Effects of biochar on soil contaminated by metals and metalloids from slag disposal of an old environmental liability in Ribeira Valley, Brazil.

Contamination with potentially toxic metals and metalloids (PTMs) in mining areas poses significant environment and human health risks. Using biochar as an amendment can be a cost-effective and eco-friendly method to reduce PTM bioavailability in contaminated soils, thus lowering plant uptake. This study investigated biochar derived from the organic fraction of municipal solid waste (OFMSW) at three pyrolysis temperatures (300, 500, and 700 °C) and two application rates (1% and 5%, w/w) for the remediation of slag-contaminated soils from an old environmental liability in the Ribeira Valley (Brazil). The results showed Zn > Pb > Cu > As > Co > Cr > Cd > Ni pseudo-total concentrations in slag with concentrations of As, Cu, Pb, and Cd posing greater environmental risks due to their toxicity. The biochar addition exerted limited effects on chemical fractionation, likely due to soil alkalinity, and BC300 5% increased As availability. A 1% biochar addition improved maize (Zea mays) growth, whereas 5% BC500 and BC700 were phytotoxic. The highest bioconcentration factor (BCF) values were observed for Cr, Cu, Ni, and Zn, which are all essential nutrients for plants; however, translocation factor (TF) from roots to shoots was generally low. A combination of BCF and TF < 1 suggested mechanisms limiting PTM uptake and translocation in plants. Pb showed a high ecological risk potential (E), with hazard quotients (HQ) exceeding 1 for the slag. BC700 5% provided the most promising E for As, Pb, and Zn; however, it proved toxic to maize, highlighting the need for multidisciplinary research and biochar's potential in site remediation. Further treatments are necessary for enhancing the retention efficiency or exploring combinations with other organic or inorganic amendments.