Degradation mechanism of cyanide in water using a UV-LED/H₂O₂/Cu²⁺ system

In this study, we developed a UV-LED/H₂O₂/Cu²⁺ system to remove cyanide, which is typically present in metal electroplating wastewater. The results showed the synergistic effects of UV-LED, H₂O₂, and Cu²⁺ ions on cyanide removal in comparison with UV-LED photolysis, H₂O₂ oxidation, UV-LED/H₂O₂, and H₂O₂/Cu²⁺ systems. Cyanide was removed completely in 30 min in the UV-LED/H₂O₂/Cu²⁺ system, and its loss followed pseudo-first order kinetics. Statistically, both H₂O₂ 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 H₂O₂ (pK_a of H₂O₂ = 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/H₂O₂/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/H₂O₂/Cu²⁺ system may be an alternative method of cyanide removal from wastewaters.