Water-Solubility Enhancement of Nonionic Organic Contaminants

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Downloaded by UNIV ILLINOIS URBANA on September 4, 2013 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch010

Water-Solubility Enhancement of Nonionic Organic Contaminants Daniel E . Kile and Cary T. Chiou U.S. Geological Survey, Denver Federal Center, Denver, CO 80225

Water-solubility enhancement of selected organic solutes by dissolved organic matter was studied by using a variety of dissolved soil and aquatic humic materials, commercial humic acids, and synthetic organic compounds. The solubility enhancement of relatively water-insoluble solutes (e.g., polychlorinated biphenyls and dichlorodiphenyltrichloroethane) by natural and commercial humic materials is accounted for by a partitionlike interaction of the solutes with the nonpolar organic environment of the dissolved organic matter. Solute-solubility enhancement is minimal when poly(acrylic acid) and phenylethanoic acid are used as dissolved organic matter species. The polarity, size, and configuration of the dissolved organic matter and the hydrophilicity of the solute are controlling factors for solubility enhancement. Solubility enhancement is much greater with commercial humic acids than with the aquatic humic materials.

INTERACTIONS O F CERTAIN ORGANIC CONTAMINANTS with dissolved natural

humic substances can often significantly influence the transport and fate of the compounds in aquatic systems. A fundamental understanding of the mechanism(s) of interaction in relation to the structure and composition of dissolved humic substances is essential for assessing the behavior of organic contaminants in natural waters. A n evaluation of this effect in relation to the properties of solute and dissolved organic matter is presented in this chapter. Data on the solubility enhancement of a variety of organic solutes by different types of dissolved humic substances (of soil and aquatic origins) are used as a basis for elucidating the mechanism involved.

0065-2393/89/0219-0131$07.75/0 © 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 HUMIC SUBSTANCES

In general, solutes that are relatively insoluble in water are most sus ceptible to enhanced water solubility by dissolved organic matter. Wershaw et al. (J) found that the apparent water solubility of D D T (l,l'-(2,2,2-trichloroethylidene) bis[4-chlorobenzene]) increased more than 200 times in an aqueous solution of 0.5% (soil-derived) sodium humate, and Poirrier et al. (2) showed that an organic-mineral colloid present in a natural surface water concentrated D D T by a factor of 15,800. Boehm and Quinn (3) reported significant increases in the apparent water solubility of some highly waterinsoluble alkanes when in contact with fulvic acid derived from a marine sediment. Later, Carter and Suffet (4) and Landrum et al. (5) found significant enhancements of the concentrations of D D T and other sparingly watersoluble organic compounds in water caused by aquatic humic substances and by Aldrich humic acid. The magnitudes of enhancement were found to be dependent on the source of the organic matter and on the solution p H . Because concentrations (or apparent solubilities) in water of many spar ingly soluble organic solutes can be influenced by dissolved humic sub stances, equilibrium constants and process rates related to the solute concentrations in water may exhibit certain anomalies when the effect on solute concentration is not taken into account. For example, Hassett and Anderson (6) observed a decreased recovery of cholesterol from water due to a substantial binding of the compound to dissolved humic substances; Griffin and Chian (7) and Hassett and Milicic (8) showed lower rates of volatilization for certain PCBs (polychlorinated biphenyls). Similarly, Perdue and Wolfe (9) reported an apparent decrease in the hydrolysis rate of the 1-octyl ester of 2,4-D (2,4-dichlorophenoxyethanoate) at a relatively low concentration of dissolved Aldrich humic acid in water as a result of the sorption of the solute by dissolved humic acid. A more commonly recognized consequence of the interaction between solute and dissolved humic substance is the modification of the distribution of the organic solute to other aquatic compartments. Hassett and Anderson (10) showed that the sorption coefficients of cholesterol and 2,5,2',5'-tetrachlorobiphenyl between river sediment and water are decreased substan tially in the presence of dissolved humic substances, compared to those in the presence of pure water. Similarly, Caron et al. (JI) observed that the apparent sorption coefficient of D D T with aquatic sediments decreased in the presence of dissolved humic substances; they attributed this decrease to a strong association of D D T with the dissolved humic substances. Gschwend and W u (12) found a similar decrease of the sediment-water sorption coefficient for some polychlorinated biphenyls with an increasing concentration of particulate material in water, and they attributed this effect to a simultaneous partitioning of the solutes to the suspended particulatebound organic matter. Dissolved humic substances have also been shown to reduce the intrinsic bioconcentration factors for some polynuclear aromatic hydrocarbons (13-15).


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These studies demonstrate that interactions of many organic contami nants with naturally occurring dissolved humic substances can lead to sig nificant modifications of the apparent solubility (and thus the mobility and fate) of the compound in aquatic systems. Because enhancement of solute water solubility by dissolved humic substances is presumably the primary cause for many related modifications of solute behavior in aquatic systems, evaluation of the significance of this effect for various solutes with different types of humic substances is warranted for environmental-impact assess ments.

