Jiaxin Xu,Jing Zou,Jianying Wu,Huiping Zeng,Yixin Huang,Jingxin Yang,Chunming Gong,Siying Chen,Jun Ma
Chlorination, one of the most common oxidation strategies, performed limited degradation capacity towards many emerging organic contaminants under neutral pH conditions. In this study, 2,2′-azinobis(3-ethylbenzothiazoline)-6-sulfonate (ABTS) was discovered to possess an outstanding activation property towards free available chlorine (FAC) during the chlorination of diclofenac (DCF) among pH 6.0–9.5. ABTS radical (ABTS •+ ) primarily accounted for the elimination of DCF in the ABTS/FAC system, although hydroxyl radicals , reactive chlorine species, and singlet oxygen were also generated via the self-decomposition of FAC. ABTS acted as the electron shuttle to degrade DCF in the ABTS/FAC system, where ABTS was firstly oxidized by FAC to ABTS •+ via single electron transfer , and followed by the elimination of DCF with the generated ABTS •+ . Eight DCF degradation intermediates were identified by LC/Q-TOF/MS, and four DCF degradation pathways were proposed. Real water bodies, humic acid , and the coexistent anions of Cl − , HCO 3 − , NO 3 − , and SO 4 2− performed negligible influence on DCF removal in ABTS/FAC system. ABTS/FAC system was much superior to sole chlorination in terms of toxicity reduction and anti-interference capacity. Overall, this study innovatively introduced ABTS as the electron shuttle to enhance the oxidative capacity of FAC under neutral pH conditions and provided a new insight that the ABTS-like organic/synthetic components might play an important role in degrading emerging organic contaminants by chlorination in water treatment .
Chlorine, serving as the mainstream disinfectant, can react with dissolved organic matter (DOM) to form undeserved disinfection by-products (DBPs). Free available chlorine (FAC) concentration is crucial to ensure effective disinfection while minimizing the formation of toxic DBPs. In this study, we propose a convenient method using sodium sulfite (Na 2 SO 3 ) to reduce oxidized chlorine in FAC. The molar concentration of reduced chloride ion (Cl − ) was quantified directly by ion chromatography to reflect FAC concentration. Compared with common FAC detection techniques including DPD colorimetry, iodometry, and UV methods, this novel reduction method exhibits a lower detection limit and is more resistant to interference. Common water matrices, such as DOM and anions did not affect the method accuracy (< 3.6%). Furthermore, carbonaceous DBPs (C-DBPs) like regulated trihalomethanes and halogenacetic acids, unregulated aromatic chlorophenols, did not interfere with the determination of FAC by using this reduction method. This lack of interference can be attributed to the low redox potential of Na 2 SO 3 , which does not readily react with these C-DBPs. However, nitrogenated DBPs (N-DBPs) like dichloroacetonitrile displayed slight interference (the effect of common dichloroacetonitrile concentration in water on FAC was less than 0.0007 μM). This suggests that this method is well-suited for determining FAC in chlorination processes where the C-DBPs predominated. Overall, the reduction method enables precise determination of FAC and proves valuable in assessing residual chlorine levels in both laboratory and real disinfected water samples dominated by C-DBPs.
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