Published in Environmental Science & Technology 39, issue 18, 6896–6916, 2005, 1 which should be used for any reference to this work A New Concept Linking Observable Stable Isotope Fractionation to Transformation Pathways of Organic Pollutants ,² ²,³ § ² MARTIN ELSNER, * LUC ZWANK, DANIEL HUNKELER, AND RENEÂ P. SCHWARZENBACH Swiss Federal Institute for Environmental Science and Technology (EAWAG) Swiss Federal Institute of Technology Zurich (ETHZ), Ueberlandstrasse 133, CH-8600 Duebendorf Switzerland, and Centre of Hydrogeology, University of NeuchaÃtel, Rue Emile Argand 11, CH-2007 NeuchaÃtel, Switzerland Measuring stable isotope fractionation of carbon, hydrogen, reactions; AKIEC ) 1.02-1.03 for reductive cleavage of and other elements by Compound Specific Isotope C-Cl bonds; AKIEC ) 1.00-1.01 for CdC bond epoxidation; Analysis (CSIA) is a new, innovative approach to assess AKIEC ) 1.02-1.03 for CdC bond oxidation by permanganate. organic pollutant degradation in the environment. Central to Hence, the evaluation scheme presented bridges a gap this concept is the Rayleigh equation which relates between basic and environmental (bio)chemistry and provides degradation-induced decreases in concentrations directly insight into factors that control the magnitude of bulk to concomitant changes in bulk () average over the isotope fractionation factors. It also serves as a basis to whole compound) isotope ratios. The extent of in situ identify degradation pathways using isotope data. It is shown transformation may therefore be inferred from measured how such an analysis may be even possible in complex isotope ratios in field samples, provided that an appropriate field situations and/or in cases where AKIE values are smaller enrichment factor (bulk) is known. This bulk value, than intrinsic KIE values, provided that isotope fractionation however, is usually only valid for a specific compound is measured for two elements simultaneously (ªtwo- and for specific degradation conditions. Therefore, a direct dimensional isotope analysisº). Finally, the procedure is comparison of bulk values for different compounds and used (1) to point out the possibility of estimating approximate for different types of reactions has in general not been bulk values for new compounds and (2) to discuss the feasible. In addition, it is often uncertain how robust and moderate, but non-negligible variability that may quite reproducible bulk values are and how confidently they can generally be associated with bulk values. Future research be used to quantify contaminant degradation in the field. is suggested to better understand and take into account To improve this situation and to achieve a more in-depth the various factors that may cause such variability. 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 Introduction effects (AKIE) derived from bulk values reported in In recent years Compound Specific Isotope Analysis (CSIA) environmentally oriented studies. Starting from basic rate has undergone a rapid development toward important new laws, a quite general derivation of the Rayleigh equation applications in contaminant hydrology and organic (bio)- is given, resulting in a novel set of simple equations that take geochemistry. With CSIA, the relative abundance of the heavy into account the effects of (1) nonreacting positions and (hE) and light (lE) isotopes of a given element E is determined (2) intramolecular competition and that lead to position-specific in molecules of a given compound, expressed by the ratio ) h l ) AKIE values rather than bulk enrichment factors. R E/ E. Such bulk isotope ratios ( ratios averaged over the bulk compound) can be measured by gas chromatog- Reevaluation of existing literature values result in bulk raphy-isotope ratio mass spectrometry (GC-IRMS) and are consistent ranges of AKIE values that generally are in good reported as difference in per mil δhE with respect to an agreement with previously published data in the (bio)- international reference standard (1, 2): chemical literature and are typical of certain degradation reactions (subscripts C and H indicate values for carbon R - R (hE/lE) - (hE/lE) ) - ) - h ) ref ‚ ) ref ‚ and hydrogen): AKIEC 1.01 1.03 and AKIEH 2 23 for δ E ( ) 1000½ ( ) 1000½ R h l oxidation of C-H bonds; AKIE ) 1.03-1.07 for S 2- ref ( E/ E)ref C N (1) ++ ++ * Corresponding author phone: 1 416 978 0825; fax: 1 416 - 978 3938; e-mail: [email protected]. Present address: Stable Presently, online GC IRMS analysis is possible for the Isotope Laboratory, Department of Geology, University of Toronto, elements hydrogen (2H/1H), carbon (13C/12C), nitrogen (15N/ 22 Russell Street, Toronto, Ontario M5S 3B1, Canada. 