Photochemical degradation of thiocyanate by sulfate radical-based advanced oxidation process using UVC KrCl-excilamp

The degradation of thiocyanates (SCN-) by UV-C-activated persulfate (PS) in the presence of ferric ion (Fe3+) was investigated. As a source of monochromatic far-UV-C irradiation (222 nm), mercury-free KrCl excimer lamp was used. Results showed that compared with direct photolysis, UVC/ PS and PS/ Fe3+, the combined UVC/PS/Fe3+ treatment had the highest initial reaction rate omega(0) and removal efficiency. 99.99% conversion of thiocyanates (100 mg/L of initial concentration) was achieved in 40 min. The addition of Fe3+ in the UVC/ PS treatment was found to reduce energy consumption (calculated as amount of oxidized thiocyanates per consumed electrical energy) by 4.5 times, while only a 30% difference between direct photolysis and UVC/ PS was observed. The high efficiency of the UVC/ PS/ Fe3+ process revealed a synergistic effect (synergy index f = 1.98). The effect of the initial SCN, PS, and Fe3+ molar ratios and UV-C exposure time on SCN removal in UVC/ PS/Fe3+ was further investigated. It was found that at molar ratios [S2O82-]:[SCN-] = 3:1 and [S2O82-](:)[Fe3+] = 1:0.1, effective decomposition of SCN in a wide initial concentration range (from 50 to 500 mg/L or 0.86-8.6 mM) can be achieved. The strong role of (OH)-O-center dot and SO4 center dot- in the removal of SCN- was confirmed by the addition of radical scavengers. It was demonstrated that the presence of Cu2+ in simulated gold mine wastewater effluents neutralizes the inhibitory effects that S2O32- and NH4+ have on the degradation process.