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Isotopic methods give clues about the origins of trace metals and organic pollutants in the environment
Trends in Analytical Chemistry, Vol. 38, 2012
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Isotopic methods give clues about the origins of trace metals and organic pollutants in the environment Philippe Ne´grel, Michaela Blessing, Romain Millot, Emmanuelle Petelet-Giraud, Christophe Innocent Recent advances in techniques and methodologies in analytical chemistry help to address problems of environmental contamination. This is particularly true for isotope determinations of metals using multi-collector inductively coupled plasma mass spectrometry (MS) and on-line coupling of capillary gas chromatography and isotope-ratio MS, or compound-specific stable isotope analysis. We explore the methodologies and applications using the isotope signatures of inorganic or organic contaminants, which offer good potential for distinguishing potential sources within contamination plumes and possibly even estimating different source inputs at the catchment scale. Moreover, for organic pollutants, such methods provide unique information for deciphering their origin and studying degradation processes. ª 2012 Elsevier Ltd. All rights reserved. Keywords: Degradation; Environment; Inorganic element; Isotope; Isotopic determination; Isotopic signature; Organic molecule; Organic pollutant; Pollution; Trace metal
1. Introduction Philippe Ne´grel*, Michaela Blessing, Romain Millot, Emmanuelle Petelet-Giraud, Christophe Innocent BRGM, Metrology Monitoring Analysis Division, Avenue C. Guillemin, BP 36009, 45060 Orle´ans Cedex 02, France
*
Corresponding author. E-mail: [email protected]
Geology deals with the composition, the structure and the processes of formation of rocks and sediments, and covers topics such as mineral-resource exploration, evaluation of water resources, environmental geochemistry, natural risk assessment and geotechnical engineering [1–3]. Geology coupled with environmental science is building the knowledge base for a sustainable Earth, and geoscientists are increasingly involved in developing approaches for prevention or remediation of environmental pollution [4]. Since the signing of the ‘‘Treaty of Maastricht’’ in 1992, protecting the environment from pollution and conserving water resources for future generations has become an important task for the institutions of the European Community [5–7]. European environmental programs foster actions for reducing emissions in all
sectors, including industry, agriculture, transport and domestic pollution. New techniques and methodologies in analytical chemistry, geophysics, microbiology, geochemistry, hydrogeology, and informatics have emerged for addressing environmental contamination [8]. In the wake of a growing concern about environmental pollution, geosciences are evolving into a multidisciplinary field of research for understanding how pollutants behave, studying their transport and fate in natural environments, developing powerful remediation technologies, and finally helping to find solutions for a sustainable Earth and to protect natural resources for future generations. Environmental contamination is caused by a wide range of pollutants (e.g., metals, hydrocarbons, organic micro-pollutants, solvents, and pesticides) [4,9]. Identifying pollution sources and tracking their transport pathways is of fundamental interest, not only in terms of the ‘‘polluter
0165-9936/$ - see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:http://dx.doi.org/10.1016/j.trac.2012.03.017
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pays’’ principle that is now generally applied by environmental legislation. One of the profound changes in Earth sciences has been the addition of new analytical tools (e.g., isotopes) to complement classic descriptive ones. In this isotopegeology framework [10–12], isotope hydrology and isotope hydrogeology have become great challenges [13,14]. Isotope geology began with dating geologic events through various isotope systematics [15], and then progressed into isotope hydrology and isotope hydrogeology that are mainly based on studying variations in the contents of naturally occurring isotopes of elements found in abundance in the EarthÕs environment [16]. Within this framework, recent improvements in analytical techniques with high-grade analytical tools have made isotopic analyses available for an increasing number of elements and compounds present at low concentrations (e.g., lead, nitrates and chlorinated compounds) [17–20]. This is particularly true for the multi-collector inductively coupled plasma mass spectrometry (ICP-MS) instrument for isotope determination of metals [21] and for the gas chromatography isotoperatio MS (GC-IRMS) technique that has become known as compound-specific stable-isotope analysis (CSIA) [22]. Both offer a great opportunity for tracing pollution and pathways (transport or degradation) of specific elements and compounds. The objective of this article is to promote such techniques by illustrating recent methodologies and applications.
2. General consideration of the isotope-ratio measurements Mass spectrometers with different types of ion source are the principal instruments for measuring isotope ratios of a chemical element. In the vacuum source of a mass spectrometer, an ion beam is produced before being separated according to its mass/charge ratio by a combination of electromagnetic fields, including acceleration by an electric field and mass separation by a magnetic field. The separated ion beams are finally detected and measured (e.g., Faraday cups and multi counting) before being expressed in terms of their relative abundance. 2.1. The mass spectrometer Precise measurements of even small variations in the natural isotopic abundance demand specialized mass spectrometers. Three main mass-spectrometer types are mainly used for isotope-dedicated studies: (1) gas-source stable-isotope ratio mass spectrometers (typically called IRMS); (2) solid-source mass spectrometers (usually called TIMS, thermo-ionization mass spectrometry); and,
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(3) plasma-source mass spectrometers with multi-collection (called CP-MS-MC). The IRMS instruments determine the isotope ratios of light elements (13C/12C, D/H, 15N/14N, 18O/16O, 34S/32S) using two different types of inlet system – a ‘‘dual’’ inlet (DI) or a ‘‘continuous gas flow’’ inlet (CF). In both cases, the sample must be converted into gaseous form (i.e. CO2, H2, N2, CO or SO2) before analysis. With the DI-IRMS technique, the sample needs to be converted into simple gases by an off-line method. The DI system allows double injection for alternately measuring the unknown gas sample and a reference or standard gas sample with known isotopic composition under identical circumstances. The CF-IRMS system uses a helium carrier gas to carry the analytes on-line (i.e. the reference gas subsequently followed by the sample gas) into the ion source of the mass spectrometer. The CF technique involves coupling an IRMS to a range of on-line sample-preparation devices for automated gas conversion for measuring isotope ratios. The significant improvement of the CF-IRMS in analytical performance was its capability to provide isotopic measurements at a level of accuracy comparable to that of conventional DI-IRMS. However, the advantages of CF-IRMS are higher sensitivity and greater productivity with an only slightly reduced precision compared to conventional DI-IRMS systems. The TIMS technique involves the thermal ionization of an element in a vacuum source at a temperature of around 1000C. After purification of the matrix, the sample is loaded on rhenium, tungsten, tantalum or even platinum filaments as an evaporated salt. This filament is heated in the mass-spectrometer vacuum source to a temperature sufficient to ionize the salt. The MC-ICP-MS technique uses the ICP-MS plasma method for vaporizing samples coupled with the MS technique to separate isotopes. The sample solution is introduced into the spectrometer source as a spray with a micro-nebulizer coupled to a spray chamber of the spectrometer, where it is vaporized and ionized by Ar plasma. Compared to the TIMS technique, the advantages of the MC-ICP-MS are a better ionization rate and a great sensitivity, allowing the determination of the isotopic ratios of a wide variety of chemical elements, with a precision comparable to, or better than, that of TIMS. Furthermore, the plasma ionization overcomes the filament-heating procedure of the TIMS technique that is time consuming but is needed for proper ionization of some refractory elements (e.g., Th and Hf). 2.2. Some technical aspects of the analytical procedures As mentioned earlier, IRMS instruments require sample conversion into simple gases before determining the relative abundance of stable isotopes. In comparison to DI-IRMS, which requires off-line sample preparation, the
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CF-IRMS technique offers the possibility of on-line coupling with automated sample-transformation devices [e.g., gas bench (GB), elemental analysis (EA), thermocombustion EA (TC/EA), or, more recently, GC and liquid chromatography (LC)]. The GB, EA and TC/EA techniques are for the determination of ‘‘bulk’’ stable-isotope ratios (BSIA) (i.e. the isotope ratio derived from the substance as a whole) [14]. To measure the isotope ratios of individual organic compounds present in contaminant mixtures (CSIA), a chromatographic separation is required before combustion or thermal conversion. Compound-specific carbon-isotope and nitrogen-isotope analyses of volatile organic compounds are performed by combustion using a GC-C-IRMS coupling. Hydrogen and oxygen compound-specific isotope ratios of volatile compounds can be measured by high-temperature transformation using GC-TC-IRMS. CSIA measurements of non-volatile compounds require the application of LC-IRMS interface techniques, which today are only available for the determination of carbon-isotope ratios. Details on these points, as well as on some other recent analytical developments (e.g., CSIA of chlorine, bromine and sulfur) can be found in a recent review by Hofstetter and Berg [22]. The MC-ICP-MS and TIMS techniques require a preparatory chemistry phase that is necessary for separating the chemical element, target of the isotope measurements, from the rest of the sample matrix. This usually takes place in a cleanroom under positive pressure of filtered air, allowing all phases of sample preparation (e.g., solid dissolution) to be performed under clean conditions. Chemical separation is done by using ion-exchange or chelating resins and purified reagents in order to avoid any contamination of the sample. The isotopic ratio directly measured by the MC-ICP-MS or TIMS techniques is biased, resulting from a physical phenomenon known as instrumental mass fractionation, so it does not correspond to the true isotopic composition. As the mathematical laws of mass fractionation are not well defined, three different empirical laws are used in TIMS measurements to overcome this uncertainty: (1) a linear law: (M1/M2)corrected = (M1/M2)measured · [1 + f(m1 m2)] (2) a power law: (M1/M2)corrected = (M1/M2)measured · (1 + f)m1 m2 (3) an exponential law: (M1/M2)corrected = (M1/M2)measured · (m1/m2)f, which is thought to best describe the physical process [3,15] where M1 and M2 are the two isotopes of respective masses m1 and m2, and f is the mass-fractionation factor. The type of mass-fractionation process differs for TIMS and MC-ICP-MS. For TIMS, during the initial stage of analysis, the lighter isotopes are preferentially evaporated, resulting in a measured isotopic composition that is lighter than the true value, while, at the end of the
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analysis, the measured composition is heavier than the true value. The mass fractionation can be evaluated and corrected according to the certified isotopic composition of standard materials, which is the case for Pb-isotopic measurements by TIMS. This method implicitly implies that analytical parameters should be as similar as possible for both standard and sample analyses. Instrumental mass-fractionation effects are much more easily handled in the case of other elements (e.g., Sr), for which there is generally only one isotopic ratio of interest. The isotopic ratio to be determined (e.g., the 87Sr/86Sr ratio for Sr) is normalized to another ratio (e.g., the 86Sr/88Sr ratio that is considered to be 0.1194). The reproducibility of the isotope-ratio measurements was tested through replicate analyses of an international standard (NBS987 for Sr). The last method involves adding a certified solution of the element with a different isotopic composition to the sample. This method, usually known as the ‘‘double-spike technique’’, also provides the most reliable correction of mass fractionation. Unlike TIMS, MC-ICP/MS is operated at steady state, so the mass-fractionation process is not time-dependent, the heavy isotopes being transmitted with a better efficiency. As mass fractionation for MC-ICP/MS is usually much more important than for TIMS, only the exponential law provides a consistent correction. Three analytical techniques for correcting the instrumental mass-fractionation process are available. The samplestandard bracketing method, using normalization to a certified material is the simplest, but, in case of matrix effects, the corrected values may be biased. Standard bracketing is therefore normally not considered a suitable method when precise isotopic compositions are required. The second method is the chemical-standard addition of an isotopically-certified standard of another element having an atomic mass comparable to that of the element of interest. This is typically adapted for Pb-isotope or Zn-isotope measurements where the sample is doped with Tl or Cu, respectively. In this method, it is assumed that the matrix effects in the sample result in similar differences in the degree of mass fractionation compared to a chemically pure solution. The third method to correct for mass fractionation is the double-spike technique, which was previously developed for the TIMS technique.
