Application of Nontraditional Stable-Isotope Systems to the Study of Sources and FATE of METALS in the ENVIRONMENT New instrumentation has opened the door to stable-isotope analysis of heavier elements for environmental forensic applications.
DOMINIK J. WEISS MARK REHKÄMPER IMPERIAL COLLEGE LONDON AND THE NATUR AL HISTORY MUSEUM, LONDON RONNY SCHOENBERG LEIBNIZ UNIVERSITY OF HANNOVER (GERMANY) MIKE McLAUGHLIN JASON KIRBY CSIRO LAND AND WATER AND THE UNIVERSITY OF ADELAIDE (AUSTR ALIA) PETER G. C. CAMPBELL INRS-ETE, UNIVERSITY OF QUEBEC (CANADA) TIM ARNOLD JOHN CHAPMAN KATE PEEL IMPERIAL COLLEGE LONDON SIMONE GIOIA UNIVERSITY OF SÃO PAULO (BR AZIL) © 2008 American Chemical Society
T
he release of metals into the environment from anthropogenic activities poses major risks to ecosystems and human health. Sources and fate need to be fully understood so that remediation is appropriately targeted and emissions are maintained at sustainable levels. In many circumstances, multiple sources, natural and anthropogenic, contribute to accumulations at any given site. This leads to problems in developing emission control strategies and remediation targets for contaminated sites. In addition, biogeochemical processes such as redox transformations affect toxicity, bioavailability, and mobility of pollutants in ecosystems. The U.S. EPA lists elements such as Cd, Cr, Cu, Pb, Hg, Ni, Se, Ag, Tl, and Zn as priority pollutants under the Clean Water Act. February 1, 2008 / Environmental Science & Technology ■ 655
Until recently, detection of isotope variations for elements with masses >40 amu was difficult,
The mass spectrometer J. J. Thomson developed the first mass spectrometer a century ago. Even though the original instruments bear little resemblance to today’s MC ICPMS systems, the underlying principles are remarkably similar and simple. Then, as today, elements had to be converted into charged ions. Cavendish L abor atories
The potential of isotope abundance analyses for the study of metal pollutants has been demonstrated in the case of Pb. Isotope-ratio measurements successfully distinguished sources of Pb in groundwater, aerosols, soils, and human blood (1). Unique relative Pb isotope abundances for sources are produced by the radioactive decay of the parent nuclides, depending on the initial parent-to-product elemental ratios and time. Unfortunately, apart from Pb, none of the abovementioned elements has a single isotope that undergoes such a radiogenic ingrowth from a radioactive parent nuclide, and some elements, such as As, consist of a single nuclide. Some stable isotopes, such as 53Cr and 107Ag, underwent ingrowth from short-lived parent nuclides, but these became extinct shortly after the birth of our solar system, and the isotopes formed have since been mixed into the terrestrial reservoirs by the dynamic interaction of crust and mantle.
The ions are steered and focused by the magnetic, electrostatic, and radio-frequency fields used as lenses within the mass analyzer. The ionized molecule’s mass and charge together determine its trajectory. H
if not impossible. B
The differences in the relative, mass-dependent abundances of stable isotopes have the potential to elucidate sources and fate of contaminants in the biosphere (2). A drawback is that the range in isotope variations of an element is generally negatively correlated with its atomic mass. Until recently, detection of variations for elements with masses >40 amu was difficult, if not impossible. However, since the development of multicollector inductively coupled plasma mass spectrometry (MC ICPMS) in the mid-1990s, numerous groups have identified significant isotopic variability and fractionation for elements up to U, thereby supporting theoretical predictions (3). These studies, which often addressed problems in earth science, such as the development of proxies for early life or changes in oxygenation in ancient oceans, offered new insights into the biogeochemistry of these nontraditional stable-isotope systems (4). In this article, we elaborate on the potential use of nontraditional stable-isotope systems to trace sources, fate, and behavior of metals in the environment, especially for environmental forensic applications. We provide a basic review of instrumentation and isotopic fractionation mechanisms as well as summaries for six priority contaminants (Cr, Cu, Zn, Se, Cd, and Hg): their isotope systematics, fractionation processes, and environmental applications. We present the emerging big picture and discuss the potential future implications and applications of nontraditional stable-isotope systems in determining the source, fate, and behavior of contaminants in the environment. 656 ■ Environmental Science & Technology / February 1, 2008
D F
A
E C
G
A schematic of a generic MC ICPMS instrument is shown here. The main components are (A) ICP torch and coil, (B) turbomolecular pumps, (C) sampler and skimmer cone, (D) lens stacks, (E) electronic analyzer, (F) magnetic mass analyzer, and (G) multicollector array. Depending on the instrumental design, an electrostatic analyzer can be substituted with a hexapole collision cell (H).
