Singlet oxygen generation for selective oxidation of emerging pollutants in a flow-by electrochemical system based on natural air diffusion cathode.

Abstract

The decay of free radicals involved in side reactions is one of the challenges faced by electrochemical degradation of organic pollutants. To this end, a non-radical oxidation system was constructed by a natural air diffusion cathode (ADC) and a Ti-based dimensional stable anode coated by RuO2 (RuO2-Ti anode) for cathodic hydrogen peroxide activation by anodic chlorine evolution. The ADC fabricated by the carbon black of BP2000 produced a stable concentration of hydrogen peroxide of 339.94mg L-1 (current efficiency of 73.4%) without aeration, which was superior to the cathode made by the XC72 carbon black. The flow-by ADC-RuO2 system consisted of an ADC and a RuO2-Ti anode showed high selectivity to aniline (AN) compared to benzoate (BA) in a NaCl electrolyte, whose degradation efficiencies were 97.72% and 1.3%, respectively. Rapid degradations of a mixture of emerging pollutants and AN were also observed in the ADC-RuO2 system, with pseudo-first-order kinetic constants of 0.51, 1.29, 0.89, and 0.99min-1 for Bisphenol A (BPA), tetracycline (TC), sulfamethoxazole (SMX) and AN, respectively. Quenching experiments revealed the main reactive oxygen species for the pollutant degradation was singlet oxygen (1O2), which was also identified by the electron spin resonance (ESR) analysis. Finally, the steady-stable content of 1O2 was quantitatively determined to be 6.25 × 10-11M by the method of furfuryl alcohol (FFA) probe. Our findings provide a fast, low energy consumption and well controlled electrochemical oxidation method for selective degradation of organic pollutants. H2O2 generated on an air diffusion cathode by naturally diffused O2, reacts with ClO- produced from chloride oxidation on the RuO2-Ti anode to form singlet oxygen (1O2). The electrochemical system shows an efficient oxidation to electron-rich emerging pollutants including bisphenol A, tetracycline, sulfamethoxazole and aniline, but a poor performance on the electron-deficient compounds (e.g., benzoate).