Ferrihydrite reduces the bioavailability of copper and cadmium and phosphorus release risk in hydroxyapatite amended soil

Abstract

Hydroxyapatite (HAP) can effectively decrease bioavailability of trace metals in soils, but the phosphorus (P) release risk was not fully known. In this study, the impacts of ferrihydrite (FH) on passivation of copper (Cu) and cadmium (Cd) and P release risk in soil amended by HAP were investigated under flooding conditions. Results showed that the combinations of HAP and FH increased soil pore water pH by 0.74–2.24 units and reduced phosphate by 71.1–99.5% than single HAP. Soil pore water Cu2+ and Cd2+ in HAP plus 5% FH treated soils were decreased by 96.8–98.6% and 86.6–98.6% than those of single HAP. The combinations of HAP and FH significantly decreased CaCl2 extractable Cu and Cd by 55.9–99% and 13–72.2%, respectively. Moreover, the combinations of HAP and FH significantly decreased exchangeable Cu 47.1–91.5%, but only FH5 decreased exchangeable Cd by 69.4% than the single HAP. Amorphous iron oxide in FH treated soils was 4.06–20 times higher than control, but some of them were transformed into crystalline iron oxide after 42-d flooding incubation. The FH significantly decreased soil labile P by 42.8–78.8% in the presence of HAP and increased stable P by 23.1–33.5% than the single HAP. This study suggested that the addition of FH can not only promote the passivation of HAP on Cu and Cd in the soil, but also reduce the P release risk. Overall, this study highlighted the potential value of the combination of HAP and FH in the passivation of Cu and Cd contaminated soils.