Organic Chemicals : An Environmental Perspective - Chapter 3 doc

Neilson, Alasdair H. "Partition: Distribution, Transport, and Mobility" Organic Chemicals : An Environmental Perspective Boca Raton: CRC Press LLC,2000 ©2000 CRC Press LLC 3 Partition: Distribution, Transport, and Mobility SYNOPSIS The dissemination of a xenobiotic after discharge into the environment is determined by its partition between the water, the soil and sediment, and the atmospheric phases, and its potential for concentration in biota. These processes determine both the biological impact of the xenobiotic and the extent of its dissemination. Procedures for determining the partition of xenobiotics into biota are discussed, and attention is drawn to complicat- ing factors, including the association of xenobiotics with macromolecules, and to the important interdependence of metabolism and bioconcentration. Surrogate procedures for evaluating bioconcentration potential that use physicochemical partition coefficients are outlined and their intrinsic limitations are pointed out. Such systems are unable to take into account the important issues of metabolism in biota and the structure of biological lipid membranes. Procedures for determining the distribution of xenobiotics between aqueous and solid phases are presented. The desorption of xenobiotics from the soil and sediment phases is discussed, and a brief account is given of interaction mechanisms between xenobiotics and components of solid matrices. Attention is drawn to the phase heterogeneity of the water mass in many natural systems and to the role of both particulate and dissolved matter in the distribution and dissemination of xenobiotics in lakes and rivers. Brief comments are devoted to the partitioning of xenobiotics between the aquatic phase and the atmosphere and to the significance of atmospheric transport on a global scale. A discussion of monitoring strate- gies is presented together with brief comments on the complexities in evaluating biomagnification. It is emphasized throughout that partitioning involves a complex set of molecular interactions, that these are reversible to varying degrees, and that attention should be directed both to the structure of the xenobiotic and to the ecosystem to which the results are to be applied. Equations used for correlating partition coefficients with physicochemical parameters have been presented and some of their limitations have been noted. ©2000 CRC Press LLC Introduction With the availability of suitable analytical procedures, the next question that should be addressed is the distribution of xenobiotics among the various phases after their discharge into the environment. This information provides a basis for deciding upon the ultimate fate of these compounds — particularly those that are not readily degradable — whose dissemination, persistence, and toxicity have aroused the greatest environmental concern. The distribution of xenobiotics is determined on the one hand by physicochemical equilibria and on the other by chemical or biologically mediated reactions, some of which may result in essentially irreversible associations between the xenobiotic and organic or inorganic components of the aquatic and sediment phases. The distribution of xenobiotics is therefore a function of many inter- acting factors and it is to a discussion of these that this chapter is devoted. The most-detailed discussions are devoted to the aquatic, and soil and sediment phases, although attention is also directed to the atmosphere because of its established significance in the global dissemination of many xenobiotics. In this chapter, the term aquatic phase will be taken to include the water phase together with biota (e.g., algae and fish) and particulate material (seston), while the term aqueous phase will be applied in a more restricted sense to the water phase alone. A valuable overview of the global dissemination of persistent organic compounds has been given (Wania and Mackay 1996), and application of fugacity models to the distribution of PAHs (Mackay and Callcott 1998). Attention should also be directed to the different physiology and biochemistry of the organisms as well as to their trophic level; important details of food chains are, however, noted only tangentially in this account. The partitioning of organic compounds between the aqueous and the sediment phases and between the aqueous phase and particulate matter including algae is important for a number of rather different reasons: 1. It determines the exposure of biota to a potential toxicant initially discharged into the aqueous phase (Section 3.2) and the extent to which it is justifiable to correlate observed biological effects with measured concentrations of the toxicant. 2. It has a significant bearing on the persistence of a xenobiotic which is discussed in greater detail in Section 4.6.3. 3. An assessment of the ultimate fate of xenobiotics — and of putative metabolites — requires estimates of their concentration and distribution in all environmental compartments. 