Arsenate and cadmium co-adsorption and co-precipitation on goethite.

Arsenate (As(V), AsO4(3-)) and cadmium (Cd) are among the toxic elements of most concern. Their sorption behaviors on goethite were studied by batch experiments (pH edges, isotherms and kinetics) and X-ray diffraction (XRD). Arsenic coordination environment was explored by X-ray absorbance fine structure (EXAFS) analysis. Sorption isotherms of both As(V) and Cd on goethite could be divided into the adsorption-dominated and precipitation-dominated parts, while their sorption showed different pH-dependency and sorption reversibility. Cadmium adsorption was enhanced in the presence of AsO4(3-), which could be explained by the decrease in the electrostatic potential due to the sorption of AsO4(3-) and the formation of a ternary Cd-As(V)-goethite complex. Based on the EXAFS study, AsO4(3-) adsorbed on goethite mainly formed bidentate-binuclear complex. The high loadings of Cd changed the As(V)-Fe distance and its coordination number. However, Cd did not affect the As(V) adsorption amount in the adsorption-dominated region. When As(V) and Cd formed co-precipitates, their sorption amounts were both increased. The formation of co-precipitates decreased the mobility of Cd but increased the mobility of As(V) because less As(V) was sorbed on goethite through surface complexation. This study will provide better understandings on As(V) and Cd transport and useful information on their remediation strategies.