Analytical aspects of MC ICPMS MC ICPMS instruments are composed of three major components: an ion source, a mass analyzer, and a detection unit (box above). The ion source is a hightemperature plasma, which enables the ionization of elements with high ionization potential. The mass analyzer consists of different focusing setups that reduce the kinetic-energy spread of ions entering the mass spectrometer and focus the ion beam, as well as an electromagnet that separates the ions according to their mass. Finally, the detection unit consists of a collector array, which measures ion beams
Mechanisms of fractionation
Notation
The basics of stable-isotope fractionation are well established and reviewed elsewhere (2, 9), but the most important principles are sumisotopic ratio of sample – isotopic ratio of standard δi/jXA,B = ×1000 marized below and in the boxisotopic ratio of standard es on this page and the next. Because isotope variations in isotopic ratio of sample – isotopic ratio of standard nature are small, differences εi/jXA,B = �10,000 isotopic ratio of standard are expressed as per mill or per 10,000 deviations relative where δ i /j X A,B and εi /j X A,B are the δ and ε values, respectively, of the to a reference standard in δ or element X in phase A or B for the isotope pair i/j, for example, δ66/64 Zn ε units, respectively, where δ in the case of 66 Zn/64 Zn; in general, the heavier isotope i is used in the > 0 is considered heavy and δ numerator. < 0 light, if the heavier isotope The partitioning of stable isotopes of a certain element between is in the numerator. The refertwo substances, A and B, is described by the isotopic fractionation ence standard can either be factor α: certified for isotopic ratios or for concentrations, or simply RA αA–B = be any industrial single-eleR B ment solution. The partitioning of two where RA and R B are the ratios of the heavy to light isotopes in the molstable isotopes of an element ecules or phases A and B, respectively. The fractionation factor, or the between two substances or isotopic contrast of a certain element between two substances, varies phases is described by the mainly as a function of temperature and reflects equilibrium or kinetic parisotopic fractionation factitioning (see box on p 658). tor α. Isotopic partitioning Because α is very close to 1, we derive the very useful relation: is controlled by equilibrium or kinetics. The two process103 ln αA–B ≈ δi/jX A – δi/jX B ≡ Δi/jX A–B es are described by different fractionation laws, which are where Δi/j X A–B is the fractionation between phases A and B. expressed by the exponent (β) that relates the fractionation factors for two isotope ratios with different mass of different isotopes at the same time, leading to a contrasts, such that α2/1 = α3/1β and βequil ≠ βkinetic. precision down to ±0.001% for ratios. A thorough When possible, data are commonly plotted in threedescription of current mass spectrometers is given isotope space. The resulting curves are referred to elsewhere (5). Some instruments are capable of high as terrestrial mass-fractionation lines. The level of resolution (m/∆m = 10,000), whereas for particularly precision achieved with MC ICPMS allows us sometroublesome elements (e.g., 56Fe), isobaric interfer- times to characterize the fractionation laws because ences are eliminated, but this approach results in lower sensitivity. Another method to deal with inMass spectrometers terferences is to use a hexapole collision cell as a reaction cell (6). favor transmission of Laser ablation sample introduction has advantages that include the reduction of spectral interheavy isotopes, and ferences, increased sample throughput, and the possibility of in situ spatially resolved analysis. this effect needs These advantages come at the price of lower accuracy and precision and the need for higher sample