3. Inorganic elements 3.1. Background and issues In recent years, isotope geochemistry has experienced an unprecedented development of applications for tracing metal pollution in the environment as the result of instrumental developments in the field of isotopic analysis. The MC-ICP-MS instrument has enabled accurate, precise measurements at low levels for non-traditional http://www.elsevier.com/locate/trac
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isotope systems, including some metals, metalloids and non-metals [23–25]. This technological development has generated many applied research programs for studying the origin, the transfer and the fate of these elements in water, soil, sediment and the atmosphere. The measurement of isotopic variations for metals, metalloids and non-metallic elements in different geochemical reservoirs allows not only the study of processes or mechanisms of transfer of these elements between these reservoirs, but also the quantification of fluxes and the characterization of physical and chemical reactions that can generate these isotopic fractionations. Thus, isotope geochemistry uses contrasts inherited from the history of the Earth (terrestrial geochemical differentiation of different reservoirs) to study the geochemical transfer of elements between land surface and deep reservoirs [10,12]. However, even if it is a powerful tool, the isotopic tracing of a single element can provide information on only sources and processes affecting this specific element. Conversely, the multi-isotopicapproach, integrating analysis of several isotopic systems on a given geochemical object, has a much greater potential for environmental studies [26]. 3.2. Origin of pollution, its impact and the isotopic approach Environmental pollution arises primarily from anthropogenic industrial activity, although other activities (e.g., intensive agriculture or domestic use) can also cause pollution. For many years, the increasing environmental level of certain pollutants in water and soil has been of major concern. It was shown that, in most cases, this pollution is caused by environmental releases from industrial or agricultural activities. In the long term, these pollutants have a strong impact on the environment and the health of exposed populations. Due to technological and analytical developments, many of these pollutants can be measured at very low levels. However, it has been shown that some pollutants in air, water, sediments and soil can have a natural origin (natural pollutants, also called geogenic elements) and are often found in areas where they are naturally concentrated [27,28]. At geological time-scales, natural geological processes (e.g., volcanic eruptions, erosion of mountain ranges, or hydrothermal circulation) can significantly accumulate these pollutants in specific areas. Finally, at the convergence of anthropogenic and geogenic accumulations, the leaching of mine tailings can also enrich pollutants in water, including those for irrigation and aquaculture purposes. The isotopic compositions of the pollutants may help to characterize emission sources or may be tracers of physical and chemical processes leading to isotopic fractionation. It is obvious that both phenomena can be superimposed. For tracing sources (i.e. the origin and the quantification of fluxes), lead isotopes may be the best 146
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known example. The use of different isotope ratios of lead (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) can reveal the presence of one or more sources in the environment. The range of isotopic variations for Pb in terrestrial materials is so important that the isotopic imprint left by the geological history of Pb (segregation of reservoirs and evolution over geological time) is incorporated into the pollution source. Thus, the isotopic composition of lead can be used as a fingerprint for discriminating different sources of pollution. Major applications have focused on the impact of mineralization processes [29–31], urban pollution [32–35] and the origin of pollutants in soils and ecosystems [36–39]. In this context, Notten et al. [34] investigated two polluted locations in the floodplains of the rivers Meuse and Rhine, plus a reference location for lead-isotope ratios, determined in soil, litter, leaves, snails, rainwater and airborne particulate matter, to trace the origin of Pb in a soil–plant–snail food chain. They determined the anthropogenic Pb source in the soils of all locations as being derived from deposition of Pb-polluted river sediments during flooding and from the atmosphere that contributed substantially to Pb pollution. The anthropogenic component is a mixture of various sources related to industrial activities along the Meuse and Rhine rivers. They concluded that the lead pollution in plant leaves and snails can be explained from a mixture of river-sediment Pb and atmospheric Pb from various transfer routes, even if the main contributor of lead in plant leaves and snails is from atmospheric pollution, despite the current low concentrations of Pb in the atmosphere. Notwithstanding the origin of pollution, the occurrence of possible impacts on snails due to the pollution of heavy metals (e.g., Pb) along the food chain is another step to be investigated. Regarding the atmosphere, Widory et al. [32] studied the origin of particles in Paris through lead isotopes. The emissions of different pollution sources (e.g., industrial sources and road traffic) and the natural background were differentiated by characterizing the lead-isotope ratios. Different samples of atmospheric aerosols were collected in two sites, one in a location with heavy road traffic, and the other in downtown Paris that represented the pollution baseline for the city. The isotope ratios of these samples showed that road traffic was not the only lead source and that an industrial origin is preponderant, while, in some samples, there was a natural input. Comparing these data with those obtained in the past in the Paris atmosphere traced the evolution of lead sources from road traffic to an industrial source, mainly due to the decrease of lead content in gasoline. Concerning the process than can generate fractionation in the isotope ratios of some elements, the main mechanisms involved are physico-chemical reactions [e.g., redox transformations, dissolution or precipitation of minerals and/or of newly formed phases (oxyhydroxides)] during weathering, but also adsorption and
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desorption mechanisms on surfaces. In addition, there are the effects related to microorganisms (bacteria) that, during their development, can incorporate some metals into their metabolism and cause isotopic fractionation in their growth media. 3.3. Innovations in analytical methodologies for measuring isotopes The pioneering work of estimating the average isotopic composition of pollutants during the 1940s and 1950s was done by using TIMS, the only appropriate technology available at the time [21]). TIMS analyses are still commonly performed in the case of Pb, given its large isotopic variations. Recent studies have involved TIMS analysis for Se and Cd, using an isotopic tracer that can correct the instrumental mass bias (e.g., the double or multi-spike) [40]. MC-ICP-MS is now used for accurate, precise measurements of the isotopic compositions of many elements whose (small) variations follow the laws of stable-isotope fractionation, unlike Pb, which follows the laws of radioactive decay [15]. The analysis of solutions used for MC-ICP-MS (rather than solid deposits evaporated on a very fine filament for TIMS) allows quick switching from one type of solution to another – to insert a standard solution between each sample: method of sample-standard bracketing – and thereby enables the control of the mass discrimination and its drift over time. An alternative analytical method widely used in MCICP-MS is to add an element with an atomic mass close to that of the analyte to the analyte solution. This corrects for both mass discrimination and measurement bias induced by the presence of traces of elements from the chemical matrix of the sample (matrix effect). This
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technique is widely used for the combined measurement of Cu and Zn isotopes, but also for Pb using TlMS [41,42]. The different applications in the field of isotope geochemistry have been compiled from the literature of the past 10 years (ISI Web of Knowledge research conducted in March 2011). Fig. 1 shows the number of publications on isotopic systematics for both analytical development and applications in geochemistry. It is clear that lead isotopes cover a major part of the publications of the past 10 years. Furthermore, all 45 publications on lead concern only applications. The analytical developments of lead-isotope measurements mostly predate the 2000s [15,23]. The various fields of application of lead isotopes are shown in Fig. 2 for the atmosphere, soil and sediment, and water during the period 2004–10. It is worth noting in Fig. 1 that quite a few scientific publications also relate to metals (e.g., Zn, Cu and Hg) [41–44]. For example, Borrok et al. [38] discussed the isotopic variations of dissolved copper and zinc in stream waters affected by mining in six historical mining districts of the United States and Europe. Their study highlighted that the isotopic signatures of dissolved Zn and Cu were heterogeneous in the streams investigated and that there was no evidence of a unique isotopic composition for the continental crust. They concluded that: (1) if the Cu and Zn isotopic signatures for some key streams are unique, then these metal-isotope signatures may be tracked and interpreted for space and time-scales in sedimentary records; and, (2) when the pool of dissolved metals is significantly impacted, Cu-isotopic and Zn-isotopic signatures may be used for monitoring biogeochemical processes impacting surface waters.