4. The dissemination of xenobiotics (Section 3.5) initially discharged, for example, into the aquatic phase may take place in several of the phases — within the water mass including suspended particulate ©2000 CRC Press LLC matter, in the sediment phase to which the compound is sorbed, or via the atmosphere — and alterations in the structure of the xenobiotic may take place during transport within all of these phases. Possibly the greatest attention, however, traditionally has been directed to the concentration of organic compounds from the aqueous phase into biota. This effort has been motivated by the consistent recovery of many compounds of industrial interest such as PCBs and the more persistent agrochemicals such as DDT (and its metabolite DDE), mirex, and aldrin from samples of fish, birds, and marine mammals such as seals, whales, and polar bears. In a few cases, a plausible correlation has been established between injury to biota and exposure to a toxicant, and this is discussed in a wider perspective in Chapter 7, Section 7.7.2. Only two examples of such correlations will therefore be given here as illustration: 1. Exposure of bottom-dwelling fish to concentrations of PAHs in contaminated sediments in Puget Sound, Washington and the incidence of disease including hepatic neoplasms (Malins et al. 1984). This is discussed, with emphases on karien flatfish by de Maagd and Vethaak (1998). 2. Exposure of fish-eating herring gulls ( Larus argentatus ) in the Great Lakes and the incidence of porphyria in the gulls (Fox et al. 1988). However, even though exposure of biota to xenobiotics does not necessarily result in toxification of these organisms, the possibility that such compounds could enter the food chain and could therefore ultimately be consumed by the final predator — humans — has awakened serious concern over the dissemination of such compounds. A good example is provided by the concern over possible adverse effects on human health, including reproduction, that could result from the consumption of fish from the Great Lakes that may be heavily contaminated with organochlorine compounds including PCBs (Swain 1991). It should be appreciated that the concentration of a xenobiotic in biota is a dynamic process and represents a balance between uptake and elimination and that, as discussed in Sections 3.1.2 and 3.1.3, elimination may involve both the unchanged xenobiotic and its metabolites. Depuration therefore provides both a mechanism for the detoxification of the xenobiotic and its return — either unaltered or in the form of metabolites — to the aquatic phase, and a means of its dissemination within the water mass; this aspect is discussed in Section 3.5.2. Aquatic ecosystems are highly heterogeneous and comprise at least three apparently distinct phases: the aqueous phase, seston, the sediment phase and the biota. None of these phases should, however, be considered as an independent entity: for example, probably most sediments have a rich biota consisting of microorganisms together with a spectrum of higher organisms ©2000 CRC Press LLC such as oligochaetes and amphipods, and the exposure of sediment-dwelling biota to toxicants is significantly determined by exposure to the interstitial water in the sediment phase. The distribution of an organic compound initially discharged into the aquatic environment is therefore exceedingly complex and is determined by the dynamics of a number of partition processes between (1) the aquatic phase and biota including, for example, microalgae, higher plants, invertebrates, and fish; (2) the aquatic phase and the sediment phase; and (3) the sediment and sediment-dwelling biota. Almost all of these involve potentially reversible partitions all of which should be taken into consideration; they may be mediated, for example, by chemical desorption processes from the sediment phase or by depuration and elimination from biota. It should also be appreciated that few — if any — of these distributions are in true equilibrium; this fact should be borne in mind especially in extrapolating the results of laboratory experiments to natural ecosystems. In addition, the situation is complicated by the fact that none of these phases is truly homogeneous. Even the aquatic phase is heterogeneous and often contains particulate matter including inorganic material, and both soluble and insoluble organic matter originating from aquatic biota and terrestrial plants. In addition, components of the sediment phase may have originated from atmospheric transport and deposition; the quantitative importance of all of these distribution processes has therefore received increasing attention. As a result, intensive investigations have increasingly been directed to factors whose quantitative significance had not been fully appreciated previously. A few examples may be given to illustrate some of the important issues: 1. The sorption to particulate matter in the water column, and the