to be corrected. concentrations (7). Mass spectrometers favor transmission of heavy isotopes, and this effect needs to be corrected (8). they produce lines with different slopes. Figure 1 The simplest technique is standard sample bracket- shows the results of a study assessing the extent and ing, in which temporal drift in mass bias is presumed nature of Cd isotope fractionation during evaporato be constant over a short time period. A dopant tion of Cd metal into a vacuum (10). element may be used to correct directly for massKinetic and equilibrium fractionation can differ bias shifts and to quantify mass fractionation. The significantly for the same reaction. This has been double-spike technique resolves instrumental and demonstrated for hematite precipitation, in which natural mass fractionation in the same sample and equilibrium fractionation between dissolved ferric possible fractionation during sample preparation. Fe and Fe2O3 (∆56/54Feequil = 0.1‰ ± 0.2‰) is smaller Because heavy stable-isotope variations in nature are very small, the isotope compositions of samples are expressed as either per mil (δ) or per 10,000 (ε) deviations from the composition of a reference standard:
February 1, 2008 / Environmental Science & Technology ■ 657
Mass-fractionation mechanisms
δi/jX A = δi/jX B + ∆i/jX A–B δi/j X
This produces a straight line in terms of A or δi/j XB as a function of the fraction B produced and Δi/j X A–B is constant for 0 ≤ fA ≤ 1. Kinetic fractionation occurs when the reaction is unidirectional and reaction rates are mass-dependent. In that case, ik ≠ jk. Fractionation exists because bonds with the lighter isotope and lower atomic mass are broken faster. Rayleigh distillation occurs in the case of a reaction (kinetic or equilibrium) where phase B does not continue to exchange with phase A. Rayleigh fractionation is best described for changes in δi/j X A values for the individual components: than kinetic fractionation (∆56/54Fekinetic = 1.1‰ ± 0.2‰) (11). Stepwise isotopic fractionation via various kinetic or equilibrium processes can lead to large isotope variations during the cycling of an element in ecosystems (12). A very important fractionation process—Rayleigh distillation—occurs when reaction products are irreversibly separated from the reactant reservoir, potentially leading to extreme fractionation. This isotope effect is manifested during the reduction of Cr(VI) in groundwater. Cr(VI) fractionates upon reduction to Cr(III), and because Cr(VI) is soluble and Cr(III) is particle-reactive, Cr(III) is removed from the system, leaving the remaining dissolved Cr highly fractionated (Figure 2).
Stable-isotope biogeochemistry of selected elements In this section, we discuss the stable-isotope biogeochemistry of Cr, Cu, Zn, Se, Cd, and Hg, because of the significant amount of work performed with these elements in the environmental context. Sb, Ni, and Tl, also priority pollutants, have been studied, but the work has focused to date on geological and planetary problems. The isotopic patterns observed for these elements, however, fit well into the emerging overall picture for nontraditional stable-isotope systems, as discussed further below. For each element, we look first at the isotope systematics and then at fractionation processes and environmental applications. 658 ■ Environmental Science & Technology / February 1, 2008
4 3
l-A
2
Ra
1 0 –1 –2
0
0.2
Ra l-B
where i is the heavier isotope, j is the lighter isotope, and K is the equilibrium constant. When phases A and B are allowed a complete isotopic exchange during the process (closed-system equilibrium), δi/j X A at a given fraction of B remaining is:
where δi/j X is defined for A or B and δi/j Xinit is the initial isotopic composition. Because the product is progressively isolated, large changes in Δi/j X in the remaining component may occur.