Figure 1. Statistics of scientific publications by isotopic system during the past 10 years (ISI Web of Knowledge research conducted in March 2011), split between analytical papers and application papers.
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Figure 2. Example of various fields of application of lead isotopes (e.g., the atmosphere, soil and sediment and water) during the period 2004–10.
Finally, Fig. 1 shows that the aforementioned elements are followed by Hg and, to a lesser extent, other elements (e.g., Cd, Se, Te, Ni, Tl, Sn and Sb). To date, the few publications on W and Mo relate to method developments. 4. Organic molecules and pollution Industrial, agricultural and household use of organic chemicals (e.g., aromatic hydrocarbons, petroleum products, chlorinated solvents, pesticides, and detergents) significantly contributes to environmental pollution [4,6]. Environmental contaminants are discharged to water, spread to soils, or released into the atmosphere, by point sources or diffuse sources. Most of these organic pollutants ultimately enter aqueous systems and pose a severe threat to drinking-water resources [9]. Limiting the input of contaminants and sustainable water-quality management require thorough understanding of contaminant behavior in soil, sub-soil and groundwater systems. Classic approaches, in general based on the observation of contaminant concentrations, have shown their limitations for identifying the sources, the fate and the behavior within a contaminant plume. Combining classical investigation tools with isotopic characteriza148
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tion of organic chemicals (as for the aforementioned metals) can overcome these limitations and help to gain insight into the behavior of contaminants in the environment. 4.1. Isotope approach to organic molecules in environmental studies The isotopic composition of organic compounds, commonly a characteristic feature of their origin, may distinguish and allocate different pollution sources [17]. In addition, the isotope approach is gaining increased attention for assessing the different processes that affect compounds after their release into the environment. This is in particular true for in situ remediation strategies, where biodegradation reactions are a key factor for the remediation of contaminated groundwater. In situ strategies are cost-effective and little-intrusive remediation options that rely on the controlled use of naturallyoccurring attenuation (biodegradation) and/or enhanced biotic or abiotic degradation mechanisms of contaminants in the sub-surface, or monitored natural attenuation (MNA). Classic methods, based on a massbalance approach of contaminant concentrations in groundwater or soil, often do not provide conclusive evidence and fail to identify or to quantify degradation mechanisms. To study the fate of organic pollutants and
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thus to apply the MNA concept, especially at complex field sites, require reliable investigation tools in addition to standard methods (e.g., identification and quantification of pollutants and their metabolites, hydrogeological modeling, or microbiological methods). The analysis of isotope ratios of individual organic compounds (CSIA) is such a complementary tool that allows the identification of in situ degradation reactions and/or the determination of pollution sources. 4.2. Recent developments in analytical methodologies Due to its potential for environmental chemistry applications, the analysis of stable isotopes has undergone considerable development within this field of research in the past 20 years [45,46]. CSIA by on-line GC-IRMS made it possible to measure precisely even small isotopic variations of stable elements within individual organic pollutants (e.g., 13C/12C, D/H, 15N/14N, 18O/16O), with values given in & deviation relative to an international reference. While carbon-isotope-ratio analyses are by far the most frequently applied, analyses of isotope ratios of hydrogen, nitrogen or oxygen are much less common. In recent years, CSIA has become a powerful investigation tool for allocating contaminant sources, identifying in situ transformation processes, characterizing the degradation mechanisms involved, and monitoring and modeling transport and transfer of organic pollutants in the environment [47–49]. Fig. 3 shows the number of international publications in the field of environmental science and how CSIA applications for organic pollutants have developed since 1990. 4.3. Concept of isotope fractionation All organic molecules contain stable isotopes in a certain ratio (e.g., 13C/12C, D/H, and 15N/14N). These isotopic ratios can change according to the different processes that affect organic molecules during their transport and
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transformation in the environmental compartments of soil, air and groundwater [38]. Processes (e.g., dilution, evaporation, sorption/desorption) and other physical transport effects that act on the molecule as a whole just decrease contaminant concentrations without changing their chemical structure. These processes do not, or hardly, change the isotopic composition of an organic compound within the range of analytical error. By contrast, the transformation of a molecule, by biodegradation through biotic metabolism or abiotic transformation reactions (e.g., chemical), is commonly associated with a significant change in its isotopic composition. This kinetic isotope fractionation is due to different reaction rates caused by different binding strengths of molecules containing light and heavy isotopes at the reactive position. All chemical reactions, whether biotic or abiotic, involve this kinetic effect, where molecules containing light isotopes at the bondbreaking position react faster than those containing heavy isotopes. Molecules containing light isotopes are therefore preferably degraded and the remaining contaminant fraction becomes enriched in heavier isotopes over the course of the reaction. The isotopic enrichment can be expressed using the Rayleigh equation [Equation (1)]: Rt =R0 ¼ ðct =c0 Þ
ð1=a1Þ
ð1Þ
where R0 describes the initial isotopic composition (e.g. C/12C) of the contaminant, Rt the isotopic composition of the remaining contaminant after time t, ct/c0 the concentration change or the fraction of the contaminant remaining after time t, and a is the isotopic fractionation factor. Using a range of appropriate a-values for a field site allows estimating the degree of intrinsic biodegradation [50]. Isotopic fractionation factors a (also known as enrichment factors e = (a-1), expressed in &) are experimentally determined in laboratory microcosms or
13
Figure 3. Instrumental developments and number of compound-specific stable-isotope analysis (CSIA) studies of organic pollutants published in international literature over the past 20 years (search performed by ISI Web of Knowledge in June 2010).
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Table 1. Summary of the type of molecules, elements and application field of compound-specific stable-isotope analysis (CSIA) studies during the past 20 years Laboratory batch studies Degradation processes
Other processes (incl. production)
Chlorinated aliphatic hydrocarbons (PCE, TCE, VC, DCA etc.)
CCC
Aromatic hydrocarbons (BTEX – benzene, toluene etc.)
CC HH CC HH CC H C H C CC H NN
CC H CC H C H C H C
Oxygenated fuel additives: ethers (MTBE, ETBE etc.) Saturated aliphatic hydrocarbons (CH4, n-alkanes) Polycyclic aromatic hydrocarbons (PAHs – naphthalene etc.) Aromatic chlorinated compounds (PCBs, chlorobenzene etc.) Nitroaromatic compounds (TNT, atrazine, isoproturon etc.)
Field applications (sources and fate of pollutants) Soil
Water CCC
CC
CCC HH CC H C
CCC
C
C C
C
N
N
Carbonyl compounds (acetaldehyde, formaldehyde, ketones)
Atmosphere
C
CC H CC H
CC H
C, H, N = CSIA studies for carbon, hydrogen and nitrogen, respectively. XXX 6 50, XX 6 20 et X 6 5 = number of international publications for the different applications.
abiotic batch reactors, where the (bio)degradation of an organic compound is performed under controlled laboratory conditions. These laboratory-derived fractionation factors may be used for calculating the amount of in situ biodegradation based on the measured isotopic changes of the contaminant measured in the field, if the geochemical and microbiological site conditions are close to the conditions of the microcosm experiments. As the kinetic isotope-fractionation effect depends strongly on the type of reaction mechanism, it can also be used for identifying the degradation processes that are involved [49]. 4.4. How CSIA can be applied in environmental studies 4.4.1. Studies of degradation of organic molecules. The answers given by compound-specific carbon-isotope analysis in environmental studies (e.g., source allocation) and sustainable contaminated-site management (e.g., degradation studies) are complementary, but often crucial, compared to those given by classical investigation tools. The method has been applied for investigating contaminated soil and groundwater systems, as well as atmospheric pollution, and there are many published studies (Table 1). One of the standard examples is to study in situ biodegradation by stable carbon-isotope monitoring in groundwater polluted by chlorinated ethenes, mainly perchloroethene (PCE) and trichloroethene (TCE) [45,50]. Reductive dechlorination of PCE shows a significant difference between the isotopic values (d13C) of the initial compound, the remaining compound and its 150
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degradation products. Another example is given for aromatic hydrocarbons {e.g., for the degradation of toluene under anaerobic conditions, involving a significant shift in its isotopic composition; the remaining fraction of toluene is enriched in heavy (13C) isotopes during the reaction [51]}. Fig. 4 shows both examples. In the same way, laboratory experiments (microcosms/batch) studying the influence of biotic and abiotic degradation on isotope ratios have shown that the rate of isotopic fractionation is sufficiently different to be used as an indicator for in situ attenuation mechanisms for aromatic hydrocarbons (BTEX), chlorinated solvents (e.g., PCE, DCA, and VC), oxygenated fuel additives (e.g., MTBE, and ETBE), short-chained n-alkanes (
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Figure 4. Isotopic fractionation induced by degradation (A) during different stages of biotic reductive dehalogenation of PCE under anaerobic conditions, while carbon-isotopic compositions of PCE in the sterile control do not change (horizontal line), and (B) during toluene biodegradation in a sulfate-reducing microcosm. The d13C increases during degradation (enrichment in heavy carbon isotope 13C in the remaining compound fraction). Figures taken from [43] (A) and [40] (B).
