CHEMICAL as a Tool for
T
he persistence of a chemical in the environment is a key parameter for registering new chemicals, performing risk assessment on existing chemicals, and identifying chemicals of particular concern within international accords such as the Stockholm Convention on Persistent Organic Pollutants (POPs) (1) and the UN Economic Commission for Europe Convention on Long-Range Transboundary Air Pollution (2). But various environmental organizations, authorities, and individuals worldwide have defined persistence in different ways, to the point of confusion. For example, the Stockholm Convention states that POPs are “chemicals that remain intact in the environment for long periods, become widely distributed geographically, accumulate in fatty tissue of living organisms and are toxic to humans and wildlife” (1). In other words, a POP is persistent, bioaccumulative, and toxic (PBT) and is subject to long-range transport (LRT). The wording of that definition inadvertently inflates the expectation of what persistence should reveal about a chemical. If a chemical is in reality PBT and subject to LRT but is designated only as persistent, this conveys the idea that for a chemical to be labeled as persistent it must also be demonstrably toxic and bioaccumulative and undergo LRT. Although this is clearly not the intended meaning of the term persistence, it remains a popular misconception. In this article, we provide a sounding board to aid the environmental community in finding a new approach to credibly measure persistence. We take the established concept of multimedia modeling as the most appropriate vehicle for predicting persistence and propose a novel system that provides compartmental transformation rates of chemicals for use in such models. The central hypothesis is that the inherent properties of chemical reactivity underpin the rate of transformation of a substance, whether in a test tube or in the environment and whether these transformations are catalyzed abiotically or enzymatically. An experimental system for providing indicative measures of reactivity is outlined in this article and illustrated with specific reactions.
A primer on persistence In regulatory contexts, persistence is almost exclusively defined with the single-medium approach. In practice, a chemical is classified as either persistent or nonpersistent on the basis of whether its degradation half-life in any one environmental compartment, such as air, soil, or water, exceeds a specified threshold value (3). Indeed, the Stockholm Convention also defines the persistence element of a POP in these terms.
480A n EnvironmEntal SciEncE & tEchnology / dEcEmbEr 1, 2005
© 2005 american chemical Society
Estimating Persistence These definitions aside, the persistence of a chemical in the environment is widely accepted as dependent on its dynamic partitioning between the various environmental compartments as well as its degradation rate within each compartment (4–6). Considerable advances have been made in developing models for expressing the theoretical overall persistence of chemicals, and various endpoints have been suggested (e.g., residence time, joint persistence, and persistence in a temporal remote state). These advances reflect the many important issues surrounding the concept of persistence as well as their implications for environmental health (7–9). Although models of persistence have progressed, the attention given to the quality of input data has been mixed. For example, techniques for measuring the physicochemical properties of chemicals have improved (10), and more sophisticated methods for describing their partitioning processes exist (11). However, the lack of accurate compartmental degradation rates remains the key weakness for all modeled predictions of persistence (6). To rectify this, substantial progress has been made in certain fields— for example, the measurement and prediction of atmospheric degradaA proposed experimental tion rates (12). However, regulatory approach for measuring authorities, academics, and those in the chemical industry admit to overthis key environmental reliance on ready biodegradability factor tests (RBTs) for determining persistence (13). Originally designed to assess whether synthetic deterNICHOL AS GREEN gents would adequately degrade in Å KE BERGM A N a wastewater treatment plant (14), STOCK HOLM UNIV ERSIT Y these tests are of questionable value for determining the degradation potential of chemicals in the natural environment. Furthermore, RBTs are considered to be overly conservative and do not provide degradation rates, merely pass/fail results (13). Even so, RBTs still form the bedrock on which persistence is assessed for most chemicals. It is time to consider completely new approaches. Environmental fate models require very crude simplifications of the environmental system to provide a manageable framework for studying and predicting the fate of chemicals, in which the basic environmental components—air, water, soil, and sediment—are treated uniformly. Although this approach may severely limit accuracy, the resolution is still sufficient for the output to be useful. dEcEmbEr 1, 2005 / EnvironmEntal SciEncE & tEchnology n 481A
pHoTodisc / WHo / coREL
REACTIVITY
Webster et al. argue that any new system for assessing persistence must have three imperatives (15). First, the method must be defined, standard, and transparent. Second, the half-lives used should be viewed as properties of the chemical and, hence, independent of variations in environmental conditions. Third, a set of standard environmental conditions must be defined. The distribution and partitioning functions of fate models fulfill these three criteria, as does our proposed approach. In the strategy we propose in this article, chemical reactivities are taken to be the most appropriate properties of a chemical on which to base compartmental half-lives. Consequently, the standard environmental conditions that need to be defined are the reactive power of each environmental compartment. The environment, as represented by fate models, has predefined physical and physicochemical characteristics; to this list, chemical characteristics are now added. The persistence of a chemical can then be established according to whichever endpoint in whichever model is deemed appropriate by the user. However, for regulatory purposes, specific, agreed upon versions of these should ideally be adopted. We also discuss how the inherent chemical reactivities of chemicals might be measured and what further work would be required to define the reactive power of the environment.