Δi/j XA–B
Kequil = iK/jK = (ik f /i k b)/( jk f /j k b)
f (α–1) = (1000 + δi/jX) / (1000 + δi/jX init)
δi/j XA or δi/j XB (per mil)
Equilibrium fractionation happens during isotope exchange when the forward and backward reaction rates (kf, kb) of the isotope that lead to isotope redistribution are identical:
Eq-A Eq-B
0.4 0.6 Fraction of B (fB)
0.8
1
The above diagram shows the extent of isotopic fractionation during a reaction where phase A reacts to phase B, such as adsorption, redox reaction, precipitation, complexation, and so on, under (1) equilibrium conditions in a closed system (Eq-A and Eq-B, solid lines) and (2) unidirectional conditions (Ral-A and RalB, dotted lines). The initial isotope ratio of the system is 0‰ (only phase A is present), and the fractionation factor is 1.5‰; thus αA–B = 1.0015. (Adapted with permission from Ref. 4.)
Cr (Z = 24, A r = 51.9961, stable isotopes at 50, 52, 53, and 54 amu)
Isotope systematics. δ 53/52Cr values of basalts fall within the range of ±0.05‰ relative to the NIST SRM 979 Cr standard. Groundwaters at various locations in the U.S. exhibit a significantly larger variation in isotope compositions, ranging from 1.1‰ to 5.8‰ (13). Fractionation processes and environmental applications. Reduction of the highly toxic Cr(VI) to Cr(III) is accompanied by a large mass-dependent isotope fractionation (∆53/52CrCr(III)–Cr(VI) = –3.4‰), probably kinetically controlled (14). Equilibrium adsorption of Cr(VI) onto γ-Al2O3 and goethite, in contrast, causes only small fractionations (∆53/52Cradsorbed–soluble ≤ 0.04‰) (15). Ellis et al. (14) measured δ53/52Cr in various plating baths and found a mean value of ~0.34‰. Groundwater samples from two sites of contamination with plating wastes (Putnam, Conn., and Berkeley, Calif.) with low concentrations of Cr(VI) were enriched in the heavy isotope relative to the plating baths (14). This would be in line with a reduction process that preferentially removes light isotopes from the solution (Figure 2).
Cu (Z = 29, A r = 63.546, stable isotopes at 63 and 65 amu) Isotope systematics. The isotopic composition of δ65/ 63Cu in igneous-hosted chalcopyrite samples falls within a narrow range of ±0.5‰ relative to the NIST SRM 997 Cu standard (16). Large variations,
FIGURE 1
Isotopes to elucidate fractionation mechanisms Three isotope plots of Cd isotopes measured in residual samples and starting material during an evaporation experiment with pure Cd metals (using the δ´-notation, where δ´i Cd = 1000 ln[(i Cd/ 114 Cd) i 114 C,D,E )/( Cd/ Cd) A ] and i = 106 or 110). (a) Residual samples C, D, and E and the starting material A define a fractionation line in δ´106 Cd vs δ´110 Cd isotope space. (b) Close-up of (a) shows the results obtained for samples C and E. Shown are the reference lines for mass-dependent isotope fractionation according to the kinetic and equilibrium fractionation laws (β values). The measured β value is intermediate between those expected for equilibrium and kinetic fractionation. (Adapted with permission from Ref. 10.) (a) 0
–40
D
Residual material
–60 –80
Zn (Z = 30, Ar = 65.409, stable isotopes at 64, 66, 67, 68, and 70 amu) Isotope systematics. A comprehensive summary of δ66/64Zn values relative to the Lyon JMC 03-0749L Zn standard in biological, geological, and environmental materials was compiled recently (20). The total range found so far is from –1‰ to 1.5‰, and basalts have a narrow range of 0.2‰ to 0.5‰.
A Starting material
–20
δ'106Cd
between –3.0‰ and 5.7‰, are found in sediments, biological material, and secondary ore minerals. Fractionation processes and environmental applications. Reactions that change the oxidation state cause the largest isotope shifts. This is observed in natural mineral assemblages in which oxidized minerals are isotopically heavier than primary Cu(I) minerals (16) and during experimental precipitation of the Cu(I) mineral covellite from aqueous Cu(II), with Δ65/63CuCu(II)–CuS of 3.06‰ ± 0.14‰ (17). Leaching of Cu sulfides with sulfuric acid led to slightly enriched 65Cu solution relative to the mineral but to no measurable fractionation with nitric acid (18), and slow precipitation of malachite from a chloride solution resulted in greater fractionation than from a nitrate solution. These findings hint at the importance of solution speciation. Biogenic uptake of Cu by azurin and yeast proteins showed the Cu protein depleted by 0.98–1.53‰. Bacterial cells further acted as a sink for heavy Cu because of precipitation of Cu minerals on the cell surface (18). A recent study of stream water from an area polluted by acid mine drainage found enrichment in 65Cu, with δ65/63Cu ranging from 1.03‰ to 3.76‰. The highest value occurred at the mine-shaft inflow with high Cu concentration, and the δ65/63Cu values decreased downstream with decreasing concentration (19).