prediction of natural degradation reactions of BTEX, MTBE and chlorinated compounds [57–59]. 4.4.2. The concept of source discrimination. The isotope compositions of organic hydrocarbons depend on their chemical precursors and/or the method of synthesis involved in their production, so pollutants of various origins may be discriminated by their isotope signature. It has been shown that organic chemicals (e.g., BTEX) from different manufacturers could be discriminated on the basis of their d13C values. This observation was also made for chlorinated solvents of different manufacturers and sources, n-alkanes of different origins, aromatic compounds present in different gasoline samples, and MTBE [47]. The use of carbon-isotope analysis seems to be a valuable tool for identification of pollution sources for PAHs that comprise 2–5 aromatic rings on which microbial degradation did not result in significant isotopic fractionation [60]. Much work has already been done in this field (e.g., after the Erika oil spill in France, where the application of this analytical technique to oil residues sampled on the northern French Atlantic Coast confirmed that the contamination by these petroleum products was linked to the Erika tanker spill) [61]. The same principle can be adapted to PCBs, where compound-specific d13C analysis may be a useful complementary tool for tracing these molecules in the environment [62]. Various authors have shown that carbon-isotope ratios of certain organic hydrocarbons can be used for tracking these pollutants in the environment (e.g., in order to discriminate contamination sources in urban air or groundwater) [63,64]. 4.4.3. Multi-isotope approach for studying pollution in the environment. The broad application field of CSIA, first
restricted to carbon isotopes, has expanded even more since instruments for compound-specific hydrogen-isotope analysis became available in 1998. For example, it was shown that gasoline samples from different origins could clearly be discriminated if compound-specific d13C values and dD analyses of specific organic compounds (e.g., BTEX, n-alkanes or MTBE) are combined [65]. In addition, it was reported that the analysis of compoundspecific dD values may be a complementary tool for tracing PAH compounds in the environment [66]. Moreover, according to different isotopic fractionation effects observed in laboratory experiments, isotope studies can be used for distinguishing between different degradation pathways of a compound, especially if the multi-dimensional isotope approach is applied. CSIA applications for the determination of compoundspecific nitrogen-isotope ratios by GC-C-IRMS are still relatively rare. For example, it was shown for pesticides atrazine and isoproturon that distinct isotopic fractionation is observed during different microbial and abiotic degradation pathways. Thus, a clear differentiation of different degradation processes was shown for these herbicides, based on three-dimensional isotope analysis (d13C, dD and d15N) [67,68]. For chlorine-isotope analysis of halogenated organic compounds, several promising analytical approaches by TIMS, GC-MS and IRMS are currently under development [22].
5. Summary and perspectives The application of isotope signatures of inorganic or organic contaminants provides valuable and sometimes unique information for deciphering their origin and studying their degradation processes in the environment.
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However, there are limiting and challenging aspects of the methods that should be critically addressed by the scientific community. Finally, since most of these techniques come from research, the connection to local authorities, stakeholders and environmental consultants is often weak. Bringing together the different approaches, including their, sometimes controversial, economic and social aspects, might be one of the most challenging tasks in the near future in creating integrated risk-assessmentbased contaminated-site management. Protecting and managing surface and groundwater resources is the most essential task of safeguarding drinking-water supplies. However, organic contaminants, which derive from industry, oil spills, improper disposal and/or leaking storage tanks, landfill leachates, household use, motor-vehicle emissions, agricultural fertilizers and pesticides, pose a threat to soil and freshwater resources. Industrial activity and agricultural practices may have a significant impact on vulnerable aquifers and thus on drinking-water supply. By introducing the EU Water Framework Directive [5], and the EU Groundwater Directive on the protection of groundwater against pollution and deterioration [69], the European Commission has made water protection one of its priority tasks. As a framework for water protection and management, it commits EU Member States to identify and to analyze European waters and finally to adopt management plans and programs for measures adapted to each water body. Within the WFD context, the application of high-grade analytical tools (e.g., MCICP-MS or CSIA) offers great potential for distinguishing various potential sources within contamination plumes and maybe even estimating different source inputs at the catchment scale [70]. In addition, the possibility of CSIA for evaluating in situ transformation reactions is of fundamental importance for studying the fate of organic pollutants, especially for evaluation of remediation strategies that rely on monitoring natural and/or engineered attenuation of organic contaminants in soil and groundwater systems. Future studies will help to develop these high-grade analytical tools into a powerful, generally accepted investigation framework for groundwater characterization and contaminant hydrology, and to define and apply monitoring tools [9]. Groundwater-monitoring programs are required to provide a coherent, acceptable overview of water status with regard to pollution and its origins and to define trends in pollutant concentrations [71], especially for organic molecules, as defined by Terry et al. [72] for 1,3-dichloropropene and metabolites in groundwater in five EU countries (UK, France, Italy, Spain and Greece) through the analysis of 5000 groundwater samples, but without any source tracing due to the low values of observed contamination levels.
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Acknowledgments This study was supported by the ‘‘Association RECORD – REseau COope´ratif de Recherche sur les De´chets’’ Grant n 09-0138/1A Isotopic methods for identifying the origin of metal and organic pollutants in the environment: state of the art and critical approach for implementation and by the BRGM Research Division through the METR09 Isotopes research program. The authors wish to acknowledge B. Couffignal and R. Gourdon from RECORD for their support during this study. Suggestions by A. Damien, C. Veronneau, D. Bilbault, H. Roussel, L. Cimolino and M. Brugioni from the RECORD steering group significantly improved the quality of this manuscript. We are grateful to H.M. Kluijver for proofreading and editing the English text. References [1] A.R. McBirney, Igneous Petrology, Jones and Bartlett, Boston, MA, USA, 2007. [2] P.A. Allen, Earth Surface Processes, Blackwell Science, John Wiley & Sons, Hoboken, NJ, USA, 1997. [3] G. Faure, Principles and Applications of Geochemistry (2nd Edition), Prentice Hall, Upper Saddle River, NJ, USA, 1998. [4] C.A.J. Appelo, D. Postma, Geochemistry, Groundwater and Pollution (2nd Edition), A. A. Balkema Publishers, Leiden, The Netherlands, 2005. [5] European Union, Water Framework Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy, Offic. J. Eur. Commun. L327 (2000) 1. [6] P. Quevauviller, U. Borchers, C. Thompson, T. Simonart, The Water Framework Directive: Ecological and Chemical Status Monitoring, Water Quality Measurements Series, Wiley, Chichester, W. Sussex, UK, 2008. [7] European Union, Strategic Environmental Assessment Directive 2001/42/EC of the European Parliament establishing a framework on Environmental Impact Assessments, Offic. J. Eur. Commun. L197 (2001) 30. [8] P.J.T. Morris, From Classical to Modern Chemistry: The Instrumental Revolution, Royal Society of Chemistry, Cambridge, UK, 2002. [9] P. Quevauviller, A.M. Fouillac, J. Grath, R. Ward, Groundwater Monitoring, Water Quality Measurements Series, Wiley, Chichester, W. Sussex, UK, 2009. [10] C.J. Alle`gre, Earth Planet. Sci. Lett. 86 (1987) 175. [11] R. Bowen, Isotopes in the Earth Sciences, Elsevier Applied Science, Amsterdam, The Netherlands, 1988. [12] G.J. Wasserburg, Earth Planet. Sci. Lett. 86 (1987) 129. [13] P. Fritz, J.C. Fontes, Handbook of Environmental Isotope Geochemistry, Vols. 1–5, Elsevier, Amsterdam, The Netherlands, 1980. [14] P.K. Aggarwal, J.R. Gat, K.F.O. Froehlich, Isotopes in the Water Cycle; Past, Present and Future of a Developing Science, Springer, Berlin, Germany, 2005. [15] G. Faure, Principles of Isotope Geology, John Wiley & Sons, Chichester, W. Sussex, UK, 1986. [16] W. Kloppmann, Actual. Chim. 267 (2003) 44. [17] G. Charalampides, O. Manoliadis, Environ. Int. 28 (2002) 147. [18] C.E. Dunlap, E. Steinnes, A.R. Flegal, Earth Planet. Sci. Lett. 167 (1999) 81. [19] D. Widory, E.P. Giraud, Ph. Ne´grel, B. Ladouche, Environ. Sci. Technol. 39 (2005) 539.
