Simultaneous removal of sulfamethoxazole and Cr(VI) from water by pulsed discharge plasma

Abstract

Real aquatic matrices are frequently burdened by intricate co-contamination from organic micropollutants and toxic heavy metals, creating an urgent demand for environmentally benign technologies capable of eliminating both contaminant classes in a single step. Pulsed discharge plasma (PDP) generates a cocktail of oxidative and reductive transient species that can concurrently oxidise organic compounds and reduce metallic ions. This study investigated the simultaneous PDP-driven degradation of sulfamethoxazole (SMX) and reduction of hexavalent chromium (Cr(VI)). The influences of electrical parameters, solution chemistry, and water-quality characteristics on the coupled removal efficiency were systematically evaluated. Maximal SMX elimination (50 mg·L⁻¹) was attained in the presence of 2 mg·L⁻¹ Cr(VI), whereas optimal Cr(VI) reduction (0.7 mg·L⁻¹) was achieved upon spiking with 15 mg·L⁻¹ SMX. Elevated applied voltage, pulse frequency, duty cycle, and gas-flow rate, together with acidic pH and low conductivity, markedly promoted the synchronous detoxification. Conversely, naturally occurring anions (Cl⁻, HCO₃⁻, SO₄²⁻) and humic acid (HA) exerted inhibitory effects. Scavenger experiments confirmed that the formation and subsequent fate of hydroxyl radicals (·OH), superoxide radicals (·O₂⁻), singlet oxygen (¹O₂), hydrated electrons (e⁻), and hydrogen atoms (·H) all played mechanistic roles in the dual-removal process. LC-MS and DFT analyses further enabled the proposal of three distinct SMX transformation pathways within the PDP system. Moreover, ECOSAR-based toxicity assessments indicated a substantial decrease in effluent ecotoxicity following treatment. Collectively, these findings establish PDP as a promising green technology for the concurrent abatement of antibiotic residues and toxic chromium from water.