Biogeochemical processes and trace element mobility in alkaline waste affected soils

High pH leachates can mobilise oxyanion forming elements from alkaline waste, such as iron and steel slags and bauxite ore processing residue, into the wider environment. Red mud is a highly alkaline waste product from bauxite ore processing. It contains elevated concentrations of oxyanion forming elements such as Al, As, V and Mo that are mobilised at high pH together with other trace metals including Cu and Ni. The red mud spill at Ajka, Hungary released 1 million m3 of caustic red mud into the surrounding area. As part of the initial clean-up, some thinner red mud deposits (< 5 cm) were ploughed into fields to prevent dust formation whereas wetland areas were left untreated. This study used aerobic and anaerobic batch microcosms together with XAS spectroscopy, (HPLC)-ICP-MS and solid phase extraction techniques to investigate the effects of red mud addition to unaffected soils with respect to biogeochemical processes and trace element release. Experiments were designed to be analogous to soil conditions following the remediation efforts. The results showed that the effect of red mud addition to soils was highly dependent upon soil properties, experimental pH, and total organic carbon (TOC) content. As and V were found to be persistently mobile under both aerobic and anaerobic conditions. Red mud addition to soils with a high TOC content mobilised high aqueous concentrations of organic matter complexing metals such as Cu, Ni and V under anaerobic conditions. Gypsum addition to red mud affected soils showed a reduction in aqueous oxyanion concentrations compared to soils with red mud addition alone indicating that it is potential ameliorant for red mud contaminated soils. The results showed that the extensive remediation efforts by the Hungarian authorities were justified but that red mud should be used with caution as a soil amendment.