Redox transformation of arsenic by magnetic thin-film MnO2 nanosheet-coated flowerlike Fe3O4 nanocomposites

- Magnetic MnO2 nanosheet-coated flowerlike Fe3O4 nanocomposites were synthesized.
- The ratio of Mn/Fe was a key factor influencing the uptake of arsenic in aqueous solution.
- Redox transformation of arsenic in liquid-solid phase were simultaneously explored.
- MnO2 acted as an oxidant, while Fe3O4 played the role as arsenic species adsorbent.

Inorganic arsenic cycling in the natural environment is significantly controlled by manganese/iron oxides, and studying the redox transformation of arsenic in both aqueous and solid phase is therefore essential to understand its environmental toxicity and predict its environmental behavior. Herein, the synthesized magnetic thin-film MnO2 nanosheet-coated flowerlike Fe3O4 nanocomposites have been used to simultaneously investigate the potential redox transformation of arsenic in aqueous phase and arsenic speciation on the solid phase interface. The results showed the initial Mn/Fe ratio has serious influenced the morphologies, textural properties and uptake of arsenic in aqueous solution. The maximum adsorption capacities of As(III) and As(V) by MF2 were 76.73 mg/g and 120.50 mg/g, respectively. After 10 h reaction, the total arsenic concentration in both As(III)-MF2 system and As(V)-MF2 system far below than that of 10 μg/L, which was setted the maximum guideline concentration value in drinking water by World Health Organization (WHO). HPLC-ICP-MS revealed that in aqueous solution part of As(III) was oxidized into As(V), while As(V) was not reduced into As(III). X-ray absorption near edge structure (XANES) and XPS analysis from solid phase further confirmed that the MnO2 nanosheet mainly acted as an oxidant, while flowerlike Fe3O4 only played the role as an arsenic species adsorbent. Extended X-ray absorption fine structure (EXAFS) analysis indicated that both As(III) and As(V) formed inner-sphere bidentate binuclear corner-sharing (2C) complexes with an As-Fe interatomic distance of 3.32-3.34 Å on the interface of As-MF2 solid phase.

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