Assessing water matrix influence and toxicity reduction of crystal violet and reactive black 5 dyes after cold plasma-driven degradation

Abstract Crystal Violet (CV) and Reactive Black 5 (RB5), representative of the triphenylmethane and azo dye classes, are extensively used in the textile industry and are recognized as major contributors to aquatic pollution. Their structural stability and resistance to biodegradation lead to environmental persistence and potential toxicity. In this paper, a non-equilibrium cold plasma-based pin-to-plate dielectric barrier discharge (P2PDBD) source was employed for the degradation of dye solutions. The effects of plasma discharge power, dye concentration, and the chemical composition of the water matrix were systematically studied. With increasing salt concentration from 0 to 25 mg L −1 , Sodium carbonate synergistically enhanced CV degradation to near completion within minutes but inhibited RB5 by 38%, while sodium chloride, sulfate, and nitrate reduced CV degradation by 16%, 11%, and 8.26%, and RB5 degradation by 39%, 8.36%, and 7.7%, respectively. Radical scavenging experiments using methanol and isopropyl alcohol confirmed that hydroxyl radicals played a dominant role in the degradation process. The initial pH also affected degradation, with CV degrading more rapidly under basic conditions, while RB5 showed reduced efficiencies under both acidic and basic environments. Structural changes and degradation pathways were further elucidated through high-resolution mass spectroscopy, UV–Vis spectroscopy, and surface-enhanced Raman spectroscopy. Post-treatment phytotoxicity and cytotoxicity evaluations indicated that the treated water was non-toxic and, under controlled conditions, may also support early plant growth due to the presence of nitrates, suggesting that plasma-treated effluents are safe and suitable for reuse in agricultural and environmental applications.