Photocatalytic reduction of Cr(VI) by citric and oxalic acids over biogenetic jarosite

In this study, a series of bath experiments were carried out to investigate the photoreduction of Cr(VI) by small molecular weight organic acids (SOAs) over jarosite, a mineral found in acid mine drainage (AMD). The results demonstrated that jarosite 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 jarosite significantly enhanced the reduction of Cr(VI) by SOAs under the same condition. The photocatalytic reduction of Cr(VI) was strongly influenced by pH, the initial concentrations and the structures of SOAs. Of the tested two SOAs, the reaction rates of photocatalytic reduction of Cr(VI) were in the order of oxalic acid > citric 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 jarosite was proposed.