Insights\non the Contradiction between the Affinity\nand Capacity of Ferrihydrite toward As(III) and As(V): Surface Reaction\nRevisited

Ferrihydrite\nis omnipresent in nature, and its adsorption of As(III/V)\ndecides the migration of arsenic. Although As(III) is commonly recognized\nas the more mobile species of inorganic arsenic, it sometimes exhibits\nless mobility in ferrihydrite systems, which calls for further insights.\nIn this study, we elucidated the adsorption behavior and mechanisms\nof As(III/V) on ferrihydrite under different loading levels (molar\nratio As/Fe = 0–0.38), solution pH (3–10), and coexisting\nions [P(V) and Ca(II)] based on batch adsorption experiments, surface\ncomplexation modeling, density functional theory calculations, and\nX-ray photoelectron spectroscopy. Our results show that As(III) exhibits\nweaker adsorption affinity but a larger capacity compared with that\nof As(V). On ferrihydrite, As(III) and As(V) are adsorbed mainly as\nbidentate mononuclear complexes at type-a sites [Fe(OH<sup>–0.5</sup>)<sub>2</sub>] and bidentate binuclear complexes\nat type-b sites (2FeOH<sup>–0.5</sup>), respectively.\nAs the dosage increases, As(III) further forms mononuclear monodentate\ncomplexes at both surface sites, resulting in a higher site utilization\nefficiency, while As(V) does not due to repulsive electrostatic interaction.\nThe difference in surface species of As(III/V) also leads to complex\nresponses when coexisting with high concentrations of P(V) and Ca(II).\nThis study helps us to understand environmental behavior of As(III/V)\nand develop remediation strategy in As(III/V) contaminated systems.