Removal of crystal violet by ultraviolet/persulfate: Effects, kinetics and degradation pathways

Abstract

Abstract The degradation of crystal violet (CV) by the ultraviolet/persulfate (UV/PS) process was investigated in this study. The degradation processes of CV by PS and UV/PS could be appropriately described by second-order degradation kinetics, with kinetics rate constants of 3.57 × 10−3 and 9.58 × 10−3 g⋅mg−1min−1, respectively (UV lamp at 0.346 mW/cm2, CV 0 = 10 mg/L, PS = 0.5 mmol/L, pH = 7.0, and temperature = 20 °C). An increases in PS dosage and temperature can both promote the degradation of CV. However, an excessive PS dosage ( ≥ 1 mmol/L), too high a temperature ( ≥ 25 °C) and too low an initial CV concentration may not be desirable options with respect to the economic cost. In addition, better degradation efficiencies were obtained both under neutral conditions (pH = 7) and the presence of an appropriate amount of metal ions (0.1 mmol/L), such as Cu 2 + , Zn 2 + , and Fe2＋. Economical cost comparison revealed that UV/PS has a better economy as compared to UV/H2O2 and UV/TiO2. Free radical quenching experiments revealed that the contributions of ⋅OH and SO⋅ 4 − to the CV removal by UV/PS process were 19.7% and 26%, respectively (UV lamp at 0.346 mW/cm2, CV 0 = 10 mg/L, PS = 0.5 mmol/L, pH = 7.0, and temperature = 20 °C). The CV degradation pathways of UV/PS were proposed and can be summarized as follows: demethylation, decarboxylation, deamination and ring-opening reaction following the oxidative cleavage of the double bond of CV, after which small molecules such as H2O, CO 3 2 − , NO 3 − were eventually generated.