Insight into the interaction mechanisms of toxic cadmium and lead with calcite/phosphate composite solids

The phosphorus (P) fertilizer in farmland soils can form composite solids with the calcium (Ca) carbonate minerals. However, the effects of these composites on chemical fractions and migration trends of heavy metal pollutants remain unclear. Thus, in this study, batch experiments, modified Tessier sequential extraction method, and spectral approaches were combined to explore the immobilization mechanisms of calcite/phosphate (Ca/P(x)) composites for toxic cadmium (Cd) and lead (Pb). The Ca/P(x) composites exhibited ultrafast Cd and Pb sorption kinetics (10 min) and high sorption ratios (> 95 % with sorption amounts of ∼ 1 mg/g for Cd and ∼ 50 mg/g for Pb) without competitive actions between them. The sorption of Pb and Pb/Cd facilitated the release of Ca and P from Ca/P(x) composites, while the coexisting natural organic matter (NOM) exhibited an inhibitory effect. The retention mechanisms included the formation of binary/ternary surface complexes and heavy metal-incorporated chlorapatite phases. Pb facilitated the formation of more stable Cd species in the Ca/P(x) + Pb/Cd + NOM systems. In contrast, Cd did not affect the proportions and thermodynamic stabilities of Pb species in all the sorption systems. NOM inhibited the precipitation of calcium-lead chlorapatite but promoted ternary Ca/P(x)-Pb-NOM complexation. This study provides insight into the environmental fate of Cd, Pb, Ca, and P in heavy metal-polluted farmland soils and provides a scientific basis for rationally formulating soil remediation strategies.