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

## TL;DR

This study shows how adding specific soil amendments can reduce lead availability in urban soils, depending on local soil chemistry.

## Contribution

A site-specific framework for Pb immobilization using amendments tailored to soil geochemistry is developed.

## Key findings

- Gypsum reduced exchangeable Pb in alkaline soils by increasing carbonate- and oxide-bound Pb.
- Biochar + lime decreased Pb availability in acidic soils through pH increase and organic-bound Pb.
- Alum effectively immobilized Pb in near-neutral soils via oxide-bound Pb and Al-hydroxide sorption.

## Abstract

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.

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.

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).

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.