dynamics and resuspension of surficial sediments; 2. The role of dissolved organic matter in the water column, accom- panied by an increased appreciation of the important distinction between truly dissolved and finely divided particulate matter that may be colloidal; 3. The significance of interstitial water both in mediating exposure particularly to sediment-dwelling biota and in diffusion of xenobiotics into the water mass; 4. The importance of partitioning between the aquatic phase and the atmosphere even for compounds with relatively low volatility, and the role of the atmosphere in mediating the long-distance transport of xenobiotics. These factors have focused attention on important new aspects of the phase partitioning of organic substances, and have indeed often revealed complexities that have merited intensive investigation and resulted in new perspec- tives. It is appropriate to note an increased awareness of possible limitations ©2000 CRC Press LLC in extrapolating data from laboratory studies to the natural environment. Two simple examples may be used as illustration — both involving PCBs. 1. Partitioning between the aquatic and particulate phase in New Bedford Harbor was not strongly correlated with values of P ow and revealed the importance of temperature (Bergen et al. 1993). 2. Studies of partitioning between the aquatic phase and algae have revealed that in natural ecosystems equilibrium is not reached in growing populations of algae so that use of P ow values is not jus- tified (Swackhamer and Skoglund 1993); this may indeed have wider implications and is discussed in greater detail subsequently. It is important not to be left with the impression that biota and sediments function solely or primarily as sinks for xenobiotics. A number of mechanisms exist for their elimination from these phases including metabolism and depuration in biota (Section 7.5), and desorption from the sediment phase (Section 3.2.2). Elimination from biota may also depend on diffusion mechanisms when the biota are in intimate contact with another phase. Two illus- trative example are given: 1. Elimination of 2,3,3 ′ -trichlorobiphenyl, DDE, and γ -hexa- chloro[ aaaeee ]cyclohexane from larvae of the midge Chironomus riparius was generally greater in sediments with higher organic content, and a significant correlation was found between the rate of elimination and the octanol / water partition coefficient of the compounds (Lydy et al. 1992). 2. Mayflies ( Hexagenia sp.) were chosen for monitoring PCBs in the upper Mississippi River on account of their long intrinsic need to be in contact with substrates at the base of the food chain (Stein- graeber et al. 1994). It is appropriate to make some comments here on the similarities between the soil and the sediment phases, since this is relevant both to the contents of this chapter and to the issues that are taken up in Chapters 4, 6, and 8. At first sight the soil and sediment phases appear to be totally different, but closer examination reveals important similarities and many of the principles set forth for aquatic systems are directly applicable, or with minor modification, to the terrestrial systems. 1. In both there may be substantial amounts of organic carbon, and sorption to and desorption from both mineral and organic components are essentially comparable. 2. Except on the surface of tropical deserts and arid lands, there is a subsurface water component of soils, and interstitial water is ©2000 CRC Press LLC important in the dynamics of sorption and desorption, and as a determinant of the bioavailability, toxicity, and persistence of xenobiotics to biota. 3. Both aerobic and anaerobic processes are important, and at greater depths the environment becomes essentially anaerobic. 4. Bioturbation is important in both environments. 5. Atmospheric deposition occurs directly onto terrestrial and aquatic environments, and from both by further partitioning may enter the sediment phase. There are important reasons for including discussions of the atmosphere since this interfaces with terrestrial plants, soil surfaces, and the aquatic environment. A holistic view must therefore take into account all of these partitions. Some specific reasons for considering the atmospheric environment include the following: a. The discharge of xenobiotics during incineration involves both “free” and particulate components, and partitions involving these in the atmosphere are extremely important (Mackay and Callcott 1998) b. The atmospheric environment is a dynamic one, and transformation products may reenter the terrestrial and aquatic environments. This is discussed in detail in Section 4.1.2. Considerable effort has been given to correlations between physicochemical properties and the various partitions. These properties themselves may also be of environmental significance. For example, quadricyclane that has been suggested as a high-performance aviation fuel, has the propensity to form microemulsions that could play a significant role in the dissemination of the compound in groundwater (Hill et al. 1997). There has also been concern that the water-soluble t -butyl methyl ether might act as a cosolvent for aromatic hydrocarbons such as BTEX that would probably occur at the same site (Poulsen et al. 1992). Field measurements in the United States suggest, fortunately, that this is not likely to pose a serious threat (Squillace et al. 1996). 