Degradation mechanism of cyanide in water using a UV-LED/H 2 O 2 /Cu 2+ system.

In this study, we developed a UV-LED/H2 O2 /Cu2+ system to remove cyanide, which is typically present in metal electroplating wastewater. The results showed the synergistic effects of UV-LED, H2 O2 , and Cu2+ ions on cyanide removal in comparison with UV-LED photolysis, H2 O2 oxidation, UV-LED/H2 O2 , and H2 O2 /Cu2+ systems. Cyanide was removed completely in 30 min in the UV-LED/H2 O2 /Cu2+ system, and its loss followed pseudo-first order kinetics. Statistically, both H2 O2 and Cu2+ ions showed positive effects on cyanide removal, but Cu2+ ions exhibited a greater effect. The highest cyanide removal rate constant (k = 0.179 min-1 ) was achieved at pH 11, but the lowest was achieved at pH 12.5 (k = 0.064 min-1 ) due to the hydrolysis of H2 O2 (pKa of H2 O2  = 11.75). The presence of dissolved organic matter (DOM) inhibited cyanide removal, and the removal rate constant exhibited a negative linear correlation with DOM (R2  = 0.987). The removal rate of cyanide was enhanced by the addition of Zn2+ ions (from 0.179 to 0.457 min-1 ), while the co-existence of Ni2+ or Cr+6 ion with Cu2+ ion reduced cyanide removal. The formation of OH radicals in the UV-LED/H2 O2 /Cu2+ system was verified using an aminophenyl fluorescence (APF) probe. Cyanate ions and ammonia were detected as the byproducts of cyanide decomposition. Finally, an acute toxicity reduction of 64.6% was achieved in the system within 1 h, despite a high initial cyanide concentration (100 mg/L). In terms of removal efficiency and toxicity reduction, the UV-LED/H2 O2 /Cu2+ system may be an alternative method of cyanide removal from wastewaters.