Ultramafic technosols: Metal mobility and carbon sequestration in reclaimed mine tailings

Ultramafic settings have garnered increased attention as sources of nickel (Ni), a critical component of sustainable infrastructure and computer hardware. Interest in ultramafic minerals is exemplified by FPX Nickel’s Baptiste project, a large operation located outside of Fort St. James, BC. Ultramafic rock has the potential to be carbonated when exposed to sufficient CO2, as the silicate anion within the mineral lattice is replaced with a carbonate group. This creates the exciting possibility of carbon sequestration through enhanced rock weathering. However, extraction from these settings leads to the generation of ultramafic mine tailings, a nutrient poor and heavy metal rich parent material. Its currently unclear what the optimal reclamation strategy is for these settings, and how reclamation might affect carbon sequestration. To test these questions, two randomized complete block design experiments were conducted. One experiment featured a modified leaching column design, which tested for trace metal mobility and plant growth response to reclamation treatments. The other experiment looked at how reclamation strategies affect the sequestration of carbon, with particular attention given to organic carbon occlusion and mineral carbonation. It was found that adding 12.5% compost by mass to the tailings significantly improved plant growth, and lead to the formation of mineral-associated organic carbon (MAOC) within one year. Chromium was the only trace metal that leached in quantities above the CCME guidelines for irrigation water, signalling a potential risk that requires further investigation. These findings will contribute to develop a reclamation protocol for ultramafic tailings across the world and potentially abet the Baptiste Project’s goal of carbon neutrality.