Ambient-temperature atmospheric microwave plasma for synergistic removal of sulfamethoxazole and Cr(VI): Performance and mechanistic insights

The coexistence of recalcitrant antibiotics and toxic heavy metals in water poses serious ecological risks and challenges conventional treatment approaches. This study explores the simultaneous removal of sulfamethoxazole (SMX) and hexavalent chromium (Cr(VI)) using an ambient-temperature atmospheric microwave plasma (ATAMP) system, evaluating both removal efficiency and underlying mechanisms. Optimizing operating parameters to maintain stable plasma jet performance maximized reactive species generation, enhancing pollutant degradation. On this basis, the effects of water matrix constituents were systematically investigated, including the individual interference of common inorganic anions (Cl − , CO₃ 2− , SO₄ 2− ), representative metal cations (Cu 2+ , Zn 2+ ), and dissolved organic matter (sodium humate and peptone), as well as the treatment performance in mixed matrix and real water samples (tap water, Yudai River water, and Tuojiang River water). At 50 mg/L SMX, the addition of 20 mg/L Cr(VI) increased SMX degradation from 80.18% to 91.81%, while 50 mg/L SMX enhanced Cr(VI) reduction from 67.49% to 89.68%, indicating a synergistic effect. Targeted scavenger experiments identified hydroxyl radicals (OH ) as the primary oxidant for SMX, and hydrogen radicals (H ) and hydrated electrons (e − ₐ q ) as key reductive species for Cr(VI), with 1 O₂ and O₂ − modulating both pathways. Density functional theory (DFT) and Fukui analyses pinpointed reactive sites on SMX, while, in the presence or absence of Cr(VI), UHPLC-Q-TOF-MS revealed intermediates and seven potential degradation pathways. Cr(VI) preferentially consumes reductive radicals, suppressing some S N cleavage routes and enhancing oxidative transformations, generating a broader range of intermediates. Ecotoxicity evaluation using ECOSAR confirmed minimal acute and chronic risks of the oxidative products. These results demonstrate that ATAMP effectively removes SMX and Cr(VI) from the investigated co-contaminated water system, providing a promising approach for their simultaneous treatment in wastewater.