Aquatic Humic Substances - ACS Publications - American Chemical

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Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch018

Bioavailability and Toxicity of Metals and Hydrophobic Organic Contaminants John F. McCarthy Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, T N 37831-6036

The effect of humic substances on the availability and toxicity of organic and inorganic contaminants in the aquatic environment is reviewed. Organic contaminants associated with humic substances appear to be essentially unavailable for uptake by amphipods, daphnids, andfish.Acute toxicity of these compounds is also diminished proportionally. Because the affinity of organic solutes for binding to humic substances is related to their hydrophobicity, the effect of humic substances is significant only for compounds with octanol-water partition coefficients greater than 10 . In most cases, association of toxic metals with humic substances reduces the uptake and toxic effects of the contaminants. However, complex interactions among the toxicants, humic ligands, other transition metals and major cations in solution, and the carrier proteins on biological membranes make it difficult to generalize and predict any reduction in accumulation and toxicity of metals. Humic substances may have secondary effects on biota uptake and accumulation of toxicants through their role in altering the transport and fate of pollutants. 4

BINDING OF ORGANC I OR N IORGANC I CONTAMN IANTS

to humic substances, or to other dissolved or colloidal organic matter in aquatic systems, can alter the availability of the contaminants for uptake by biota. It is generally thought that humic substances alter bioavailability by altering the concentration of a contaminant that is in a physicochemical form capable of traversing mem-

0065--2393/89/0219-0263$06.00/0 © 1989 American Chemical Society

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.


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AQUATIC H U M I C SUBSTANCES

branes. Association of metal or organic solute with the humic macromolecule masks the chemical properties of the contaminant and thus alters the normal biochemical interaction of the contaminant with the membrane. This physicochemical interaction also changes the toxicity of the contaminant, because the toxic effect of a pollutant is directly related to the dose incorporated by the organism. The relationship between dose and adverse response is not always straightforward. Contaminants can be sequestered in pharmacokinetically inactive compartments within the animal (e.g., stored in lipid deposits that are isolated from sites of toxic action) or complexes can be formed with specific proteins, such as metallothioneins, which prevent met als from entering tissues and promote excretion of toxic metals. In this chapter, I review current knowledge of how reversible inter action of humic substances with organic and inorganic contaminants alters the biological availability and toxic effect of the pollutants. The nature of the interaction of the humic substances with the contaminants and the factors that influence the quantitative distribution of the contaminants between a bound and free form are not discussed here; these are the topics of other chapters. In much of this discussion, the general term "dissolved or colloidal organic matter" ( D O M ) will be used to identify organic components of natural systems that alter the physicochemical properties and the bioavailability of organic or inorganic contaminants. This term includes both aquatic humic and fulvic substances, although the affinity of different components of natural D O M to bind organic or inorganic solutes may be quite different. Metal contaminants are discussed first; then follows a discussion of the effects of humic substances on the availability and toxicity of organic contaminants.

Effect of DOM on Bioavailability and Toxicity of Metals Because of their polyelectrolytic nature, humic and fulvic substances are capable of complex associations with metals (J, 2). The presence of D O M has often been reported to alter the bioavailability and toxic effects of metals in aquatic systems; however, interactions are complex and not easily de scribed or generalized. Some examples of the many and often conflicting reports can illustrate the difficulties in attempting to interpret how the in teractions of metals with D O M affect the accumulation and toxcity of metals. Humic acids have been reported to increase the uptake of cadmium by mussels (3) and rainbow trout (4), but to decrease accumulation in phytoplankton (5, 6), algae (7), Daphnia magna (8), and corn roots in water culture (9). D O M slightly increased uptake of americium and plutonium by phytoplankton (JO), but decreased uptake of several other metals, including mercury and zinc by fish (4), and zinc, chromium, and cobalt by algae (7). D O M had no measurable effect on accumulation of copper by a polychaete



18.

