Exploring the interference mechanisms of surface and aqueous complexes with groundwater arsenate and arsenite adsorption

Abstract

Ca²⁺, Mg²⁺, and HCO₃^- are extremely common coexisting ions with arsenic (As) in geogenic As-polluted groundwaters. Although extensive research has improved our knowledge of groundwater As removal techniques and mechanisms, there is still a lack of a definite explanation of the distinct influences of Ca²⁺ and Mg²⁺ on As immobilization. Furthermore, the question of whether the occurrence of metal-As aqueous complexes has positive or detrimental effects on As adsorption is still open, which hinders our ability to predict the effectiveness of groundwater As removal. The goal of our present work was to investigate the molecular-level interference mechanisms of Ca²⁺, Mg²⁺, and HCO₃^- on arsenic adsorption with batch/column filtration experiments and spectroscopic techniques. The results showed that the co-presence of Ca²⁺ and As significantly increased As(V) and As(III) adsorption by 22.1 and 12.2% in batch studies and by 20.1 and 16.7% in column adsorptive filtrations, which could be explained by forming a ternary Ca-As-TiO₂ complex. Without the surface complex, Mg²⁺ only had a slightly positive effect on As removal. Co-existence of Ca²⁺ and HCO₃^- prevented the generation this surface complex, which significantly decreased the elimination of As(III). Inversely, the As(V) ternary complex and adsorption were not interfered by HCO₃^-. Moreover, an aqueous Ca-As(V) complex was detected without surface, which facilitated generation of the surface complex and As(V) adsorption. The results of this work clarified the distinct effects of Ca²⁺ and Mg²⁺ and As(V) and As(III) adsorption, which was critical in predicting the As adsorption efficiency in column filtration processes.

Keywords: Adsorption; Aqueous complex; Column adsorptive filtration; Groundwater arsenic; Surface complex; TiO2.