Electrochemical Transformation of Trace Organic Contaminants in the Presence of Halide and Carbonate Ions

Electrochemical treatment on anodes shows promise for the oxidation of organic contaminants in industrial wastewater and reverse osmosis concentrate from municipal wastewater recycling due to the high conductivity of the matrix and the concomitant low energy demand. The effect of background electrolyte composition (Cl(-), HCO3(-), and NH4(+)) on the formation and fate of electrochemically produced heterogeneous (HO(•)ads and Cl(•)ads) and homogeneous (HOCl and HOBr) oxidants was evaluated on Ti-IrO2 and boron-doped diamond (BDD) electrodes using a suite of trace organic contaminants that exhibited varying reactivity with HO(•), CO3(•-), HOCl, and HOBr. The contributions of adsorbed and bulk oxidants to contaminant degradation were investigated. Results show that transformation rates for most contaminants increased in the presence of chloride and trace amounts of bromide; however, elevated concentrations of HCO3(-) often altered transformation rates due to formation of selective oxidants, with decreases in reactivity observed for electron-poor contaminants and increases in reactivity observed for compounds with amine and phenolic moieties. Using this information, rates of reactions on anode surfaces and measured production and loss rates for reactive homogeneous species were used to predict contaminant removal in municipal wastewater effluent. Despite some uncertainty in the reaction mechanisms, the model accurately predicted rates of removal of electron-rich contaminants but underestimated the transformation rates of compounds that exhibited low reactivity with HOCl and HOBr, possibly due to the formation of halogen radicals. The approach employed in this study provides a means of identifying key reactions for different classes of contaminants and for predicting the conditions under which anodic treatment of wastewater will be practical.