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Isotopic methods give clues about the origins of trace metals and organic pollutants in the environment Philippe Ne´grel, Michaela Blessing, Romain Millot, Emmanuelle Petelet-Giraud, Christophe Innocent Recent advances in techniques and methodologies in analytical chemistry help to address problems of environmental contamination. This is particularly true for isotope determinations of metals using multi-collector inductively coupled plasma mass spectrometry (MS) and on-line coupling of capillary gas chromatography and isotope-ratio MS, or compound-specific stable isotope analysis. We explore the methodologies and applications using the isotope signatures of inorganic or organic contaminants, which offer good potential for distinguishing potential sources within contamination plumes and possibly even estimating different source inputs at the catchment scale. Moreover, for organic pollutants, such methods provide unique information for deciphering their origin and studying degradation processes. ª 2012 Elsevier Ltd. All rights reserved. Keywords: Degradation; Environment; Inorganic element; Isotope; Isotopic determination; Isotopic signature; Organic molecule; Organic pollutant; Pollution; Trace metal
1. Introduction Philippe Ne´grel*, Michaela Blessing, Romain Millot, Emmanuelle Petelet-Giraud, Christophe Innocent BRGM, Metrology Monitoring Analysis Division, Avenue C. Guillemin, BP 36009, 45060 Orle´ans Cedex 02, France
*
Corresponding author. E-mail: [email protected]
Geology deals with the composition, the structure and the processes of formation of rocks and sediments, and covers topics such as mineral-resource exploration, evaluation of water resources, environmental geochemistry, natural risk assessment and geotechnical engineering [1–3]. Geology coupled with environmental science is building the knowledge base for a sustainable Earth, and geoscientists are increasingly involved in developing approaches for prevention or remediation of environmental pollution [4]. Since the signing of the ‘‘Treaty of Maastricht’’ in 1992, protecting the environment from pollution and conserving water resources for future generations has become an important task for the institutions of the European Community [5–7]. European environmental programs foster actions for reducing emissions in all
sectors, including industry, agriculture, transport and domestic pollution. New techniques and methodologies in analytical chemistry, geophysics, microbiology, geochemistry, hydrogeology, and informatics have emerged for addressing environmental contamination [8]. In the wake of a growing concern about environmental pollution, geosciences are evolving into a multidisciplinary field of research for understanding how pollutants behave, studying their transport and fate in natural environments, developing powerful remediation technologies, and finally helping to find solutions for a sustainable Earth and to protect natural resources for future generations. Environmental contamination is caused by a wide range of pollutants (e.g., metals, hydrocarbons, organic micro-pollutants, solvents, and pesticides) [4,9]. Identifying pollution sources and tracking their transport pathways is of fundamental interest, not only in terms of the ‘‘polluter
0165-9936/$ - see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:http://dx.doi.org/10.1016/j.trac.2012.03.017
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pays’’ principle that is now generally applied by environmental legislation. One of the profound changes in Earth sciences has been the addition of new analytical tools (e.g., isotopes) to complement classic descriptive ones. In this isotopegeology framework [10–12], isotope hydrology and isotope hydrogeology have become great challenges [13,14]. Isotope geology began with dating geologic events through various isotope systematics [15], and then progressed into isotope hydrology and isotope hydrogeology that are mainly based on studying variations in the contents of naturally occurring isotopes of elements found in abundance in the EarthÕs environment [16]. Within this framework, recent improvements in analytical techniques with high-grade analytical tools have made isotopic analyses available for an increasing number of elements and compounds present at low concentrations (e.g., lead, nitrates and chlorinated compounds) [17–20]. This is particularly true for the multi-collector inductively coupled plasma mass spectrometry (ICP-MS) instrument for isotope determination of metals [21] and for the gas chromatography isotoperatio MS (GC-IRMS) technique that has become known as compound-specific stable-isotope analysis (CSIA) [22]. Both offer a great opportunity for tracing pollution and pathways (transport or degradation) of specific elements and compounds. The objective of this article is to promote such techniques by illustrating recent methodologies and applications.
2. General consideration of the isotope-ratio measurements Mass spectrometers with different types of ion source are the principal instruments for measuring isotope ratios of a chemical element. In the vacuum source of a mass spectrometer, an ion beam is produced before being separated according to its mass/charge ratio by a combination of electromagnetic fields, including acceleration by an electric field and mass separation by a magnetic field. The separated ion beams are finally detected and measured (e.g., Faraday cups and multi counting) before being expressed in terms of their relative abundance. 2.1. The mass spectrometer Precise measurements of even small variations in the natural isotopic abundance demand specialized mass spectrometers. Three main mass-spectrometer types are mainly used for isotope-dedicated studies: (1) gas-source stable-isotope ratio mass spectrometers (typically called IRMS); (2) solid-source mass spectrometers (usually called TIMS, thermo-ionization mass spectrometry); and,
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(3) plasma-source mass spectrometers with multi-collection (called CP-MS-MC). The IRMS instruments determine the isotope ratios of light elements (13C/12C, D/H, 15N/14N, 18O/16O, 34S/32S) using two different types of inlet system – a ‘‘dual’’ inlet (DI) or a ‘‘continuous gas flow’’ inlet (CF). In both cases, the sample must be converted into gaseous form (i.e. CO2, H2, N2, CO or SO2) before analysis. With the DI-IRMS technique, the sample needs to be converted into simple gases by an off-line method. The DI system allows double injection for alternately measuring the unknown gas sample and a reference or standard gas sample with known isotopic composition under identical circumstances. The CF-IRMS system uses a helium carrier gas to carry the analytes on-line (i.e. the reference gas subsequently followed by the sample gas) into the ion source of the mass spectrometer. The CF technique involves coupling an IRMS to a range of on-line sample-preparation devices for automated gas conversion for measuring isotope ratios. The significant improvement of the CF-IRMS in analytical performance was its capability to provide isotopic measurements at a level of accuracy comparable to that of conventional DI-IRMS. However, the advantages of CF-IRMS are higher sensitivity and greater productivity with an only slightly reduced precision compared to conventional DI-IRMS systems. The TIMS technique involves the thermal ionization of an element in a vacuum source at a temperature of around 1000C. After purification of the matrix, the sample is loaded on rhenium, tungsten, tantalum or even platinum filaments as an evaporated salt. This filament is heated in the mass-spectrometer vacuum source to a temperature sufficient to ionize the salt. The MC-ICP-MS technique uses the ICP-MS plasma method for vaporizing samples coupled with the MS technique to separate isotopes. The sample solution is introduced into the spectrometer source as a spray with a micro-nebulizer coupled to a spray chamber of the spectrometer, where it is vaporized and ionized by Ar plasma. Compared to the TIMS technique, the advantages of the MC-ICP-MS are a better ionization rate and a great sensitivity, allowing the determination of the isotopic ratios of a wide variety of chemical elements, with a precision comparable to, or better than, that of TIMS. Furthermore, the plasma ionization overcomes the filament-heating procedure of the TIMS technique that is time consuming but is needed for proper ionization of some refractory elements (e.g., Th and Hf). 2.2. Some technical aspects of the analytical procedures As mentioned earlier, IRMS instruments require sample conversion into simple gases before determining the relative abundance of stable isotopes. In comparison to DI-IRMS, which requires off-line sample preparation, the
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CF-IRMS technique offers the possibility of on-line coupling with automated sample-transformation devices [e.g., gas bench (GB), elemental analysis (EA), thermocombustion EA (TC/EA), or, more recently, GC and liquid chromatography (LC)]. The GB, EA and TC/EA techniques are for the determination of ‘‘bulk’’ stable-isotope ratios (BSIA) (i.e. the isotope ratio derived from the substance as a whole) [14]. To measure the isotope ratios of individual organic compounds present in contaminant mixtures (CSIA), a chromatographic separation is required before combustion or thermal conversion. Compound-specific carbon-isotope and nitrogen-isotope analyses of volatile organic compounds are performed by combustion using a GC-C-IRMS coupling. Hydrogen and oxygen compound-specific isotope ratios of volatile compounds can be measured by high-temperature transformation using GC-TC-IRMS. CSIA measurements of non-volatile compounds require the application of LC-IRMS interface techniques, which today are only available for the determination of carbon-isotope ratios. Details on these points, as well as on some other recent analytical developments (e.g., CSIA of chlorine, bromine and sulfur) can be found in a recent review by Hofstetter and Berg [22]. The MC-ICP-MS and TIMS techniques require a preparatory chemistry phase that is necessary for separating the chemical element, target of the isotope measurements, from the rest of the sample matrix. This usually takes place in a cleanroom under positive pressure of filtered air, allowing all phases of sample preparation (e.g., solid dissolution) to be performed under clean conditions. Chemical separation is done by using ion-exchange or chelating resins and purified reagents in order to avoid any contamination of the sample. The isotopic ratio directly measured by the MC-ICP-MS or TIMS techniques is biased, resulting from a physical phenomenon known as instrumental mass fractionation, so it does not correspond to the true isotopic composition. As the mathematical laws of mass fractionation are not well defined, three different empirical laws are used in TIMS measurements to overcome this uncertainty: (1) a linear law: (M1/M2)corrected = (M1/M2)measured · [1 + f(m1 m2)] (2) a power law: (M1/M2)corrected = (M1/M2)measured · (1 + f)m1 m2 (3) an exponential law: (M1/M2)corrected = (M1/M2)measured · (m1/m2)f, which is thought to best describe the physical process [3,15] where M1 and M2 are the two isotopes of respective masses m1 and m2, and f is the mass-fractionation factor. The type of mass-fractionation process differs for TIMS and MC-ICP-MS. For TIMS, during the initial stage of analysis, the lighter isotopes are preferentially evaporated, resulting in a measured isotopic composition that is lighter than the true value, while, at the end of the
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analysis, the measured composition is heavier than the true value. The mass fractionation can be evaluated and corrected according to the certified isotopic composition of standard materials, which is the case for Pb-isotopic measurements by TIMS. This method implicitly implies that analytical parameters should be as similar as possible for both standard and sample analyses. Instrumental mass-fractionation effects are much more easily handled in the case of other elements (e.g., Sr), for which there is generally only one isotopic ratio of interest. The isotopic ratio to be determined (e.g., the 87Sr/86Sr ratio for Sr) is normalized to another ratio (e.g., the 86Sr/88Sr ratio that is considered to be 0.1194). The reproducibility of the isotope-ratio measurements was tested through replicate analyses of an international standard (NBS987 for Sr). The last method involves adding a certified solution of the element with a different isotopic composition to the sample. This method, usually known as the ‘‘double-spike technique’’, also provides the most reliable correction of mass fractionation. Unlike TIMS, MC-ICP/MS is operated at steady state, so the mass-fractionation process is not time-dependent, the heavy isotopes being transmitted with a better efficiency. As mass fractionation for MC-ICP/MS is usually much more important than for TIMS, only the exponential law provides a consistent correction. Three analytical techniques for correcting the instrumental mass-fractionation process are available. The samplestandard bracketing method, using normalization to a certified material is the simplest, but, in case of matrix effects, the corrected values may be biased. Standard bracketing is therefore normally not considered a suitable method when precise isotopic compositions are required. The second method is the chemical-standard addition of an isotopically-certified standard of another element having an atomic mass comparable to that of the element of interest. This is typically adapted for Pb-isotope or Zn-isotope measurements where the sample is doped with Tl or Cu, respectively. In this method, it is assumed that the matrix effects in the sample result in similar differences in the degree of mass fractionation compared to a chemically pure solution. The third method to correct for mass fractionation is the double-spike technique, which was previously developed for the TIMS technique.
