Effect of biochar amendments on the mobility and (bio) availability of As, Sb and Pb in a contaminated mine technosol

Reclaiming degraded and contaminated soils with biochar amendments has become increasingly popular. Biochar, a fine-grained and porous biocarbon produced by the slow pyrolysis of biomass under oxygen limited conditions is foreseen as a possible solution to reduce metal(loid)s availability and phytotoxicity and simultaneously to improve the soil quality. Mining activities are one of the main concern generating potentially toxic elements (PTE) such as metal(loid)s-rich wastes and technosols. The effects of biochar addition (2% and 5%) on a former mine technosol characterized by high levels of PTE were investigated in laboratory during short-term experiments where technosols were potted and cultivated with dwarf beans (Phaseolus vulgaris L.) as indicator plants. Soil and soil pore water (SPW) physico-chemical properties were determined together with total dissolved concentrations of As, Sb and Pb in SPW sampled by Rhizons and available concentrations as measured by DGT (Diffusive Gradients in Thin-Films). Additionally indicator plant biomass and PTE concentrations in their organs (roots, stem and leaves) were determined. The results of the present study indicate that addition of 2 and 5% biochar to contaminated technosols increased soil and SPW pH, electrical conductivity (EC) and soil water content (SWC), decreased the labile concentration of Pb while increasing the As and Sb solubility. Indicator plants cultivated on biochar amended mine technosols showed a smaller biomass and higher PTE concentration levels in the various organs.