Sulfite activation by ultrasound for the degradation of emerging contaminants: The dependence of pollutant property and mechanistic study

The easy recombination of •OH radicals in ultrasonic processes (US) always leads to the inadequacy in degrading relatively hydrophilic pollutants. Herein, sulfite was activated by ultrasound and the US/S(IV) process was for the first time applied to degrade emerging contaminants (ECs) with different physicochemical properties. Kinetic analysis and density functional theory (DFT) calculation showed that LogP of ECs was the dominant factor affecting the degradation efficiency in both US and US/S(IV) processes, and the addition of S(IV) more significantly improved the degradation of compounds with low LogP, high EHOMO and low ΔE (ΔE = ELUMO – EHOMO). The main mechanism of sulfite activation was attributed to •OH rather than the heat. The recombination of •OH could be well inhibited by sulfite and more •OH and SO4•− radicals could be derived from SO3•− in the bulk phase with the presence of O2. Based on DFT and LC/MS analysis, the reactive sites of sulfamethoxazole (SMX) upon the attack from •OH and SO4•− were identified and three degradation pathways were proposed. The primary intermediates of SMX could be efficiently degraded into smaller compounds in US/S(IV) process. Moreover, US/S(IV) process showed stronger adaptability to a broad pH range and complex water matrix than US process. This work may provide new insights into the sulfite activation technologies and a promising alternative for efficient degradation of ECs with different physicochemical properties.