Influence Mechanisms of Chloride Ions on Degradation of Phenol by Thermally Activated Persulfate

Abstract

Chloride ion (Cl−) widely exists in groundwater and can react with free radicals to generate reactive chlorine species, which may react with pollutants in groundwater to create new environmental risks. To study the effect and mechanism of Cl− on the remediation of phenol-contaminated groundwater by persulfate (PS)-based advanced oxidation processes, the effects of temperature, PS concentration, pH, and Cl− concentration on the degradation of phenol were investigated using a thermally activated PS system. The spectral characteristics of the system during phenol degradation were investigated by excitation emission matrices-parallel factor analysis (EEM-PARAFAC). Moreover, the quantities and species of chlorinated toxic by-products were identified by gas chromatograph-mass spectrometer, and the degradation mechanism was revealed. The results showed that: (1) Degradation of phenol was promoted by increasing the reaction temperature and PS concentration, and the degradation process was in accordance with the pseudo-first-order kinetics model. (2) The presence of Cl− accelerated the degradation of phenol by thermally activated PS, and the degradation efficiency increased with the increase of Cl− concentration. When Cl− concentration was kept at 10, 25 and 50 mmol/L, the degradation rate of phenol reached 100% after 5 h of reaction. (3) The fluorescence characteristics of the reaction solution can be divided into four fluorescence components (C1, C2, C3 and C4) during the degradation of phenol. Fluorescence intensity of C1 and C2 components decreased more significantly in the presence of Cl−. C3 and C4 components are assigned to the degradation products of phenol, and the fluorescence intensity of C3 increased first and then decreased during the reaction. (4) Seven chlorinated toxic by-products (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, chlorohydroquinone, 3,5-dichlorocatechol, 2,3-dichloro-2-methylbutane, and 2-chloro-4-methyl-2-pentanol) were detected. The degradation mechanism was proposed based mass spectrometry analysis, including hydroxylation/oxidation and chlorination process. The study showed that Cl− can improve the remediation efficiency of thermally activated PS for phenol-contaminated groundwater, but also induce the generation of chlorinated toxic by-products due to the chlorination process.