Co-adsorption of Cd(II) and Sb(III) by ferrihydrite: a combined XPS and ITC study

Abstract

Heavy metal and metalloid commonly coexist in soils and sediments, and interact frequently with various minerals. The coexistence of Sb and Cd is commonly observed in Sb mine area, but their co-adsorption behaviors to soil minerals still remain poorly understood. This study aimed to elucidate the co-adsorption characteristics of Cd(II) and Sb(III) by ferrihydrite (Fh) under anoxic condition. Batch experiments were performed to determine the sorption capacity of Cd(II) and Sb(III) in both single and binary systems. The major functional groups that were responsible for Cd(II) and Sb(III) sorption were determined by X-ray photoelectron spectroscopy (XPS), while the thermodynamic sorption mechanisms were elucidated using isothermal titration calorimetry. Cd(II) sorption on Fh increases with increasing pH levels (4–8) whereas Sb(III) sorption shows less variation with pH level variations. The Langmuir adsorption capacity is 55.54 mg/g for Cd(II) and 188.19 mg/g for Sb(III). In Cd–Sb binary systems, Cd(II) sorption is significantly diminished whereas Sb(III) uptake is close to single Sb(III) sorption. XPS indicates the Fe–OH groups are mainly responsible for the binding of Cd and Sb, possibly through the formation of inner-sphere complexes. This hypothesis is further confirmed by the positive entropy (ΔS) after Cd and/or Sb binding. A larger ΔS in the binary Cd–Sb titration than in their single titrations implies the formation of a ternary Fh–Sb–Cd complex, which results in a higher disorder of the sorption system. The presence of Sb(III) reduces Cd(II) sorption whereas Cd(II) has a negligible effect on Sb(III) sorption to ferrihydrite; moreover, Sb(III) and Cd(II) might form surface ternary complexes in binary systems. These new findings have important implications for predicting the sequestration, migration, and fate of Cd and Sb in soils.