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首页> 外文期刊>Environmental Science & Technology >A new concept linking observable stable isotope fractionation to transformation pathways of organic pollutants
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A new concept linking observable stable isotope fractionation to transformation pathways of organic pollutants

机译：将可观察到的稳定同位素分馏与有机污染物转化途径联系起来的新概念

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摘要

Measuring stable isotope fractionation of carbon, hydrogen, and other elements by Compound Specific Isotope Analysis (CSIA) is a new, innovative approach to assess organic pollutant degradation in the environment. Central to this concept is the Rayleigh equation which relates degradation-induced decreases in concentrations directly to concomitant changes in bulk (= average over the whole compound) isotope ratios. The extent of in situ transformation may therefore be inferred from measured isotope ratios in field samples, provided that an appropriate enrichment factor (epsilon(bulk)) is known. This epsilon(bulk) value, however, is usually only valid for a specific compound and for specific degradation conditions. Therefore, a direct comparison of epsilon(bulk) values for different compounds and for different types of reactions has in general not been feasible. In addition, it is often uncertain how robust and reproducible epsilon(bulk) values are and how confidently they can be used to quantify contaminant degradation in the field. To improve this situation and to achieve a more in-depth understanding, this critical review aims to relate fundamental insight about kinetic isotope effects (KIE) found in the physico(bio)chemical literature to apparent kinetic isotope effects (AKIE) derived from epsilon(bulk) values reported in environmentally oriented studies. Starting from basic rate laws, a quite general derivation of the Rayleigh equation is given, resulting in a novel set of simple equations that take into account the effects of (1) nonreacting positions and (2) intramolecular competition and that lead to position-specific AKIE values rather than bulk enrichment factors. Reevaluation of existing epsilon(bulk) literature values result in consistent ranges of AKIE values that generally are in good agreement with previously published data in the (bio)chemical literature and are typical of certain degradation reactions (subscripts C and H indicate values for carbon and hydrogen): AKIE(C) = 1.01-1.03 and AKIE(H) = 2-23 for oxidation of C-H bonds; AKIE(C) = 1.03-1.07 for S(N)2-reactions; AKIE(C) = 1.02-1.03 for reductive cleavage of C-Cl bonds; AKIE(C) = 1.00-1.01 for C=C bond epoxidation; AKIEC = 1.02-1.03 for C=C bond oxidation by permanganate. Hence, the evaluation scheme presented bridges a gap between basic and environmental (bio)chemistry and provides insight into factors that control the magnitude of bulk isotope fractionation factors. It also serves as a basis to identify degradation pathways using isotope data. It is shown how such an analysis may be even possible in complex field situations and/or in cases where AKIE values are smaller than intrinsic KIE values, provided that isotope fractionation is measured for two elements simultaneously ("two-dimensional isotope analysis"). Finally, the procedure is used (1) to point out the possibility of estimating approximate epsilon(bulk) values for new compounds and (2) to discuss the moderate, but non-negligible variability that may quite generally be associated with epsilon(bulk) values. Future research is suggested to better understand and take into account the various factors that may cause such variability.
机译：通过化合物特异性同位素分析（CSIA）测量碳，氢和其他元素的稳定同位素分馏是一种评估环境中有机污染物降解的新颖创新方法。该概念的中心是瑞利方程，该方程将降解引起的浓度降低直接与相应的体积变化（=整个化合物的平均值）同位素比联系起来。因此，只要已知适当的富集因子（ε（散装）），就可以从现场样品中测得的同位素比率推断出原位转化的程度。但是，此ε（大）值通常仅对特定化合物和特定降解条件有效。因此，直接比较不同化合物和不同类型反应的ε（本体）值通常是不可行的。此外，通常不确定ε和bulk值的鲁棒性和可再现性，以及它们可用于量化野外污染物降解的可信度。为了改善这种情况并获得更深入的了解，此重要的评论旨在将有关物理（生物）化学文献中发现的动力学同位素效应（KIE）的基本见解与源自ε的表观动力学同位素效应（AKIE）相关联（环境指标研究中报告的值）。从基本速率定律开始，给出了Rayleigh方程的相当一般的推导，从而得出了一组新颖的简单方程，其中考虑了（1）非反应位置和（2）分子内竞争的影响，并导致了位置特异性AKIE值而不是体积富集因子。重新评估现有的epsilon（bulk）文献值会产生一致的AKIE值范围，该值通常与（生物）化学文献中以前发表的数据吻合良好，并且是某些降解反应的典型特征（下标C和H表示碳和氢）：用于CH键氧化的AKIE（C）= 1.01-1.03和AKIE（H）= 2-23;对于S（N）2-反应，AKIE（C）= 1.03-1.07; AKIE（C）= 1.02-1.03，用于还原性切割C-Cl键；对于C = C键环氧化，AKIE（C）= 1.00-1.01；对于由高锰酸盐进行的C = C键氧化，AKIEC = 1.02-1.03。因此，提出的评估方案弥合了基础化学与环境（生物）化学之间的鸿沟，并提供了对控制整体同位素分馏因子大小的因素的深入了解。它也可作为使用同位素数据识别降解途径的基础。如果同时测量两个元素的同位素分馏，则表明在复杂的现场情况和/或AKIE值小于固有KIE值的情况下，这种分析甚至是可能的（“二维同位素分析”）。最后，该程序用于（1）指出估计新化合物的近似epsilon（bulk）值的可能性，以及（2）讨论通常与epsilon（bulk）相关的中等但不可忽略的变异性价值观。建议进行进一步的研究，以更好地理解和考虑可能导致这种可变性的各种因素。

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