A mechanism study of light-induced Cr(VI) reduction in an acidic solution

The mechanisms of photo-catalytic reduction of Cr(VI) were investigated in acidic solutions with and without Fe(III). In a system without Fe(III), no Cr(VI) reduction was observed in dark conditions; conversely, under light conditions, the reduction reaction rate increased to 0.011 and 0.020 μM min^-1 at pH 2 and pH 1, respectively, indicating the occurrence of Cr(VI) photo-reduction. The Cr(VI) photo-reduction reaction was induced by the photolysis of water molecules, leading to O₂ production. Upon the addition of Fe(III), the photo-reduction rate of Cr(VI) was significantly enhanced due to the formation of Fe(II), which is the photolytic product of FeCl₂⁺ and the electron donor for Cr(VI) reduction. However, with the same concentration of FeCl complexes, a strong inhibition of Cr(VI) reduction at pH 2 was observed, compared with pH 1. A possible explanation is that FeOH²⁺ becomes predominant with increasing pH and that its photolytic product, the OH free radical, is an oxidant for Fe(II) and Cr(III) and can compromise Cr(VI) reduction. The kinetic result of each photo-reduction reaction pathway shows zero-order kinetics, suggesting that the photolysis reaction of H₂O or FeCl²⁺ is the rate-determining step in each pathway. The results also show the potential of developing a homogeneous photo-catalytic method to treat Cr(VI)-containing water.