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Metals and Hydrophobic Organic Contaminants 265

(11), nor cadmium by D. magna (12). The presence of D O M also altered the toxicity of metals, but not necessarily in direct relation to the accumu lation of the metals in organisms (13). Toxicity decreased for cadmium in Atlantic salmon and algae (14); for copper in Atlantic salmon (15), algae (5), and D . magna (12, 16); and for zinc in daphnids (17). Toxicity of cadmium to D . magna and copper to D . pulex (16) increased in the presence of humic acid, but only in hard water (12, 18). Several mechanisms have been proposed to account for the diverse and seemingly conflicting results. It is generally assumed that the free metal ion is the chemical species responsible for the biological effects of metals. The biological uptake and toxicity of the metals are expected, therefore, to be altered by interactions of the metal with inorganic and organic ligands, which alter the concentration of the ionic species (e.g., 5, 19-22). The major proc esses that control chemical speciation of metals in natural waters (precipi tation, formation of complexes with inorganic and organic ligands, and adsorption by particulate material) can be modeled to permit calculation of the concentration of free ionic metal (23, 24). However, direct measurement of the free metal ion concentration during accumulation or toxicity experi ments failed to confirm a direct relationship between free ion concentration and biological effect (e.g., 4, 8, 17). The models and measurements of solution chemistry may fail because they neglect possible changes in solution equilibria at the interface between the water and the biological membrane (gill or gut). Competitive effects between the D O M - m e t a l complex and the carrier proteins in biological membranes responsible for active transport of metals into the organism, or changes in binding affinity of these membrane ligands due to competitive binding of other transition metals or major cations in solution, could alter the amount of metal translocated across biological membranes into the or ganism. Enhanced uptake of metals could be due to passive diffusion of organic-metal complexes. Association of metals with relatively low-molec ular-weight organic ligands could make the metal more lipid-soluble and thus increase permeability through membranes by a mechanism other than carrier-mediated transport of the free ionic metal species (3, 25). Interpretation of uptake and toxicity results can be further complicated by biochemical processes within the test organism. D. magna excrete organic compounds with metal-binding activity similar to that of humic and fulvic acids (26). The release of agents that form complexes may affect the results of toxicity tests by reducing metal activities during incubation. Furthermore, relationships between the accumulated body burden of metal and adverse toxic effects may be obscured by the protective action of internal metalbinding proteins, such as metallothioneins, which are produced at higher levels within organisms chronically exposed to nonlethal levels of metals (e.g., 27).


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Effect of DOM on Bioavailability and Toxicity of Organic Contaminants The effect of humic substances on the biological uptake and toxicity of organic contaminants appears much more consistent and predictable than their effect on metal toxicants. Available data suggest that association of organic con taminants with D O M reduces the uptake and toxicity of the contaminant. The interaction of the organic contaminant discussed in this chapter is limited to the reversible association of the organic solute with the humic macromolecule. This is to be distinguished from the covalent incorporation of an organic solute into the chemical structure of the humic molecule by oxidative coupling. The detoxification of a pollutant by copolymerization with humic macromolecules is discussed by Bollag in ref. 28. The reversible association of organic solutes with D O M appears to result from the solvophobic partitioning of hydrophobic solutes from the polar aqueous environment into the more nonpolar domain of the humic macromolecule (29). The chemical nature of the solute does not change because of the association; the partitioning is simply an entropy-driven equilibration of the solute between a polar and nonpolar phase similar to that described for the partitioning of organic chemicals between water and octanol, or between water and the organic coatings of sediment particles (30, 31). The equilibrium or steady-state concentrations of solute in the two phases (freely dissolved in water or bound to D O M ) can be described quantitatively by a distribution coefficient, K , analogous to the octanol-water partition coef ficient (K ) or the carbon-referenced sediment partition coefficient ( K J (32): dom

ow

Kdoo, = C /C [DOM] dom

(1)

d

where and C are the concentrations of solute associated with D O M (mol/kg of carbon) or freely dissolved in water (mol/L of water), respectively, and [ D O M ] is the concentration of D O M (kg of carbon/L). The affinity of an organic solute for associating with D O M is inversely related to the water solubility and directly related to the K of the solute (29, 33, 34). Because aqueous solubility is related to the molecular surface area of the solute (35), contaminants with higher molecular weights, such as polycyclic aromatic hydrocarbons (PAHs) with three or more rings, or polychlorinated biphenyls (PCBs), have a high affinity for associating with the D O M . A linear rela tionship has been described, for example, between the log K of a series of PAHs and their log K s for Aldrich D O M [log K = (1.03 log KJ 0.5] (36). d

