Stability of in situ immobilized lead and zinc in rhizosphere soil of phosphorus amended metal contaminated soils.

The in situ immobilization of Pb and Zn via the formation of sparingly soluble Pb- and Zn-P minerals has been regarded as one of the most environmentally friendly and economical option to remediate Pb and Zn contaminated soils. However, some uncertainties remain unclear in the use of P amendments to immobilize excess Pb and Zn in soils. This work was initiated to investigate some of these uncertainties including the long-term stability of the immobilized Pb-and Zn-P in the rhizosphere soil, factors contributing to the poor efficiency Pb-and Zn-P formation in soils, and the interaction between plants, immobilized Pb- and Zn-P and soil biota, specifically arbuscular mychorrizal fungi. A one-year long batch dissolution study revealed that organic acids (Oxalic, citric, and malic acid) abundant in rhizosphere soil were able to dissolve pyromorphite (a synthetic Pb-P mineral) and liberate Pb and P into solution. Similarly, tall fescue plants grown with pyromorphite as a sole source of P were able to solubilize pyromorphite and consequently accumulate Pb in their biomasses. In an incubation experiment, the formation of target Pb- and Zn-P minerals after P amendment was significantly reduced in the presences of oxalic and citric acids. The results suggest that organic acids in rhizosphere soils are capable of releasing Pb from Pb- and Zn-P minerals, and are significant factors contributing to the poor efficiency of P amendments to effectively immobilize high levels of Pb and Zn in contaminated soils. In a complementary study, an alternative metal immobilization technique by co-pyrolysis of a contaminated soil with woody biomass revealed great potential to further exploit pyrolytic processes to immobilize metals such as Zn and Cd in contaminated soils with highly resilient charred biomass. --P. ii.