-7]. The oil sector generates significant oily
waste from upstream and downstream operations, encompassing solid and liquid forms [8]. Since
the Torrey Canyon supertanker sank in the British seas in the 20th century, the transportation of
oil worldwide and the response to oil spills have experienced significant changes [9].
In recent years, terrestrial and marine hydrocarbon contamination in temperate regions,
including northern Russia, Canada, Alaska, and the Qinghai-Tibet Plateau, has grown to be a
severe environmental issue as it endangers human health, depletes natural resources and disrupts
the economy [10-12]. Annually, 3.2 million tonnes of oil are emitted into the environment from
all sectors [13]. The vast bulk of this oil results from commercial and industrial activity. The Gulf
War of 1991 saw the most significant oil spill in history, with 4 and 8 million barrels of oil spilling
into the Gulf and the Kuwaiti Desert, respectively [14]. In oil production projects in

1

Newfoundland, Canada, 381 spills from production platforms occurred between 1997 and 2010
[15]. Oil may spill into the ocean when tankers fill or empty their cargo or during any oil industry
activity or accident. Natural disasters may also trigger spills that cause oil to percolate into the
oceans [16].
The petroleum refining sector is at the forefront of the fight to implement practical and
ecologically friendly methods for environmental preservation because it is considered one of the
significant sources of pollution, both directly and indirectly, through oil exploitation and
automobile exhaust gases [17]. Therefore, oil and gas operators are responsible for treating the
wastewater from oilfield reservoirs using proper technological methods, and they can gain
financially from increased oil production through water flooding. In addition, water from
processed oil can be released into the environment or utilized for other projects because of its
acceptable quality.
1.2.

Objectives and significance of the study
To date, a variety of techniques have been employed to contain the spread of oil leakage and

to treat oily wastewater, including in situ burning [18], mechanical collection [19], biodegradation
[20], use of an electric field [21], and chemical dispersants [4]. Nevertheless, conventional
methods suffer from drawbacks, including insufficient separation, limited selectivity, high
toxicity, reduced recyclability, high operational expenses, lengthy time requirements, and the
potential for reintroducing pollutants [22].
Adsorption has lately emerged as the primary technology for treating oily wastewater owing
to its simplicity, cost-effectiveness, accessibility, and environmentally friendly attributes [23].
Recently, many studies have focused on the conventional and modern adsorption approaches used
in treating oily wastewater. For example, Medeiros et al. [24] compared various methods for

2

treating oily waste streams employed by industries. These methods include membrane separation
technology, advanced oxidation techniques, and the utilization of microorganisms. The pros and
cons of emerging approaches in treating oil-contaminated effluents were also discussed. Tian et
al. [25] thoroughly examined and compared cutting-edge pretreatment and advanced treatment
methods for petrochemical wastewater. They discussed the mechanisms to remove organic
pollutants from petrochemical wastewater through these treatment systems. Additionally, the study
explored harnessing bioenergy from petrochemical wastewater to reduce pollutants while
facilitating bioenergy generation. Challenges such as slower processing times and high operational
costs make these technologies undesirable.
Cost-effective and environmentally friendly adsorbents have recently been the focus of
research to cut costs and minimize negative environmental impacts. Employing these adsorbents
for treating oily wastewater provides economic advantages, diminishes environmental harm,
guarantees adherence to regulations, and boosts sustainability efforts. Therefore, the aim of this
research project was to develop a cost-effective, eco-friendly adsorbent that will overcome the
drawbacks of the abovementioned techniques in treating petrochemical wastewater.
To achieve the aim, the specific objectives of this work were:
(1) To determine the treatability of oily wastewater using an adsorbent based on walnut shells
(WS);
(2) To modify WS as adsorbents and determine their adsorption potential; and
(3) To identify the adsorption mechanism by examining the appropriate model.
The study focuses on a two-step modification of WS to create an effective adsorbent. By
optimizing the modification process, the study aims to enhance the oil removal efficiency of WS,
offering a sustainable and eco-friendly remedy for wastewater management. The various

3

experimental parameters for the study were (1) adsorbent dosage (AD), (2) initial oil concentration
(IC), (3) reaction time (RT), and (4) salinity. Some crucial questions the study seeks to answer are
(1) What modifications to WS most effectively boost their oil adsorption capability in wastewater
treatment, (2) What are the comparative advantages of modified WS compared to other adsorbents
in the treatment of oily wastewater, and (3) What are the environmental and economic advantages
of utilizing WS-based adsorbents for wastewater treatment in the oil and gas sectors. The study is
crucial for developing a cost-efficient, environmentally sustainable adsorbent derived from WS to
treat oily wastewater. It enhances resource optimization by converting agricultural waste into a
practical solution and fosters advances in sustainable water treatment.
1.3.

Organization of thesis
This thesis is structured into five chapters. Chapter 1, the current chapter, introduces the

background of marine oil spills, detailing their major contributors and the conventional methods
used to manage these spills, along with the limitations of these techniques. An overview of
adsorption is provided, followed by the problem definition and the research objectives.
Additionally, the chapter outlines the overall structure of the thesis.
Chapter 2 delivers an in-depth literature review of recent advancements in wastewater
treatment technologies, with a particular focus on oily wastewater treatment through adsorption.
The chapter discusses various adsorbents, the key factors influencing the adsorption process, and
identifies research gaps requiring further exploration.
Chapter 3 describes the experimental setup, including the procedures for adsorbent
development, oily wastewater preparation, and the materials and equipment used. It also details
the experimental design and explains the rationale behind the chosen methodology.

4

Chapter 4 presents the results related to adsorbent development and evaluates the removal
efficiencies of the adsorbents. It includes a characterization of the surface morphology and
chemical composition of the materials, as well as findings on the optimal adsorbent dosage and
the outcomes of adsorption isotherm studies.
Chapter 5 summarizes the research findings, emphasizing the potential advantages of
developing environmentally friendly adsorbents. It concludes with recommendations for future
research directions. The references supporting this thesis are listed at the end.

5

Chapter 2 Literature Review
2.1. Characteristics of petroleum wastewater
Oil has elevated concentrations of biochemical oxygen demand (BOD) and chemical oxygen
demand (COD), hydrocarbons, heavy metals, volatile organic compounds, oil and grease
constituents, total dissolved salts, ammonia, nitrates, sulfides, waste catalysts, greasy sludge, and
other unwanted substances [26]. Petroleum effluent is a harmful pollutant to the environment since
it contains large quantities of polycyclic aromatics, which are highly poisonous and act as
contaminants in different quantities [27, 28]. Petroleum aliphatic hydrocarbons (up to C10) and
well-known aromatic compounds, including benzene, toluene, and ethylbenzene, were found to be
the primary constituents of the organic contaminants in petroleum refinery effluent [29].
Asphaltenes and substances with oxygen, nitrogen, and sulfur are other forms of hydrocarbons that
can be found in petroleum wastes. Generally, aromatic and aliphatic molecules in oily wastewater
often account for up to 75% of petroleum hydrocarbons, with trace metals, oxygenated
compounds, heterocyclic compounds, halogenated compounds, etc., making up the remaining 25%
[26, 30, 31]. Table 2.1 below presents the characteristics of petroleum wastewater.
Table 2.1: Characteristics of petroleum wastewater
Parameters References
[32]
COD (mg/L) 500-

[33]

[34]

[35]

[36]

[37]

[38]

[39]

21000

7896

1343

450

80-120

364

800

1000
Sources

Industrial processes (spills, leaks, runoff from roads), Surfactants from cleaning
operations, Chemical additives and emulsifiers from industrial processes, Textile
industries (chemicals and dyes)

BOD (mg/L) -

8000

3378

846

6

174

40.25

-

350

Sources

Agricultural runoff (from plant debris and organic fertilizers), Biodegradable
lubricants, Organic residues from food processing (sugars, starches, proteins),
Waste from biological treatment processes, Organic compounds (alcohols,
phenols, ketones)

Total

90-300

2580

-

74

150

22.8

105

100

suspended
solids (TSS)
Sources

Oil and grease in particulate form, Industrial processes (particles from machining,
grinding, cutting), Urban runoff (including debris, litter, and sediment from roads
and construction sites), Natural sources (erosion, organic matter decomposition,
sedimentation)

pH

6.5-6.8

2.5

8.48

7.0

6.7

8.0

7.6

6.5-8.5

Sources

Chemical reactions (organic matter oxidation, hydrolysis of compounds),
Corrosion and leaching, Cleaning agents and detergents, Industrial processes
(metal cleaning, plating, processing operations), Atmospheric deposition

Oil

& 400-

Grease

1140

-

240

870

-

946

3000

1000

(mg/L)
Sources

Oil and gas production (oil drilling, extraction and refining operations), Industrial
processes (manufacturing, metalworking, food processing, and chemical
production), Agricultural activities (the use of lubricants, hydraulic fluids, and
machinery that can leak or spill oil), Marine activities (Ships, boats, and offshore
platforms can release oil and grease into water bodies), Vehicle maintenance (Auto
repair shops, car wash facilities, and gas stations can produce oily wastewater from
vehicle maintenance activities, including oil changes, engine cleaning, and fuel
spills)

Total

3000-

dissolved

5000

37000

-

-

solids (TDS)
(mg/L)

7

-

-

-

-

Sources

Agricultural runoff (use of Fertilizers, pesticides, and herbicides), Mining
operations (dissolution of minerals and metals from ore processing and waste rock
disposal), Industrial processes (use of chemicals and solvents), Oil and gas
production (oil and gas exploration, drilling, and production), Municipal sources
(use of detergents, cleaning agents, and personal care products in households)

2.2.

Human and environmental health impacts of petroleum wastewater
There are various possible entry points for oily wastewater into surface and groundwater,

including pipeline or tanker truck accidents, wastewater storage pond or tank leaks at well pads or
dumpsites, and subsurface fluid migration through damaged well casings [40]. The petroleum
industry discharges substantial volumes of pollutants into the environment, including
hydrocarbons, heavy metals, phenols, and other hazardous compounds, through wastewater [30].
Small or large amounts of crude oil or refined oils, such as gasoline or diesel, have the potential to
contaminate water bodies and coastal regions, leading to an oil spill, a form of pollution. This
occurs due to harmful compounds in the leaked oil, posing significant health risks to oil workers,
residents, marine life, animals, and the overall habitat in both the short and long term.
Oil pollution rapidly spreads across the water's surface, covering extensive areas and
forming an air barrier threatening aquatic life. Oily wastewater further damages marine habitats
due to substances such as `ammonia and hydrogen sulfide, which reduce dissolved oxygen levels
in water bodies to levels insufficient for the survival of aquatic life. [41, 42]. The inevitable
outcome is the wide-ranging death of aquatic organisms when the dissolved oxygen threshold limit
falls below 2 mg/L [43]. Oil spills in marine environments could hinder the exchange of gases
between phytoplankton and the ocean, reducing dissolved oxygen levels. This, in turn, could
trigger significant fluctuations in oxygen levels and temperatures across various components of
the sea, potentially resulting in the death of phytoplankton and weakening the aquatic food chain
8

[44]. The aromatic substances released during oil spills can modify hatched fish, accumulating
carcinogens in fish or shellfish. This diminishes aquatic organisms' economic value and threatens
human health [45]. Moreover, wetlands and mangroves suffer harm when spilled oil infiltrates
coastal wetlands and protected marine life zones, reducing biodiversity [46]. In addition to
inflicting severe damage on the natural aquatic environment and ecosystems in coastal regions, oil
spills threaten crew members' lives and result in substantial economic losses and the destruction
of resources [47, 48]. Furthermore, hydrocarbon contamination in marine environments leads to
the extinction of fish, birds, and various marine species, resulting in a dramatic decline in aquatic
resources. The consequences of the spills become notably more severe for operators if the oil leak
results in flames or explosions [49]. Oil spills, therefore, need to be cleaned up immediately after
the spill [50].
2.3.

