Fe(II)/PAA process is an emerging advanced oxidation process for the degradation of naproxen (NAP) with ●OH, carbon-centered radical (R-O●) and high-valent iron species (Fe(IV)) as the main reactive species. Cl−, commonly found in water matrices, may react with these active species and alter their contribution to the degradation of NAP. The experimental results in this study demonstrated that the introduction of Cl− (0.1-10 mM) slightly promoted the degradation efficiency of NAP from 90% to 99.9%, but the pseudo-secondary kinetics reaction rate (kobs) significantly accelerated with the obviously enhanced yield of R-O●. It is noteworthy that when the Cl− concentration reached 1 mM, the exposure dose of R-O● (CTR-O●) was more than 12 times that of CTCl● and more than 2 times that of CT●OH. Combining with probe experiments, electron paramagnetic resonance (EPR) spectrum and density functional theory (DFT) calculations, Cl− could promote the generation of Fe(IV), R-O●, and ●OH in the Fe(II)/PAA system, thereby accelerating the degradation of NAP. Hydroxylation, acetylation, and chlorination intermediates were all detected during the oxidation of NAP, and their integrated biological toxicity was lower than the parent toxicity. The new knowledge obtained in this study that the presence of Cl− can influence the conversion of reactive species which provides an important basis for further studies on Fe(II)/PAA advanced oxidation systems, and draws attention to the effect of coexisting anions, especially the water matrix Cl−, on the oxidation of pollutants.

Effect of water matrix chloride ion on the reactive oxidizing species transformation in Fe(II)/peracetic acid system

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