Effect of operating conditions and water matrix on the performance of UV combined electrochemical process for treating Chloride-containing solution and its reaction mechanism

Abstract

The performance of ultraviolet combined electrochemical (EC) process for treating chloride-containing (UV/EC/Cl-) solution was evaluated based on the N, N-Diethyl-m-toluamide (DEET) degradation. The influences of operating conditions and water matrix on the DEET degradation were investigated, and the reaction mechanism of UV/EC/Cl- process was discussed based on the analysis of DEET degradation pathway, the detection of the steady-state concentrations of active free radicals and the analysis of free radicals scavenging. The observed reaction rate constant of DEET degradation in UV/EC/Cl- process (0.093 min−1) is significantly higher than that of UV/Cl2 process (0.036 min−1), and the synergistic effect on the DEET degradation is remarkable in UV/EC/Cl- process. The current and chloride concentration have remarkable impact on the DEET degradation, while the initial pH, reaction temperature, and initial DEET concentration have slight influence on the DEET degradation. The impact of alkalinity on DEET degradation is more significant than that of ammonia, while sulfate and phosphate have slight effect on the DEET degradation. The active points attacked by free radicals on DEET molecule were predicted basing on density functional theory. During the DEET degradation, small molecular products are produced through a series of reactions such as hydrogen abstraction, substitution, dealkylation, and ring-opening. The steady-state concentrations of HO• and Cl• in UV/EC/Cl- process (1.57 × 10-13 M and 2.37 × 10-14 M, respectively) are higher than those of UV/Cl2 process (1.23 × 10-13 M and 1.07 × 10-14 M, respectively), and HO• and Cl• play key roles in the DEET degradation during the UV/EC/Cl- process.