Compartment transformation rates and chemical reactivity A common expression of persistence from multimedia fate models is the residence time at steady state, as represented by
Eq1
1
�OR =
n
� kc' c =1
mc M
(1)
where OR is the reaction residence time of a substance in the total environment, kc is the pseudofirst-order rate constant of the reaction in medium c, D Eq2 m isk' the = kcamount [R]c = � kof the dc[R d]csubstrate in medium c, and c c d =1 M is the total amount of the substrate in all n media being considered. For Equation 1 to be usable, one must assume that a transformation in any one medium (although not in the combined environment) D The compromise between the ' = Eq3 is pseudo-first-order. kci [Rid]c �ofkidthis veracity assumption and its convenience is d =1 generally accepted as necessary (16). Other model endpoints require alternative expressions of Equation 1. However, each consists of a degradation term (i.e., chemical transformation) and a distribution –bx [Rxiand ]c = ae Eq4 term, each of these terms can be treated with 1 of simplicity. In this article, we focus �OR = levels Eq1 varying n mc on the transformation expression, which is required kc' � for all such models. M c =1 The transformation rate constant of a chemical � in one compartment can be expressed by ' =
Eq5
Eq2
kci
�kxi[Rxi]cdx
x=0
D
kc' = kc [R]c = � kdc[Rd]c
(2)
d =1
where kc is�the second-order rate constant for trans–bx
kci' = �si x a e dx = cisi Eq6 formation of the substrate by the reagents of medix=0
D ' = Science 482A n Environmental kci kid[Rid]&c Technology / december 1, 2005 d =1
Eq3
�
um c, [R] c is the concentration of the transformation reagents in medium c, kdc is the second-order rate constant for transformation by a reagent d in medium c, and [Rd ] c is the concentration of that reagent in that compartment. D is the number of different reagents in compartment c. The middle expression in Equation 2 implies that a single rate constant holds for all reagents in the compartment, while the right-hand expression provides rate constants for different reagents. The number of reagents delineated is a compromise between the effectiveness (accuracy) of the measure of persistence and the amount of information needed. The established approach to degradation delineates the following transformation systems and divides them into separate compartments (given in parentheses): air (vapor-phase hydroxyl radical reaction), water (aerobic microbial degradation, hydrolysis, and photolysis), soil (aerobic microbial degradation and hydrolysis), and sediment (anaerobic microbial degradation). The reason this provides poor-quality compartmental transformation rates may be either because some important types of reagent are being overlooked or because the methods for measuring these degradation rates are not appropriate for giving representative values for the targeted environmental compartment. Alternatively, the complexity of the environmental system may be so great that the degradation process cannot be simplified to a manageable level without impractical levels of error being introduced. In our proposed system, we attempt to encompass all possible reactions in each medium. These reactions are categorized according to their class— irrespective of their method of execution, specific reagents involved, or whether they are catalyzed abiotically or with microbial enzymes. We consider five classes of reactions to cover all possible reactions: oxidations; reductions; direct photolyses; reactions with radicals; and a single class that covers hydrolysis, substitution, and elimination (hse) reactions. Each class is considered in each medium, although some combinations will be negligible (e.g., photolyses in sediment, reductions in the atmosphere). The level of simplicity adopted is that which provides a single, homogeneous value for the reactivity of any environmental medium toward each of the five reaction categories. The reactive power of the