The visible-light-driven photo-reduction of Cr(VI) by formic acid in aqueous solution

To study the applicability and mechanism of the visible-light-driven photo-reduction of Cr(VI) is important since it will elucidate the chromium cycle in natural circumstances, and since this reduction reaction is important in treating wastewater contaminated by Cr(VI). Herein, we conducted an in-depth mechanism study for this process using formic acid as electron donor. The positive dependence of the Cr(VI) conversion on the incident light intensity and the good match between the apparent quantum yield (0.0039-0.39%) and the light absorption spectrum (550-365 nm) suggests a light-induced mechanism. The light-induced charge transfer occurs through the ligand-to-metal charge transfer state of the complex which forms between one dichromate molecule and two formic acid molecules through symmetrical or asymmetrical configuration as demonstrated by DFT simulation. EPR analysis further proves that this reaction occurs through a stepwise process with Cr(V) and Cr(IV) as the intermediates. The practical use of this photochemical procedure with broad electron donor scope and wide pH, reaction temperature, and initial Cr(VI) concentration ranges, is proven by the application potential evaluation. The efficient reduction of Cr(VI) by formic acid under sunlight further proves the applicability of this photochemical procedure for the treatment of Cr(VI) contaminated wastewater. Development of the present study into both the treatment of Cr(VI) contaminated wastewater, and other visible-light-driven photo-redox reactions between metals and organics are the subject of further research and results can be expected in the near future.