3. Inorganic elements 3.1. Background and issues In recent years, isotope geochemistry has experienced an unprecedented development of applications for tracing metal pollution in the environment as the result of instrumental developments in the field of isotopic analysis. The MC-ICP-MS instrument has enabled accurate, precise measurements at low levels for non-traditional http://www.elsevier.com/locate/trac
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isotope systems, including some metals, metalloids and non-metals [23–25]. This technological development has generated many applied research programs for studying the origin, the transfer and the fate of these elements in water, soil, sediment and the atmosphere. The measurement of isotopic variations for metals, metalloids and non-metallic elements in different geochemical reservoirs allows not only the study of processes or mechanisms of transfer of these elements between these reservoirs, but also the quantification of fluxes and the characterization of physical and chemical reactions that can generate these isotopic fractionations. Thus, isotope geochemistry uses contrasts inherited from the history of the Earth (terrestrial geochemical differentiation of different reservoirs) to study the geochemical transfer of elements between land surface and deep reservoirs [10,12]. However, even if it is a powerful tool, the isotopic tracing of a single element can provide information on only sources and processes affecting this specific element. Conversely, the multi-isotopicapproach, integrating analysis of several isotopic systems on a given geochemical object, has a much greater potential for environmental studies [26]. 3.2. Origin of pollution, its impact and the isotopic approach Environmental pollution arises primarily from anthropogenic industrial activity, although other activities (e.g., intensive agriculture or domestic use) can also cause pollution. For many years, the increasing environmental level of certain pollutants in water and soil has been of major concern. It was shown that, in most cases, this pollution is caused by environmental releases from industrial or agricultural activities. In the long term, these pollutants have a strong impact on the environment and the health of exposed populations. Due to technological and analytical developments, many of these pollutants can be measured at very low levels. However, it has been shown that some pollutants in air, water, sediments and soil can have a natural origin (natural pollutants, also called geogenic elements) and are often found in areas where they are naturally concentrated [27,28]. At geological time-scales, natural geological processes (e.g., volcanic eruptions, erosion of mountain ranges, or hydrothermal circulation) can significantly accumulate these pollutants in specific areas. Finally, at the convergence of anthropogenic and geogenic accumulations, the leaching of mine tailings can also enrich pollutants in water, including those for irrigation and aquaculture purposes. The isotopic compositions of the pollutants may help to characterize emission sources or may be tracers of physical and chemical processes leading to isotopic fractionation. It is obvious that both phenomena can be superimposed. For tracing sources (i.e. the origin and the quantification of fluxes), lead isotopes may be the best 146
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known example. The use of different isotope ratios of lead (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) can reveal the presence of one or more sources in the environment. The range of isotopic variations for Pb in terrestrial materials is so important that the isotopic imprint left by the geological history of Pb (segregation of reservoirs and evolution over geological time) is incorporated into the pollution source. Thus, the isotopic composition of lead can be used as a fingerprint for discriminating different sources of pollution. Major applications have focused on the impact of mineralization processes [29–31], urban pollution [32–35] and the origin of pollutants in soils and ecosystems [36–39]. In this context, Notten et al. [34] investigated two polluted locations in the floodplains of the rivers Meuse and Rhine, plus a reference location for lead-isotope ratios, determined in soil, litter, leaves, snails, rainwater and airborne particulate matter, to trace the origin of Pb in a soil–plant–snail food chain. They determined the anthropogenic Pb source in the soils of all locations as being derived from deposition of Pb-polluted river sediments during flooding and from the atmosphere that contributed substantially to Pb pollution. The anthropogenic component is a mixture of various sources related to industrial activities along the Meuse and Rhine rivers. They concluded that the lead pollution in plant leaves and snails can be explained from a mixture of river-sediment Pb and atmospheric Pb from various transfer routes, even if the main contributor of lead in plant leaves and snails is from atmospheric pollution, despite the current low concentrations of Pb in the atmosphere. Notwithstanding the origin of pollution, the occurrence of possible impacts on snails due to the pollution of heavy metals (e.g., Pb) along the food chain is another step to be investigated. Regarding the atmosphere, Widory et al. [32] studied the origin of particles in Paris through lead isotopes. The emissions of different pollution sources (e.g., industrial sources and road traffic) and the natural background were differentiated by characterizing the lead-isotope ratios. Different samples of atmospheric aerosols were collected in two sites, one in a location with heavy road traffic, and the other in downtown Paris that represented the pollution baseline for the city. The isotope ratios of these samples showed that road traffic was not the only lead source and that an industrial origin is preponderant, while, in some samples, there was a natural input. Comparing these data with those obtained in the past in the Paris atmosphere traced the evolution of lead sources from road traffic to an industrial source, mainly due to the decrease of lead content in gasoline. Concerning the process than can generate fractionation in the isotope ratios of some elements, the main mechanisms involved are physico-chemical reactions [e.g., redox transformations, dissolution or precipitation of minerals and/or of newly formed phases (oxyhydroxides)] during weathering, but also adsorption and
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desorption mechanisms on surfaces. In addition, there are the effects related to microorganisms (bacteria) that, during their development, can incorporate some metals into their metabolism and cause isotopic fractionation in their growth media. 3.3. Innovations in analytical methodologies for measuring isotopes The pioneering work of estimating the average isotopic composition of pollutants during the 1940s and 1950s was done by using TIMS, the only appropriate technology available at the time [21]). TIMS analyses are still commonly performed in the case of Pb, given its large isotopic variations. Recent studies have involved TIMS analysis for Se and Cd, using an isotopic tracer that can correct the instrumental mass bias (e.g., the double or multi-spike) [40]. MC-ICP-MS is now used for accurate, precise measurements of the isotopic compositions of many elements whose (small) variations follow the laws of stable-isotope fractionation, unlike Pb, which follows the laws of radioactive decay [15]. The analysis of solutions used for MC-ICP-MS (rather than solid deposits evaporated on a very fine filament for TIMS) allows quick switching from one type of solution to another – to insert a standard solution between each sample: method of sample-standard bracketing – and thereby enables the control of the mass discrimination and its drift over time. An alternative analytical method widely used in MCICP-MS is to add an element with an atomic mass close to that of the analyte to the analyte solution. This corrects for both mass discrimination and measurement bias induced by the presence of traces of elements from the chemical matrix of the sample (matrix effect). This
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technique is widely used for the combined measurement of Cu and Zn isotopes, but also for Pb using TlMS [41,42]. The different applications in the field of isotope geochemistry have been compiled from the literature of the past 10 years (ISI Web of Knowledge research conducted in March 2011). Fig. 1 shows the number of publications on isotopic systematics for both analytical development and applications in geochemistry. It is clear that lead isotopes cover a major part of the publications of the past 10 years. Furthermore, all 45 publications on lead concern only applications. The analytical developments of lead-isotope measurements mostly predate the 2000s [15,23]. The various fields of application of lead isotopes are shown in Fig. 2 for the atmosphere, soil and sediment, and water during the period 2004–10. It is worth noting in Fig. 1 that quite a few scientific publications also relate to metals (e.g., Zn, Cu and Hg) [41–44]. For example, Borrok et al. [38] discussed the isotopic variations of dissolved copper and zinc in stream waters affected by mining in six historical mining districts of the United States and Europe. Their study highlighted that the isotopic signatures of dissolved Zn and Cu were heterogeneous in the streams investigated and that there was no evidence of a unique isotopic composition for the continental crust. They concluded that: (1) if the Cu and Zn isotopic signatures for some key streams are unique, then these metal-isotope signatures may be tracked and interpreted for space and time-scales in sedimentary records; and, (2) when the pool of dissolved metals is significantly impacted, Cu-isotopic and Zn-isotopic signatures may be used for monitoring biogeochemical processes impacting surface waters.