3.1 Partitioning into Biota: Uptake of Xenobiotics from the Aqueous Phase 3.1.1 Direct Measurements of Bioconcentration Potential 3.1.1.1 Outline of Experimental Procedures For aquatic organisms, bioconcentration is the accumulation of a chemical from the aqueous phase; exposure takes place only via the water although the ©2000 CRC Press LLC compound may exist either in the dissolved form or associated with dissolved organic material. It is therefore distinguished from bioaccumulation that includes all modes of uptake including that of particulate matter; this is discussed more fully in a later section. Bioconcentration factors (BCF) may be calculated by either of two procedures: the basic assumption in both is that the uptake and depuration are governed by first-order kinetics although deviations may occur that may be accounted for by the induction of enzymes for metabolism of the xenobiotic. In practice, a number of additional factors may be involved including the tox- icokinetics of different organs and possible interference from growth of the test organism if the compound is only slowly accumulated. In one method, concentrations in the biota and in the surrounding medium are measured after a steady state has been reached, and the ratio of the two concentrations is used to obtain the BCF value. In the other, rates of uptake and elimination of the xenobiotic are measured and the ratio is used to calculate concentrations in the biota: the BCF is then calculated from these values. The experimental difficulties of maintaining a constant substrate concentration and achieving a steady state have been overcome by using a procedure based on iterative integration of the experimental data (Gobas and Zhang 1992). The possible complications resulting from metabolism of the test compound are discussed in Section 3.1.5, and more fully in Chapter 7, Section 7.5. In laboratory experiments using fish, exposure takes place primarily by uptake through the gills directly from the aquatic phase (Pärt 1990), and the bioconcentration factor may be estimated by either or both of the procedures outlined above. Both procedures have been evaluated in experiments in which guppy ( Poecilia reticulata ) were exposed to a series of organophospho- rus pesticides that are metabolized only slowly. It was shown that there was a linear relation between the BCFs and the ratios of the uptake and elimination rates within the logarithmic range of 2.6 and 4.7 (de Bruijn and Hermens 1991). Although in this case the two procedures produced essentially identi- cal results, some discrepancy would be expected if the compounds were metabolized to a significant extent and the metabolites were subsequently eliminated from the fish. This is discussed more fully in Section 3.1.5. Exposure to the xenobiotic generally extends over a period of days or weeks and even for up to several months; either semistatic or flow-through systems may be used, and analytical control of the concentrations of the test substrate should be maintained. After exposure, fish are generally maintained in a xenobiotic-free environment to allow excretion of toxicants or their metabolites to take place. A variety of different fish including rainbow trout ( Oncorhynchus mykiss syn. Salmo gairdneri ), fathead minnows ( Pime- phales promelas ), guppy ( Poecilia reticulata ), zebra fish ( Brachydanio rerio ), and medaka ( Oryzias latipes ) have been employed, even though it has been clearly established that fish have highly effective metabolic potential for a wide range of compounds (Sections 3.1.5 and 7.5.1) and that this metabolic potential varies with the species. Different BCF values may therefore be found in experiments using different fish; for example, for a restricted range of ©2000 CRC Press LLC chlorobenzenes using fathead minnows, green sunfish ( Lepomis cyanellus ), and rainbow trout, experimental values for rainbow trout were the lowest (Veith et al. 1979a), and this might plausibly be correlated with their established metabolic capability. It should also be appreciated that the disposition of xenobiotics within the organisms may differ significantly; for example, a number of neutral organochlorine compounds are accumulated in the central nervous system (CNS) of cod ( Gadus morhua ) but not in that of rainbow trout, and for hexachlorobenzene it has been shown that whereas it is the xenobiotic itself that is present in the CNS system, it is metabolites that are found in cere- brospinal fluid (Ingebrigtsen et al. 1992). Low concentrations of the test compound are generally employed, and