Treatment technologies
Different physical, chemical, and biological technologies have been employed for oily

wastewater treatment. These technologies are discussed below.
2.3.1. Physical treatment methods
Physical techniques for treating oily wastewater include gravity separation, dissolved air
flotation (DAF), and centrifugation. Physical separation is the primary technology for oily
wastewater treatment in petroleum refineries [51]. Gravity separation, which uses the density
difference between oil and water, is the most fundamental technique for treatment. A higher
contrast improves the separation effect in density between the oil and water [52]. Gravity
separation is the first separation step for dispersed and floating oil. In the initial treatment stages,
gravity separation is effective in separating oil and water for compliance with environmental
regulations. Widely employed in extensive facilities, the gravity separation approach is flexible

9

and is frequently integrated with other treatments to attain optimal outcomes in the oil and gas
sector. However, it is unsuitable for separating emulsified oil because it has several disadvantages:
it requires ample space, has limited separation capacity, is challenging to operate and manage, and
provides poor treatment results [53, 54].
The DAF technique is well-recognized for treating oily wastewater. In DAF, pressurized
air is applied to the bottom of an open basin, where the air bubbles rise to the top, carrying
contaminants [55]. The treatment of oily wastewater has considerable potential since flotation
devices have a high processing capacity, produce less sludge, and enhance separation efficiency
[56]. Chemicals that encourage coagulation and enhance floc size for simple separation are
commonly used in DAF units. At this stage, heterogeneous components of the effluent, such as
suspended solids, colloids, dispersion, and immiscible liquids, are eliminated [53, 57].
Conventional DAF produces microbubbles between 20 and 100 microns in size. Oil droplets are
attracted to the microbubbles, which increase the buoyant properties of the droplet and cause it to
rise. Pressure and air saturation in the wastewater control the DAF process. The pressure must be
brought down to atmospheric levels with excess dissolved gas for the microbubbles to form and
float to the system’s surface [54]. The DAF method can remove emulsified oil and produce treated
water of higher quality than the gravity separation method [58]. The technology requires fewer,
shallower treatment facilities and operates with improved surface loading [59]. However, the DAF
technology has a few drawbacks, including high operational costs (bubbles must be produced
continuously).
Centrifugation relies on centrifugal force to partition oil from water. Wastewater is
subjected to spinning within a centrifuge, prompting the denser oil to migrate outward, facilitating
its collection [24]. The primary benefits of centrifugation include less space and operation time

10

demands, making it well-suited for small workshops [60]. This approach excels at removing
minute oil droplets effectively. Contemporary centrifugation technology enables processing
substantial wastewater volumes at minimal operational expenses and with lower capital investment
than gravity settling requirements. Centrifugation has been extensively employed for separating
oil, water, and solids within oil sludge on a large scale. However, due to limitations in efficiently
separating highly viscous oil sludge, Li et al. [61] investigated a physical approach utilizing froth
flotation and centrifugation to improve oil and solid material separation at moderate temperatures.
The findings indicated that, under specific conditions (flotation duration of 25 min, an impeller
speed of 1580 rotations per min, sludge to flotation liquid ratio of 1:12, tap water as the flotation
liquid at 33°C), the oil recovery and the oil content of residue were 93.7% and 3.1%, respectively.
2.3.2. Chemical treatment methods
Chemical technologies have been commonly employed for the treatment of oily
wastewater. In chemical treatment, reactions involve oil particles, suspended sediments, COD, and
various contaminants. Coagulation (followed by flocculation, a physical method), adsorption, and
electrochemical technology are frequently used to treat oily wastewater [62, 63].
Coagulation and flocculation have also been applied for oily wastewater treatment.
Coagulation/flocculation is cost-effective and eco-friendly due to its simplicity, affordability, and
consistent effectiveness [64]. In oily wastewater treatment, coagulants initially work to counteract
the negative charges on oil particles, diminishing the electrostatic repulsion within the electric
double-layer, a process known as destabilization. Subsequently, these destabilized particles
undergo development and gradually combine into larger clusters known as flocs. These flocs are
then separated from the water phase through free precipitation or air flotation [65]. Coagulants
commonly utilized in treating oily wastewater include inorganic flocculants such as aluminum

11

sulfate, poly aluminum chloride, and polymeric ferric sulfate, as well as organic polymeric
coagulants such as polyacrylamide, polyethylene oxide, and polyvinyl alcohol [66]. Daud et al.
[67] utilized aluminum sulfate, poly aluminum chloride, ferric chloride, and ferric sulfate as
coagulants to determine the effectiveness of coagulation and flocculation for the removal of
suspended solids (SS), color, COD, and oil and grease from biodiesel wastewater, employing a
conventional jar test setup. The results showed that the effect of coagulant dosage on SS, color,
COD, and oil and grease removal showed a similar trend, and poly aluminum chloride was found
to be superior and was observed at a reasonably lower amount of coagulant (300 mg/L). The
effectiveness of oil removal was enhanced by as much as 99% by applying zinc silicate and anionic
polyacrylamide, with suspended particle concentrations reduced to below 5 mg/L [32]. These
results indicate that coagulation and flocculation is effective as a pre-treatment for treating
biodiesel wastewater.
Chemical oxidation is one of the most effective ways to remove COD and petroleum
hydrocarbons from oily wastewater. Chemical oxidation uses redox reactions to convert organic
molecules into readily biodegradable substances directly or indirectly. This intricate process,
called electrochemical oxidation, involves producing oxidants that oxidize the contaminants found
in oily wastewater after introducing an electric current [68]. This process results in the
mineralization of pollutants, decomposition, and the ease of creating airtight circulation without
secondary pollution [62, 69]. The successful application of chemical oxidation in oil treatment
involves using reactive substances to decompose contaminants. This approach efficiently treats
intricate mixtures, decreases organic content, and improves water quality. Nevertheless, challenges
arise, such as the costs associated with chemicals and the need for meticulous management to avert
environmental repercussions and guarantee safety.

12

The electrolysis method is another approach for treating oily wastewater, leading to a timedependent reduction in COD. This process involves three key stages: efficiently reducing
contaminants in the oily wastewater and cleaning. Initially, oil components are directly oxidized
at the electrode, facilitated by either the metal oxide or hydroxyl radicals at the electrode surface.
Subsequently, oil components are indirectly oxidized by newly generated intermediate oxidizing
agents. Finally, electro-flotation induces the aggregation of suspended oil droplets in the third step
[70-72]. Electrochemical catalysis offers an efficient method for treating oily wastewater [73]. Ma
and Wang designed a treatment technique for treating oily waste based on a pilot-scale system
with multiple cathodes and anodes [73]. The cathode contained iron and a catalyst with a large
surface area that contained noble metals, and the anode comprised active metal and graphite. By
adopting the electrochemical technique, they recorded a reduction in BOD and COD of about 90%
and a reduction in suspended solids of 99% [74]. The successful application of electrolysis in oil
treatment involves utilizing electric currents to degrade contaminants and improve oil and water
separation. This electrolysis method proves effective for emulsified oils, decreasing reliance on
chemicals and enabling precise control. Although there are challenges like energy consumption
and equipment maintenance, ongoing advancements are making these systems increasingly
practical.
One of the traditional methods available for marine oil spill clean-up is dispersant
application, a chemical agent introduced into the spill to break the oil into many tiny droplets using
sea energy [75]. These abundant tiny oil droplets typically persist within the water column and
disperse naturally. Once in the water column, processes such as biodegradation, dissolution, and
potential sedimentation act upon these minute oil droplets. Of these processes, biodegradation,
also called microbial degradation, is predominant [75]. Due to wave actions, Chemical dispersants

13

accelerate the speed and extent of oil dispersion and dilution [76]. These dispersants encompass
surfactants and active agents that reduce the interfacial tension between oil and water, breaking
down oil slicks into minuscule droplets, solvents, and other compounds. Dispersants facilitate the
formation of tiny oil droplets, enabling swift entrainment into the water column without
resurfacing, coalescing, or reaching coastal regions [77]. Surface dispersant applications are
considered particularly effective in eliminating floating oils, mainly when deployed in remote
areas, addressing significant spill incidents, and minimizing health risks to at-sea responders
compared to alternative response techniques. During the early 1960s, dispersants saw widespread
use in numerous countries. However, following the Torrey Canyon incident in 1967, where the
application of dispersants led to significant environmental harm, several nations have imposed
restrictions on their usage. Subsequently, researchers developed less hazardous chemical
dispersants, safer than those employed during the Torrey Canyon spill [78].
2.3.3. Biological treatment
Biological treatment, also known as bioremediation, is the process by which biological
agents such as microbes, biosurfactants, and enzymes convert complex organic pollutants into
simpler organic molecules [79]. Bacteria are the most active agents for degrading critical
petroleum hydrocarbon components, contributing significantly to the ability of the environment to
break down oil spills while producing the useful byproduct lipase [80]. Microbial degradation is
the foremost and highly efficient natural approach for treating petroleum hydrocarbon
contaminants from the environment [81, 82]. Their research delved into the substantial
biodegradation of alkyl aromatics within marine sediments before the noticeable biodegradation
of the crude oil's n-alkane profile [83]. They identified microorganisms as the agents behind the

14

degradation of alkyl aromatics. One of the crucial limitations to the biodegradation of oil pollutants
in the environment is the low accessibility of these pollutants to microorganisms.
Removing or breaking down petroleum hydrocarbon molecules from soil presents
challenges due to their capacity to adhere to soil components [84]. Microbial metabolism entails
harnessing the microbial decomposition of crude oil to convert organic substances into nutrients
for the microbial organic components or to propagate into additional microorganisms, thereby
eliminating organic matter from wastewater [85]. Ma et al. [86] isolated eight strains displaying
significant petroleum-degrading capabilities from surface water and mud of an oil terminal,
employing crude oil as a carbon source. Subsequently, they selected two highly effective
petroleum-degrading yeasts through enrichment culture. Following five days of mixed culture, the
petroleum degradation rate escalated to 51%. Using the activated sludge process, organic matter
is broken down by microorganisms using dissolved oxygen in the aeration tank [87]. In a recent
study, Uma et al. [88], explored distinct soil bacteria sourced from a hydrocarbon-contaminated
area, aiming to treat synthetic oily bilge water and produce biopolymer using the activated sludge
process. Their experiments revealed optimal organic removal (81%) and the highest
polyhydroxyalkanoate yields from cell dry mass (81%) when the substrate concentration reached
5000 mg/L of soluble COD.
Biofilm technology has also been applied for oily wastewater treatment. Due to its costeffectiveness, capability to handle high volume loads, absence of sludge, biological detoxification,
enhancement of wastewater biodegradability, generation of methane as a clean energy source, and
other advantages, the anaerobic biofilm reactor has been widely used [89, 90]. The moving bed
biofilm reactor (MBBR), sequential batch biofilm reactor (SBBR), up-flow anaerobic sludge bedanaerobic biological filter (UASB-BF), and microporous membrane bioreactor (MBR) have been

15

identified as some of the most popular technologies [91, 92]. Emadian et al [93] treated diluted
bilge water using a hybrid up-flow anaerobic sludge blanket bioreactor. Their findings
demonstrated that with 8 h of hydraulic retention time and a 0.6 g COD/d organic load rate, the
COD elimination rate was 75%.

