The behavior of chromium and arsenic associated with redox transformation of schwertmannite in AMD environment

Schwertmannites are metastable and often contaminated by chromium and arsenic. However, when schwertmannite is subjected to reducing conditions, the effect of reductive dissolution of schwertmannite coupled to the potential reduction of Cr(VI) or As(V) on the behavior of chromium and arsenic are less known. The present study systematically explored Fe(II)-induced transformation of schwertmannite with adsorbed CrO2-4 or AsO3-4 at pH 6.5, and the consequent redistribution of chromium and arsenic. The results show that adsorption of oxyanions obviously facilitated the release of schwertmannite-bound SO(4)(2- )from both the mineral surface and the lattice structure via SO42--exchange. Adsorbed chromate and arsenate both inhibited the Fe(II)-induced transformation of schwertmannite. CrO2-4 -schwertmannite persisted as the dominant mineral phase for up to 30 d as a consequence of Fe(II) consumption and coverage of Cr(III)-Fe(III) hydroxide. In contrast, AsO3-4-schwertmannite transformed to poorly crystalline goethite via an intermediate lepidocrocite in 4 mM Fe(II) treatment. Both sorption and redox processes of Cr(VI) imprisoned chromium on schwertmannite. Transformation of AsO3-4-schwertmannite caused no re-mobilization of arsenic into solution, indicating the repartition to secondary minerals. Reduction of Cr(VI) to Cr(III) by Fe(II) would greatly lower the environmental hazard of chromium, and the persistence of As(V) would limit the toxicity of arsenic. These findings will help to understand the geochemical behavior of the important toxic Cr(VI) and As(V) specifically in the environmental systems with cyclic redox conditions. (C) 2019 Elsevier Ltd. All rights reserved.