Geochemical fate of lead in contaminated residential soils following application of amendments for lead immobilization

Background: Lead (Pb) persists in urban soils, where its partitioning among geochemical fractions governs mobility, bioavailability, and human health risk. These fractions are strongly controlled by soil physicochemical properties, necessitating site-specific remediation strategies. Methods: This study developed a site-specific Pb immobilization framework linking amendment selection to soil geochemistry and sustainability considerations. Pb-contaminated residential soils from three U.S. cities, San Antonio (alkaline), Baltimore (acidic), and Detroit (near-neutral), were treated with gypsum, biochar + lime, and alum, respectively. Changes in Pb speciation were tracked using sequential extraction over 7, 30, and 90 days. Results: All amendments significantly reduced exchangeable Pb (F1) and increased less mobile fractions (F2–F3). Gypsum reduced F1 by ∼30% in San Antonio soils with minimal pH change, coincident with increased carbonate- and oxide-bound Pb. Biochar + lime reduced F1 by ∼50% in Baltimore soils, driven by a 0.4–0.8 pH increase and enhanced carbonate- and organic-bound Pb (F2–F4). Alum reduced F1 by ∼28% in Detroit soils, with transient pH shifts and strong increases in oxide-bound Pb (F3). Conclusion: Despite contrasting soil chemistries, all treatments achieved rapid and statistically significant Pb stabilization via distinct mechanisms, including Ca²⁺-facilitated precipitation, pH-driven surface complexation, and Al-hydroxide sorption. This work provides a mechanistic, transferable framework for tailoring low-cost, in situ amendments to local soil geochemistry to durably reduce Pb bioavailability and exposure risk in urban residential soils.