Photocatalytic reduction of Cr(VI) by small molecular weight organic acids over schwertmannite

In this study, a series of bath experiments was carried out to investigate the photoreduction of Cr(VI) by small molecular weight organic acids (SOAs) over schwertmannite, a mineral found in acid mine drainage (AMD). The results demonstrated that schwertmannite or SOAs alone was unable to effectively transform Cr(VI) to Cr(III) even if exposed to an illumination of mimic solar light. However, an addition of schwertmannite significantly enhanced the reduction of Cr(VI) by SOAs under the same condition. For example, 100 μM Cr(VI) was almost completely removed within 50 min in the presence of both schwertmannite (0.6 g L−1) and oxalic acid (300 μM) at pH 3.0. The photocatalytic reduction of Cr(VI) was strongly influenced by pH, the initial concentrations and the structures of SOAs. Of the tested three SOAs, the reaction rates of photocatalytic reduction of Cr(VI) were in the order of oxalic acid > citric acid > tartaric acid. The reaction obeyed to zero-order kinetics with respect to Cr(VI) with excess SOAs. A possible mechanism for photoreduction of Cr(VI) by SOAs over schwertmannite was proposed. Fe(III) on the surface of schwertmannite was dissolved by SOAs, and then Fe(III)–SOA complexes with high photochemical activity formed. Further, Fe(II) together with organic acid radicals, CO2- and O2-, was generated through a metal–ligand-charge-transfer pathway (MLCT), leading to a rapid reduction of Cr(VI).

► Schwertmannite was obtained through oxidation of FeSO4 by A. ferrooxidans.
► Schwertmannite was an efficient photocatalyst for Cr(VI) reduction by organic acids.
► Schwertmannite photoreduction of Cr(VI) depended on the structures of organic acids.