Identifying the existence and molecular structure of the dissolved HCO₃-Ca-As(V) complex in water

Calcium (Ca²⁺) and bicarbonate (HCO₃^-) ions co-exist with arsenic (As) in natural water systems, while Ca-based materials such as lime and cement are widely used to immobilize As(V) in contaminated solids. In this paper, a new dissolved ternary complex, HCO₃-Ca-As(V), was discovered and its molecular structure was identified. The results from the batch experiments showed that adding As(V) to the solutions containing Ca²⁺ and HCO₃^- increased the dissolved Ca concentration from 4.8 to 73.2 mg/L at pH 11. Both infrared and X-ray absorption spectroscopy indicated the presence of dissolved HCO₃-Ca-As(V) complex. Based on the quantitative geometric information obtained from the spectroscopic results, the molecule of (OH)OC-O-(OH₂)₄Ca-O₂-As(OH)₂ was identified by the density functional theory (DFT) calculation. Although Ca²⁺ and As(V) can form complex without HCO₃^-, the presence of HCO₃^- further enhanced the stability of the dissolved Ca complex, as evidenced by the lower binding energy (BE) of HCO₃-Ca-As(V) (-329.1959 kJ/mol) than Ca-As(V) (4.7171 kJ/mol). The discovery of dissolved HCO₃-Ca-As(V) complex is important for understanding the mobility of As(V) in natural water, and the possible release of As(V) in contaminated solids treated with Ca-based materials.