Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China.
Key Laboratory of Photochemistry, Institute of Chemistry Chinese Academy of Sciences, Beijing National Laboratory for Molecular Sciences, Beijing 100190, China.
Environ Sci Technol. 2023 Jul 25;57(29):10849-10859. doi: 10.1021/acs.est.3c02458. Epub 2023 Jul 10.
Photolysis of free chlorine is an increasingly recognized approach for effectively inactivating microorganisms and eliminating trace organic contaminants. However, the impact of dissolved organic matter (DOM), which is ubiquitous in engineered water systems, on free chlorine photolysis is not yet well understood. In this study, triplet state DOM (DOM*) was found to cause the decay of free chlorine for the first time. By using laser flash photolysis, the scavenging rate constants of triplet state model photosensitizers by free chlorine at pH 7.0 were determined to be in the range of (0.26-3.33) × 10 M s. DOM*, acting as a reductant, reacted with free chlorine at an estimated reaction rate constant of 1.22(±0.22) × 10 M s at pH 7.0. This study revealed an overlooked pathway of free chlorine decay during UV irradiation in the presence of DOM. Besides the DOM's light screening ability and scavenging of radicals or free chlorine, DOM* played an important role in the decay of free chlorine. This reaction pathway accounted for a significant proportion of the decay of free chlorine, ranging from 23 to 45%, even when DOM concentrations were below 3 mg L and a free chlorine dose of 70 μM was present during UV irradiation at 254 nm. The generation of HO and Cl from the oxidation of DOM* by free chlorine was confirmed by electron paramagnetic resonance and quantified by chemical probes. By inputting the newly observed pathway in the kinetics model, the decay of free chlorine in UV-irradiated DOM solution can be well predicted.
光解自由氯是一种越来越被认可的有效灭活微生物和去除痕量有机污染物的方法。然而，溶解有机物（DOM）的影响，它在工程水系统中无处不在，对自由氯光解的影响尚不清楚。本研究首次发现三重态 DOM（DOM*）会导致自由氯衰减。通过使用激光闪光光解，在 pH 值为 7.0 时，确定了自由氯对三重态模型敏化剂的猝灭速率常数在（0.26-3.33）×10 M s 范围内。DOM作为还原剂，以估计的反应速率常数 1.22（±0.22）×10 M s 在 pH 值为 7.0 时与自由氯反应。本研究揭示了在存在 DOM 的情况下，UV 照射过程中自由氯衰减被忽视的途径。除了 DOM 的光屏蔽能力和自由基或自由氯的清除外，DOM在自由氯衰减中也起着重要作用。这种反应途径占自由氯衰减的很大一部分，范围从 23%到 45%，即使在 254nm 紫外光照射下，DOM 浓度低于 3mg/L 且存在 70 μM 的自由氯剂量时也是如此。通过电子顺磁共振和化学探针证实了 DOM*被自由氯氧化生成 HO 和 Cl。通过在动力学模型中输入新观察到的途径，可以很好地预测 UV 辐照 DOM 溶液中自由氯的衰减。