ow

w

d o m

dom

B i n d i n g o f O r g a n i c C o n t a m i n a n t s to D O M . The association of a hydrophobic organic contaminant (HOC) with D O M alters the bioavaila bility and toxicity of the H O C . Boehm and Quinn (37) observed that uptake



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of hexadecane by the clam, Mercinaria mercinaria, increased significantly when the natural organic matter in seawater was removed by filtration through activated charcoal, but the uptake of phenanthrene was unaffected. The accumulation of the five-ring P A H , benzo[a]pyrene (BaP), by D. magna decreased by over 95% in the presence of 20 mg C / L of a commercial (Aldrich) humic acid (38). While Leversee et al. (39) also observed a decrease in BaP uptake by D. magna (25% decline with 2 mg C / L of Aldrich humic acid), they reported that the same concentration of humics had little effect on uptake of anthracene, dibenzanthracene, and dimethylbenzanthracene. Although they reported that the humic acid increased uptake of another five-ring P A H , 3-methylcholanthrene (3-MC) (39), this was not confirmed by a subsequent study that demonstrated that 3 - M C uptake by D. magna decreased with increasing concentrations of Aldrich humic acid (40). Leversee et al. (39) also found that removal of natural D O M from streamwater by photooxidation ( D O M decreased from 5.5 to 0.2 mg C / L ) increased uptake of BaP by 40%. The uptake of 2,4',5-trichlorophenol by Atlantic salmon fry (Salmo solar) was 30% lower when exposures were conducted in humic lake water (7 mg C / L ) , compared with exposures in water from a nonhumic lake (41). Bioaccumulation of the less hydrophobic compound tetrachloroguaiacol was reduced by 14% in the presence of humic water, but humics had no significant effect on accumulation of the even less hy drophobic contaminants lindane and trichlorphenol (41). Similarly, the ad dition of 10 m g / L of Aldrich D O M had only a slight effect on the accumulation of bis(tributyltin) oxide (TBT) by the mussel, Mytilus edulus (42). In general, D O M has been reported to have had an inhibitory effect on the uptake of very hydrophobic contaminants, but little effect on compounds with K s below 10 . This relationship may be related to the fraction of the total contaminant that binds to the D O M , which can be calculated: o w

4

fraction of H O C bound to D O M

=

1 + UDOM]

(2)

On the basis of the direct relationship between K and K (33, 36) and equation 2 (40), it can be estimated that only a few percent of the phen anthrene, lindane, trichlorophenol, or T B T would have been bound to the D O M under conditions of the aforementioned studies. o w

d o m

Bioavailability of H O C Bound to D O M . Direct physical meas urement of the amount of H O C bound to the D O M during toxicokinetic studies has confirmed that contaminant bound to D O M is essentially un available for uptake by aquatic organisms. Uptake is therefore reduced i n proportion to the fraction of the H O C bound to the D O M . The uptake and elimination of BaP and naphthalene were measured in bluegill sunfish, Lepomis macrochirus (34), in the presence and absence of Aldrich D O M .



268


In the presence of 20 mg C / L of D O M , 97% of the BaP was bound to D O M (measured by equilibrium dialysis) (43, 36). Uptake was reduced by about 95%, compared with uptake in the absence of D O M . Only 2% of the less hydrophobic compound naphthalene was bound to the same concentration of D O M , and uptake of naphthalene was not significantly affected by the presence of D O M (34). The kinetics of accumulation of a series of two- to five-ring PAHs by D. magna was measured in the presence of several con centrations of Aldrich D O M (40) (Figure 1). Uptake and accumulation de creased in proportion to the fraction of P A H bound to the D O M . Both kinetic and steady-state analyses of the data confirmed that P A H bound to D O M was not taken up by the D . magna. ORNL-DWG 84-1471