Table 2.2: Oily wastewater treatment technologies
Technique
Dissolved
floatation
Microfiltration

air

Dynamic membrane

Source
of
oily
wastewater
Synthetic industrial
effluent
Prepared
oily
wastewater
Prepared
oily
wastewater
Synthetic
oilseawater mixtures
Synthetic
oilseawater mixtures

*Graphene
oxide
alginate aerogel
*Sodium alginate –
nano
fibrillated
cellulose
Electroflotation
Industrial
oily
wastewater
Electrocoagulation
Biodiesel wastewater

Treatment effect

References

>90% oil removal

[57]

99% oil removal

[94]

99% oil removal

[95]

90% oil removal

[96]

99.65% oil removal

[97]

99.71% oil removal

[98]

98.42% oil and grease
removal
Electrocoagulation
Biodiesel wastewater 97.8% oil and grease
removal
Electrochemical
Oily bilge wastewater 93.2% oil and grease
reactor
removal
Electrocoagulation
Oily bilge wastewater 95.6 ± 0.2% oil and
grease removal
Biological aerated Oilfield wastewater
84-90%
*PAHs
filter
before desalination
removal
Batch stirred tank
Oilfield produced
>60%
crude
oil
Bioreactor
Water
removal
Membrane
Synthetic petroleum
>97% hydrocarbon
sequential
batch Wastewater
removal
reactor
16

[99]
[100]
[101]
[102]
[103]
[104]
[105]

MBR and MBBR

Shipboard slop
wastewater

Pseudomonas strain

70-85%
of
total [106]
petroleum
hydrocarbon removal
oily 95 ± 1.5% oil removal [107]

Synthetic
wastewater
AGR
Brewery wastewater
Anoxic-oxic MBR
Refinery wastewater
Aerobic
granular Emulsified
diesel
reactors
wastewater
Microelectrolysis
Synthetic wastewater
using Fe-C fillers
containing diesel
Microelectrolysis
Ship sewage
using Fe-C fillers
Halomonas venusta Span-Twenkerosene/water
Bacillus subtilis
Waste crude oil
Pseudomonas
Distilled water
aeruginosa
Membrane
Synthetic oil-water
technology
emulsions
Membrane
Synthetic oil-water
technology
emulsion
*PAHs = polycyclic aromatic hydrocarbons

67.39% oil removal
96.6% oil removal
90.31% oil removal

[108]
[109]
[110]

81.4% oil removal

[111]

80.1% oil removal

[112]

92% oil removal

[113]

95% oil removal
85% oil removal

[114]
[115]

98.8% oil and grease [116]
removal
> 98% oil
[117]

2.4. Adsorption
Adsorption is a process of mass transfer wherein solutes or removable substances are
transferred from a fluid phase to the surface of a solid phase. It represents a primary mechanism
whereby molecules or ions adhere to a solid surface. This phenomenon allows for the accumulation
and concentration of substances through intermolecular interactions. Understanding the intricacies
of adsorption is crucial in various fields, from catalysis and environmental remediation to drug
delivery and gas separation.
There exist two forms of adsorption: chemical, also known as chemisorption, which occurs
due to chemical reactions between the adsorbent and the adsorbate, resulting in the formation of
covalent or ionic bonds; and physical adsorption, wherein the rise in adsorbate concentration at
17

the interface is driven by non-specific van der Waals forces, irrespective of the substance's nature.
Chemisorption is selective, typically irreversible, and involves a considerable heat impact ranging
from tens to hundreds of kJ/mol. On the other hand, physical adsorption is weakly specific and
reversible [118, 119]. Adsorbate migration typically occurs in three stages: (1) movement of the
adsorbate to the outer shell of the adsorbent, (2) diffusion within the pores of the adsorbent
material, and (3) the process of solute adsorption and desorption [120]. The adsorbate, adsorbent,
and matrix properties influence the speed at which these processes occur. Adsorption isotherms
are utilized to determine the maximum adsorption capacity of the material. The shape of an
adsorption isotherm is determined by plotting the number of molecules adsorbing per unit area of
the interface against either the gas pressure or the equilibrium concentration of the liquid solution
[121]. The most commonly utilized models for assessing pollutant adsorption include the
Langmuir and Freundlich isotherms [122].
Desorption, or the transfer of adsorbate ions from the adsorbent surface to the solution,
frequently occurs in conjunction with adsorption. One can assess the reversibility of adsorption
based on the amount of adsorbate that is desorbed from the absorbent: the reversibility of the
adsorption process increases with the amount of desorbed adsorbate. [123, 124].
Two main approaches to running the adsorptive processes are batch flow and continuous
flow methods. Batch adsorption is frequently employed on a laboratory pilot size to treat minute
quantities. These systems consist of a batch reactor with an adsorbent colloidally suspended in an
adsorbate combined over a predetermined period. Until equilibrium is attained, the effects of
variables such as temperature, adsorbate concentration, amount of adsorbent, agitation speed, and
adsorbent particle size are also assessed [125]. In general, industrial-scale processes employ
continuous adsorption systems. The fixed bed column of continuous flow is the most widely

18

utilized approach for adsorbate and adsorbent interaction in this approach. Studies of continuous
adsorption systems assess the effects of variables on column adsorptive processes, including initial
adsorbate concentration, the flow of adsorbate, column bed height, pH, adsorbent particle size,
system temperature, breaking points, and exhaustion [126].
2.4.1. Wastewater treatment by adsorption
Adsorption is considered a highly effective approach for wastewater treatment due to
numerous qualities that render it more efficient than similar methods [127]. The United States
Environmental Protection Agency (USEPA) acknowledges the adsorption process as one of the
finest and most remarkable wastewater treatment methods, citing its remarkable removal
efficiency, which can attain up to 99.9% [128]. Removing dye from aqueous solutions is an
everyday use for this separation method. Adsorption is particularly useful in the textile, leather,
dyeing, cosmetics, plastics, food, and paper industries since water recovery is crucial. To remove
methylene blue dye from an aqueous solution, fly ash and red mud underwent chemical and heat
treatment of 800 °C overnight [129]. The findings indicated that subjecting fly ash and red mud to
heat treatment reduced their adsorption capacity. Conversely, acid treatment using HNO3 had
varying effects on both materials: it enhanced the adsorption capacity of the fly ash while
diminishing that of red mud. In addition, Shi et al. [130] grafted sunflower stalks chemically with
quaternary ammonium groups to enhance their adsorption capacity. The findings revealed that the
maximum adsorption capacities for two dyes Congo red and direct blues on modified sunflower
stalks were reported as 191 mg/g and 216 mg/g, respectively. These values were at least four times
higher than those observed on the untreated sunflower stalks.
Extensive research on algal biochar as a bio-sorbent has been demonstrated to remove
heavy metals and other pollutants [131]. Algal biochar has superior bio-sorption abilities. As a

19

result, it is considered suitable for wastewater treatment applications such as eliminating different
inorganic, ammonium-N, and organic contaminants from waste effluents [132]. For example, Cole
employed biochar in 2017 to isolate dissolved phosphorous and nitrogen from municipal
wastewater, while Jung used biochar to sift phosphate ions in 2016 [133]. Several traditional
methods are available for removing heavy metals from water, such as chemical precipitation,
solvent extraction, membrane filtration, ion exchange, electrochemical removal, and coagulation.
However, these methods have drawbacks, including limited efficiency, delicate working
conditions, toxic sludge availability, high energy consumption, inadequate removal, and expensive
disposal [134, 135]. Since adsorption tends to have the most significant impact on the transport,
toxicity, and biological availability of heavy metals in aqueous environments (mainly at their trace
concentrations) and is simple to use and affordable, it has been widely employed [136-138]. In the
sorption of the binary components Cu2+ and Pb2+, Kongsuwan et al. [139] studied the use of
activated carbon produced from eucalyptus bark. Adsorption was found to be the primary
mechanism for the uptake of both heavy metals, with the maximum adsorption capacities for Cu 2+
and Pb2+ being 0.45 and 0.53 mmol/g, respectively. Božić et al. [140] used sawdust from beech,
linden, and poplar trees as adsorbents to remove heavy metal ions from synthetic solutions. The
maximum adsorption capacity (7 – 8 mg/g) was attained between a pH of 3.5 and 5.0. As a result,
adsorption-based wastewater treatment presents a potentially effective way to combat water
contamination. Many industries have found success in the effective removal of pollutants using
adsorbent materials.

20

2.4.2. Factors affecting adsorption
Several factors influence the effectiveness of adsorption processes. These factors are
outlined below. Understanding these factors is essential to optimize adsorption processes and
create effective treatment plans.
2.4.2.1. Effect of pH
The pH of a solution significantly influences the efficiency of adsorption. The pH level
impacts the ionization of adsorbates and their surface charge, affecting how well they interact
[141]; and, this influences the effectiveness and the adsorbent's capacity. Moreover, the pH of the
solution can modify both the adsorbent and the chemical composition of the solution. It also
impacts the surface charge and ionization degree of the adsorbed ions [142, 143]. Cruz-Lopes et
al. [144] found that the quantity of lead adsorbed onto soils increased as the pH increased. They
proposed that this enhancement in adsorption resulted from a reduction in H+ concentration and
an elevation in negative charges due to the deprotonation of OH˗ groups. Changes in pH were
recognized to exert different effects on adsorption, contingent upon the pollutant type and the
functional groups on the adsorbent surface. For instance, prior research indicates that the
elimination of oil from produced wastewater using castor oil and modified castor oil can be
influenced by pH variations [145]. Adsorption is more effective at lower pH because as pH rises,
more organic compounds are removed from produced wastewater, which increases competition
with other ions for available adsorption sites. A solution's pH strongly influences the efficiency of
adsorption processes. Surface charges can be managed to improve adsorption efficiency by
understanding and maximizing the pH of the solution, which opens the door for more effective
adsorption-based treatments and environmental remediation methods.

21

2.4.2.2. Effect of initial oil concentration
The oil concentration influences the overall adsorption efficiency, which also impacts the
distribution of oil molecules in equilibrium and the available adsorption sites [146]. Understanding
this relationship is essential for developing efficient oil removal strategies and improving
adsorption-based treatment techniques. Since adsorption is a mass transfer phenomenon, the
concentration of the pollutants is crucial because it affects the force that drives the contaminants
out of the solution and onto the adsorption site. Investigating the effectiveness of coconut coiractivated carbon in cleaning up oil spills through adsorption, it was observed that raising the initial
concentration of crude oil enhanced the adsorption capacity but reduced the percentage of crude
oil removed [146]. This outcome stemmed from the presence of active sites on the surface of the
adsorbent. A low initial crude oil concentration of 3888 mg/L and a heightened ratio of surfaceactive sites facilitated more significant interaction between natural oil molecules and the adsorbent,
resulting in the occupation of more active sites on its surface. Consequently, this led to a substantial
adsorption capacity of 971.925 mg/g and an impressive percentage removal of crude oil amounting
to 99.99228%. Another study found that the percentage of oil removed decreased from 96% to
84.6% when the initial oil concentration decreased from 276 mg/L to 103 mg/L [128]. This trend
can be attributed to the significance of external mass transfer in the process. As the initial
concentration of oil increases, the adsorption of the pollutant accelerates onto the surface.
However, Ewis et al. [147] observed a negative correlation between initial oil concentration and
the amount of emulsified oil removed from produced water, with the amount of pollutant removed
decreasing as initial oil concentration increased. The oil concentration at the beginning of the
adsorption process profoundly influences its effectiveness, emphasizing the importance of
optimizing initial conditions for efficient oil removal in environmental applications.

