Ascorbic Acid-Enhanced CuO/Percarbonate Oxidation: Insights Into the pH-Dependent Mechanism

Herein, the decontamination performance and likelihood of a shift in the reaction mechanism of CuO-activated percarbonate (SPC) in the presence of ascorbic acid (AA) (i.e., CuO/SPC/AA process) under varying pH conditions were explored. Experimental results revealed that sulfamethazine (SMT) could be well degraded over a wide pH range (3.0–9.0). Mechanistic investigations suggested that the hydroxyl radical (^•OH) was the major reactive oxygen species (ROS) at acidic pH, whereas Cu(III) played a dominant role in SMT degradation at neutral and alkaline pHs. Alternatively, ^•OH was determined to be the secondary reactive species generated from the hydrolysis of Cu(III) at neutral and alkaline pHs. Moreover, the role of AA and coexisting Na₂CO₃ was explored. It was concluded that the regeneration of Cu(I) played an important role in mediating the copper redox cycle under all of the studied pHs, which mainly benefited from reductive AA, and the available superoxide radical (O₂^•–) and deprotonated form of H₂O₂ could also contribute to this process at neutral and alkaline pHs. The coexisting Na₂CO₃ exerted a negligible impact on SMT degradation at acidic and neutral pHs, while a slight inhibitive effect was observed at alkaline pH. Simultaneously, the potential practicability of the CuO/SPC/AA process was evaluated. This work deepens the understanding of copper-catalyzed heterogeneous activation of percarbonate, widening the application of the reductant for water decontamination.