Phosphate recycled as struvite immobilizes bioaccessible soil lead while minimizing environmental risk

Phosphorus (P) usage and water quality concerns are associated with using phosphate-based amendments to remediate lead (Pb) contaminated soils in urban areas. Struvite has gained increasing attention in the last decade to recover P from point sources while being used as a novel P fertilizer with reduced P loss risk, but its potential for serving as a P amendment for Pb immobilization has not yet been evaluated. This study evaluated the potential of using struvite to maximize Pb immobilization in alkaline urban soils while minimizing P loss risk by comparing the influences of three P amendments (triple superphosphate [TSP], struvite, phosphate rock [PR]) and two particle sizes (0.5 vs 2–4 mm) on Pb immobilization efficacy (relative change in Pb concentrations of in vitro physiologically based extraction test [PBET] test, Pb_PBET), P loss risk (water extractable P, P_w) and the translocation of Pb from soil to plant. The P amendments were added to two Pb-contaminated soils under urban agricultural use with contrasting clay contents (280 mg kg⁻¹ with 4% clay versus 727 mg kg⁻¹ with 14% clay) at a rate commonly used for Pb immobilization based on the soil elemental molar ratio of 4:1 (P/[Cd + Zn + Pb]). Greater immobilization efficacy of Pb (up to 19%) occured for granular (2-4 mm diameter) than ground (<0.5 mm) struvite. For a given particle size, Pb immobilization and plant tissue Pb decreased in the order of PR ≥ struvite > TSP, but the order reversed for P loss risk, with up to 177-fold greater P_w for TSP than PR and struvite. Greater immobilization for granular PR and struvite could be caused by lower dissolution rates which may better synchronize P dissolution and desorption of Pb. In addition to avoiding secondary contamination compared to conventional P amendments (e.g., Cd content), granular struvite can optimize trade-offs among soil Pb immobilization, crop Pb health risk, and P loss risk.