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Arsenic (As) is a toxic trace metalloid found naturally in the earth’s crust. Due to weathering of arsenic-bearing minerals, volcanic activity or anthropogenic activities, high quantities of As can be moved to the surface and into the surrounding soils. Leaching of arsenic from polluted areas to nearby water bodies poses a risk to local water supplies.In polluted soils, arsenic is mainly present in its inorganic forms arsenite (AsIII) and arsenate (AsV). AsV is less toxic and easily immobilized by poorly crystalline iron (Fe) and manganese (Mn) oxides while AsIII is more toxic and mobile. Arsenate is dominantly present under oxic conditions, meanwhile AsIII usually prevails under anoxic conditions. Manganese oxides are known to rapidly oxidize AsIII to AsV under anoxic conditions, however, the presence of aqueous FeII inhibits their oxidation capacity [1]. Iron oxides such as magnetite are strong sorbents of arsenite, yet AsIII oxidation only occurs in the presence of aqueous FeII [2]. In nature, Mn and Fe often substitute for one another but the reactivity of mixed minerals towards As under anoxic conditions remains largely unknown.Here, we studied arsenite oxidation during sorption to jacobsite ((MnII,MnIII)FeIII2O4) by reacting 1g/L jacobsite with 26µM AsIII at pH 7 (50 mM MOPS buffer) under anoxic conditions for 28d. The mineral was reacted pure or after equilibration for one hour with 1mM aqueous FeII, MnII and CaII before spiking with AsIII to investigate the role of competing divalent cations. Aqueous samples were collected over time to measure the As and cation concentrations with ICP-MS. The oxidation state of As, Fe and Mn in the solid phase was measured with their respective K-edge XANES after 6h, 48h, 7d and 28d.After 6h, ca. 85% of the AsIII was removed from solution in the cation-free, MnII- and CaII-reacted treatments, while 99% of the arsenite was removed in the FeII-reacted treatment. Results from linear combination fitting (LCF) of the As K-edge XANES spectra showed the near complete oxidation of AsIII to AsV within 7d in the cation-free (97% AsV) and CaII-reacted treatment (93% AsV). In comparison, AsIII oxidation was slower for the jacobsite equilibrated with MnII (83% AsV after 7d), but still near complete after 28d (96% AsV). Equilibration of jacobsite with aqueous FeII significantly inhibited As oxidation and after 28d only 40% of the AsIII was oxidized to AsV. Results from the LCF of the Mn K-edge XANES spectra revealed reduction of structural MnIII by 15% after the reaction with aqueous FeII which might cause the reduced oxidation capacity of jacobsite.Our results demonstrate the strong As sorption capacity of jacobsite under anoxic conditions. However, the presence of other redox-active elements in soil solution may hinder the oxidation of arsenite on the jacobsite surface. These results have implications for remediation approaches seeking to remove As from contaminated soil to provide safe water supplies. [1] Pettit Mock, R. et al. 2019. ACS Earth Space Chem. 3 (4), 550-561.[2] Gubler, R. & ThomasArrigo, L. 2021. J. Hazard. Mat. 402, 123425.