Formic acid motivated photocatalytic reduction of Cr(VI) to Cr(III) with ZnFeO nanoparticles under UV irradiation.

UV-light irradiated photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution using ZnFeO nanoparticles in the presence of formic acid was reported. X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and UV-Vis diffuse reflectance spectroscopy (DRS) were employed to characterize ZnFeO nanoparticles. The photocatalytic activity of pure ZnFeO under UV irradiation was significantly low. However, the Cr(VI) reduction efficiency on nano-sized ZnFeO in the presence of 0.40% formic acid reached 95.4% within 4 h. Herein, the effect of pH, photocatalyst amount, initial concentration of Cr(VI) and formic acid concentration on the photocatalytic reduction of Cr(VI) was investigated. The results indicated that the photocatalytic reduction of Cr(VI) decreased with increase in the initial concentration of Cr(VI), photocatalyst dosage and pH. The reduction rate constant declined from 0.017 min to 0.0023 min with the increase in initial concentration of Cr(VI) from 5 to 25 mg L. However, the reduction rate constant sharply increased from 0.000075 min to 0.0127 min with the increase in formic acid concentration from 0.05% to 0.40%. The formic acid could capture the photogenerated holes, and eventually formate (HCOO) ions could be converted into carbon dioxide radicals (•CO). Because of more negative redox potential for •CO radicals, Cr(VI) species could easily be reduced to Cr(III) under UV irradiation. The pseudo-first-order kinetic reaction was confirmed for this reduction process. A tenable mechanism for the photocatalytic Cr(VI) reduction has also been demonstrated.