Kinetics of arsenic oxidation and adsorption on phenol-rich organic matter-carbonate complexes

The mobility and availability of arsenic (As) is closely related to carbonate minerals, which are ubiquitous and stable in various geologic environments. These minerals commonly co-precipitate with organic matter (OM) to form OM-carbonate complexes. However, their roles in controlling As speciation and immobilization remain poorly understood. This study investigated the coupled kinetics of As(III) oxidation and adsorption on phenol-rich organic matter (PROM)-carbonate complexes. We found that reactive species, such as hydrogen peroxide (H₂O₂) and hydroxyl radical (•OH), were generated in PROM-carbonate complex suspensions and effectively oxidized As(III) at the solid-solution interface, with H₂O₂ serving as the dominant oxidant. The resulting As(V) was largely adsorbed by the carbonate fraction, while surface oxygen-containing functional groups provided additional sorption sites at high As loadings. X-ray absorption spectroscopy revealed that As primarily formed corner-sharing complexes between AsO₄ tetrahedron and Ca octahedron, alongside coordination with phenolic hydroxyl groups from PROM. Mg²⁺ incorporation into the calcite lattice enhanced As adsorption by strengthening electrostatic interaction, increasing site density, and facilitating substitution of carbonate groups by As oxyanions. A process-based kinetic model showed that Mg²⁺ incorporation increased the apparent rate constants of As(III) oxidation from 0.28 to 1.41 M^-1 s^-1 by H₂O₂ and from 0.71 to 2.61 × 10⁹ M^-1 s^-1 by •OH, respectively, by promoting As(III) surface enrichment, H₂O₂ cleavage, and As(V) removal. These findings highlight the pivotal role of commonly overlooked PROM-carbonate complexes in regulating As transformation kinetics, and facilitate the evaluation of As mobility and fate in natural environments.