Figure 1. The time course of uptake of PAH shown for D. magna exposed to aqueous solutions of (a) BaP, (b) benzanthracene, and (c) anthracene in the presence of different concentrations of Aldrich DOM. The presence of 1560 mg C/L of DOM did not decrease the accumulation of naphthalene (data not shown). The mean concentrations in the animals ( + SE offour replicates from each time point) are plotted. The concentration of DOM and the fraction of PAH bound to DOM (f) are indicated. The body burden of 3-methylcholanthrene accumulated after a 30-h exposure in the presence of 0.15-15 mg C/L of DOM decreased in proportion to the fraction of contaminant bound to the DOM (data not shown). (Reproduced with permission from ref. 40. Copyright 1985 Elsevier Science Publishers.) In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

Metah and Hydrophobic Organic Contaminants 269


MCCARTHY

EXPOSURE TIME (h)

Figure 1.—Continued.



270


Landrum et al. (44) measured the change in the rate of uptake by the amphipod, Pontoporeia hoyi, of a variety of H O C s (several PAHs, D D T , diethylhexyl phthalate, and two PCBs) in the presence of different concen trations of Aldrich D O M . The change in the uptake rate was compared with the amount of H O C bound to the D O M (measured by a reverse-phase separation method) (33). Biological partition coefficients were derived from the toxicokinetics, based on an assumption that only freely dissolved H O C was being taken up by the amphipods. The partition coefficients derived from the reverse-phase method and from the toxicokinetics correlated well, supporting the conclusion that H O C s bound to D O M were not available to the organisms (44). Bioconcentration of tetrachloro- and octachlorodibenzop-dioxins ( T C D D and O C D D , respectively) by rainbow trout and fathead minnows was orders of magnitude lower than would have been predicted from well-established regression correlations between bioconcentration and the hydrophobicity of the compounds (45). The low accumulation of at least the O C D D was attributed to binding of the dioxins to natural D O M in the exposure water. Analyses using the reverse-phase separation method indi cated that 15% of the T C D D , but apparently all of the O C D D , was associated with the D O M .

Effect of H O C - D O M Interaction on Toxicity.

The reduction in

accumulation of H O C s due to their association with D O M results in a cor responding reduction in their toxicity. The presence of humic material in hibited the effect of several organic mutagens, as measured in vitro by using an Ames mutagenesis bioassay system (46). The in vivo toxicity of D D T to D . magna was reduced in the presence of natural D O M and was directly related to the lower body burden accumulated due to association of the D D T with the D O M . The toxicity and body burden of lindane, which has a low affinity for binding to D O M , were not affected (47).

Possible Mechanisms for Reduced Accumulation.

The available

evidence indicates that the association of H O C s with natural D O M reduces the uptake and accumulation of those contaminants by aquatic organisms. It appears that D O M prevents transport of the H O C through biological membranes. D O M does not reduce bioaccumulation by changing the rate of metabolism or eliminating the contaminant. In bluegill sunfish exposed to P A H , rate coefficients for the elimination of the contaminant were identical in the presence and absence of D O M , and the rate of biotransformation of the BaP within the fish was unaffected by the presence of D O M (34). As sociation of H O C with D O M has a direct effect on the efficiency with which the solute is taken up from water passing over the gills of rainbow trout. Measurements using a metabolic chamber demonstrated that the reduction in the uptake efficiency was equal to the fraction of H O C bound to the D O M and that only dissolved compound was translocated across the gill membranes



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Metals and Hydrophobic Organic Contaminants

271

(48). Most likely, the impaired uptake involves a charge-exclusion or sizeexclusion mechanism. H O C s are translocated across the gills by a passive diffusion through the lipophilic membranes. Polar solutes are not readily transported by this mechanism, but generally require active transport by specific ligands in the membrane. The polar character of the polyelectrolyte humic macromolecule undoubtedly overwhelms the lipophilic properties of the H O C s and controls the transport of those contaminants. Uptake of the humic macromolecule (and the associated HOCs) would be minimal because of both the D O M ' s polar nature and the large molecular size of the macro molecule, which would inhibit or prevent its passage through the membrane.

