Mechanisms of efficient As(III) and As(V) removal by Ni-coprecipitated hausmannite nanocomposites

Being one of the low-valence manganese oxides ubiquitous in surficial environments, hausmannite has high adsorption and redox reactivities towards various contaminants. Natural hausmannite often contains various other cations, such as Ni2+, but it is unclear yet on the crystal chemistry of the foreign cations in the hausmannite structure and as-induced changes in the mineral physicochemical properties and reactivities. In this study, Ni-containing hausmannites were synthesized by Ni2+ coprecipitation with Mn2+, and then were characterized by powder XRD, FTIR, HRTEM, Ni K-edge XAFS, and wet chemical analysis. Then As(V) adsorption and As(III) oxidation behaviors on these nanocomposites were investigated by batch experiments and reaction mechanisms were deciphered by As K-edge XAFS. These nanocomposites have higher points of zero charges and larger specific surface areas than the pristine hausmannite. The dopant mainly exists as alpha-Ni(OH)(2) coatings on the hausmannite surfaces. Compared to the pure hausmannite, these Ni-containing nanocomposites have enhanced As(V) adsorption capacity per unit mass but reduced adsorption affinity and density; while they can also quickly remove As(III) from waters by oxidation, adsorption and fixation, though the initial oxidation rate is slightly reduced. Arsenic immobilized on the mineral surfaces is As(V) and binds with active Mn/Ni sites with an average As-Mn/Ni distances of 3.28-3.30 angstrom, which can be assigned to bidentate binuclear complexes. These results not only provide new insights into the mediations of As geochemical behaviors by low valence Mn oxide minerals, but also deepen our understanding of the interaction mechanisms of various transition metals with these minerals.

Automated summary

This study investigates the effects of nickel ions on the physicochemical properties and reactivities of hausmannite. The synthesized Ni-containing nanocomposites exhibit higher adsorption capacity and larger specific surface areas compared to pure hausmannite. These nanocomposites can quickly remove arsenic from water through oxidation, adsorption, and fixation. Arsenic immobilized on the mineral surfaces forms bidentate binuclear complexes with active Mn/Ni sites.