Fractionation during evaporation
E/C –100
(b)
–120 –60
–50
–40
–30
–20
0
–95 –96
Biogeochemical processes
–97
lead to significant
–98
isotopic variations in
–99
the environment.
–100
Fractionation processes and environmental applications. Uptake of free Zn2+ by plants and algae species favors the light isotope, probably because of the kinetic diffusion across cell membranes. Adsorption induces small fractionation: organic tissue preferentially adsorbs the heavy isotope from solution, as experiments with diatoms and plant roots suggest, and small positive and negative fractionations were found during adsorption onto iron oxides, depending on pH and crystal structure. Small fractionation by diffusion (~0.3‰) has been confirmed in aqueous solutions with faster transport of the lighter isotope (21). Attempts to trace pollutant sources have been promising. Selected rainwater samples from two adjacent areas in Southern France corresponded to the isotopic signature of a Zn-containing chemical widely used in that area. Similarly, Zn isotope ratios
–10
C
E ß kinetic = 2.036
–101 –102 –103 –50
ß equil. = 2.075 –49
δ'110Cd
–48
–47
in lichens around an ore processing and mining site in Russia were similar to those in ore-bearing granites but not to those in host rocks (Figure 3), suggesting that the Zn in the environment was derived from the mining and mineral processing rather than from local soil dust (22). Although the isotopic composition measured in lichens in Metz, France, yielded a fairly homogeneous range, with most lichen samples indistinguishable from urban aerosols and flue gases, lighter Zn was found in samples dominated by February 1, 2008 / Environmental Science & Technology ■ 659
FIGURE 2
FIGURE 3
Isotopes to elucidate and quantify biogeochemical processes
Isotopes to identify and quantify sources
A Rayleigh fractionation curve highlights the effect of removal of reduced Cr via adsorption on particulates on the isotopic composition of the remaining unreduced Cr(VI). Experimental fractionation factors were determined with magnetite and pond sediment (α = 0.9965) and estuarine sediment (α = 0.9967) as the reducing agents. We can calculate the extent of Cr(VI) reduction in groundwater with the relationship δ 53 Cr = [(δ53 Crinit + 1000)f (α – 1)] – 1000. (Adapted with permission from Ref. 14.)
(a) Summary of δ66/64 Zn data relative to JMC 030749L for geological samples and lichens found around an ore processing and mining site in Russia. The ratios in the host rocks are less than in the ore-bearing granite intrusion, which in turn shows a range similar to that of the Zn found in the lichens. This suggests that the Zn in the lichens is sourced directly from the mining activities at the site. (Adapted with permission from Ref. 22.) (b) Zinc isotopic composition vs 1/Zn for selected lichens, which yielded low 206 Pb/ 207Pb ratios, suggesting a significant component of old Pb from leaded gasoline. Gray areas represent the urban–industrial field. (Adapted from Ref. 29.)