Environmental Significance of Association of HOCs with D O M . Figure 2 presents a conceptual model for calculating the effect of humic substances on the bioaccumulation of H O C s in aquatic systems. The total amount of contaminant in a particle-free system can be viewed as being distributed among dissolved, bound, and biotic compartments. The accu mulation of the H O C by the organism can be described: dCJdt

= kC x

A

- fc C 2

+

a

kC 3

(3)

dom

where C is the concentration of H O C in the animal (mol/g), k and k are the rate coefficients (h" ) for uptake of dissolved and bound H O C , respeca

x

3

1

DISSOLVED UPTAKE

(k ) t

DEPURATION

(k ) 2

HOC IN ANIMAL (C ) a

UPTAKE

(k ) 3

SORBED TO DOM

Figure 2. Conceptual model used for multicompartment analysis. The HOC in the system is viewed as being distributed among three compartments: dissolved in water (Cd); bound to DOM (Cam); and incorporated by the animal (C ). Rate coefficients describe the steady-statefluxof HOC among the compartments. (Reproduced with permission from ref 40. Copyright 1985 Elsevier Science Publishers.) A


272


1

tively, and k is the rate coefficient ( h ) for elimination of H O C from the animal. The potential of an H O C accumulating in an aquatic organism is often quantified as a bioconcentration factor (BCF), which is the ratio of the steady-state distribution of the H O C between the animal and the water. In almost all toxicological and regulatory contexts, it is generally assumed that all of the nonparticulate contaminant in the water is dissolved and available for uptake. However, i f D O M is present in the system, only a fraction of the H O C is freely dissolved, while the remainder is bound to D O M and much less bioavailable. Thus, the B C F achieved in the presence of D O M ( B C F ) will be less than that reached in the absence of the sorbent. If, as the evidence suggests, H O C bound to D O M is totally unavailable for uptake (k = 0), the B C F will depend directly on the fraction of H O C that is freely dissolved in the water (/ )


2

d o m

3

d o m

:

free

(4) (5) This conceptual model has been used to develop a structure-activity rela tionship (SAR) between a physicochemical property of the H O C (its K ) , the amount of D O M present in the system, and the bioconcentration of the H O C from the water (Figure 3) (40). The B C F of an H O C is logarithmically related to the hydrophobicity of the H O C , and several regression equations have been reported to predict the B C F from the aqueous solubility or of the solute (49-51). Figure 3 was based on the observed B C F s of the different PAHs in the experiment with D . magna shown in Figure 1, and on K calculated from the K , based on regression correlations for the same compounds (36). The S A R illustrates the significance of D O M in al tering the accumulation of H O C with K s greater than 10 . These very hydrophobic compounds have a high affinity for accumulating in aquatic organisms, but also have a high affinity for binding to D O M . The presence of environmentally realistic concentrations of D O M (1-10 mg C / L in lakes and rivers, and up to 50 mg C / L in wetlands and swamps) (52) will bind a substantial fraction of the contaminant and diminish its potential for accu mulation by aquatic organisms and movement through food chains to hu mans. Unfortunately, there is a large degree of variability in the affinity of D O M from different sources of water to bind H O C s . The total concentration of dissolved organic carbon in a water is not a good predictor of the capacity of that water for binding organic contaminants. Qualitative differences in the nature of the organic carbon have large effects on its affinity for binding H O C . The K for binding of individual H O C (measured by the reversephase method) differed by 2 orders of magnitude for D O M from waters taken from different locations within the Great Lakes (53, 54). The affinity of dif ferent water sources for binding D D T (measured by equilibrium dialysis) also varied by orders of magnitude (55). There is an obvious need to uno w