22

2.4.2.3. Effect of adsorbent dosage
The quantity of adsorbent used is critical in determining its capacity to adsorb a certain
amount of substance under given operational conditions. Research findings indicate that there is a
direct correlation between adsorption capacity and dosage [148, 149]. This suggests that as the
dosage increases, the adsorption capacity also rises. This can be attributed to the expansion of the
adsorbent's surface area and the availability of more sites for adsorption [150]. In their study of
the crystal violet dye adsorption by Acacia nilotica sawdust-activated carbon, Gupta and Lataye
[151] found that the percentage of dye removal increases as the adsorbent dosage increases up to
8 g/L. Moreover, the removal percentage increases only marginally when the adsorbent dosage is
raised from 8 to 20 g/L. Beyond this point, dye removal stays relatively constant and is not
influenced by further increases in the adsorbent dose. It has been reported that increasing the
adsorbent dosage from 0.05 g to 0.1 g increased the percentage of oil removal from 18% to 28.4%
[147]. Similar outcomes were attained by utilizing walnut shells to adsorb organic material [152],
where an increase in dosage from 0.5 g to 3 g increased the oil recovery percentage from 39.6%
to 62.3%. However, contradictory findings were reported from other studies, with their results
indicating that the adsorbent dose increase led to a reduction in the adsorption capacity [153, 154].
This was attributed to the saturation of the adsorbent surface, the clustering of adsorbent particles
at higher dosages, and the electrostatic repulsion between the adsorbate and the functional groups
on the surface of the adsorbent [155].
2.4.2.4. Effect of temperature
In environmental remediation and separation approaches, the impact of temperature on
adsorption processes is a crucial aspect to consider. Understanding the thermodynamics of the
process and improving efficiency can benefit from looking at how temperature affects adsorption.

23

The adsorption process mainly depends on temperature since it alters the adsorption capacity of
the adsorbent [156]. The degree of adsorption increases with temperature if the adsorption process
is endothermic [157]. This might result in the molecules becoming more mobile and adsorption
active sites as the temperature rises. Conversely, when the adsorption process is exothermic, an
increase in temperature leads to a decrease in adsorption. This phenomenon occurs because as the
temperature increases, the adsorption forces between the adsorbate species and the active sites on
the adsorbent surface weaken, thereby reducing the extent of adsorption [158]. The research on
the use of Imperata cylindrica for oil adsorption revealed that the adsorption capacity increased as
the temperature increased from 20 to 60 °C [159]. Similarly, a rise in temperature from 20 °C to
70 °C led to an increase in the amount of lithium adsorbed, which went from 12.5 mg/g to 17 mg/g
[160]. Additionally, crude oil adsorption capacity and the percentage removal increased in direct
proportion to temperature [146]. This increase was explained by reinforcing the physical bond
between the active sites of crude oil molecules and the adsorbent, which occurs as hydrogen bonds
and van der Waals forces diminish at higher temperatures. Moreover, the decrease in solution
viscosity and the increase in crude oil's solubility in water contributed to the heightened diffusion
rate of crude oil molecules through the external boundary layer and the internal pores of the
adsorbent particles as temperature increased [161, 162].
2.5.

Adsorbents for wastewater treatment
Various adsorbents and their mechanisms have been extensively studied in the literature for

wastewater treatment. These encompass a range of materials such as natural substances, industrial
and agricultural residues, waste products, biomass materials, and more. When selecting an
adsorbent for pollutant removal, crucial considerations include the concentration and nature of
pollutants in the wastewater, the adsorbent's capacity for adsorption, and its effectiveness [163].

24

Adsorbents should also be easily regenerated, affordable, non-toxic, and readily available.
Adsorbents for oily wastewater treatment are described below.
2.5.1. Natural adsorbents
Many naturally occurring substances have similar characteristics to those of an adsorbent.
Although plentiful, these materials have demonstrated successful applications in removing various
pollutants, such as heavy metal ions, organic compounds, and colorants from wastewater. Such
materials include chitin, clay, zeolite, peat moss, coal, and wood [164]. Zeolites are structured
crystalline aluminosilicates with microporous characteristics and a well-organized pore structure.
These materials find widespread use in environmental protection endeavors, particularly in
adsorbing volatile organic compounds and heavy metal ions. Their adaptability in pore size,
notable hydrophobic properties, excellent thermal stability, and ease of surface modification
contribute to their effectiveness in various environmental applications [165, 166]. It was found
that zeolites typically exhibit an average specific surface area of 800 m2/g, with a maximum of up
to 1769 m2/g. The average pore volume of zeolites is approximately 0.66 cm 3/g, reaching a
maximum of 1.02 cm3/g. Regarding adsorption capacity, the average saturated toluene adsorption
on zeolites is around 137.2 mg/g, with a maximum capacity of up to 288.4 mg/g [167].
Chitin and chitosan, biopolymers used in fiber production, have gained notable recognition
for their unique characteristics and decomposing ability. These fibers exhibit superior removal
efficiency and strength compared to activated carbon, garnering considerable attention in various
applications [168]. Chitin, a naturally sourced renewable substance, holds promise for multiple
industrial applications, including its potential as an adsorbent in sectors such as wastewater
treatment [169]. Chitosan has garnered significant interest as a potent biosorbent due to its costeffectiveness compared to activated carbon. Its abundant amino and hydroxyl functional groups

25

contribute to a robust adsorption capacity for various aquatic contaminants [170, 171]. This
biopolymer presents an attractive alternative to current biomaterials due to its physicochemical
attributes, chemical stability, heightened reactivity, remarkable chelation characteristics, and
superior selectivity towards pollutants [172]. Natural chitosan has been subjected to numerous
physical and chemical alterations to enhance its capacity to bind diverse contaminants. Various
forms of chitosan, such as membranes, microspheres, gel beads, and films, have been produced
and assessed for their effectiveness in treating wastewater. Previous studies extensively detail the
structures of chitin and chitosan, and the diverse methods employed for their physical and chemical
modifications [172-175].
The use of clay minerals as alternative inorganic adsorbents offers many benefits. The
colloidal characteristics and negative charge layers enhance their ability to adsorb cations and
organic compounds [176]. Clay-based adsorbents are a relatively new group recognized for their
catalytic properties [177, 178]. Clay-based adsorbents offer versatility in adsorbing various
contaminants, and various modification techniques can tailor them for selective sorption. These
benefits encompass their widespread availability, ease of application, simple recoverability, costeffectiveness, non-toxic nature, and suitability for a broad spectrum of contaminants [179]. Clay
minerals, characterized by their significant surface area and robust sorption capacity, demonstrate
excellent structural and chemical resilience when adsorbing organic and inorganic contaminants.
The surface charge chemistry of clay directly influences the adsorption mechanism. Due to the
mechanism of surface sorption, clay has received increased attention as an adsorbent for the
treatment of different wastewater streams. Additionally, several clays, including bentonite [180],
kaolinite [181], illite [182], sepiolite [183], montmorillonite [184], and pyrophyllite [185], are
effective at removing pollutants. Several mechanisms, such as physical adsorption, micro-

26

precipitation involving ion exchange at permanently charged sites, and the formation of complexes
at edge sites with hydroxyl groups, along with chemisorption, have been documented for the
sorption of metal ions by clay minerals [186, 187].
Wood is considered a non-renewable, inexpensive natural adsorbent that can be disposed
of by burning [188]. Recently, there has been exploration into using wood across various energy
storage domains, solar steam generation, and wastewater treatment. Wood, composed of cellulose
microfibrils within a porous lignin matrix, capitalizes on its hierarchical and porous structure for
these applications [189]. Wood was studied as an adsorbent for the removal of telon blue by Poots
et al. [188] to develop a cost-effective adsorbent. The adsorbent was used after being sieved into
various size ranges and investigated without pretreatment. The kinetics of the process was shown
to be particle size dependent, with small particle sizes of 150 – 250 µm having the lowest kinetics
(>3h) and big particle sizes of 710-1000 µm having the highest kinetics (>6h). A long contact
period of 8 h, as well as a low adsorption capacity, were observed in this study. Additionally, the
same wood adsorbent was employed to remove another dye (basic), astrazone blue [190].
However, it was shown that for this system, equilibrium adsorption required a shorter contact time
of 2 h and had a greater adsorption capacity of 100.1 mgg-1 for particles between 150 and 250 µm.
2.5.2. Agricultural waste
Several investigations have shifted attention towards utilizing agricultural waste materials,
including stone fruits, nuts and husks, peels, wood and forest residues, and plants, as cost-effective
and eco-friendly biosorption materials for wastewater treatment. These materials are favored due
to their unique chemical composition, widespread availability, renewable nature, and affordability
[23, 191-193]. The primary constituents of these agricultural wastes consist of cellulose,
hemicelluloses, and lignin. However, they may contain additional functional groups such as

27

hydroxyl, aldehyde, carbonyl, carboxyl, phenolic, and ether. These functional groups can interact
with pollutants present in wastewater through diverse specific binding mechanisms and
interactions [194, 195]. They can serve as appealing alternatives to traditional adsorbents [196].
Nevertheless, compared to synthetic commercial adsorbents, raw agricultural wastes often
have lower biosorption capabilities. Consequently, different pre-treatment methods have been
utilized to augment the sorption properties of raw biomass, thereby enhancing their effectiveness
and applicability for wastewater treatment [197]. These operations primarily consist of physical
and chemical processes involving organic compounds, oxidizing agents, bases, organic and
mineral acids, and organic compounds [198]. Chemical agents such as calcium hydroxide, sodium
carbonate, sodium hydroxide, nitric acid, hydrochloric acid, sulfuric acid, tartaric acid, citric acid,
formaldehyde, methanol, and hydrogen peroxide are commonly employed to enhance the pollutant
adsorption capabilities of sorbents [199]. McKay et al. [200] studied the effectiveness of five lowcost adsorbents, including rice husk, bark, cotton, hair, and coal, for the adsorption of two cationic
dyes (safranine and methylene blue). All the adsorbents under study, except for coal, had more
excellent safranine monolayer adsorption capabilities than methylene blue.
In another study, barley straw treated with sodium hydroxide and the cationic surfactant
hexadecyl pyridinium chloride monohydrate (CPC) was used to remove two anionic dyes from
aqueous solution, Acid Blue (AB40) and Reactive Blue 4 (RB4) [201]. With a good removal % of
AB40 and RB4, the CPC was seen to be firmly bonded to the surface of the straw. Although the
desorption process was found to be generally ineffective, the adsorption of these dyes on modified
straw was favorable in very acidic conditions. Numerous plant parts, including plant bark, coconut
fiber, coconut shell, pine needles, neem leaf powder, and cactus leaves, have been tested for their
ability to remove chromium and have demonstrated an effectiveness of more than 90% at the ideal

28

pH level [202]. It was indicated that chromium may be removed from wastewater using natural
and modified forms of sugarcane bagasse [166]. To effectively remove copper ions from
wastewater, rice bran, rice straws, and soybean hulls are treated with citric acid [203].
2.5.3. Industrial waste
Industrial waste is one of the inexpensive adsorbents for removing heavy metals from
wastewater [204]. It only needs a minimal amount of processing to boost its adsorptive ability.
Industrial waste materials are affordable because they are widely accessible locally in huge
quantities. The feasibility of employing various industrial by-products, including fly ash, blast
furnace sludge, waste slurry, lignin, iron (III) hydroxide, and red mud, has been studied for the
removal of heavy metals, organic compounds, and dyes from wastewater. The metal hydroxide
sludge from industrial effluent from electroplating was used to create adsorbents for removing azoreactive dyes [205, 206]. For azo reactive anionic dye, these adsorbents containing many metal
hydroxides with positive charges had a maximum adsorption capacity of 48 – 62 mg/g. Fly ash is
a byproduct of combustion primarily used to manufacture cement, bricks, and other building
materials. Due to its high alumina and silica percentage and suitability for adsorption, fly ash is a
cheap adsorbent [207]. Fly ash has been found to have a porous surface with tiny, irregularly
shaped, glassy spherical particles. The features of fly ash have been altered to make it more
adsorption-friendly [208]. Using fly ash from a heat and power plant and oxides with a SiO2/Al2O3
oxide composition over 2.4, class "F" fly ash was generated [209]. The adsorbent exhibited
remarkable performance and created a new substrate with high adsorption capacity for heavy
metals in multi-cation wastewater treatment. Disposing of bagasse fly ash, a waste product from
the sugar industry, poses a persistent challenge. Previously used primarily as a filler in construction
materials, bagasse fly ash has recently been repurposed into an efficient adsorbent to remove

