Oxalate enhanced photochemical decomplexation of Cu(II)-EDTA in a Cu-Fenton system

The removal of recalcitrant copper complexes requires an initial decomplexation step to free the copper ions, enabling their subsequent removal by alkaline precipitation. The conventional Cu-Fenton processes suffer from slow kinetics due to the rate-limiting reduction of Cu(II). This study introduces an oxalate-enhanced visible-light-driven Cu-Fenton process (H₂O₂/Cu(II)/OA/VL) that overcomes this limitation through a targeted photochemical mechanism. The synergistic use of oxalic acid (OA) and visible light (VL) dramatically enhanced Cu(II)-EDTA decomplexation, increasing copper removal efficiency from 22.78 to 94.37% and accelerating the reaction rate by 11.42-fold (rate constant: 0.0217 vs. 0.0019 min⁻¹). Mechanistic investigations revealed that OA coordinates with Cu(II) to form photoactive Cu(II)-oxalate complexes. Under VL irradiation, these complexes undergo a ligand-to-metal charge transfer (LMCT) process, thereby accelerating the rate-limiting reduction of Cu(II) to Cu(I). This sustains an active Cu(I)/Cu(II) redox cycle, enabling continuous H₂O₂ activation to generate a suite of reactive species (·OH, O₂^·-, and Cu(III)). These species collectively facilitate the complete degradation of EDTA via stepwise decarboxylation, yielding low-molecular-weight acids, CO₂, and inorganic ions. The environmental viability of the process is demonstrated by a significant reduction in phytotoxicity, as evidenced by improved lettuce growth. This work provides an efficient, iron-free advanced oxidation process hinged on a well-defined photochemical Cu(I)/Cu(II) redox cycle, offering a sustainable strategy for treating recalcitrant metal-complexed wastewater.