10 Progressive removal of Cr(III) via adsorption on particles α = 0.9965 Magnetite and pond sediment
6
(a)
4 δ53/52 Crinit
2 0
1
0.8
0.6 0.4 Fraction of Cr remaining
0.2
Sources
Host rock α = 0.9967 Estuarine sediment
Orebearing granites 0
Receptor
δ53/52 Cr (per mil)
8
Lichens –0.5
Isotope systematics. Different in-house standards have been used to report Se isotope data, but these were recently scaled to the NIST SRM 3149 Se standard (23). Mafic terrestrial rocks have a 0‰ composition on average but a large variation of ±0.7‰. Sediments have a positive average composition (0.39‰ for δ82/76Se) with a variation of ±1.11‰. Sulfides from modern hydrothermal fields and black shales yield negative average values of –1.15‰ ± 2.6‰ and –0.57‰ ± 5.0 ‰, respectively. These variations in δ82/76Se suggest that surface cycling leads to significant isotopic fractionation. The results are in line with findings from Se isotope measurements in sediment, biota, and groundwater (24, 25). Fractionation processes and environmental applications. Mass-dependent fractionation of Se was reviewed recently (26). Reduction of Se(VI) and Se(IV) oxyanions is the main fractionation process in natural systems, and abiotic mediated reduction seems more frequent than biotic. However, biotic reduction likely plays a greater role in the natural environment. In contrast, sorption, precipitation, and assimilation by biota induce a small amount of Se isotopic fractionation. Figure 4 summarizes the current understanding of Se isotope fractionation systematics and illustrates the importance of redox reactions. John660 ■ Environmental Science & Technology / February 1, 2008
0.5 δ66/64Zn (α)
1
1.5
0.4 0.3
Urban/industrial field
Se (Z = 34, A r = 78.96, stable isotopes at 74, 76, 77, 78, 80, and 82 amu)
(b)
δ66/64Zn (0/00)
gasoline Pb, leading to the suggestion that light Zn is indicative of automotive sources (Figure 3).
0
0.2 0.1 0
–0.1 –0.2
Automotive circulation? 0
0.005
0.01
0.015 1/Zn
0.02
0.025
son et al. (25) explored the use of isotope ratios to trace the transport of Se-rich refinery wastewater into San Francisco Bay but found no significant difference between pollutant source and natural water samples. Herbel et al. (24) successfully applied Se isotope ratios to identify the transformation pathways and accumulation of reduced forms of Se in wetland systems.
Cd (Z = 48, Ar = 112.411, stable isotopes at 106, 108, 110, 111, 112, 113, 114, and 116 amu) Isotope systematics. Cd isotope studies use the JMC Cd solution from Münster or in-house solutions that are isotopically indistinguishable as the reference standard. The BAM-I012 Cd isotope reference is fractionated relative to JMC Cd Münster. Wombacher et
Hg (Z = 80, Ar = 200.59, stable isotopes at 196, 198, 199, 200, 201, 202, and 204 amu)
FIGURE 4
Isotope systematics Summary of proposed Se isotope systematics in the environment. Redox reactions (biological and abiotic) induce the largest fractionation. Arrows represent reactions between Se species, which are given in boldface type. Isotopic fractionation for each reaction is given in the box attached to the arrow, and in each case, lighter isotopes are enriched in the reaction products. Question mark indicates an estimated or preliminary value. Uptake in algae and higher plants alike is small. (Adapted with permission from Ref. 4.) Se(VI) Abiotic 00/00
Abiotic 7–120/00
Bacterial 3–50/00
Abiotic 6–120/00
Bacterial 6–90/00
Se(IV) ~00/00 ~10/00? 00/00
Higher plants
Algae
Se(0)
Small?
Dissolved organics
00/00
Se(–II)
Small?
Isotope systematics. Isotopic variability in δ202/198Hg in the terrestrial environment (fossil hydrothermal systems, sediments, fish tissues) is >5‰ (32, 33) and is reported relative to the NIST SRM 1641d or 3133 Hg standards. Fractionation processes and environmental applications. Differences found in hydrothermal systems have been explained by a combination of boiling of hydrothermal fluids, redox reactions involving the transformation of HgS to H2S and Hg(0), and kinetic effects associated with mineral precipitation. Initially, fractionation during ecosystem cycling was inferred from the analysis of a sediment core and an assemblage of food-web animals from Lake Ontario, but a debate over data quality ensued. However, further work showed that organic samples, containing predominantly methylmercury, appeared enriched in light isotopes compared with inorganic Hg samples, suggesting that biomethylation plays a key role in Hg fractionation in the environment. In a recent contribution, Kritee et al. demonstrated that microbes preferentially reduce lighter isotopes (34). No direct evidence exists that Hg isotopes can be used for source tracing, but significant variability was found within sediment cores. This observation could suggest that Hg isotopes are promising for source assessment (33).