29

harmful chemicals [210]. Bagasse has recently demonstrated its use in eliminating zinc and other
metals from aqueous solutions [211].
2.5.4. Nano-adsorbents
Nano-adsorbents have been effectively developed via nanotechnology for wastewater
treatment, which has become a rapidly expanding and fascinating field of research [212]. Nanoadsorbents possess diverse physical and chemical characteristics, encompassing high surface area,
chemical reactivity, conductivity, catalytic activity, selectivity, and magnetic and optical
properties. Because of the vast surface area, there are more active sites where different
contaminants interact with nano-adsorbents. Due to these characteristics, they have more excellent
adsorbent properties than their corresponding bulk materials [213-216]. In fixed bed columns, 16
metal oxide nanoparticles, including TiO2, NiO, Fe2O3, and ZrO2, were examined [217] for their
ability to remove arsenic. All nanoparticles displayed a removal capacity of about 98% except
ZrO2. The Freundlich adsorption isotherm provided good agreement with the adsorption results.
Some well-known nanomaterials employed in wastewater treatment include carbon nanotubes,
ordered mesoporous carbon, graphene, carbon nanofibers, and titania nanotubes and their
variations. Large surface areas cause a higher concentration of surface defects and make it easier
for contaminants to interact with the surface [218].
Numerous carbon-based nanomaterials have been generated due to the rising need for
nanotechnology [219]. Nanomaterials of carbon and graphene, like nanotubes, come in
functionalized and non-functionalized varieties [220]. Two crucial advantages result from the
alterations made to nanostructures made of carbon. Firstly, improvement in hydrophilicity
promotes dispersion in an aqueous medium, suggesting that better distribution provides a larger
surface area for exposure to contaminants or other undesirable species [221, 222]. The second

30

factor is an increase in the electrostatic interactions between the adsorbent and the adsorbate, which
increases adsorption capacity. If a negative charge functionalizes the surface, the electrostatic
interactions are the primary mechanism that drives the adsorption of particularly positively
charged species such as heavy metals [223]. Surface modification enhances the adsorption activity
of carbon-based nanomaterials by amplifying the presence of organic functional groups [224]. The
widely favored adsorption material is activated carbon, renowned for its outstanding physical and
chemical properties, including a vast specific surface area, many functional groups, chemical
stability, strong mechanical strength, and resilience against acids and alkalis [225, 226]. Activated
carbon has an average surface area of 1404 m2/g and a specific surface area ranging from 478 to
3167 m2/g [167]. The range of the pore volume, which runs from 0.41 to 1.75 cm3/g, is 0.72 cm3/g
on average. The average adsorption capacity for volatile organic compounds is 460.0 mg/g,
ranging from 170.0 to 737.0 mg/g. Non-zerovalent iron was impregnated on activated carbon and
utilized to adsorb the arsenic contaminant As (III) [227]. The nanocomposite removed roughly
1.997 mg/g in 2.0 mg/L of arsenic solution at a pH of 6.5. The authors suggested this adsorbent as
a prime candidate for treating drinking water contaminated with arsenic [228]. The ability of
coconut shell-activated carbon to remove the dye methylene blue with the help of magnetic
particles was examined [229]. The uptake of methylene blue hampered the encapsulated activated
carbon on regenerated cellulose beads and only reached complete adsorption after 90 min. The dye
removal efficiency of granular coconut shell activated carbon obtained 99.7% within 2 min. This
suggests that the dye took a lengthy time to enter the beads' interior [229].
Because of their unique one-dimensional structures, remarkable mechanical strength,
superior electronic properties, and wide-ranging applications, carbon nanotubes have emerged as
a captivating addition to the carbon family, attracting considerable research attention. They

31

encompass single-walled and multi-walled varieties, distinguished by their number of layers. The
exceptional thermal and chemical stability of carbon nanotubes has prompted their adoption as
carriers for catalysts [230], in addition to the potential for creating improved composites [231] and
nanoscale electrode devices [232]. Structurally, they are the perfect adsorption material due to
their enormous specific surface areas and hollow and layered structures. The adsorption
characteristics of carbon nanotubes for various toxic substances, including lead, cadmium, and
1,2-dichlorobenzene, were determined. The results indicate that carbon nanotubes are superb and
effective adsorbents for eliminating these hazardous substances from water [195]. The adsorption
of small molecules [233-235], heavy metal ions [236-238], radionuclides [239, 240], and organic
compounds [241, 242] on various carbon nanotubes (CNTs) (single-walled or multiwalled, closedor open-ended CNTs), has already been the subject of numerous experimental studies. From a
theoretical and practical perspective, the research of CNT adsorption properties is significant
because it may provide fresh insight into the adsorption mechanism in complex systems.
Effective adsorbents can be produced widely at a reasonable cost using functionalized
graphene [243]. This arises from the two-dimensional layer structure of the materials, their
expansive surface area, significant pore volume, and content of surface functional groups.
Inorganic nanoparticles help prevent the adsorbent from aggregating. Research has shown that the
equilibrium data were well suited to the Langmuir model, while the kinetic data followed a pseudosecond-order model [244]. Additionally, the graphene nanocomposite adsorbent of a monocomponent system had a maximum Cd2+ adsorption capacity of 91.29 mg g-1. Similar outcomes
for the maximal adsorption capacity for cadmium biosorption were previously obtained [245].

32

2.5.5. Bio-sorbent
Growing interest and research into using inexpensive adsorbents have led to several
successful applications of these affordable, naturally occurring, and easily accessible organic
adsorbents in treating industrial effluents using adsorption processes [246]. Biosorbents offer
numerous advantages, including affordability, high efficacy, reduced production of chemical or
biological sludge, the capability for biosorbent regeneration, and the potential for metal recovery
[247]. Bio-sorbents, as their name suggests, originate from biological origins. With specific
functional groups on their surfaces, these materials are inclined to attract and bind contaminants
[248]. Magnetic-activated carbon and magnetic biosorbents were created to remove lead, with
adsorption capabilities ranging from 11.6 mg/g to 35.7 mg/g. Pb2+, Cr6+, and Cu2+ removal
efficiencies on biosorbents modified with zero-valent iron reached 99.4%, 66.7%, and 99.1%,
respectively, on zero-valent iron-modified biosorbents through complexation with -OH/-COOH
functional groups or delocalized π electrons [212]. A potential technique for producing biosorbents with practical sorption capabilities has been shown in another work using modified walnut
shells. High inorganic acid concentration modifications increase the sorption capability. These biosorbents can also be employed in the low wastewater demineralization method [249].
Table 2.3: Adsorption performances of different adsorbents for wastewater treatment
Adsorbent
Activated
carbon
multi-walled
carbon
nanotubes
Graphene
TiO2
nanotubes/CNTs

Adsorbate

Adsorption
capacity
(mg/g)
Crystal violet 84.11
dye
Reactive Red 95-352.1
M-2BE
textile dye
Cd
106.3
Pb
192-588
33

Removal
efficiency
(%)
85-90

Optimal
pH

References

10.0

[250]

-

2.0

[251]

-

pH 6.0 ± 0.1 [252]
[253]

Few-layered
graphene oxide
(FGO)
Fe3O4
nanoparticles
Activated
carbon
Bentonite
Kaolin
Kaolinite
Montmorillonite
Natural
bentonite
Open burnt clay
Fungal
and
bacterial
biomass
Modified lemon
leaf
Citrus
limetta
residue
Modified biogas
residue
Waste-based
bio-sorbent
Eggshell waste
Modified
sawdust
Wheat straw
Fly
ash
(geopolymer)
Coal fly ash
Chromium
Leather waste

Pb(ii)

842-1850

-

7-8.1

[254]

Pd, Cd, Cu

369.0–523.6

98

7.0

[255]

Pb2+

294.1

-

-

[256]

Malachite
Green
Methylene
Blue
Methylene
Blue
Methylene
Blue
Diazo dye

-

>90

9.0

[257]

52.76

-

6.0

[258]

102.04

-

6.0-10.0

[259]

289.12

-

11.0

[260]

74.5

>95

10.0

[261]

Congo Red
VOCs

374-620

98
-

3.0
-

[262]
[263]

Cationic dye

36.10

70

5.0

[264]

Fluoride

12.6

74-76.5

4.0

[265]

Nitrate

64.12

>82

5.0-9.0

[266]

Fluoxetine

21.86–233.5

-

7.0-9.0

[267]

Cu
Cr (VI)

142.86
8.84

93.17
100

6.0
2.0

[268]
[269]

Cu2+
Anionic
surfactant
Anionic
surfactant
Anionic
surfactant

7.05
714.3

-

5.0
2.0

[270]
[271]

2.95

96

-

[272]

423

>90

4.0-8.0

[273]

34

2.6.

Oily wastewater treatment using adsorption.
A variety of treatment approaches, including physical methods (filtration, electrodialysis,

flotation, and adsorption), chemical techniques (precipitation, chemical oxidation, and
electrochemical technologies), and biological methods (activated sludge, aerated biological filters,
microbial capacitive desalination, and microalgae-based treatment) have been used for oily
wastewater treatment [274-277]. However, most of these techniques are either overly complicated,
expensive, or ineffective since they are inefficient at removing smaller oil droplets and emulsions
[278, 279]. The adsorptive methods have been utilized extensively because they have low
production costs and good selectivity, are applicable in low concentrations, and may be employed
in batch and continuous processes. Adsorbent materials can also be regenerated and used in
subsequent adsorption cycles [280, 281]. Hence, in recent years, sorption has emerged as one of
the most efficient methods for completely removing spilled oil under ambient conditions.
Adsorption is frequently employed as a secondary removal step after a crude oil removal method,
like skimming [282, 283]. However, the adsorbent parameters, such as surface area, pore size, pore
diameter, and functional groups, considerably impact the method's effectiveness.
To remove PAHs from wastewater, some carbon-based nanomaterials have caught the
interest of researchers. Specific properties of carbon-based materials, such as hydrophobicity,
electrical conductivity, high surface area, mechanical resistance, and low production costs, make
it easier to use them in adsorption techniques to treat effluents [284]. The removal of
dibenzothiophene from petroleum distillates through activated carbon in a continuously packed
bed column was explored [285]. In this study, activated carbon was used as an adsorbent in a
continuous mode to desulfurize model diesel that only included dibenzothiophene. According to
the findings, the adsorbent has an approximate 16 mg kg-1 adsorption capacity for adsorbate at a
35

flow rate of 3 ml/min and a dosage of 2.0 g. Furthermore, the Langmuir isotherm model was an
ideal fit for the adsorbent's adsorption behavior. Abdullahi et al. showed a simple method to
generate hydrophobic graphene oxide (GO) films functionalized with alkylamines that can be used
as sorbents to remove oil from water mixtures [286]. Through nucleophilic substitution and
amidation processes, GO and three distinct alkylamines (hexylamine, dodecyl amine, and
hexadecylamine) were combined to create the hydrophobic alkylamine-functionalized graphene
oxide sorbents. The water contact angle of the top-performing C16-FGO surface, which has been
functionalized

with

hexadecylamine,

was

141.08±1.65°,

demonstrating

exceptional

hydrophobicity. Additionally, the C16-FGO has shown great effectiveness in the separation of the
oil-water mixture as well as excellent performance in oil adsorption. Its reusability was evaluated
to demonstrate the applicability of the produced sorbent in more complicated situations. Most
recently, a heat-localization device based on reduced GO and silver nanoparticles was presented
by Wang's group for the adsorption of oil particles using a simple coating process [287]. The
adapted sensor exhibited photothermal properties and hydrophobicity, resulting in a notable solarsteam evaporation efficiency of 86.8% and outstanding adsorption capacity for organic solvents
and low-viscosity oils ranging from 54.0 to 123.0 g/g. The adsorbent decreased the oil’s viscosity
to achieve a high heavy crude oil adsorption capacity (68.0 g/g). Moreover, Zhuo and co-workers
introduced a hierarchical nano-adsorbent composed of carbon nanotubes. They reduced GO to
enhance the adsorption of heavy crude oil with the assistance of solar light heating [288]. The
developed rough and oleophilic surface-based adsorbent reduced the oil diffusion resistance to
speed up the adsorption of heavy crude oil via capillary force. Compared to other oil adsorbents
previously reported, the proposed adsorbent provided an exceptional efficiency of heavy crude oil
adsorption, up to 267 g g-1, within 10 min.