whole modeled environment is then described by the elements of a 5 × 4 matrix, as shown in Figure 1. If further compartments are desired (e.g., vegetation), a broader matrix can be produced. The reactivity of the environment is a crucial part of the equation for the transformation of a compound. For any one of the five reaction categories, in any one compartment, a number of reagents can bring about the transformation of a substrate. Furthermore, the reactivity of each reagent varies depending on its microenvironment. Taken en masse, a compartment may consist of reagents that provide a continuum of low to high reactivity over a large range. Without actually defining each reagent and its strength under any given environmental conditions, we can still consider the inte-
Eq2
Eq2
D
= � kdc[Rd]c kc =effect kc [R]cthe grated reagents have on the transformation d =1 of a substrate. On the basis of Equation 2, the transformation rate constant for a substance in medium c due to reactions of class i may be written as '
Eq3
' = kci
D
� kid[Rid]c
(3)
Eq3
d =1
where kci is the pseudo-first-order rate constant for transformation of a substance through reactions Eq4 of class i in compartment c; Rid is a reagent acting = ae –bx [Rxi ]c reaction Eq4 through class i; D is the number of reagents of that class, each with a different strength of reactivity; [Rid ] c is the concentration of reagent Rid in medium c; and kid is the second-order rate constant for the reaction of reagent Rid with a given substrate. � Eq5 In this kxi[Rxi]cdxmicrobial degradation is viewed kci' = approach, � Eq5 as a reaction, x = 0 such as hydrolysis, performed by a reagent, such as water, on a substrate (the chemical). Under these catalytic conditions, the strength of the reagent greatly increases. This is accommodated � Eq6 within' the complete –bx suite of reagents of that class dx = cisi Eq6 (e.g.,khse ci = �si x a e in a compartment, and results in xreactions) =0 a higher value for the reactive power of the compartment than would exist if only abiotic reactions were considered. Thus, anaerobic microbial degradation will form part of the reductive and hydrolytic power of the sediment compartment and aerobic microbial degradation will form part of the oxidative and hydroof1 the soil and water compartments. �ORpower Eq1 lytic = n mc Becausekthe � c' Mstrength of the reagent—not its idenc =1 tity or nature—is important in the calculations of Equation 3,1 we can consider the D reagents to have Eq1 �OR = reactiven strengths varying from zero to infinity. mc kc' the assumption that the abundance of � We make M D c =1 Eq2 reagents increasing strength, so kc' = kc will [R]c =decrease � kdc[Rd]with c 1 that very weakd =reagents are plentiful and powerful reagents are relatively scarce. In our approach, we that aDfrequency diagram for the abundance Eq2 assume ' = = kdc[Rd]c c kc [R]c of� ofkreagents a given strength would follow an expod =1 nential Ddecrease with increasing reactive strength. ' = Eq3 This kci ]c the most appropriate function for � knot id[Rid may be d =1 all classes of reactivity, and other functions could be decided upon instead. D case, we can write Eq3 k' In = the k exponential [R ] ci
Eq4
�
d =1
id
id c
[Rxi ]c = ae –bx
(4)
where [R xi ] c is the concentration of all reagents of strength x–bx operating through the class of reaction [Rxi ]c = ae Eq4 i in compartment c, and a and b are constants of � scale. We can then rewrite Equation 3 in terms of kaxi[R kci' =of � xi]cdxstrength rather than of a given Eq5 reagents given x=0 identity: �
Eq5 Eq6
Eq6
kci' =
� �
(5)
kxi[Rxi]cdx x = 0� kci' = si x a e–bxdx = cisi =0 where kxix is the second-order rate constant for trans-
formation of a given substrate through reaction class � i with reagents of strength x. kci' = �si x a e–bxdx = cisi Simplistically, the magnitude of kxi depends on x=0 the strength x of the reagent and on the susceptibility of the substrate (si ) to this mode of reaction i.
