Effects of Mn or Al incorporation on the structure, composition, and As(III) adsorption of oxidized green rust

The trace cations could incorporate into the structure of oxidized green rust (Ox-GR), a layered reactive Fe(III) oxyhydroxide formed from rapid oxidation of GR, but the effects of cation incorporation on the mineralogical properties and surface reactivity of Ox-GR remain unknown. Here, we synthesized Mn- or Al-incorporated Ox-GR by oxidation of sulfate-bearing GR and determined their structure, elemental composition and distribution, and As(III) adsorption using macroscopic batch experiments and spectroscopic analyses. The presence of Mn or Al favored sulfate accumulation in Ox-GR, with some sulfate being homogeneously distributed in the interlayer, others adsorption on the mineral edge sites. Majority of Mn and Al entered the layer structure of Ox-GR as Mn(III) and Al(III) through isomorphous substitution, leading to the increased d-spacing of (001) plane but slightly decreased d-spacing of a-b planes. The Al incorporation remarkably reduced the structural ordering degree and Fe octahedral layers of Ox-GR through inhibiting the crystal-growth of GR. Compared with the Mn incorporation, the Al incorporation led to a more pronounced structural variation of Ox-GR, ascribed to its higher isomorphic substitution amount. The incorporation of Mn or Al both promoted As(III) adsorption per mineral mass, predominantly due to the increase of sulfate content and/or specific surface area, and the incorporated Mn(III) could oxidize As(III). As(III) adsorption on Ox-GR involved both surface sulfate and > Fe-OH/OH2 groups exchange, forming a bidentate-binuclear inner-sphere surface complexation. These new insights into the structure and reactivity of Ox-GR are essential to understanding environmental behavior of GR and its derivative Ox-GR in artificial and environmental settings. •Presence of Mn or Al favored sulfate accumulated in the interlayer and adsorbed on edge sites of Ox-GR.•Majority of Mn and Al incorporated into the layer structure, thus increasing d(001)-spacing but decreasing da-b plane-spacing.•Incorporation of Mn or Al promoted As(III) adsorption of Ox-GR, and the incorporated Mn(III) could oxidize As(III).•As adsorption on Ox-GR involved a bidentate-binuclear inner-sphere complex by exchanging both Fe-OH and sulfate groups.