The emerging picture al. (27) and Cloquet et al. (28) provided isotope data for igneous rocks, chemical and detrital sediments, and Cd-rich minerals formed at low temperatures. These natural samples display δ114/110CdJMCMünster values from –0.40‰ to 0.48‰, with variations almost insignificant relative to the reproducibility. Fractionation processes and environmental applications. Common inorganic processes do not generate substantial isotope effects (27). In contrast, significant fractionation by evaporation and condensation is inferred from laboratory experiments, meteorites, and anthropogenic samples (10). The fractionated (light) isotope compositions of industrial standards are probably related to the preparation of purified Cd metal by distillation. Cloquet and co-workers (28, 29) identified small but significant isotope variations in anthropogenic samples that most likely relate to industrial evaporation and condensation processes. Analyses of seawater and cultured phytoplankton suggest that biological use generates significant isotope effects (30). Cd-rich intermediate water from the North Pacific is characterized by δ114/110Cd of ~0.32‰, whereas depleted upper water column samples from the Atlantic and Arctic oceans yield δ values of ~0.6‰. The difference between intermediate water and the biologically active upper water column is in line with fractionation occurring during biological use by phytoplankton. This is supported by data from phytoplankton cultures depleted relative to the initial culture medium by ~1.4‰ for δ114/110Cd (31).
Although the isotopic variability of elements heavier than 40 amu in igneous materials is small, biogeochemical processes lead to significant variations in the environment. These are up to 5‰/amu, which compares favorably to the current measurement precision for MC ICPMS.
The differences in the relative, mass-dependent abundances of stable isotopes have the potential to elucidate sources and fate of contaminants in the biosphere. Redox reactions (inorganic and biologically mediated) induce the largest fractionation. This makes isotopes a powerful tool for detecting, monitoring, and quantifying redox processes during the biogeochemical cycling of elements such as Cr, Cu, Fe, Se, Hg, Sb, Tl, and even U (35). Biological processes induce smaller, but distinct, variations for Cd, Fe, and Zn, which opens up the possibility of quantifying and elucidating nutrient acquisition and translocation processes in microorganisms and plants. Inorganic processes such as adsorption, disFebruary 1, 2008 / Environmental Science & Technology ■ 661
TA B L E 1
Direction and magnitude of isotope fractionation during different biogeochemical processes. Entries are expressed as Δ i/j X A–B = δi/j X A – δi/j X B. Small refers to fractionation 1‰/amu. This table should be taken only as a guide, with respect to direction and to magnitude. Process
Chemical
Type
Complexation
Dissolution
Precipitation
Adsorption
Redox reaction Ion exchange Biological
Fractionation pair
∆ A–B
A
B
(‰/amu)
Weak ligand
Free metal
Complexed metal
Negative
Small
Strong ligand
Free metal
Negative
Large
Congruent, proton-promoted Incongruent, proton-promoted Incongruent, ligand-promoted Incongruent, microbe-promoted Abiotic, equilibrium Abiotic, kinetic On organic surface On inorganic surface Biological and nonbiological On ion-exchange resin
Solution
Complexed metal Solid
Solution
Solid
Small
Solution
Solid
None, negative Negative
Solution
Solid
Negative
Small
Solution
Solid
Positive
Small
Solution Solution
Solid Solid
Positive Negative
Small to large Small
Solution
Solid
Small to large
Oxidized
Reduced
Positive, negative Positive
Free metal
Positive to negative Positive
Large
Solution Solution
Resin-bound metal Plants and algae Protein Solid
Small to large Small to large
Residue Solution
Vapor Source
Positive Negative, positive Positive Negative
Nutrient uptake Protein–metal Precipitation
Physical
Comment
Evaporation Diffusion
Solution
Biologically mediated Aqueous
