Plasma-Induced Degradation Pathways of Crystal Violet and Reactive Black 5 Dyes: Environmental Influences and Toxicity Evaluation

Crystal Violet (CV) and Reactive Black 5 (RB5) are the widely used dyes in the textile industry, and are significant contributors to water pollution [1], [2]. This study explores the application of non-thermal plasma as a sustainable method for degrading these pollutants, focusing on the underlying mechanisms and the influence of various water constituents. Degradation efficiency was evaluated under different plasma power levels, dye concentrations, and water chemistries. At 36.2 W plasma power and 15 minutes of treatment, nearly complete degradation of 50 mg L⁻¹ CV and 100 mg L⁻¹ RB5 was achieved, along with notable toxicity reduction. Carbonate ions enhanced CV degradation but hindered RB5 removal, while chloride, sulphate, and nitrate consistently suppressed degradation of both dyes. Hydroxyl radicals were identified as the dominant reactive species. pH had a significant impact, with CV degrading faster in basic media, whereas RB5 showed reduced degradation in both acidic and basic conditions. Advanced analytical techniques like high-resolution mass spectrometry, surface-enhanced Raman spectroscopy, and UV-visible spectroscopy were used to identify degradation intermediates and propose degradation pathways. Additionally, phytotoxicity and cytotoxicity assays were conducted, revealing that after plasma degradation, the treated solution was free from toxicity, making it suitable for agricultural applications.