The role of surface hydroperoxyl copper (I) in degradation of Congo red pollutant using copper oxide composites

The use of CuO for degradation of organic pollutants suffers from a number of shortcomings, viz., low surface area, high H₂O₂ demand which causes high Cu(II) leaching, and the need for strong UV illumination. The present work aims at embedding CuO in silicate (CuO.SiO₂) and aluminosilicate (CuO.NaAlSiO₈) composites to reduce both Cu(II) leaching and adsorption of anionic dyes such as Congo red (CR) which is thought to give more opportunity for the reaction of H₂O₂ with the surface. CuO.SiO₂ (0.07%) and CuO.NaAlSiO₈ (14.4%) were found to have lower adsorption of CR than raw CuO (23.1%) with more efficient degradation efficiency in combination with H₂O₂ (89.0, 86.5 and 71.0%, respectively). The kinetics of degradation of CR was investigated as a function of H₂O₂ concentration and absence of illumination. Amalgamation of CuO with silica or aluminosilicate resulted in lower Cu leaching and lower dependence on H₂O₂ concentration. For example, 9.8 mM H₂O₂ gives 72% degradation within 210 min using CuO.NaAlSiO₈. The observed trend of rate constants (k₁, min⁻¹) at 97.9 mM H₂O₂ and 50 °C, which followed the order CuO (1.31 × 10^–2) > CuO.NaAlSiO₈ (8.17 × 10^–3) > CuO.SiO₂ (3.84 × 10^–3), reflected the limitations imposed by silica and aluminosilicate on the reach of CR to the surface which may resulted in more efficient attack of H₂O₂ on the surface even at lower H₂O₂ concentrations. DFT- B3P86 calculations revealed high favorability of Cu(I) complex with hydroperoxyl radical (HO₂^*), providing the active surface sites for degradation of CR.