solution, precipitation, and diffusion invoke small fractionations unless coupled with redox reactions or biological processes. Complexation leads to significant fractionation between free and complexed compounds in aqueous solution. Table 1 provides a general guide to the effects that biogeochemical processes have on isotopic fractionation. Source tracing with the new isotope systematics is promising for Cd, Cu, and Zn; a quantitative approach to material fluxes within the urban and natural environments should be feasible for these elements (box on p 663; 36). Distinct isotopic differences can occur between host rocks and ore-forming intrusions, and these can be exploited to study pollution sources and dispersal derived from mining activities. The isotopic composition of ore-bearing 662 ■ Environmental Science & Technology / February 1, 2008
Extent of fractionation
None
Small
Large
Small
Small to large Small
minerals is likely to be variable on a deposit scale, and it is unlikely that we can use isotope signatures to identify ore provenances on a global scale, as was previously done for Pb. Industrial processes such as smelting produce isotopically light elements in the vapor phase that, when emitted into the environment, contrast with the heavy signature of natural sources. This seems to provide a tool for tracing industrial sources, as shown for Cd and Zn. The relatively small isotopic variability between various reservoirs and postdepositional fractionation during element cycling make source tracing less effective on large scales. Nontraditional stable-isotope systems identify and quantify sources and biogeochemical processes (especially redox and biological reactions) of
Isotopes to quantify source contributions If the isotope values of the sources are known, we can calculate the contribution of each source using an end-member mixing model. δt = fAδA + (1 – fA) δB where δt is the total δ value, δA and δB are the isotope values of the sources A and B, respectively, and fA is the fraction of the total contributed by source A. Rearranging for fA gives A
=
δ t – δB δA – δB
This allows the use of a stable-isotope approach for quantification of material fluxes in natural and anthropogenic systems (see, for example, Ref. 36). contaminant metals in the environment and provide a great new tool to solve problems that cannot be studied with conventional techniques. However, future work needs to calibrate fractionation processes against variables of interest. Better precision of isotope-ratio determinations will improve the signal-to-noise ratios and allow the study of the physical–chemical nature of fractionation processes. Given recent findings for isotope systems like Sb, Ni, and Tl within the cosmo- and geochemical context, application of these systems to environmental problems is a promising prospect. Dominik J. Weiss is a senior lecturer of environmental geochemistry and Mark Rehkämper is a reader of isotope geochemistry in the department of earth science and engineering at Imperial College London; both are associate scientists at the Natural History Museum, London. Ronny Schoenberg is a senior research fellow of geochemistry at the Institute of Mineralogy at Leibniz University of Hannover (Germany). Mike McLaughlin is a chief research scientist in the Environmental Biogeochemistry research theme at CSIRO Land and Water in Adelaide (Australia) and a professor in the School of Earth and Environmental Sciences of the University of Adelaide. Jason Kirby is a research scientist in the Advanced Analytical Techniques in Biogeochemistry Section at CSIRO Land and Water in Adelaide. Peter G. C. Campbell holds a Canada Research Chair in Metal Ecotoxicology at the Institut National de la Recherche Scientifique (Centre Eau, Terre, et Environnement) at the University of Quebec (Canada). Tim Arnold and Kate Peel are postgraduate students and John Chapman is a postdoctoral researcher in the department of earth science and engineering at Imperial College London. Simone Gioia is a research scientist at the Instituto de Geociencias at the University of São Paulo (Brazil). Address correspondence about this article to Weiss at
[email protected].
Acknowledgments We thank the International Copper Association, the International Zinc Association, the International Lead Zinc Research Organization, and the Nickel Producers Environmental Research Association for supporting this review. D. J. W. dedicates this paper to Alain and Marie-France; Kerry, Pedrolito, and Martha; Pedro, Gustavo, and Arelys; and thanks Bill Williams, Christoph Cloquet, Jean Carignant, Yigal Erel, Alan Matthew, Nadya Teutsch, Boaz Luz, John Seth, Gideon Henderson, Derek Vance, Baruch Spiro, Derek Large, and Ed Boyle for stimulating discussions.
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