Mineral transformation of structural Fe(II) hydroxides with O2, Cu(II), Cr(VI) and NO2− for enhanced arsenite sequestration

Structural Fe(II/III) hydroxides on the shell of nano-zerovalent iron (nZVI) are reactive sites for As(III) adsorption. Well-controlled structural Fe(II) hydroxides (SFH) is prepared to serve for As(III) removal in present study. With oxygen As(III) removal by SFH occurred efficiently but the removal efficiency was greatly reduced under anoxic conditions. The mineral transformation from SFH to lepidocrocite probably contributed to the robust removal efficiency of As(III). It is found that Cu(II), Cr(VI) and NO 2 − in aqueous solution could act as oxidants and promote the mineral transformation of SFH. A more efficient As(III) removal was therefore observed without oxygen. The removal efficiency decreased in the order O 2 /SFH   ≈   Cu(II)/SFH   >   Cr(VI)/SFH   ≈   NO 2 − /SFH at near neutral pH. The removal efficiency was less dependent on the initial pH for O 2 /SFH and Cu(II)/SFH systems than Cr(VI)/SFH and NO 2 − /SFH systems. Distribution of As on solid phases was able to describe the removal mechanism of As(III), and the results showed that the mineral transformation promoted the precipitation process and inhibited outer-sphere complexation, thus leading to enhanced As removal. With Cu(II), Cr(VI) and NO 2 − , the generation of oxidants from SFH transformation drove As(III) oxidation, and contributed to total As removal.