Synergy of cold plasma and sulfate radicals in the treatment of dye-contaminated wastewater; mechanistic study of the degradation mechanism using DFT

Abstract

Organic dyes including azo and xanthene dyes are used widely, and ultimately discharged into wastewaters. Owing to the drawbacks of conventional water treatment, different advanced oxidation processes (AOPs) have been combined with nonthermal atmospheric pressure plasma (NTAPP) as sustainable alternatives for reclaiming dye-polluted wastewater. Herein, the synergy of NTAPP and sulfate radicals were utilized for treating real colored wastewater. Individual and combined systems of NTAPP and UV-C activated sulfate and hydroxyl radicals were evaluated based on their removal efficiencies for two model dyes: Rhodamine B (Rh.B) and Congo red (CR). The combined system of NTAPP and UV-C activated persulfate (PSUVP) was the most efficient with >80 % degradation of both dyes in 10 min and showed synergy factors of 2.55 and 5.71 for Rh.B and CR degradation, respectively. Moreover, PSUVP was highly efficient in TOC removal from Rh.B (90 %) and CR (67 %) solutions. The determining oxidants in Rh.B and CR degradation were the singlet oxygen and sulfate radical, respectively. The DFT analysis revealed that higher number of acidic hydrogens were available in CR compared to Rh.B and this might be one reason for observing the influential role of SO4∙- in CR degradation. Although the background constituents had negligible effects on Rh.B degradation, the degradation efficiency of the PSUVP system decreased by 30 % in real wastewater containing 50 ppm CR. The treatment of real colored wastewater using PSUVP resulted in 94 % TOC removal. Despite the toxicity of the PSUVP (toxicity unit ≥1), which can be mitigated by diluting the treated wastewater before discharge, this system exhibited high efficiency in decolorization and degradation of organic components. Thus, the proposed system has promising potential for the treatment of industrial effluents with high TOC.