particular care should be exercised with compounds displaying even subliminal toxic effects at the concentrations used during exposure; for example, the value of 39,000 for the BCF of 2,3,7,8- tetrachlorodibenzo[1,4]dioxin at the concentration where rainbow trout were least affected may well be too low, since the corresponding value of the less toxic 2,3,7,8-tetrachlorodibenzofuran increased from 2455 at a concentration of 3.93 ng/l to 6049 at a concentration of 0.41 ng/l (Mehrle et al. 1988). Virtually any aquatic organism may, of course, be used and, for example, common mussels ( Mytilus edulis ) have been used for investigating the uptake of a restricted range of neutral organochlorine compounds (Ernst 1979), the crustacean Daphnia pulex for the uptake of azaarenes (Southworth et al. 1980), and freshwater mussels ( Anodonta anatina ) for the uptake of chlorophenolic compounds (Mäkelä et al. 1991). Attention is drawn (Section 3.1.5) to the differences that may be observed between fish and bivalves, and this may plausibly be attributed to the relatively lower metabolic capacity of bivalves (Livingstone and Farrar 1984). This is noted in the wider context of toxicity and metabolism in Chapter 7, Section 7.5.2. The design of the uptake experiments themselves and the analytical deter- minations are straightforward: specific analysis may be carried out for the compounds being examined (together with their metabolites) or advantage may be taken of, for example, 14 C-labeled substrates. A number of important limitations in the numerical significance of the values obtained in laboratory studies have been pointed out (Oliver and Niimi 1985) and these are worth emphasizing: 1. Uptake of the xenobiotic may be so slow that the length of exposure is insufficient to attain a steady state. 2. The molecules may be too large for uptake, for example, via the gills of fish so that BCF values are negligible, and uptake in the environment is dominated by uptake via the food; this is discussed in greater detail below. 3. The xenobiotic is metabolized by the biota and this results in erroneously low concentrations in the biota and hence low BCF values; the interdependence of bioconcentration and metabolism in fish is considered in Section 3.1.5, and in a wider context in ©2000 CRC Press LLC Section 3.5.2, while additional details of metabolism by fish are given in Section 7.5.1. These limitations have been systematically explored for 34 halogenated compounds in rainbow trout, and they were shown to be particularly relevant to making realistic predictions of the concentrations in wild biota. Indeed, the striking incidence of DDE (the principal transformation product of DDT) in environmental samples is consistent with its bioconcentration in field samples to a degree greatly exceeding that predicted from laboratory measurements (Oliver and Niimi 1985). Significant differences in measured BCF values may also result from the design of the experiments and from the inevitable biological variability in the test organism. For example, BCF values for 2,3,4,5-tetrachloroaniline in guppy ( P. reticulata ) increased with increasing exposure time or increasing concentration of the test compound (de Wolf et al. 1992b), and log BCF values on a lipid basis for the same trichloroaniline isomer obtained in the same laboratory over a period of time using different strains of guppy ranged from 2.61 to 3.21 for 2,3,4-trichloroaniline and from 2.88 to 3.40 for 2,4,5-trichloroaniline (de Wolf et al. 1993). The significant role of the lipid content of the test organism is discussed in detail later. It cannot therefore be too strongly emphasized that all of these consider- ations should be critically evaluated in discussions of bioconcentration potential. 3.1.1.2 The Molecular Size and Shape of Xenobiotics and the Role of Lipid Content of Biota Increasing evidence points to the specific role of lipids in determining bioconcentration potential, and two different kinds of situation may be clearly distinguished. It should be clearly appreciated, however, that the term lipid is used for a class of structurally diverse compounds united by a single physicochemical property (solubility in organic solvents). They include, for example, neutral glyceryl triesters and glyceryl galactosides, zwitterionic phosphate diesters of glycerol and ethanolamine, and diesters of glycerol and inositol. Some compounds such as hexabromobenzene, octachlorodibenzo[1,4]dioxin, and tetradecachloroterphenyl are accumulated by fish only to a minor extent, presumably due to the size and configuration of the molecules (Bruggeman et al. 1984). Such compounds have been termed superhydrophobic since they have values of log P ow > 6, but it has been shown on the other hand that many of these compounds have — possibly unexpectedly — only low lipid solubility and that this decreases with increasing P ow (Chessells et al. 1992). In the case of decachlorobiphenyl, it has been suggested that only 3% of the substrate in the aqueous phase is available to guppy ( P. reticulata ) and this results in a BCF value that is between 10- and 100-fold lower than would be predicted on the basis of the P ow value of the compound (Gobas et al. 1989). Consistent with the overall role of values of P ow , the tetra- and pentabrominated diphenyl ethers [...]