Direct degradation of methylene blue by unactivated peroxymonosulfate: reaction parameters, kinetics, and mechanism

Advanced oxidation processes (AOPs) are efficient methods for water purification. However, there are few studies on using peroxymonosulfate (PMS) to remove pollutants directly. In this study, about 76% of methylene blue (MB) was removed by PMS directly within 180 min through a non-radical pathway, verified by scavenging tests, electron paramagnetic resonance and kinetic calculations. Additionally, the effects of PMS dosage, MB concentration, temperature, initial pH and competitive anions were determined. High PMS dosage, temperature and pH promoted MB degradation (from 76 to 98%) while MB concentration showed no effect on MB removal. Besides, MB degradation followed pseudo-first-order kinetic with rate constants of 0.0082 to 0.3912 min(-1). The second-order rate constant for PMS reaction with MB was 0.08 M-1 s(-1) at pH 3-6, but increased dramatically to 4.68 M-1 s(-1) at pH 10.50. Chlorine could be catalysed by PMS at high concentration Cl- and degradation efficiency reached 98% within 90 min. High concentration of bicarbonate accelerated MB removal due to the high pH value while humic acid showed a marginal effect on MB degradation. Furthermore, TOC removal rate of MB in the presence of chloride reached 45%, whereas PMS alone caused almost no mineralisation. This study provides new insights into pollutant removal and an additional strategy for water purification.

Automated summary

This study demonstrates the direct removal of methylene blue (MB) using peroxymonosulfate (PMS) as an advanced oxidation process. The efficiency of MB removal reached 76% within 180 minutes. Factors such as PMS dosage, temperature, and pH influenced the degradation of MB, while MB concentration showed no effect. The reaction rate constant of PMS with MB varied with pH, and PMS could catalyze chlorine degradation at high chloride concentration.