D
kc' = kc [R]c = � kdc[Rd]c d =1
FIGURE 1
Defining reactivity D
The E defines the reactivity of the environmental compart' =matrix kci � kid[R id]c ments, d = 1for example, the oxidative power of the atmosphere (ox–air ). Hydrolysis, substitution, and elimination reactions are abbreviated as hse. Air
εox–air [Rxi ]c = ae –bx εred–air εh�–air E= εrad–air εhse–air
Water
Soil
εox–water εred–water εh�–water εrad–water εhse–water
εox–soil εred–soil εh�–soil εrad–soil εhse–soil
Sediment εox–sed εred–sed εh�–sed εrad–sed εhse–sed
Oxidation Reduction Photolysis Radical hse
�
�= 0kxi[Rxi]cdxk The ratex constant kci' =
xi will increase linearly with increasing reagent strength x; this gives a slope proportional to si . Therefore, Equation 5 becomes �
kci' =
�si x a e–bxdx =
x=0
cisi
(6)
Integration of the function between zero and infinity gives an expression for kci that is directly proportional to si, and where ci is a constant that represents the reactive power of compartment c for reactions of class i. The value of ci to be used in calculations with Equation 6 is derived from the constants of the exponential function in Equation 4 (specifically, a/b2). Therefore, the pseudo-first-order rate constant for the transformation of a substance in compartment c, through reactions of class i, can be expressed as the product of the inherent susceptibility of the substance to that class of reaction (e.g., oxidation) and the reactive power of that compartment to perform that class of reaction. The susceptibility of the substance to reaction (its reactivity) can be measured through organic chemistry and is discussed further in the following section. Values for ci cannot be measured and must be derived collectively by experts on the basis of a broad spectrum of information. Some suggestions on how to do this are given in the final section. In a framework for assessing the persistence of a wide array of chemicals, the environment must be fixed with regard to both its distribution parameters and its reactivity. Compartmental transformation rates for the chemical can be derived from multiplication of a 1 × 5 matrix containing the substance reactivity constants by the environmental power matrix, as shown in Figure 2. Thus, for example, a chemical with a high reactivity toward photolyFIGURE 2
Computing transformation rates from inherent reactivity and reactive power C is a matrix of compartmental transformation rates for a substrate with inherent reactivities expressed in matrix S. (sox sred sh� srad shse) * E = (kair kwater ksoil ksed) S
C
december 1, 2005 / Environmental Science & Technology n 483A
sis (high sh) would still have negligible photolytic degradation in the sediment compartment because the photolytic power of the sediment is negligible (h–sed ~ 0). Hence, the photolysis of the chemical in sediment will provide a negligible contribution to its total reactivity in the sediment compartment. When hse reactions are considered, the value given to the reactive power of the sediment compartment, for example, must reflect not only the abundance of various reagents (generally nucleophiles) but also the increased reactivity of these reagents under the catalytic conditions found in the sediment compartment, including enzyme-mediated reactions. These compartmental transformation rates would be input into existing distribution models to establish a measure of persistence. Thus, the persistence of any chemical in the defined, standard environment can then be calculated from its physicochemical properties—octanol–water partition coefficient Kow, solubility, and vapor pressure— plus five substance reactivity values, for reactions involving oxidation (sox), reduction (sred), photolysis (sh), radicals (srad), and hse (shse). Figure 3 provides a conceptual overview of this strategy. FIGURE 3
Proposed strategy for determining persistence Within a fixed environment, the persistence of a substance can be evaluated on the basis of inherent chemical and physicochemical properties of the substance. Persistence
Partitioning
Transformation
Fixed Physical properties of the environment
Fixed Reactive properties of the environment
Measured Physicochemical properties of the substance
Measured Chemical reactive properties of the substance
