Comparing the efficacy of VUV and UVC/S2O82- advanced oxidation processes for degradation and mineralization of cyanide in wastewater

• CN-CN- oxidation by UVC/S2O82- and VUV advanced processes was investigated.
• Both UVC/S2O82- and VUV processes oxidized CN-CN- to much less noxious compounds.
• VUV process completely oxidized cyanide at a much less fluence than UVC/S2O82-.
• A pathway was proposed for CN-CN- oxidation in the VUV process.
• VUV process oxidized CN-CN- and simultaneously reduced Cr(VI) to Cr(III) in real sample.

The present study was designed to evaluate cyanide degradation and mineralization in an industrial wastewater by vacuum ultraviolet (VUV) and ultraviolet/persulfate (UVC/S2O82-) advanced processes. The effect of operational conditions such as cyanide concentration, air/O2 blowing, reaction time, S2O82- concentration, and presence of Cr(VI) were studied. The possible intermediates of the reaction were also tracked. It was observed that under optimal condition, complete destruction of 50 mg/L cyanide was reached in 15 min and 50 min for VUV-air and UVC/S2O82- processes, respectively. Tracking of the generated by-products revealed that VUV and UVC/S2O82- processes completely oxidized and mineralized cyanide to much less noxious compounds, mainly ammonia and carbonate. Application of VUV process for chromium electroplating wastewater (68 mg/L cyanide and 24 mg/L Cr(VI)) showed that complete oxidation of cyanide along with 72% reduction of Cr(VI) to Cr(III) was reached in a short reaction time of 18 min. Accordingly, the homogenous VUV advanced process was found to be an efficient chemical-less method appropriate for simultaneous oxidation of cyanide and reduction of Cr(VI) in industrial wastewater.