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

Qiantao Shi; Shujuan Zhang; George P Korfiatis; Christos Christodoulatos; Xiaoguang Meng

doi:10.1016/j.scitotenv.2020.138216

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

Sci Total Environ. 2020 Jul 1:724:138216. doi: 10.1016/j.scitotenv.2020.138216. Epub 2020 Mar 30.

Authors

Qiantao Shi¹, Shujuan Zhang², George P Korfiatis², Christos Christodoulatos², Xiaoguang Meng³

Affiliations

¹ Center for Environmental Systems, Stevens Institute of Technology, Hoboken, NJ 07030, United States. Electronic address: qshi4@stevens.edu.
² Center for Environmental Systems, Stevens Institute of Technology, Hoboken, NJ 07030, United States.
³ Center for Environmental Systems, Stevens Institute of Technology, Hoboken, NJ 07030, United States. Electronic address: xmeng@stevens.edu.

PMID: 32272407
DOI: 10.1016/j.scitotenv.2020.138216

Abstract

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.

Keywords: Arsenate; Bicarbonate; Calcium; Dissolved Ca complex.