Figure 1. Statistics of scientific publications by isotopic system during the past 10 years (ISI Web of Knowledge research conducted in March 2011), split between analytical papers and application papers.
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Figure 2. Example of various fields of application of lead isotopes (e.g., the atmosphere, soil and sediment and water) during the period 2004–10.
Finally, Fig. 1 shows that the aforementioned elements are followed by Hg and, to a lesser extent, other elements (e.g., Cd, Se, Te, Ni, Tl, Sn and Sb). To date, the few publications on W and Mo relate to method developments. 4. Organic molecules and pollution Industrial, agricultural and household use of organic chemicals (e.g., aromatic hydrocarbons, petroleum products, chlorinated solvents, pesticides, and detergents) significantly contributes to environmental pollution [4,6]. Environmental contaminants are discharged to water, spread to soils, or released into the atmosphere, by point sources or diffuse sources. Most of these organic pollutants ultimately enter aqueous systems and pose a severe threat to drinking-water resources [9]. Limiting the input of contaminants and sustainable water-quality management require thorough understanding of contaminant behavior in soil, sub-soil and groundwater systems. Classic approaches, in general based on the observation of contaminant concentrations, have shown their limitations for identifying the sources, the fate and the behavior within a contaminant plume. Combining classical investigation tools with isotopic characteriza148
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tion of organic chemicals (as for the aforementioned metals) can overcome these limitations and help to gain insight into the behavior of contaminants in the environment. 4.1. Isotope approach to organic molecules in environmental studies The isotopic composition of organic compounds, commonly a characteristic feature of their origin, may distinguish and allocate different pollution sources [17]. In addition, the isotope approach is gaining increased attention for assessing the different processes that affect compounds after their release into the environment. This is in particular true for in situ remediation strategies, where biodegradation reactions are a key factor for the remediation of contaminated groundwater. In situ strategies are cost-effective and little-intrusive remediation options that rely on the controlled use of naturallyoccurring attenuation (biodegradation) and/or enhanced biotic or abiotic degradation mechanisms of contaminants in the sub-surface, or monitored natural attenuation (MNA). Classic methods, based on a massbalance approach of contaminant concentrations in groundwater or soil, often do not provide conclusive evidence and fail to identify or to quantify degradation mechanisms. To study the fate of organic pollutants and
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thus to apply the MNA concept, especially at complex field sites, require reliable investigation tools in addition to standard methods (e.g., identification and quantification of pollutants and their metabolites, hydrogeological modeling, or microbiological methods). The analysis of isotope ratios of individual organic compounds (CSIA) is such a complementary tool that allows the identification of in situ degradation reactions and/or the determination of pollution sources. 4.2. Recent developments in analytical methodologies Due to its potential for environmental chemistry applications, the analysis of stable isotopes has undergone considerable development within this field of research in the past 20 years [45,46]. CSIA by on-line GC-IRMS made it possible to measure precisely even small isotopic variations of stable elements within individual organic pollutants (e.g., 13C/12C, D/H, 15N/14N, 18O/16O), with values given in & deviation relative to an international reference. While carbon-isotope-ratio analyses are by far the most frequently applied, analyses of isotope ratios of hydrogen, nitrogen or oxygen are much less common. In recent years, CSIA has become a powerful investigation tool for allocating contaminant sources, identifying in situ transformation processes, characterizing the degradation mechanisms involved, and monitoring and modeling transport and transfer of organic pollutants in the environment [47–49]. Fig. 3 shows the number of international publications in the field of environmental science and how CSIA applications for organic pollutants have developed since 1990. 4.3. Concept of isotope fractionation All organic molecules contain stable isotopes in a certain ratio (e.g., 13C/12C, D/H, and 15N/14N). These isotopic ratios can change according to the different processes that affect organic molecules during their transport and
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transformation in the environmental compartments of soil, air and groundwater [38]. Processes (e.g., dilution, evaporation, sorption/desorption) and other physical transport effects that act on the molecule as a whole just decrease contaminant concentrations without changing their chemical structure. These processes do not, or hardly, change the isotopic composition of an organic compound within the range of analytical error. By contrast, the transformation of a molecule, by biodegradation through biotic metabolism or abiotic transformation reactions (e.g., chemical), is commonly associated with a significant change in its isotopic composition. This kinetic isotope fractionation is due to different reaction rates caused by different binding strengths of molecules containing light and heavy isotopes at the reactive position. All chemical reactions, whether biotic or abiotic, involve this kinetic effect, where molecules containing light isotopes at the bondbreaking position react faster than those containing heavy isotopes. Molecules containing light isotopes are therefore preferably degraded and the remaining contaminant fraction becomes enriched in heavier isotopes over the course of the reaction. The isotopic enrichment can be expressed using the Rayleigh equation [Equation (1)]: Rt =R0 ¼ ðct =c0 Þ
ð1=a1Þ
ð1Þ
where R0 describes the initial isotopic composition (e.g. C/12C) of the contaminant, Rt the isotopic composition of the remaining contaminant after time t, ct/c0 the concentration change or the fraction of the contaminant remaining after time t, and a is the isotopic fractionation factor. Using a range of appropriate a-values for a field site allows estimating the degree of intrinsic biodegradation [50]. Isotopic fractionation factors a (also known as enrichment factors e = (a-1), expressed in &) are experimentally determined in laboratory microcosms or
13
Figure 3. Instrumental developments and number of compound-specific stable-isotope analysis (CSIA) studies of organic pollutants published in international literature over the past 20 years (search performed by ISI Web of Knowledge in June 2010).
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Table 1. Summary of the type of molecules, elements and application field of compound-specific stable-isotope analysis (CSIA) studies during the past 20 years Laboratory batch studies Degradation processes
Other processes (incl. production)
Chlorinated aliphatic hydrocarbons (PCE, TCE, VC, DCA etc.)
CCC
Aromatic hydrocarbons (BTEX – benzene, toluene etc.)
CC HH CC HH CC H C H C CC H NN
CC H CC H C H C H C
Oxygenated fuel additives: ethers (MTBE, ETBE etc.) Saturated aliphatic hydrocarbons (CH4, n-alkanes) Polycyclic aromatic hydrocarbons (PAHs – naphthalene etc.) Aromatic chlorinated compounds (PCBs, chlorobenzene etc.) Nitroaromatic compounds (TNT, atrazine, isoproturon etc.)
Field applications (sources and fate of pollutants) Soil
Water CCC
CC
CCC HH CC H C
CCC
C
C C
C
N
N
Carbonyl compounds (acetaldehyde, formaldehyde, ketones)
Atmosphere
C
CC H CC H
CC H
C, H, N = CSIA studies for carbon, hydrogen and nitrogen, respectively. XXX 6 50, XX 6 20 et X 6 5 = number of international publications for the different applications.
abiotic batch reactors, where the (bio)degradation of an organic compound is performed under controlled laboratory conditions. These laboratory-derived fractionation factors may be used for calculating the amount of in situ biodegradation based on the measured isotopic changes of the contaminant measured in the field, if the geochemical and microbiological site conditions are close to the conditions of the microcosm experiments. As the kinetic isotope-fractionation effect depends strongly on the type of reaction mechanism, it can also be used for identifying the degradation processes that are involved [49]. 4.4. How CSIA can be applied in environmental studies 4.4.1. Studies of degradation of organic molecules. The answers given by compound-specific carbon-isotope analysis in environmental studies (e.g., source allocation) and sustainable contaminated-site management (e.g., degradation studies) are complementary, but often crucial, compared to those given by classical investigation tools. The method has been applied for investigating contaminated soil and groundwater systems, as well as atmospheric pollution, and there are many published studies (Table 1). One of the standard examples is to study in situ biodegradation by stable carbon-isotope monitoring in groundwater polluted by chlorinated ethenes, mainly perchloroethene (PCE) and trichloroethene (TCE) [45,50]. Reductive dechlorination of PCE shows a significant difference between the isotopic values (d13C) of the initial compound, the remaining compound and its 150
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degradation products. Another example is given for aromatic hydrocarbons {e.g., for the degradation of toluene under anaerobic conditions, involving a significant shift in its isotopic composition; the remaining fraction of toluene is enriched in heavy (13C) isotopes during the reaction [51]}. Fig. 4 shows both examples. In the same way, laboratory experiments (microcosms/batch) studying the influence of biotic and abiotic degradation on isotope ratios have shown that the rate of isotopic fractionation is sufficiently different to be used as an indicator for in situ attenuation mechanisms for aromatic hydrocarbons (BTEX), chlorinated solvents (e.g., PCE, DCA, and VC), oxygenated fuel additives (e.g., MTBE, and ETBE), short-chained n-alkanes (
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Figure 4. Isotopic fractionation induced by degradation (A) during different stages of biotic reductive dehalogenation of PCE under anaerobic conditions, while carbon-isotopic compositions of PCE in the sterile control do not change (horizontal line), and (B) during toluene biodegradation in a sulfate-reducing microcosm. The d13C increases during degradation (enrichment in heavy carbon isotope 13C in the remaining compound fraction). Figures taken from [43] (A) and [40] (B).