d o m

o w

4

o w

dom


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6000

0

0

M

(mg

c/u

50

Figure 3. Structure-activity relationship (SAR) between K ^ , [DOM], and predicted BCF, based on equations 4 and 5. The left edge of thefigure describes the direct rehtionship between the hydrophobicity of the contaminant and its observed BCF in D. magna when no DOM is present (Figure 1). For a less hydrophobic contaminant, such as naphthalene (front edge offigure,hg Kdom = 3), the BCF is low in the absence of DOM, and the presence of increasing amounts of DOM has little effect. However, for a very hydrophobic compound, such as BaP (back edge offigure,log Kdom = 6), even small amoun of DOM bind a substantial fraction of the contaminant, and this results in a large decrease in bioaccumulation. The observed BCF in the presence of DOM (Figure 1) agreed well with the BCF predicted in thisfigure.(Reproduced with permission from ref. 40. Copyright 1985 Elsevier Science Publishers.)


274



derstand the chemical and structural properties of D O M that underlie its attraction to H O C and to develop methods for easily predicting the organic complexation capacity of natural waters. Delineation of these properties will greatly improve capabilities to predict the bioaccumulation and potential for food chain transfer of the higher molecular weight, more environmentally persistent, organic contaminants—such as PAHs, PCBs, and dioxins—that pose the greatest risks to human health.

Conclusions Aquatic humic and fulvic substances can have a profound effect on the bio availability and toxicity of both organic and inorganic contaminants. When contaminants interact with the D O M by chelation (for metals) or solvophobic partitioning (for H O C ) , the original chemical properties of the contaminant are altered and their transport and bioavailability are often dominated by the properties of the humic molecule. Available evidence indicates that H O C s that bind to humic macromolecules become essentially unavailable for uptake through biological membranes, possibly due to exclusion of the humic molecule from the membrane on the basis of the molecular size or net electrical charge of the humic substance. This interaction appears con sistent and readily predictable because it is based on a thermodynamic gradient favoring the partitioning of a water-insoluble solute from an aqueous environment into the nonpolar domain of a second physicochemical phase, the humic substance. Interactions of D O M with metals are far more complex. While the "average" properties of an "average" humic or fulvic molecule can be modeled as having two binding sites with different stability constants (56), a humic molecule is a heterogeneous array of specific sites capable of a range of interactions with metals or with competing cations. Experimental evidence for the effect of D O M on the uptake of metals is further complicated by competitive interactions with specific carrier proteins at the surface of bio logical membranes. Although the association of a metal with D O M appears, in general, to reduce the bioavailability of the metal, the effect is clearly dependent on the chemical properties of the metal, the chemical properties and amounts of competing metals or cations, the p H , and possibly the source or properties of the D O M . The alteration of the physicochemical properties of the contaminant due to its association with the humic macromolecule can also have an indirect effect on the accumulation and toxicity of the contaminant in an ecosystem. D O M can alter the dose of toxicant to which biota are exposed by altering the transport and fate of the toxicants within the environment. Water-in soluble solutes (such as HOCs) can be stabilized in solution by association with D O M and advectively transported downstream rather than concen trating on sediment particles. For example, the presence of D O M may reduce the uptake and accumulation of a P C B near a source of contamination,



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but the contaminant now stabilized in the water column may be advectively transported downstream. Dilution of the contaminant concentrations down stream can favor the reversible dissociation from the D O M and make the contaminant available for accumulation far from the site of contamination. Because humics can compete with sediment organic matter for binding of H O C s , humics may also promote removal of persistent contaminants bound to sediment. Dissociation of the H O C in the water column could make the contaminants more readily available to pelagic biota that might otherwise be physically removed from the sediment-bound contaminants. To ade quately ascertain the role of humic substances on the bioavailability and toxicity of environmental contaminants requires consideration of the phys icochemical and hydrological, as well as the toxicological, aspects of the problem.

Acknowledgments I thank M . C . Black, J. E . Breck, and B . D . Jimenez for reviewing this manuscript. Oak Ridge National Laboratory is operated by Martin Marietta Energy Systems, Inc., under contract DE-AC05-84OR21400 with the U . S . Department of Energy. Publication No. 3043, Environmental Sciences D i vision, Oak Ridge National Laboratory.

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MCCARTHY

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Aquatic Humic Substances - ACS Publications - American Chemical

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