36

Different low-cost biowaste adsorbents such as tea waste [289, 290], palm tree leaves
[291], corn silk [292], oil palm fruit [293], teff straw [294], rice husk [295], banana peels [296],
rape straw [297], and mangosteen peels [298], have been used to remove different contaminants
from wastewater. These adsorbents have attracted attention because of recent progress in
modification methods, which create a large surface area and distinctive physicochemical and
biological characteristics [299-301]. For instance, sugarcane bagasse and surplus biomass from
rice straw were used as cost-effective adsorbents to successfully remove PAHs from oil industrial
effluent [302]. The findings suggested a considerable interaction between bio-adsorbents and
PAHs due to the non-spontaneous characteristics of the bio-adsorptive process, excellent affinity
for the adsorbate, and endothermic nature. Barley straw, an agricultural byproduct, was chemically
treated with a cationic surfactant known as hexadecylpyridinium chloride monohydrate (CPC) to
remove emulsified canola oil from an aqueous solution [303]. The inclusion of CPC was found to
render the surface of barley straw non-polar, thereby significantly enhancing the adsorption
capacity of surfactant-modified barley straw for oil removal from water. The kinetic investigation
showed that the equilibrium period was short, and the isotherm study showed that the Langmuir
model adequately described the oil adsorption. The Langmuir isotherm revealed an adsorption
capacity of 576.0±0.3 mg/g at 25 °C. Walnut shells possess distinctive qualities that render them
a viable and eco-friendly option as an adsorbent material. Their accessibility, affordability, and
impressive surface area make them well-suited for the filtration of contaminants in both water and
air [304]. Due to their high surface area and porous structure, they are capable of adsorbing oil
from wastewater. Recent studies have seen walnut shells undergo modification processes to
enhance their adsorptive properties. A hydrophilic sulfonic acid group attached to the surface of
walnut shells, creating a stable coating layer by chemically bonding it to the surface functional

37

group, was developed [305]. This resulted in a decrease in micropore size, an augmentation of
surface density, and an improvement in surface wettability, all while preserving its remarkable
super-hydrophilic underwater oleophobic characteristics and compressive strength [305]. The
results of this study revealed that the oil removal rate increased from 38.05% to 89.51% after the
modification of the walnut shells. Wood sawdust offers an abundant supply and a systematic
adsorbent suitable for various pollutants such as oil, heavy metals, and salts [306]. In recent
research, activated carbon (AC) was produced by synthesizing different agricultural waste
materials, such as bamboo and banana fibers, for oil spill treatment [307]. The study indicates that
BANA-AC achieved a maximum oil adsorption capacity of 4.418 g/g with an activation ratio of
7:1 (H3PO4) and a surface area (SA) of 2,172.234 m2/g. BAMB-AC (5:1 550 ℃) exhibited the
highest surface area at 2,760.447 m 2/g with a sorption capacity range of 3.3678 g/g. The findings
suggest that further surface modifications of synthesized AC from various biomass sources could
significantly enhance remediation efficiencies, given the correlation between SA and adsorption
capacity. This research introduces a promising approach for addressing oil spills and potentially
removing pharmaceuticals and heavy metals in wastewater treatment.
One technique used recently to treat wastewater is adsorption using clay. The substance
has a large surface area, is cheap, and is abundant. Clay also exhibits significant mass transfer rates
[308]. Clays have been subjected to physical and chemical alterations to produce materials for
practical applications. Clay's properties can be altered through adsorption and intercalation of
organic polymers. The modification process, known as pillarization, frequently involves
intercalating cationic species to act as pillars, supporting the mineral layers and creating a porous
material. This porous structure is advantageous for adsorbing organic molecules and other
environmental uses [309]. One type of clay (kaolin) and one clay mineral (montmorillonite) were

38

chosen and analyzed about transmembrane pressure and solution pH [310]. In addition, clay
isotherms and adsorption rates were studied to understand the properties and behavior of the hybrid
process. The findings revealed an 80% removal effectiveness for phenol and a 40 mg g-1 maximal
adsorption capacity.
Adsorptive procedures are promoted as an easy, quick, and effective PAH remediation
solution compared to other remediation technologies. For the removal of PAHs, a variety of
adsorbents can be utilized, including activated carbon [311], organo-bentonite [312], zeolites
[313], mesoporous materials based on silica [314], and nanomaterials based on graphene [315]. It
is crucial to emphasize that research into adsorption's kinetics, equilibrium, and thermodynamics
is required when creating an adsorbent material.

39

Table 2.4: Adsorption performances of various low-cost adsorbents for treating oily wastewater
Adsorbent

Adsorbate

Adsorption
capacity
(mg/g)

Rice husk

Naphthalene,

63.6,

phenanthrene,

and104.5

Treatment
efficiency
(%)

50.4, -

Optimal
pH

References

-

[316]

4.0

[317]

pyrene
Biochar

Pyrene

and 187.27

benzo(a)pyrene

and 93.5 and

80.00

87.9

Bamboo powder Petroleum ether

-

99.3

4.6

[318]

Bentonite

PAHs

-

88

7.5

[319]

Bentonite

Diesel oil

81.65

-

6.5

[320]

Iron

Diesel oil

54.04

67

6.5

[147]

Synthetic resins

Emulsified oil

11.86

>98

2.0-10.0

[321]

AC/Bentonite

Diesel oil

48

-

2.5-11.0

[322]

Montmorillonite Dodecane

25.84

66.02

-

[323]

Zeolite

Diesel oil

600

-

-

[324]

Sawdust

Heavy crude oil

4100-6400

-

7.0

[325]

Peat

Diesel oil

9000-12000

99.21

-

[326]

Fly ash

Diesel oil

700-900

-

-

[327]

Oxide/Bentonite

40

Raw

Posidonia Crude oil

5300

99.98

7.45

[328]

6400

95.5

7.0

[325]

4860

-

6.0-7.0

[329]

oceanica fiber
Sawdust
Coconut

Heavy crude oil
coir- Crude oil

activated
carbon
Banana peel

Gas oil

5310

-

-

[330]

Wheat Straw

Crude oil

6910

-

7.7 ± 0.1

[331]

Saw Dust

Crude oil

4500-8500

99.8

-

[332]

Barley Straw

Crude oil

7000-12000

-

-

[333]

Sawdust

Motor oil-water

28.32

-

-

[334]

Nanosilica

Gasoline-water

-

99

-

[335]

emulsion
Sepiolite

Crude oil

-

99

7.0

[336]

Bentonite

Crude oil

-

96

5.0

[337]

Date palm

Crude oil

1330-1425

-

-

[338]

Banana peel

Crude oil

194

̴100

10.0

[339]

Pomegranate

Crude oil

555

92

9.5

[340]

Activated date- Phenols

from 88

78

peel

pit

petroleum waste

41

[341]

Silica NPS

Oil saltwater

-

93

5.0

[342]

Alumina NPS

Oil saltwater

185.76

-

7.0

[343]

42

Chapter 3 Materials and Methods
3.1. Materials
3.1.1. Walnut shells (WS)
Ground WS is a durable, chemically inert, non-toxic, and biodegradable substance mainly
utilized as an abrasive. The shell constitutes a significant portion of the nut, almost 50%. It has
diverse applications, including as a component in cosmetics and dental goods, for polishing, and
as a filtration medium [304]. Their availability as a renewable resource can be considered an extra
benefit. The shell of the walnut fruit is a lignocellulosic agricultural byproduct consisting of
cellulose (18%), hemicellulose (36%), and lignin (37%) [344]. The proportion of each component
may vary slightly based on the walnut's source, plant genetics, the cultivation environment, and
the processing circumstances of the fruit. The primary components of WS closely resemble those
found in wood biomass, including lignin, cellulose, hemicellulose, and tannins, which can be
extracted and utilized to produce various chemicals and materials [345].
WS is an efficient filtration medium for separating crude oil from water [346]. Currently, WS
filtration processes generate water from oil fields, refinery effluent, direct spray, caster water from
steel mills, quench water from ethylene plants, copper concentrate decant, and cooling water [347].
WS exhibits exceptional surface properties for coalescence and filtration, along with outstanding
resistance to attrition. Research has been conducted to assess the efficacy of WS filters as a
substitute for traditional sand filters in removing free oils and suspended particles [347]. WS-based
adsorbents have been extensively investigated in various adsorption experiments, particularly for
extracting heavy metal ions. Moreover, they have undergone thorough testing to remove organic
substances, with methylene blue dye frequently employed as a benchmark representation [348]. In
a typical example, Ashrafi et al. [351] eliminated green and Congo red dyes from an aqueous

43

solution with WS modified with NaOH. The modified adsorbent exhibited a 2.3-fold increase in
specific surface area. Also, Zhou et al. utilized a commercial activated carbon generated from WS
to remove diazinon herbicides from aqueous solutions [349]. The activated carbon was
subsequently treated with phosphoric acid to enhance adsorption efficiency [349]. Although
research exists on using WS-based adsorbents as a substitute for traditional methods, the literature
lacks comprehensive data regarding their oil sorption capability and recovery. This study would
facilitate comparing and selecting a sorbent for a particular application. This work aims to assess
the oil sorption capacity of the proposed WS-based adsorbent.
Table 3.1: Main materials
Name of material

Specification

Source

Conventional heavy crude oil Ar

Natural

Sciences

and

Engineering

Research Council of Canada
Walnut shells

Ar

Interactivia, USA

Sodium hydroxide

Ar

Thermo Fisher Scientific, USA

Tetrabutylammonium

98 +%

Thermo Fisher Scientific, USA

Tetrachloroethylene

≥ 99 %

Thermo Fisher Scientific, USA

Silica gel

Ar

Sigma Aldrich, USA

Sodium sulfate

99 %

Sigma Aldrich, USA

Xylene

Ar

Thermo Fisher Scientific, USA

bromide

44

Table 3.2: Main Apparatus
Name of instrument
Mikro-Bantam

Purpose

Manufacturer

laboratory Grinding adsorbent

processing mill

USA

Planetary Ball Mill
Scanning

Hosokawa Micron Powder Systems,

Grinding adsorbent
Electron Surface

IKA, USA

structure Hitachi, Japan

Microscope (SEM)

analysis

Furnace

Carbonization

Thermo Fisher Scientific, USA

Oven

Drying of materials

Thermo Fisher Scientific, USA

Electronic Balance

Mass measurements

Sartorius, Germany

Fourier transform infrared Chemical
spectroscopy (FTIR)

analysis

FTIR

Oil

properties Anton Paar, Canada
concentration PerkinElmer, UK

measurement
Homogenizer

Oil/water

emulsion IKA, USA

preparation

3.2.

