Are biosolids safe for agricultural applications? Ecotoxicity, bioavailability, and plant uptake of heavy metals

The agricultural reuse of municipal biosolids is constrained by heavy metal contamination. This study assessed the ecotoxicity, speciation, and bioavailability of metals in biosolids from ten sewage treatment plants across India. Metal concentrations in matured biosolids decreased in the order: Zn > Cr > Cu > Ni > Pb > Cd > Mo > As > Se. Wheatgrass cultivation in soils amended with 2%, 5%, and 10% (w/w) biosolids was used to evaluate growth response, metal uptake, and stability. Metal mobility, quantified using the Immobilization Ratio (IR), was highest for Cd and Zn (0.208-0.29), indicating greater bioavailability, and lowest for Cu and Pb (0.45-0.55), reflecting higher stability. Biosolid application enhanced plant growth, with 10% amendment producing maximum biomass and shoot length. Over time, declining soil and plant metal concentrations indicated active uptake, with Cr showing the highest accumulation, followed by Mo > Zn > Ni, while As, Se, Cd, and Pb were minimally absorbed. Cr and Mo uptake remained substantial even at lower application rates, underscoring the role of dose in metal transport. Findings highlight that biosolid amendments modify metal bioavailability and plant uptake patterns, and that the IR index is a useful tool for guiding safe land application to limit heavy metal transfer into the food chain.