... 4-hydroxybenzoic, cinnamic, coumaric (4-hydroxycinnamic), caffeic (3, 4-dihydroxycinnamic), ferulic ( 3- methoxy-4-hydroxycinnamic), and vanillic ( 3- methoxy-4hydroxybenzoic) acids It has been shown that at plausible concentrations of these (< 100 µg/l) and at pH 5.6, there is competition between these and 2,4-dichlorophenol for sorption sites on soil organic matter (Xing and Pignatello 1998) 7 Probably greatest... bioavailability and hence diminished toxicity A number of organisms and a range of toxicants have been examined: for example, Salmo salar and a range of organochlorine compounds including both chlorophenolic and neutral compounds (Carlberg et al 1986), Oncorhynchus mykiss (syn S gairdneri) and benzo[a]pyrene and 2,2′,5,5′-tetrachlorobiphenyl (Black and McCarthy 1988), and Diporeia sp (syn P hoyi) and PAHs and... (Ugrekhelidze et al 1997) 3 Hybrid poplars are able to transport and metabolize various xenobiotics: (a) trichloroethene is metabolized to trichloroethanol and trichloroacetate (Newman et al 1997), (b) atrazine is metabolized by reactions involving dealkylation and hydrolytic dechlorination to yield 2-hydroxy-4,6-diamino-1 ,3, 5-triazine (Burken and Schnoor 1997) ©2000 CRC Press LLC 3. 1.4 Surrogate Procedures... their type and composition, and a study using phenanthrene and atrazine with soils differing in content of organic carbon and clay, and different pH showed that organic solvent extractability does not provide a good measure of the bioavailabity to microorganisms (Chung and Alexander 1998) Such procedures provide rather a measure of the total concentration of the analyte including both sorbed and freely... range of water solubilities ranging from cyclohexanol (37 .5 g/l) to anthracene (0.0488 mg/l) has been developed (Etzweiler et al 1995) The following relations have been proposed (Mackay 1982 ): Liquids: log Pow = 3. 25 - log Xl where Xl is the molar solubility (mol.m 3) in water Solids: log Pow = 3. 25 - log Xs + 2.95 (1 -Tm/T) where Xs is the molar solubility of the solid, Tm is the melting point, and... and C9 dicarboxylic acids Prahl and Carpenter 19 83 Peterman and Delfino 1990 Pereira et al 1988 Swackhamer and Armstrong 1986 Czuczwa and Hites 1986; Macdonald et al 1992 Remberger et al 1988 Krone et al 1986 Carter and Hites 1992 Fernandez and Bayona 1992 Stephanou 1992 There are a number of important environmental consequences resulting from the partitioning of xenobiotics into the sediment phase:... toluene and trichloroethene to soil was dependent on the nature of specific organic components (Garbarini and Lion 1986), and the partition of pyrene to dissolved organic humic material was influenced by its structure and was dependent on factors other than the total content of organic carbon (Gauthier et al 1987) The same structural dependence holds for association between xenobiotics and the organic constituents... out: (1) a solution of the test substance in octanol is equilibrated with water and the concentration in the octanol phase is determined, (2) the octanol phase is ©2000 CRC Press LLC loaded onto a column packed with Chromosorb W, and (3) octanol-saturated water is pumped through the column and the solute collected in a Sep-Pak cartridge for analysis A possibly more expedient procedure is the slow-stirring... Aquatic Plants Concentration of xenobiotics into aquatic plants may also be important and presents another redistribution pathway for xenobiotics The uptake of a few agrochemicals has been investigated using the aquatic plant Hydrilla verticillata (Hinman and Klaine 1992), although only low levels of atrazine, chlordane, and lindane were accumulated These plants have moreover only low levels of lipids and... equal significance, and its significance is attested by the structural range of organic compounds that have been recovered from contaminated sediments Many of these compounds such as PAHs, PCBs, and PCDDs are widely distributed and only selected — and more or less random references — have been provided here Some of these compounds certainly enter ecosystems as a result of long-distance transport but, . "Partition: Distribution, Transport, and Mobility" Organic Chemicals : An Environmental Perspective Boca Raton: CRC Press LLC,2000 ©2000 CRC Press LLC 3 Partition: Distribution, Transport,. P ow - 1 .32 0 • Mollusks log BCF = 0.844 log P ow - 1. 235 • Daphnids log BCF = 0.898 log P ow - 1 .31 5 As implied in Sections 3. 1.1 and 3. 1 .3, it should be noted that such equations cannot,. diffusion mechanisms when the biota are in intimate contact with another phase. Two illus- trative example are given: 1. Elimination of 2 ,3, 3 ′ -trichlorobiphenyl, DDE, and γ -hexa- chloro[