Reductive dehalogenation pathways and inhibition mechanisms of 2,5-dichloronitrobenzene in UV/sulfite advanced reduction process

Abstract

2,5 − dichloronitrobenzene (2,5 − DCNB) and its chlorine − containing intermediates as adsorbable organic halogens (AOX), which may cause persist environment and health damages, are widely present in fine chemical wastewater. However, AOX in high − salt wastewater are hardly removed by biological process and advanced oxidation process (AOPs). Advanced reduction processes (ARPs), especially UV/sulfite process, have attracted increasing attention in past few years due to their powerful reduction potential to refractory contaminants. In this study, the kinetics, main inhibition mechanisms, and degradation pathways of 2,5 − DCNB in UV/sulfite process were studied. The apparent reaction rate (kobs) increasing from 0.0059 min−1 to 0.0628 min−1 with sulfite dosage increasing from 2.6 mM to 104.2 mM and from 0.0027 min−1 to 0.0659 min−1 with pH value increasing from 3 to 12. Relative quantitation method based on EPR was developed to prove the dominant role of hydrated electron (eaq−) in UV/sulfite ARPs. A new sulfite activating pathway in dark was discovered at lower pH, which partly inhibits the ARPs. The nonlinear increase of kobs with increasing sulfite dosage was attributed to UV scattering which is caused by density fluctuation. The inhibition of Cl− triggered by ●OH from H2O direct photolysis by UV at 185 nm from lower pressure mercury lamp. This inhibition is linearly depending on Cl− concentration. The multiple competitive inhibition of NO3− was discussed and the three pathways are “inner filter” effect, eaq− competition and oxidizing species generation. In addition, the reduction pathways of 2,5 − DCNB during UV/sulfite ARPs were predicted by intermediates identification and density functional theory (DFT) calculation. Cyclohexanol with lower toxicity was identified as final product of UV/sulfite process. This study revealed the capacity of UV/sulfite ARPs for direct dehalogenation on benzene ring and the application potential in industrial wastewater dehalogenation.