Effect of electrolyte composition on electrochemical oxidation: Active sulfate formation, benzotriazole degradation, and chlorinated by-products distribution

Abstract

Electrochemical oxidation is an effective technique for treating persistent organic pollutants, which are hardly removed in conventional wastewater treatment plants. Sulfate and chloride salts commonly used and present in natural wastewater influence the electrochemical degradation process. In this study, the effect of electrolyte composition on the active sulfate species (SO4●⁻ and S2O82⁻) formation, benzotriazole degradation-a model organic compound, and chlorinated by-products distribution have been investigated while using a boron-doped diamond (BDD) anode. Different Na2SO4:NaNO3 and Na2SO4:NaCl ratios with constant conductivity of 10 mS/cm were used in the experiments and applied anode potential was kept constant at 4.3 V vs. Ag/AgCl. The electrogenerated SO4●⁻ and S2O82⁻ formation were faster in 10:1 and 2:1 Na2SO4:NaNO3 ratios than in the 1:0 ratio. The ●OH-mediated SO4●⁻ production has prevailed in 10:1 and 2:1 ratios. However, ●OH-mediated SO4●⁻ production has hindered the 1:0 ratio due to excess chemisorption of SO42⁻ on the BDD anode. Similarly, the faster benzotriazole degradation, mineralization, and lowest energy consumption were achieved in the 10:1 Na2SO4:NaNO3 and Na2SO4:NaCl ratio. Besides, chlorinated organic by-product concentration (AOX) was lower in the 10:1 Na2SO4:NaCl ratio but increased with the increasing chloride ratio in the electrolyte. LC-MS analysis shows that several chlorinated organic transformation products were produced in 0:1 to 2:1 ratio, which was not found in the 10:1 Na2SO4:NaCl ratio. A comparatively higher amount of ClO4⁻ was formed in the 10:1 ratio than in 2:1 to 0:1 ratio. This ClO4⁻ formation train evidence the effective ●OH generation in a sulfate-enriched condition because the ClO4⁻ formation is positively correlated to ●OH concentration. Overall results show that sulfate-enriched electrolyte compositions are beneficial for electrochemical oxidation of biorecalcitrant organic pollutants.