Catalytic Roles of Mn(II) and Fe(III) in the Reduction of Cr(VI) by Mandelic Acid under an Irradiation of Simulated Solar Light.

The catalytic roles of manganese(II) [Mn(II)] and iron(III) [Fe(III)] in the reduction of hexavalent chromium [Cr(VI)] by mandelic acid, both with and without the presence of light, was investigated through a series of batch experiments. The results demonstrated that both Mn(II) and Fe(III) could markedly accelerate the photoreduction of Cr(VI) by mandelic acid; additionally, the catalysis of Fe(III) was superior to that of Mn(II). Without the presence of light, Mn(II) still enhanced the reduction of Cr(VI) by mandelic acid, but Fe(III) did not exert any impact on the reaction. When compared with the activities of phenylacetic acid, it was concluded that the catalytic roles of both Mn(II) and Fe(III) in the reduction of Cr(VI) were directly related to the alpha-OH group and not to the carboxyl group. The photoreduction of Cr(VI) by mandelic acid, assisted by Mn(II), followed pseudo-first-order kinetics at the initial stage of the reaction, but in the presence of Fe(III) it followed a pseudo-zero-order model.