Rapid arsenite oxidation by jacobsite under anoxic conditions: The role of divalent cations

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 (As^III) and arsenate (As^V). As^V is less toxic and easily immobilized by poorly crystalline iron (Fe) and manganese (Mn) oxides while As^III is more toxic and mobile. Arsenate is dominantly present under oxic conditions, meanwhile As^III usually prevails under anoxic conditions. Manganese oxides are known to rapidly oxidize As^III to As^V under anoxic conditions, however, the presence of aqueous Fe^II inhibits their oxidation capacity [1]. Iron oxides such as magnetite are strong sorbents of arsenite, yet As^III oxidation only occurs in the presence of aqueous Fe^II [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 ((Mn^II,Mn^III)Fe^III₂O₄) by reacting 1g/L jacobsite with 26µM As^III 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 Fe^II, Mn^II and Ca^II before spiking with As^III 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 As^III was removed from solution in the cation-free, Mn^II- and Ca^II-reacted treatments, while 99% of the arsenite was removed in the Fe^II-reacted treatment. Results from linear combination fitting (LCF) of the As K-edge XANES spectra showed the near complete oxidation of As^III to As^V within 7d in the cation-free (97% As^V) and Ca^II-reacted treatment (93% As^V). In comparison, As^III oxidation was slower for the jacobsite equilibrated with Mn^II (83% As^V after 7d), but still near complete after 28d (96% As^V). Equilibration of jacobsite with aqueous Fe^II significantly inhibited As oxidation and after 28d only 40% of the As^III was oxidized to As^V. Results from the LCF of the Mn K-edge XANES spectra revealed reduction of structural Mn^III by 15% after the reaction with aqueous Fe^II 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.