Synthesis of WS/NaOH
The WS-modified with sodium hydroxide (NaOH) adsorbent was synthesized through a

simple wet impregnation method. Initially, the raw WS was ground using a Mikro-Bantam
laboratory processing mill. The resulting material was further milled using a Planetary Ball Mill
Machine to obtain a fine, powder-like material. The ground material was then sieved using a
laboratory sieve shaker to get the desired particle size of 425 µm.
Subsequently, 20 g of WS was dispersed into 300 mL of deionized (DI) water and stirred
using a magnetic stirrer at 120° C and 200 rpm to ensure uniform suspension. Then, a 1 M NaOH
solution was added dropwise to the mixture over 1 h using a pipette to ensure gradual and
45

homogeneous mixing of the NaOH with the WS solution. The solution was left on the magnetic
stirrer for 24 h at a constant temperature and rpm to achieve a gel-like material. Subsequently, this
material was transferred into two 50 mL crucibles and dried in an oven for 24 h at a temperature
of 115° C. The resulting material then underwent a carbonization process via thermal treatment.
The materials were placed in a furnace at 550° C for 5 h. The obtained material was then allowed
to cool at room temperature and kept in a 20 mL bottle.

Figure 3.1: Images of (A) Walnut shells 24 h after the addition of NaOH via wet impregnation,
and (B) Walnut shells/NaOH after carbonization for 5 h
3.3.

Oil emulsion preparation
In this experiment, a 10 g sample of conventional crude oil was emulsified with 1 L of DI

water. The emulsion was prepared using a high-speed homogenizer. The oil and water mixture
was subjected to homogenization for 15-20 min at a controlled speed to ensure proper formation
of the emulsion. Following homogenization, the emulsion was allowed to cool and settle at room
temperature for 3 h to ensure phase stability. After this resting period, the emulsion was transferred
into airtight containers and stored in a refrigerator at approximately 4°C to maintain the integrity
of the emulsion for further analysis. To determine the known concentration of oil in the prepared
46

emulsion, the standardized procedure by ASTM D7066 [350] was followed strictly. To evaluate
the effect of salinity, different salt concentrations were added to the prepared oil/water emulsion.

Figure 3.2: Images of (A) prepared oil-water emulsion and (B)
oil extraction step
3.4.

Experimental design
A one-factor-at-a-time experimental design is mainly utilized when studying the outcome

of one independent variable (an experimental factor) relative to the dependent variable [351]. In
this case, all other experimental factors are held constant while one factor is changed to learn its
impact on the dependent variable. A one-factor-at-a-time design was deemed desirable for this
study for the following reasons:
1. Simplicity. Adsorption is a complex phenomenon. However, studying a single variable at
a time makes it easier to understand the impact of a particular variable without confounding
effects from other factors.
2. Control over factors. By maintaining all factors and changing only one factor, any observed
changes certainly result from the studied factor. A one-factor-at-a-time design blocks
interferences from other factors.

47

3. Easy identification of optimal conditions. With a one-factor-at-a-time experiment, it is
easier to identify at which level each parameter performs best. This is the first step in
optimizing experimental parameters and
4. When a single variable is altered, the data becomes more receptive to interpretation and
statistical analysis. Trends can be observed about the influence of the independent variable
on the dependent variable.
In this study, four independent variables, which were (1) IC, (2) AD, (3) RT, and (4) S,
were studied at different levels to understand their impacts on the dependent variable (oil removal
efficiency). The experiments were conducted in triplicate to ensure the reliability and consistency
of results. The detailed experimental design with all parameters and their specifications are
provided in Table 3.3. Adsorption isotherm and kinetic studies were conducted to evaluate the
performance and mechanisms of the adsorption process.
Table 3.3: Experimental design details
Parameters

Levels

Runs

Adsorbent dosage (g/L)

1, 2, 3, 4, 5, 6

3

Initial oil concentration

200, 300, 400, 500

3

10, 20, 30, 40, 50, 60

3

0, 15, 30

3

(mg/L)
Reaction time (min)
Salinity (g/L)

3.5.

Material characterization and adsorption studies
The adsorbent materials (raw WS and WS/NaOH) were characterized to understand their

structural and functional properties. Scanning electron microscopy (SEM) was employed to
48

examine the surface morphology, providing insights into the texture, porosity, and structural
features critical for adsorption. FTIR was utilized to identify functional groups present on the
material's surface, which play a vital role in the interaction with oily contaminants. These
characterization techniques ensured a comprehensive understanding of the adsorbent's properties
and its potential for effective adsorption.
In the adsorption studies, a known amount of the adsorbent was added to a beaker
containing 100 mL of oily wastewater with a predetermined initial concentration (IC) and salinity
(S). The solution was placed on a magnetic stirrer and agitated for a specific duration to facilitate
interaction between the adsorbent and the contaminants. After the adsorption process, samples
were collected, and the final concentrations of the oily contaminants were measured to determine
the adsorbent's efficiency. This setup enabled a controlled evaluation of adsorption performance
under varied experimental conditions. The oil removal efficiencies and adsorption capacities were
calculated with equations 1 and 2, respectively. The entire experimental procedure, including
material preparation and batch adsorption studies, has been graphically presented in Figure 3.3.

𝑅%=
𝑞

(

)
×

× 𝟏𝟎𝟎

1
1

Where R = oil removal efficiency (%) Co = initial oil concentration (mg/L), Ce =
concentration of oil at equilibrium (mg/L), qe = adsorption capacity (mg/g), m = mass of adsorbent
(g), and V = volume of solution (L).

49

Figure 3.3: Preparation steps of raw walnut shells, Walnut shells/NaOH, batch adsorption studies,
and sample analysis

50

Chapter 4 Results and Discussion
4.1.

Characterization of WS/NaOH
The surface morphology of the adsorbents was analyzed by comparing the raw WS with

the WS/NaOH sample, as illustrated in Figure 4.1, using SEM images. The SEM image of raw WS
is shown in Figure 4.1 A. It reveals a coarse, porous, and heterogeneous surface typical of lingocellulosic biomass characterized by irregularly shaped tiny particles resulting from grinding,
indicating potential for oil adsorption. The pores and fissures serve as active locations for oil
entrapment. However, the efficacy of adsorption may be constrained in its unmodified form.
Modifications to enhance the surface area and porosity can boost performance. Notwithstanding
specific irregular characteristics, the shells exhibit favorable structural qualities as an adsorbent
with potential enhancement. The SEM of WS/NaOH is shown in Figure 4.1 B. Unlike the raw WS,
the SEM image of WS/NaOH displays a significantly more porous and rougher surface,
characterized by an increase in micro-pores and surface area, which would enhance their
adsorption capacity. The treatment disintegrates surface fibers and reveals supplementary pore
structures, enhancing oil retention capacity. The modification additionally incorporates functional
groups that enhance affinity for oil compounds, rendering WS more efficient as adsorbents for the
remediation of oily wastewater.

51

(A)

(B)

Figure 4.1: SEM images of raw walnut shells (A) and walnut shells/NaOH (B)

The surface chemistry of the sorbent, together with its porous shape, significantly
influences adsorption processes. Consequently, the existence of functional groups was analyzed
using FTIR. FTIR study of unmodified WS reveals functional groups that improve adsorption for
oily wastewater treatment. The O-H groups (3200–3500 cm⁻¹) enhance the adsorption of polar
contaminants via hydrogen bonding, whereas C-H groups (2800–3000 cm⁻¹) confer hydrophobic
characteristics for non-polar compounds [352]. C=O groups (1700–1750 cm⁻¹) enhance
interactions with polar molecules, while C-O groups (1000–1300 cm⁻¹) from cellulose provide
functionality for binding ionic species. WS's hydrophilic and hydrophobic properties facilitate the
adsorption of both polar and non-polar pollutants, rendering them suitable for wastewater
treatment. On the other hand, the FTIR of WS/NaOH is presented in Figure 4.3. Preserved carbonyl
52

groups observed in 1700–1750 cm⁻¹ promote interactions with polar impurities. Still, the novel
peak at 1422 cm⁻¹ signifies augmented aliphatic C-H bending, improving hydrophobic
characteristics for non-polar compounds. The peaks at 877.98 cm⁻¹ and 700.87 cm⁻¹ indicate the
existence of aromatic structures, enhancing hydrophobicity. The peak at 500–650 cm⁻¹ may signify
metal-oxygen bonds, thus improving the adsorption of ionic pollutants [353]. Although hydroxyl
groups decrease, the modified shells successfully expand their adsorption spectrum for oily
pollutants.

Figure 4.2: FT-IR spectra of raw walnut shells

53

Figure 4.3: FT-IR spectra of Walnut shells/NaOH
4.2.

Batch Adsorption Studies
All tests were conducted at ambient temperature in batch mode. In the preliminary batch

experiment, 0.2 g of raw walnut shells (RWS) and WS/NaOH adsorbent were placed in separate
beakers containing 100 mL of oil-water emulsion and continuously stirred at 200 rpm with a
magnetic stirrer. The studies were conducted under various experimental conditions, including AD
(2 g/L), RT (40 min), IC (500 mg/L), and S (3 g/L). The oil removal efficiency of the two
adsorbents was assessed.
This study evaluated raw WS and WS/NaOH as adsorbents for oily wastewater treatment.
The results indicated that WS/NaOH exhibited superior removal efficiency, achieving 88%
compared to 81% for raw WS. Correspondingly, the adsorption capacity of WS/NaOH was 220
mg/g, higher than the 201 mg/g for the raw WS. The improved removal efficiency and adsorption
capacity of the WS/NaOH can be attributed to several factors. The NaOH treatment enhances the
surface of the shells by increasing the number of functional groups, such as hydroxyl and carboxyl,
54

which provide more active sites for adsorption [348]. It also increases the porosity of the WS,
allowing greater access to internal surfaces for oil molecules [354]. Additionally, the modification
makes the surface more hydrophilic, facilitating better interaction with the oil and higher
adsorption rates than RWS [355].
To improve the oil removal efficiency of the WS/NaOH, tetrabutylammonium bromide
(TBAB) was introduced as a co-adsorbent at a concentration of 0.04 M. The slow addition of
TBAB during the batch experiment resulted in a removal efficiency of 91% and an adsorption
capacity of 226 mg/g, both showing significant improvements over the WS/NaOH alone. TBAB,
which is a quaternary ammonium compound, enhances the surface charge of WS, promoting
electrostatic attraction between the positively charged TBAB and negatively charged oil droplets
[356]. Its hydrophobic nature also increases the interaction between the adsorbent and oil
molecules. [357]. Additionally, TBAB helps disperse the oil molecules more effectively,
increasing their contact with the adsorbent surface. These combined effects lead to the observed
increase in efficiency and adsorption capacity.

Figure 4.4: Oil removal efficiencies and adsorption capacities of raw walnut shells
(RWS), walnut shells/NaOH, and raw walnut shells/NaOH/TBAB
55

4.3.

Effect of experimental conditions on hydrocarbon adsorption

4.3.1. Effect of adsorbent dosage
This study determined the effect of the quantities of adsorbent used for dosages in the 1 –
6 g/L range for the WS/NaOH. The experimental conditions for this test were IC = 500 mg/L, RT
= 40 min, and S = 30 g/L. As shown in Figure 4.6, oil removal increased gradually from 87 % to
89 %, 91 %, 93 %, and 94 % to 95 %. These results were expected, given that increased AD
correlates with a rise in available adsorbent sites. These quantities attach additional ions to their
surfaces, increasing removal efficiency. Comparable outcomes were documented utilizing various
materials as adsorbents. [359].

