Soil dynamics and ecotoxicity of zinc extracted from black mass derived from discarded batteries

E-waste generation is rapidly increasing, creating significant environmental challenges. This study evaluates zinc (Zn) sources derived from black mass (BM) extracts, obtained by leaching black mass from Zn/C and alkaline spent batteries with glycine (Gly), citric acid (Cit), or sulfuric acid (H₂SO₄). Soil column experiments were conducted using two contrasting agricultural soils (acid and calcareous). Leaching behaviour, Zn distribution, and seed ecotoxicity were assessed. In acid soil, total Zn leached was high for all sources (~ 98%), but release dynamics differed: inorganic sources (ZnSO₄, H₂SO₄) released Zn rapidly, while complexed sources (Cit and Gly) showed more gradual release. Despite the overall high mobility, organic complexes retained slightly more plant-available Zn in the soil profile (≤ 1% of applied Zn). In calcareous soil, Gly leached up to 97% of Zn, Cit 70–80%, while inorganic forms leached < 23% and retained up to 60% of Zn in unavailable fractions. Low Zn doses from BM-derived sources promoted seedling growth, whereas higher concentrations caused phytotoxic effects. These findings highlight potential environmental risks of high Zn application and suggest that careful management is required if BM-derived Zn is used as a fertilizer, contributing to sustainable nutrient recycling and circular economy strategies.