prediction of natural degradation reactions of BTEX, MTBE and chlorinated compounds [57–59]. 4.4.2. The concept of source discrimination. The isotope compositions of organic hydrocarbons depend on their chemical precursors and/or the method of synthesis involved in their production, so pollutants of various origins may be discriminated by their isotope signature. It has been shown that organic chemicals (e.g., BTEX) from different manufacturers could be discriminated on the basis of their d13C values. This observation was also made for chlorinated solvents of different manufacturers and sources, n-alkanes of different origins, aromatic compounds present in different gasoline samples, and MTBE [47]. The use of carbon-isotope analysis seems to be a valuable tool for identification of pollution sources for PAHs that comprise 2–5 aromatic rings on which microbial degradation did not result in significant isotopic fractionation [60]. Much work has already been done in this field (e.g., after the Erika oil spill in France, where the application of this analytical technique to oil residues sampled on the northern French Atlantic Coast confirmed that the contamination by these petroleum products was linked to the Erika tanker spill) [61]. The same principle can be adapted to PCBs, where compound-specific d13C analysis may be a useful complementary tool for tracing these molecules in the environment [62]. Various authors have shown that carbon-isotope ratios of certain organic hydrocarbons can be used for tracking these pollutants in the environment (e.g., in order to discriminate contamination sources in urban air or groundwater) [63,64]. 4.4.3. Multi-isotope approach for studying pollution in the environment. The broad application field of CSIA, first
restricted to carbon isotopes, has expanded even more since instruments for compound-specific hydrogen-isotope analysis became available in 1998. For example, it was shown that gasoline samples from different origins could clearly be discriminated if compound-specific d13C values and dD analyses of specific organic compounds (e.g., BTEX, n-alkanes or MTBE) are combined [65]. In addition, it was reported that the analysis of compoundspecific dD values may be a complementary tool for tracing PAH compounds in the environment [66]. Moreover, according to different isotopic fractionation effects observed in laboratory experiments, isotope studies can be used for distinguishing between different degradation pathways of a compound, especially if the multi-dimensional isotope approach is applied. CSIA applications for the determination of compoundspecific nitrogen-isotope ratios by GC-C-IRMS are still relatively rare. For example, it was shown for pesticides atrazine and isoproturon that distinct isotopic fractionation is observed during different microbial and abiotic degradation pathways. Thus, a clear differentiation of different degradation processes was shown for these herbicides, based on three-dimensional isotope analysis (d13C, dD and d15N) [67,68]. For chlorine-isotope analysis of halogenated organic compounds, several promising analytical approaches by TIMS, GC-MS and IRMS are currently under development [22].
5. Summary and perspectives The application of isotope signatures of inorganic or organic contaminants provides valuable and sometimes unique information for deciphering their origin and studying their degradation processes in the environment.
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However, there are limiting and challenging aspects of the methods that should be critically addressed by the scientific community. Finally, since most of these techniques come from research, the connection to local authorities, stakeholders and environmental consultants is often weak. Bringing together the different approaches, including their, sometimes controversial, economic and social aspects, might be one of the most challenging tasks in the near future in creating integrated risk-assessmentbased contaminated-site management. Protecting and managing surface and groundwater resources is the most essential task of safeguarding drinking-water supplies. However, organic contaminants, which derive from industry, oil spills, improper disposal and/or leaking storage tanks, landfill leachates, household use, motor-vehicle emissions, agricultural fertilizers and pesticides, pose a threat to soil and freshwater resources. Industrial activity and agricultural practices may have a significant impact on vulnerable aquifers and thus on drinking-water supply. By introducing the EU Water Framework Directive [5], and the EU Groundwater Directive on the protection of groundwater against pollution and deterioration [69], the European Commission has made water protection one of its priority tasks. As a framework for water protection and management, it commits EU Member States to identify and to analyze European waters and finally to adopt management plans and programs for measures adapted to each water body. Within the WFD context, the application of high-grade analytical tools (e.g., MCICP-MS or CSIA) offers great potential for distinguishing various potential sources within contamination plumes and maybe even estimating different source inputs at the catchment scale [70]. In addition, the possibility of CSIA for evaluating in situ transformation reactions is of fundamental importance for studying the fate of organic pollutants, especially for evaluation of remediation strategies that rely on monitoring natural and/or engineered attenuation of organic contaminants in soil and groundwater systems. Future studies will help to develop these high-grade analytical tools into a powerful, generally accepted investigation framework for groundwater characterization and contaminant hydrology, and to define and apply monitoring tools [9]. Groundwater-monitoring programs are required to provide a coherent, acceptable overview of water status with regard to pollution and its origins and to define trends in pollutant concentrations [71], especially for organic molecules, as defined by Terry et al. [72] for 1,3-dichloropropene and metabolites in groundwater in five EU countries (UK, France, Italy, Spain and Greece) through the analysis of 5000 groundwater samples, but without any source tracing due to the low values of observed contamination levels.
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Acknowledgments This study was supported by the ‘‘Association RECORD – REseau COope´ratif de Recherche sur les De´chets’’ Grant n 09-0138/1A Isotopic methods for identifying the origin of metal and organic pollutants in the environment: state of the art and critical approach for implementation and by the BRGM Research Division through the METR09 Isotopes research program. The authors wish to acknowledge B. Couffignal and R. Gourdon from RECORD for their support during this study. Suggestions by A. Damien, C. Veronneau, D. Bilbault, H. Roussel, L. Cimolino and M. Brugioni from the RECORD steering group significantly improved the quality of this manuscript. We are grateful to H.M. Kluijver for proofreading and editing the English text. References [1] A.R. McBirney, Igneous Petrology, Jones and Bartlett, Boston, MA, USA, 2007. [2] P.A. Allen, Earth Surface Processes, Blackwell Science, John Wiley & Sons, Hoboken, NJ, USA, 1997. [3] G. Faure, Principles and Applications of Geochemistry (2nd Edition), Prentice Hall, Upper Saddle River, NJ, USA, 1998. [4] C.A.J. Appelo, D. Postma, Geochemistry, Groundwater and Pollution (2nd Edition), A. A. Balkema Publishers, Leiden, The Netherlands, 2005. [5] European Union, Water Framework Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy, Offic. J. Eur. Commun. L327 (2000) 1. [6] P. Quevauviller, U. Borchers, C. Thompson, T. Simonart, The Water Framework Directive: Ecological and Chemical Status Monitoring, Water Quality Measurements Series, Wiley, Chichester, W. Sussex, UK, 2008. [7] European Union, Strategic Environmental Assessment Directive 2001/42/EC of the European Parliament establishing a framework on Environmental Impact Assessments, Offic. J. Eur. Commun. L197 (2001) 30. [8] P.J.T. Morris, From Classical to Modern Chemistry: The Instrumental Revolution, Royal Society of Chemistry, Cambridge, UK, 2002. [9] P. Quevauviller, A.M. Fouillac, J. Grath, R. Ward, Groundwater Monitoring, Water Quality Measurements Series, Wiley, Chichester, W. Sussex, UK, 2009. [10] C.J. Alle`gre, Earth Planet. Sci. Lett. 86 (1987) 175. [11] R. Bowen, Isotopes in the Earth Sciences, Elsevier Applied Science, Amsterdam, The Netherlands, 1988. [12] G.J. Wasserburg, Earth Planet. Sci. Lett. 86 (1987) 129. [13] P. Fritz, J.C. Fontes, Handbook of Environmental Isotope Geochemistry, Vols. 1–5, Elsevier, Amsterdam, The Netherlands, 1980. [14] P.K. Aggarwal, J.R. Gat, K.F.O. Froehlich, Isotopes in the Water Cycle; Past, Present and Future of a Developing Science, Springer, Berlin, Germany, 2005. [15] G. Faure, Principles of Isotope Geology, John Wiley & Sons, Chichester, W. Sussex, UK, 1986. [16] W. Kloppmann, Actual. Chim. 267 (2003) 44. [17] G. Charalampides, O. Manoliadis, Environ. Int. 28 (2002) 147. [18] C.E. Dunlap, E. Steinnes, A.R. Flegal, Earth Planet. Sci. Lett. 167 (1999) 81. [19] D. Widory, E.P. Giraud, Ph. Ne´grel, B. Ladouche, Environ. Sci. Technol. 39 (2005) 539.
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