Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research - UFZ, Permoserstr. 15, 04318 Leipzig, Germany.
Environ Sci Process Impacts. 2020 Mar 1;22(3):617-626. doi: 10.1039/c9em00583h. Epub 2020 Jan 29.
Transfer of organic compounds between aqueous and gaseous phases may change the isotopic composition which complicates the isotopic characterization of sources and transformation mechanisms in environmental samples. Studies investigating kinetic phase transfer of compounds dissolved in water (volatilization) are scarce, even though it presents an environmentally very relevant phase transfer scenario. In the current study, the occurrence of kinetic isotope fractionation (H/H, C/C, Cl/Cl) was investigated for two volatile organic compounds (trichloroethene, TCE and trichloromethane, TCM) during volatilization from water and gas-phase dissolution in water. In addition, experiments were also carried out at equilibrium conditions. The results indicated that volatilization of trichloromethane and trichloroethene from water, in contrast to pure phase evaporation, only caused small (chlorine) or negligible (hydrogen, carbon) isotope fractionation whereas for dissolution in water significant carbon isotope effects were found. At equilibrium conditions, hydrogen and carbon isotopes showed significant differences between dissolved and gaseous phase whereas small to insignificant differences were measured for chlorine isotopes. The results confirm the hypothesis that isotope effects during volatilization of organics from water are caused by transport inhibition in the aqueous phase. The consideration of gas-phase diffusion and vapor pressure isotope effects (Craig-Gordon model) could not reproduce the measured isotopic data. Overall, this study provides an overview of the most common kinetic and equilibrium partitioning scenarios and reports associated isotope effects. As such it illustrates under which environmental conditions isotopic signatures of chlorinated volatile organics may change, or remain constant, during transfer between surface waters and air.
有机化合物在水相和气相之间的转移可能会改变同位素组成，从而使环境样品中源和转化机制的同位素特征复杂化。尽管水中溶解的化合物的动力学相转移（挥发）是一种具有重要环境意义的相转移情景，但研究这种转移的动力学同位素分馏（H/H、C/C、Cl/Cl）的相关研究却很少。在当前的研究中，研究了两种挥发性有机化合物（三氯乙烯，TCE 和三氯甲烷，TCM）在从水中挥发和气相在水中溶解时的动力学同位素分馏（H/H、C/C、Cl/Cl）的发生情况。此外，还在平衡条件下进行了实验。结果表明，与纯相蒸发相比，三氯甲烷和三氯乙烯从水中挥发仅导致氯同位素（较小）或氢、碳同位素（可忽略）的分馏，而在水中溶解时则发现了显著的碳同位素效应。在平衡条件下，水中溶解相和气相的氢和碳同位素存在显著差异，而氯同位素的差异则较小或可忽略不计。结果证实了这样一种假设，即在水中有机物挥发过程中的同位素效应是由水相中传输抑制引起的。考虑气相扩散和蒸气压同位素效应（Craig-Gordon 模型）无法重现测量的同位素数据。总的来说，本研究概述了最常见的动力学和平衡分配情景以及相关的同位素效应。因此，它说明了在地表水和空气之间转移过程中，氯化挥发性有机物的同位素特征在何种环境条件下可能发生变化，或保持不变。
