Synergistic Immobilization of Lead and Cadmium and Enhanced Phosphorus Utilization in Contaminated Soil Using...

Phosphorus (P)-modified hydrochar can immobilize lead (Pb) and cadmium (Cd) in contaminated soils, but improving P utilization while mitigating P release remains challenging. In this study, banana peel hydrochar (BPH200) was synthesized via phosphoric acid modification at 200 °C, and a coal gangue-derived zeolite (ZL) was prepared. A 100-day soil incubation experiment was conducted to evaluate the individual (1% BPH200 and 2% BPH200) and combined (1% BPH200 + 1% ZL) effects on Pb/Cd immobilization and P release risk. The immobilization performance, expressed as the reduction in toxicity characteristic leachingprocedure (TCLP)-extractable Pb and Cd, followed the order: 1% BPH200 + 1% ZL (78.61% for Pb, 67.61% for Cd) > 2% BPH200 (54.93% for Pb, 54.71% for Cd) > 1% BPH200 (27.41% for Pb, 21.13% for Cd). The P utilization efficiency, defined as the proportion of BPH200-derived P that contributed to Pb/Cd immobilization and to the increase in soil available P, showed a similar trend: 1% BPH200 + 1% ZL (73.29%) > 1% BPH200 (55.12%) > 2% BPH200 (50.22%). Temporal analysis revealed that soil Olsen-P in all treatments peaked sharply on the 10th day and then declined, with the 2% BPH200 treatment causing excessive Olsen-P levels (more than 20 mg/kg) throughout the incubation, indicating a persistent risk of P release. In contrast, co-application with ZL mitigated this risk while maintaining effective metal immobilization. These results demonstrate that ZL synergistically enhances P utilization for metal immobilization and reduces P mobility, offering a more sustainable remediation strategy for Pb- and Cd-contaminated soils.