Chiu Yin Ting T, Burns Alyssa M, Rosanka Simon, Hu Tiffany, Hennigan Christopher J, Carlton Annmarie G
Department of Chemistry, University of California, Irvine, Irvine, California 92697, United States.
Department of Chemical, Biochemical, and Environmental Engineering, University of Maryland, Baltimore County, Baltimore, Maryland 21250, United States.
ACS ES T Water. 2024 Dec 4;4(12):5399-5407. doi: 10.1021/acsestwater.4c00508. eCollection 2024 Dec 13.
Wet chemical oxidation (WCO) methods measure total organic carbon (TOC) in aqueous solutions through the formation and detection of carbon dioxide (CO). Prior research documents chloride (Cl) interference during WCO. However, the mechanism that determines WCO interference is not established. We investigate WCO and find that formic acid exhibits TOC recovery (89-108%) within measurement uncertainty in the presence of Cl, while acetic acid recovery is substantially reduced (3-67%). We postulate that chlorine radical (Cl) formation during WCO alters oxidation pathways for organic compounds with methyl groups to form stable halogenated organic species that are thus not detected as CO, reducing observed TOC recovery. We develop a kinetic model of elementary step reactions that reproduces observed TOC recoveries at multiple organic (1 and 5 ppm of C) and Cl (>0.01 M) concentrations for both acetic and formic acids. Independent experiments with pyruvic acid and different halogen salts are consistent with the proposed mechanism. Our findings provide a plausible mechanistic explanation for Cl interference in WCO-derived TOC measurements of environmental samples for which halogenated salts are present. A plausible mechanism provides a more complete understanding of how and why the TOC is biased low in environmental aquatic samples from saline environments when WCO is employed.
湿化学氧化（WCO）方法通过二氧化碳（CO₂）的生成和检测来测量水溶液中的总有机碳（TOC）。先前的研究记录了WCO过程中的氯离子（Cl⁻）干扰。然而，决定WCO干扰的机制尚未确立。我们对WCO进行了研究，发现甲酸在存在Cl⁻的情况下，在测量不确定度范围内TOC回收率为89%-108%，而乙酸的回收率则大幅降低（3%-67%）。我们推测，WCO过程中氯自由基（Cl·）的形成改变了含甲基有机化合物的氧化途径，形成了稳定的卤代有机物种，因此未被检测为CO₂，从而降低了观察到的TOC回收率。我们建立了一个基元反应动力学模型，该模型能够再现乙酸和甲酸在多种有机（1和5 ppm C）和Cl⁻（>0.01 M）浓度下观察到的TOC回收率。丙酮酸和不同卤盐的独立实验与所提出的机制一致。我们的研究结果为存在卤盐的环境样品中WCO衍生的TOC测量中的Cl⁻干扰提供了一个合理的机理解释。一个合理的机制能更全面地理解当采用WCO时，来自盐环境的环境水样中TOC为何以及如何被低估。