Measuring chemical reactivity The science of organic chemistry is well established and provides a vast knowledge base for developing the most suitable reactions for our approach. Precise reaction conditions can be set and replicated accurately in any laboratory at any time. In a treatise on comparing the reactivity of different chemicals, Adkins discusses various methods on the basis of thermodynamic or kinetic comparisons (17). Kinetics is appropriate for studies of environmental transformation, and Adkins suggests three different experimental approaches: compare rates of reaction under identical conditions; establish the severity of conditions required to bring about a set degree of transformation; or measure relative rates of simultaneous, competitive reactions. Compari484A n Environmental Science & Technology / december 1, 2005
son studies are almost exclusively conducted on sets of structurally similar compounds with the goal of understanding the quantitative effect on rates that is attributable to minor changes in chemical structure or to specific parameters of the reaction conditions (e.g., Hammett’s investigation of benzoic ester hydrolyses; 18). On the other hand, assessing persistence requires an experimental system for determining reactivity that is applicable to a very broad spectrum of unrelated chemical structures. Consequently, very general indicators of reactivity must be developed. This can be achieved at many levels of complexity, depending on the limitations of time and required accuracy. For the measurement of persistence to be standardized, the method used to determine reactivity must be the same for all chemicals under investigation. For practical purposes, the method must be inexpensive and fast—ideally, a single reaction. However, the large number of chemicals currently in use covers a very wide range of reactivities, so finding a single set of reaction conditions for all chemicals would be very difficult. Many parameters that govern the reactivity of a chemical are attributed to its molecular structure, such as stereo-electronic effects, polarity, and steric effects (19). Furthermore, the impact of each parameter on reactivity can depend on the nature of the reagent or solvent, as described, for example, in Pearson’s hard–soft, acid–base theory (20). Consequently, a single set of conditions may present a biased assessment of reactivity when various structural moieties are compared. Adkin’s second proposed approach is likely to prove more practical: A battery of experiments can be developed that represent a scale of reaction conditions and also incorporate reagents for specific parameters that influence reactivity, such as steric effects, hard–soft interactions, and solvent effects. A separate set of experiments would be needed for each of the five reaction categories, although a full battery may not be necessary for each (e.g., photolysis).
Four examples of reactions A recently published work from our laboratory illustrates the proposed system (21). Nucleophilic substitution or elimination reactions are presented for a set of pollutants, including brominated flame retardants, through reaction with sodium methoxide in a solvent mixture of methanol and dimethyl formamide (DMF). The methoxide ion is a small, strong, hard nucleophile that was chosen to enable the examination of the reactivity of relatively nonreactive chemicals toward hse reactions. By varying the solvent composition, we modified the reactivity of the nucleophile over a wide range (i.e., high reactivity in DMF, lower reactivity in pure methanol). The relative rates of transformation of the test set follow what would be expected from a knowledge of organic chemistry and from the compound’s observed behavior in the environment. Thus, DDT eliminates HCl very readily to form DDE, whereas DDE is resistant to transformation even under the highly reactive conditions of the experiment.
assessment (32). However, because the reactivity approach provides information on the chemistry of test compounds, it may prove a valuable tool for predicting anticipated environmental transformation products, particularly if a buildup of transformation products is observed in the test battery. Because establishing values to populate the environmental power matrix will be very difficult, information should be pooled from all potentially useful sources. For some values, relevant data are already available, such as sunlight irradiation information and atmospheric radical concentrations. For other values, such as reduction in air, photolysis, or oxidation in sediments, a value may not be necessary at all. For other values, an assessment of environmental concentrations of wellknown pollutants may provide useful information. For example, an analysis of relative concentrations of different phthalate esters in sediments may indicate how well sediments perform hydrolyses. Such values may be refined through the interpretation in depth of specific environmental reactions and can serve as reference values.
