Kinetics study of dinitrodiazophenol industrial wastewater treatment by a microwave-coupled ferrous-activated...

In this study, a ferrous-activated persulfate (PS) process induced by microwave (MW) energy was used to treat an industrial wastewater containing dinitrodiazophenol (DDNP). The effect of different operational parameters, such as PS dose, initial pH, MW power, and the mass ratio of ferrous to PS [i.e., n(Fe²⁺/PS)], on chemical oxygen demand (COD) removal and the pseudo first-order constant k was systematically studied. Results showed that the constant k and COD removal increased considerably with increasing PS dose, MW power, and n(Fe²⁺/PS); however, the constant k and COD removal decreased proportionally as the initial pH increased from 3 to 11. A base-activated PS system was formed when the initial pH further increased to 13, which resulted in a dramatic increase of k and COD removal. Controlled experiments demonstrated that the synergetic effect of MW, Fe²⁺, and PS were significant. The value of k was 0.2425 min^-1, COD removal was 71.42%, and biodegradability remarkably increased from 0.056 to 0.6712 during treatment by MW-Fe²⁺/PS at a PS dose of 8 g/L, MW power of 600 W, initial pH of 3, n(Fe²⁺/PS) of 0.04, and reaction time of 14 min. Spectral analyses showed that the chromophores (including the diazo group and nitro group compounds) and benzene rings were greatly decomposed. Identification tests for reactive oxygen species showed that the predominant oxidizing species in the MW-Fe²⁺/PS process was the sulfate radical, and that the hydroxyl radical played a lesser role for color number elimination. From the perspective of both treatment efficiency energy consumption, the MW-Fe²⁺/PS process was confirmed to be an economical and promising pretreatment process for DDNP industrial wastewater.