Reduction of chromate with UV/diacetyl for the final effluent to be below the discharge limit

Reduction of Cr(VI) to Cr(III) is helpful to lower the toxicity risk and also necessary for the removal of chromium from waste streams through alkaline precipitation. We compared the reduction of Cr(VI) in six UV systems with oxalic acid (OA), ethylenediaminetetraacetic acid (EDTA), salicylic acid (SA), hydroquinone (HQ), acetylacetone (AA) and diacetyl (BD) as chelating or non-chelating photo-activators. Overall, HQ, AA and BD were much more efficient than the carboxylic acids for the photo-reduction of Cr(VI). By introduction of UV to HQ system, the pseudo-first-order rate constant of Cr(VI) reduction at pH 5.1 was increased about 50 times. However, due to the formation of colloidal polymers, the UV/HQ treated solutions were dark in color and had a high turbidity (82 NTU). The effects of AA and BD on the photoreduction of Cr(VI) were similar. The UV/BD treated solution was colorless and clear with a turbidity lower than 1 NTU and a residual Cr less than 0.1 mg/L. The results demonstrate that UV/BD is a promising approach for the treatment of Cr(VI)-laden wastewater. The findings here also suggest that utilization of diketones in redox conversion of contaminants is a topic deserving further research.