Compositional Differences of Various Sorption States of Cu and Cd on Goethite in the Presence of Oxyanions: A Quantitative Study

The anionic species of antimony(V) and phosphate(V) are commonly found in the contaminated soil of mining areas, exerting a significant influence on the sorption of heavy metals and thus affecting their migration. This study quantitatively discussed the sorption mechanism of Sb and P in promoting the sorption of Cd or Cu on goethite through a series of extraction methods. In the single sorption system, the majority of Cu (87–98%) is adsorbed on goethite in the form of EDTA-extractable Cu (EF Cu, possibly inner-sphere complexes) under pH conditions of 3.5–6.5. Cadmium is primarily adsorbed in the form of EDTA-extractable Cd (49–71%), with a considerable amount of Mg(NO3)2 extractable Cd (MF Cd, possibly outer-sphere complexes) also present (25–51%), within the pH range of 4.5–7.5. The presence of either Sb or P greatly enhances the sorption of Cd and Cu on goethite, although the mechanisms differ significantly. Sb enhances the sorption of Cu mainly by increasing the amount of EF Cu (61.7–68.1% of total Cu enhancement), with less significant effects on MF Cu. Furthermore, Sb shows similar enhancing effects on both MF Cd and EF Cd. As the pH increases, the enhancing effects of Sb on various forms of Cu and Cd decrease. Phosphate mainly promotes the formation of MF Cu and MF Cd, accounting for 53.9– 80.8% (Cu) and 78.0–94.9% (Cd) of total enhancement at different pH levels. ΔMF and ΔEF Cu increase with increasing pH when P is present, while ΔMF/ΔEF Cd remains essentially constant. Based on the extracted results and characterization analysis, the main mechanism of synergistic sorption between elements was discussed, and the connection modes of elements at the goethite interface were preliminarily speculated. The results indicate that the promotion of oxyanions on the fixation of heavy metal cations is more complex than expected, making it difficult to describe using only one mechanism.