Competitive adsorption of arsenate and phosphate on hematite facets: Molecular insights for enhanced arsenic retention.

Understanding the competition for adsorption between arsenate and other common oxyanions at mineral-water interfaces is critical for enhancing arsenate retention in the subsurface environment and mitigating exposure risks. This study investigated the competitive adsorption between arsenate and phosphate on hematite facets using batch experiments, together with in-situ infrared spectroscopy, two-dimensional correlation spectroscopy (2D-COS), and ab initio molecular dynamic (AIMD) simulations. This study's findings revealed that hematite exhibited notable selectivity for arsenate over phosphate in both adsorption capacity and rate, with selectivity significantly influenced by the exposed facets of the hematite and reaction concentrations. To wit, the (001) facet exhibited stronger selectivity for arsenate than the (110) facet, and increasing reaction concentration further enhances this selectivity. This selectivity was driven by surface hydroxy structure-mediated complexation, where both surfaces primarily formed stable inner-sphere monodentate complexes with an affinity for arsenate. On the (001) surface, the available Fe2OH featured two close-spaced iron sites (Fe - Fe ≈ 2.86 Å), enabling arsenate to interact with both sites simultaneously, significantly boosting arsenate selectivity. At higher surface loadings, the (110) surface formed partially more selective bidentate binuclear complexes, further enhancing arsenate retention. These findings emphasize the critical role of interfacial complexation, particularly the formation of inner-sphere bidentate complexes and the availability of iron sites, in controlling arsenate retention. By tailoring mineral facets and optimizing reaction conditions to improve iron site availability and promote bidentate complexation, arsenate retention can be significantly enhanced in phosphate-rich aquatic environments, such as rivers and groundwater in agricultural areas.