Figure 4.5: Effect of adsorbent dosage on the adsorption of oil from oil-water emulsion
using the raw walnut shells/NaOH/TBAB
56

4.3.2. Effect of reaction time
The influence of RT on the performance of the WS/NaOH/TBAB was examined, as shown
in Fig. 4.7. The experimental conditions for this test were AD = 6 g/L, IC = 500 mg/L, and S = 30
g/L. In the first 40 min, rapid adsorption was observed for the WS/NaOH/TBAB, with a removal
efficiency of 88 %, 901 %, 93 %, and 95 % at RT 10, 20, 30, and 40, respectively. After
approximately 58 min, the adsorption reached equilibrium. At 60 min, an efficiency of 96 % was
achieved. Compared with the adsorption performance at 40 min, the removal efficiency did not
increase significantly. On the whole, the adsorption process can be divided into two steps based
on the adsorption rate (Fig. 4.7): rapid adsorption (first stage) and slow adsorption (second stage).
In the first stage, the oil molecules are quickly adsorbed because of numerous adsorption sites on
the WS/NaOH/TBAB surface. The slow adsorption rate in the second stage is due to the saturation
of adsorption sites, reduced diffusion of oil molecules, and less favorable interactions as the
surface becomes more covered.

Figure 4.6: Effect of reaction time on the adsorption of oil from oil-water
emulsion using the WS/NaOH/TBAB
57

4.3.3. Effect of initial oil concentration
To further probe the adsorption process, the effect of IC was investigated. The experimental
conditions for this test were AD = 6 g/L, RT = 60 min, and S = 30 g/L. From Figure 4.7, the results
revealed an increase in removal efficiency with higher IC, with removal rates of 91%, 93%, 94%,
and 95% observed for concentrations of 200, 300, 400, and 500 mg/L, respectively. This trend
suggests that the WS/NaOH/TBAB has a higher capacity to adsorb oils at increased
concentrations. A possible explanation for this could be that higher oil concentrations provide
more molecules for adsorption, allowing the adsorbent to more effectively remove oil from the
emulsion. As the concentration increases, there are more interactions between the oil molecules
and the adsorption sites, leading to higher removal efficiencies.

Figure 4.7: Effect of initial oil concentration on the adsorption of oil from oil-water
emulsion using the Walnut shells/NaOH/TBAB

58

4.3.4. Effect of oil/water salinity concentration
The pre-prepared oil/water emulsion was evaluated by introducing different salt
concentrations to examine the influence of salinity on its stability. Alterations in droplet
coalescence, emulsion destabilization, and phase separation were examined to assess the influence
of salinity on emulsion behavior. Regarding the influence of S, the results showed efficiencies of
86%, 93%, and 95% for S concentrations of 0, 15, and 30 g/L (Figure 4.8), representing control,
moderate, and typical seawater concentrations, respectively. The initial lower removal efficiency
at 0 g/L S (control) could be due to the absence of salts, which may reduce the stability of the oilwater emulsion and hinder oil adsorption [358]. However, the removal efficiency improves as S
increases to moderate (15 g/L) and typical seawater levels (30 g/L). This could be explained by
the role of S in enhancing emulsion stability, which facilitates better contact between the oil
molecules and the adsorbent surface.

Figure 4.8: Effect of salinity concentration on the adsorption of oil from oilwater emulsion using the Walnut shells/NaOH/TBAB
59

4.4.

Adsorption isotherm models
Under constant ambient conditions, the adsorption isotherm establishes a relationship

between the total mass of adsorbate adsorbed per unit mass of sorbent. This mathematical
formulation is employed to predict effective parameters that affect the adsorption process, together
with a quantitative evaluation of adsorption behavior in adsorption systems. The Langmuir
isotherm describes single-layer adsorption of uniformly adsorbed material with consistent energy
across all adsorbent surfaces. In contrast, the Freundlich isotherm pertains to multilayer and
heterogeneous adsorption on the adsorbent surface. Both are prominent models in solid-liquid
adsorption [359]. Experimental studies on both isotherms were conducted at room temperature.
Based on the adsorbent dosages studied in Figure 4.5, adsorption isotherms studies for the
WS/NaOH/TBAB adsorbent were expressed using Langmuir isotherm by plotting 1/qe against
1/Ce (as shown in Fig. 4.9) and Freundlich isotherm by plotting qe against Ce (as shown in Fig.
4.10). The results in Figures 4.9 and 4.10 are correlated by the Langmuir and the Freundlich
equations (see equations 3 and 4) with the resulting parameters listed in Table 4.1.

=(
𝑞

𝐾 )( ) +

2

3

/

Where Kl = Langmuir isotherm constant, Ce = oil concentration at equilibrium, qmax = maximum
adsorption capacity, KF = Freundlich constant, and n = Freundlich exponent.

60

Table 4.1: Adsorption isotherms constants for oil removal from oil-water emulsion
Langmuir

Freundlich

Kl

qmax

R2

1/n

Kf

R2

0.00786

294.117

0.9171

0.2886

36

0.9695

Figure 4.9: Langmuir isotherm adsorption of oil from oil-water emulsion

From the data results, it can be concluded that the Freundlich isotherm best fits the
experimental data, as indicated by a higher R2 value (0.9695) compared to the Langmuir isotherm
(R2 = 0.9171). This suggests that adsorption likely occurs on a heterogeneous surface, which aligns
with the Freundlich model's assumption of multilayer adsorption on varied binding sites rather
than the uniform monolayer adsorption assumed by the Langmuir model. Additionally, the

61

Freundlich parameter 1/n = 0.2886 suggests strong adsorption intensity, supporting that the
WS/NaOH possesses a heterogeneous adsorption surface. [360].

Figure 4.10: Freundlich isotherm adsorption of oil from oil-water emulsion

4.5.

Adsorption kinetic studies
The adsorption kinetics typically indicate the physical characteristics of the adsorbent.

These kinetic models, which include pseudo-first-order and pseudo-second-order, can project the
adsorption rate, mechanism, rate-limiting step, and diffusion process. The pseudo-first-order
model describes adsorption driven by physisorption, where the rate depends on the number of
62

unoccupied sites, typically fitting fast initial adsorption. In contrast, the pseudo-second-order
model assumes chemisorption, involving stronger chemical bonds and adsorption rates based on
site availability, making it more accurate for equilibrium predictions.
Based on the effect of reaction time studied in Figure 4.6, adsorption kinetic studies for the
WS/NaOH/TBAB adsorbent were expressed using pseudo-first-order by plotting ln (qe-qt) against
time (as shown in Fig. 4.11) and pseudo-second-order by plotting t/qt against time (as shown in
Fig. 4.12). All studies were carried out at room temperature. The calculated kinetic parameters of
pseudo-first and second-order models are given in Table 4.2 (using equations 5 and 6), and the
corresponding values are shown in Figures 4.10 and 4.11, respectively. It can be visually seen that
pseudo-second-order kinetics fits better with a higher R 2 of 0.9998 compared to that of pseudofirst order (0.95).

Figure 4.11: Pseudo first-order kinetic model for adsorption of oils in oil-water
emulsion

63

𝑙𝑛(𝑞𝑒 − 𝑞𝑡) = 𝑙𝑛 𝑞𝑒 − 𝐾 𝑡

4

5
Where qe = amount adsorbed at equilibrium (mg/g), qt = amount adsorbed at time t (mg/g), K1=
rate constant (min-1), t = Time (min), and K2 = rate constant (mg/g.min).

Figure 4.12: Pseudo second order kinetic model for adsorption of oil in oil-water emulsion

Table 4.2: Kinetic parameters for oil removal from oil-water emulsion
Pseudo first order

Pseudo second order

Kl

ln qe

R2

qe

K2

R2

0.8883

3.1402

0.95

81.9672

0.0083

0.9998

64

Table 4.3: Comparison of oil removal efficiency of the proposed WS/NaOH/TBAB with other
natural adsorbents
Adsorbent

Removal
efficiency
(%)
45

Reference

Crude oil

Adsorbate
Concentration
(mg/L)
-

Pomegranate peel/n-hexane

Crude oil

300

92

[340]

Activated date pit

Petroleum

950

78

[341]

Thermally modified orange

Adsorbate

[361]

peel

waste
Sawdust

Crude oil

-

96

[362]

Biochar

Pyrene

-

88

[317]

WS//Na2CO3/NaHSO3/Na2SO3

Crude oil

405

90

[363]

Animal bones/NaOH

Crude oil

1000

94

[364]

Anise residues/DI Water

Crude oil

1000

70

[364]

WS/NaOH/TBAB

Crude oil

500

96

This work

65

Chapter 5. Conclusion and Recommendations

5.1.

Conclusion
The increasing occurrence of oil spills has driven research into efficient and sustainable

oily wastewater treatment methods. Current efforts prioritize oil recovery from marine
environments, and a polishing step to treat the residual oil prior to discharging the water back to
the aquatic environment. Adsorption, particularly using natural materials, stands out as a simple,
cost-effective, and eco-friendly solution, offering a sustainable approach to addressing the
environmental impact of oil contamination. This study explored a cost-effective, eco-friendly
technique for treating oily wastewater with WS-based adsorbent. To improve WS's surface
properties, chemical composition, and adsorption potential, a synthesis mechanism via wet
impregnation by the slow addition of NaOH and carbonization method under thermal treatment
was employed to generate the proposed WS/NaOH adsorbent. The generated WS/NaOH displayed
good potential for the adsorption of oily particles from oil-water emulsion with a removal
efficiency and adsorption capacity of 88% and 220 mg/g, respectively. To further enhance the
adsorption potential of the WS/NaOH, TBAB was introduced as a co-adsorbent during the batch
test. The outcome revealed that the WS/NaOH/TBAB adsorbent is an excellent technique for the
remediation of oil-water separation with an efficiency of 96% and an adsorption capacity of 226
mg/g. The surface morphologies and chemical compositions of the WS/NaOH were examined by
SEM and FTIR, respectively. Adsorption isotherm and kinetic studies conducted in this research
revealed that the Freundlich and Pseudo second-order models best represent the experimental
results. The simplicity and economic viability of adsorption, particularly with eco-friendly
adsorbents, make it an appealing solution. By targeting residual oil in emulsions post-primary
recovery, adsorption helps protect aquatic ecosystems and aligns with the broader goal of

66

sustainable environmental management. In the face of the intricate challenges posed by oil spills,
the continual progress and application of adsorption technology presents a hopeful avenue for
alleviating the severe impact of oily wastewater on our planet’s fragile ecosystems.

5.2.

Limitations of the study

This research was subject to several limitations. First, the modification method used to enhance
the adsorption capacity is highly sensitive and thus requires much time and attention. Second, there
are significant challenges in scaling up the batch experiment to real-world applications, where
adsorbent efficiency may vary under continuous operation. Third, using highly toxic and
flammable chemicals in the modification and PAHs extraction processes raises health and safety
concerns that must be carefully addressed. Lastly, the study does not account for the impact of
other environmental contaminants commonly found in wastewater, which may interfere with the
adsorption process and reduce overall effectiveness.

5.3.

Recommendations

Based on the results obtained from this thesis, it is imperative to explore the following research
areas further to advance the development of eco-friendly and cost-effective adsorbents:

(i)

Evaluating the effectiveness of adsorption in various organic and inorganic pollutants
typically present in oily wastewater. Oily wastewater is intricate; investigating
competitive adsorption helps elucidate the influence of other compounds on oil
adsorption effectiveness, enhancing its relevance to real-world scenarios.

(ii)

Performing experiments at various pH levels and temperatures to ascertain the
influence of these variables on adsorption efficiency. In this case, actual oily
wastewater's pH and temperature ranges must be considered.
67

(iii)

The study’s outcome suggests a pathway for large-scale testing. Column adsorption
experiments with WS-based adsorbents could be tested to simulate continuous flow
conditions that mimic large-scale wastewater treatment. Batch experiments provide
valuable insights; however, column studies will illustrate the adsorbent's performance
in a realistic, scalable context, which is essential for practical application.

68

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Appendices
Appendix 1. Confirmation of Permission to Include Published Paper from the Journal of Energy,
Ecology, and Environment.

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Appendix 2. Confirmation of Permission to Include Published Paper from the co-authors.

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