UV-driven reciprocal promotion of Cr(VI) reduction by Cu(II)-EDTA: dual Cu(I)/H• pathways and Cr(V)-mediated redox enhancement

Background Efficient removal of Cr(VI) from industrial wastewater is a critical environmental issue. Conventional reduction agents suffer from low efficiency in complex matrices and produce secondary pollution, while the photochemical mechanism of Cu(II)-EDTA for Cr(VI) reduction remains unclear. Methods A UV-irradiated Cu(II)-EDTA system (UV/Cu(II)-EDTA) at 254 nm and pH 3.0 was studied for Cr(VI) reduction, with reduction pathways, intermediates, matrix adaptability and cyclic stability characterized by kinetics, electron paramagnetic resonance (EPR), quenching tests, UV-vis, and XPS. Significant findings The UV/Cu(II)-EDTA system achieved rapid Cr(VI) removal (>99% within 60 min) via two pathways: ligand-to-metal charge transfer (LMCT) generating Cu(I) for direct electron transfer, and a radical pathway dominated by H • with minor contribution from hydrated electrons (e aq ⁻). Cr(VI) presence enhanced Cu(II)-EDTA photodegradation from ∼4% to ∼15.3% via metastable Cr(V) intermediates that facilitated ligand transformation and drove Cu(I)/Cr(V)-mediated redox feedback. This system maintained high Cr(VI) removal efficiency (>95%) in diverse natural water matrices and exhibited stable performance over repeated cycles, providing a robust photochemical pathway for co-remediation of metal-ligand-contaminated wastewater.