14N), and oxygen (18O/16O). A short discussion of experi- ² EAWAG and ETHZ. mental details is given in the Supporting Information. More § University of NeuchaÃtel. information is provided by comprehensive reviews of Brenna ³ Present address: Centre de Recherche Public Henri Tudor, Centre de Ressources des Technologies de l'Environnement (CRTE), Tech- (3) and Meier-Augenstein (4) as well as in papers on the noport Schlassgoart, 66, rue de Luxembourg, B.P. 144, L-4002 Esch- original method development by Hayes and co-workers (5- sur-Alzette, Luxembourg. 7). The online coupling of IRMS to chromatographic separa- 2 tion methods has made it possible to analyze bulk isotope equations that allow calculating AKIE values from observable ratios of organic compounds in small, environmentally isotope fractionation data. A rigorous mathematical treatment relevant concentrations. In the case of carbon isotopes, is given in the Supporting Information, while the main method quantification limits have been achieved in the low features are explained in the text. Subsequently, existing bulk ppm (mg/L) range with liquid-liquid extraction (8) and literature values are reevaluated, and AKIE values are headspace analysis (9, 10), while enrichment methods such discussed for different well characterized transformation as solid-phase microextraction and purge and trap have reactions. With the proposed evaluation procedure and its lowered the limits even further to the low ppb (µg/L) range application to a broad range of major groundwater con- (10-12). As studies in contaminant hydrology have so far taminants, the paper aims to bridge a current gap between primarily focused on measurements of δ2Η and, particularly, contaminant hydrology and the classical (bio)chemical δ13C, this review will mostly deal with these two elements. literature. The way in which CSIA is currently used to assess groundwater contaminations was pioneered in studies by Origin and Magnitude of Isotope Fractionation: Kinetic Sherwood-Lollar et al. (13), Heraty et al. (14), Meckenstock Isotope Effects et al. (15), and Hunkeler et al. (16, 17). A common funda- Although in complex environmental systems, the magnitude mental assumption of these first publications is that the bulk of isotope fractionation occurring during transport and isotope fractionation associated with degradation of organic transformation of a given pollutant can depend on many pollutants appears to follow the Rayleigh equation. This factors, a few important general rules may nevertheless be classical relationship was originally derived for fractionation drawn from the chemical and biochemical literature (21- in the diffusion of gases (18) and was later also used to 23, 25, 26). When considering organic contaminants in describe kinetic isotope fractionation in geology (1, 2, 19), groundwater, usually, several atoms of a given element E marine sciences (20), chemistry (21, 22), and biochemistry (e.g., E ) H, C, N, O, S, Cl) are present at various locations (23). This equation is extremely useful, because it relates within a larger organic molecule. For this element, significant h differences in bulk isotope ratios ∆δ E directly to changes in isotope fractionation can then only be expected at those contaminant concentrations. Isotope ratios may therefore positions, where covalent bonds involving the element E be used to quantify how much degradation has occurred in directly or indirectly () adjacent to the reactive center) are natural systems. This is especially important if contaminant broken or formed during the rate-limiting step(s) of a given concentrations decrease not only due to degradation but process. Processes that act on the compound as a whole also due to dilution, sorption, and other processes that are such as advective-dispersive transport, volatilization (9, 10, not or only little fractionating. The concept has been 27-31), sorption/desorption (29, 32, 33) or binding to an confirmed in a number of studies, and comprehensive recent enzyme (33, 34) cause much smaller changes in the overall reviews about the use of CSIA to quantify contaminant isotope composition, which are often not detectable within degradation in the field have been published by Schmidt et the precision of the analytical method (9, 29, 32). al. (12) and Meckenstock et al. (24). When considering a reaction in which a specific bond is However, despite its great usefulness, several aspects broken, the elements involved in the reaction will generally remain unsatisfactory with respect to the way in which the show a normal isotope effect, that is, the molecules with the Rayleigh equation is currently applied. heavier isotope at this specific location will react more slowly