Redefining the term Many definitions of persistence already exist, but the one that encapsulates the concept described here is this: The persistence of a chemical is its longevity in the integrated background environment as estimated from its chemical and physicochemical properties within a defined model of the environment. To standardize the definition, it may be necessary to expand it by specifying the type of distribution model required to represent the fate of the test compound in the total environment (e.g., Mackay level III), by dictating the environmental input data to be used in the model (e.g., compartment volumes or bulk deposition rates), by stating a specific endpoint to be determined, by stipulating how the partitioning parameters of the chemical are to be determined (e.g., Kow), and/or by specifying the method by which compartmental transformation rates are to be assigned (e.g., as proposed here). Such specifications would, in effect, provide a practical definition of persistence, although its description would run to several pages. Persistence now has such a central role in the broader discussion of chemical safety that a unified definition is required. In the absence of a satisfactory method for predicting persistence, risk assessments frequently must rely on observed concentrations and trends in environmental matrices (as in the case of pentabromodiphenyl ether; 33). If such observational data have to be used, it demonstrates that the prodecember 1, 2005 / Environmental Science & Technology n 485A
photodisc
Furthermore, the flame retardant decabromodiphenyl ether is readily transformed, whereas the rate of transformation of lesser-brominated diphen yl ethers is inversely proportional to the degree of bromination (i.e., the electron density of the aromatic ring). This work will be similarly extended to milder, softer, and bulkier nucleophiles. Water is a very weak nucleophile and might be included to complete the spectrum of reactivity from very high (methoxide) to very mild (water). Existing data on the reaction of pollutants with water that have been generated by the Organisation for Economic Co-operation and Development (OECD) standard protocol (14) could then be included in the reactivity data set. The overall results would provide considerable information on the reactivity of a substrate toward nucleophilic attack and could be used as an indicator of its susceptibility to hse reactions. Note that these specific experiments are not designed with the expectation that they might be encountered in the environment and that the rates measured in these reactions are not intended as environmental transformation rates. The results can, nevertheless, be extrapolated to environmental compartments knowing that the reactive power there will be very much weaker. As another example, a candidate experiment for establishing the susceptibility of chemicals to reduction might be based on reactions with sodium borohydride in tetrahydrofuran and ethanol. These conditions have been used successfully in the synthesis of nonabromodiphenyl ethers from the decabrominated starting material (22). Similarly, lithium aluminium hydride was previously used for preparing hexachloronaphthalenes from perchloronaphthalene (23). Studies on the photolysis of several pollutants have also been recently published (24–26); they provide a suitable method for measuring susceptibility to photolysis with a wide range of chemicals (25). In fact, the methodology for determining sh is so far the most well developed for use as a standard procedure in our proposed approach. Ongoing studies in our laboratory are examining the oxidation reactions of the same test set of chemicals. In combination, these studies will provide an indication of the relative reactivities of this test set, and these will be used to demonstrate the overall strategy. Vapor-phase radical reactions of pollutants have received considerable attention (27, 28). Further development of chamber experiments (29, 30) or on-line mass spectrometric experiments (31) for routinely examining a wider array of chemicals than is currently possible will be important for the success of our approach to persistence. The reactivity approach proposed here considers only primary transformation of the chemical and does not address the potential persistence of transformation products. Fenner has highlighted how persistent transformation products can extend the effective exposure time of an environmental hazard, and, in the case of nonylphenol ethoxylates, they strongly influence the outcome of the persistence
cedure for environmental and human-health protection has failed because potentially hazardous chemicals are not identified proactively before their widespread distribution in the environment. We feel that basing measures of a chemical’s potential for transformation on properties of its chemical reactivity provides a systematic and transparent system for predicting its persistence. Establishing clearly defined measures of the persistence of substances would greatly improve our ability to identify chemicals of environmental concern. We hope this article stimulates a broader discussion on the role of chemical reactivity in determining persistency. To succeed, we need expertise far beyond our own. If this initiative is successful, the environmental community will have a powerful and improved tool for predicting the fate of chemicals in the environment. Nicholas Green is an associate professor and Åke Bergman is a professor at Stockholm University. Address correspondence regarding this article to Bergman at ake.
[email protected].
Acknowledgments This work was funded by the European Chemical Industry Council (CEFIC) via contract NMLRI-LUND-MISTRA0201 and by the Swedish Foundation for Strategic Environmental Research (MISTRA) via the NewS program. The authors thank the three reviewers of this paper for their detailed considerations of earlier drafts, their constructive criticism, and their helpful suggestions for improvement.
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