Theoretical Considerations A n acceptable model for the interaction between solutes and dissolved humic substances must take into account properties of the solutes and of humic substances that are consistent with the observed data. The magnitude of the solubility enhancement of a nonionic organic solute increases with a decrease in the intrinsic solubility of the solute in pure water. For example, Boehm and Quinn (3) showed that in the presence of dissolved humic substances (at dissolved organic carbon concentrations ranging from 1.8 to 17.5 mg L " ) there was a significant enhancement of the apparent solubility of relatively water-insoluble higher alkanes, but not of more water-soluble aromatic compounds. Caron et al. (11) found that a dissolved humic acid isolated from a sediment (at a dissolved organic carbon concentration of 6.95 mg L ) strongly influenced the sorption coefficient of D D T with sediments, but had little effect on the corresponding coefficient of lindane. Similarly, Haas and Kaplan (16) demonstrated that Aldrich humic acid (at a dissolved organic carbon concentration of 70 mg L " ) has a minimal effect on the apparent solubility of toluene, a compound with a relatively high water solubility compared to D D T . Thus, from the standpoint of the solute, one would expect that only nonionic organic compounds with very low water solubilities would be significantly affected by dissolved humic substances. 1

1

1

In principle, dissolved organic materials (such as humic substances at sufficient concentrations) can promote the apparent solubility of a nonionic organic solute in aqueous solution. This increased solubility can be accom plished either by changing the overall solvency of the solution (analogous to a conventional mixed-solvent effect) or through a direct solute interaction by either adsorption or partitioning. Humic substances, at the relatively low concentrations typically found in natural aquatic systems, are not likely to have a significant impact on water solvency. This point is substantiated by data showing that octanol-saturated water (containing about 600 ppm octanol) increases the water solubility of D D T by a factor of approximately 2.5 (17). A solubility enhancement of this magnitude is too low to account for the observed enhancement by dissolved humic substances at concentrations con siderably less than that of the octanol in water. Also, the pronounced sol-



134


ubility enhancement of a solute such as D D T is unlikely to be a result of complexation or specific interaction because the hydrophilic functional groups of a humic molecule would be preferentially associated with the highly polar water molecules. Considering this condition, and the fact that dissolved humic substances are relatively high-molecular-weight species, Chiou et al. (18) hypothesized that the observed solute-solubility enhancement results from a partitionlike interaction between solute and dissolved organic matter, in which the dissolved high-molecular-weight organic matter is regarded as a "microscopic" organic phase; that is, interactions between solute and dissolved maeromolecules are governed primarily by van der Waals forces. The kind of partition interaction envisioned here is similar mechanistically to that for the solubilization of relatively water-insoluble organic solutes in micelles, where a microscopic organic phase is formed through aggregation of surfactant monomers (19-23). However, the effectiveness of this partition effect with a dissolved humic material is a function of the size, polarity, configuration, and conformation of the humic molecule. On this basis, dissolved organic matter with a sufficiently large molecular size and nonpolar molecular environment would effectively promote the partitioning of a sparingly water-soluble nonionic solute, while a dissolved low-molecular-weight organic substance or a highly polar organic macromoleeule would not exhibit a strong effect. To facilitate a better understanding of the presumed partitionlike interaction of organic solutes with dissolved humic substances, some characteristics that differentiate a partition effect from an adsorption effect need to be considered. In general, the. term adsorption refers to the condensation or specific association of a solute (or a vapor) onto surfaces or interior pores of a solid (adsorbent) through physical or chemical bonding forces. Adsorption is necessarily competitive between solutes (or vapors) because of the constraints of the available surfaces or specific sites. Such condensation or specific association leads to a decrease in entropy for the solute, and therefore adsorption of the solute is accompanied by a relatively high exothermic heat. The adsorption isotherm (i.e., the plot of the amount adsorbed per unit mass of adsorbent versus the equilibrium concentration in solution) is rarely linear over a wide range of solute concentrations because surfaces or active sites in the adsorbent are seldom energetically homogeneous. In contrast, the term partition or partitioning is used to describe a model in which the solute is distributed between two immiscible or partially miscible phases (e.g., an organic solvent and water) by forces common to solution. Some distinctive characteristics are associated with equilibrium partitioning of organic solutes: the partition coefficients for different compounds between an organic solvent and water are closely related to the reciprocals of their water solubilities; the partition isotherms are highly linear over a wide range of solute concentrations (24-28); the system shows an absence of competitive effect in the concurrent partitioning of binary solutes


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(25, 26); and the equilibrium heat for solute partitioning is relatively small and constant because of the partial cancellation of heats of solution for the transfer of solute from water to the organic phase (24). Assuming that the water-solubility enhancement of a nonionic organic solute by dissolved organic matter results from a partitionlike interaction between the solute and the macromolecule, the magnitude of this effect for a solute with respect to a specific dissolved organic matter sample can be defined as S* = S

w

+ XC

(1)

0

where S * is the apparent solubility of the solute in water containing dissolved organic matter (at concentration X ) , S is the solubility of the solute in pure water, and C is the mass of solute partitioned into a unit mass of dissolved organic matter. In accordance with this model, the solute partition coefficient ( K ) between dissolved organic matter and pure water can be defined as w

0

dom

Q ^dom — 7~

(2)

Substituting equation 2 into equation 1 gives S* = S ( l + XKtJ

(3)

w

Alternatively, if C and X are defined in terms of the organic carbon content of the dissolved organic matter, equation 3 can be written as 0

S* = S ( l + X K a J

(4)

w

where K is the corresponding organic-carbon-based partition coefficient. According to equation 3 or 4, the magnitude of solute solubility enhancement is controlled by the X K (or X K ) term, and a substantial enhancement therefore takes place when the magnitude of this term is significant relative to 1. The magnitude of the partition coefficient ( K or K ) is a function of the physical properties of the solute and the nature of the dissolved organic matter. A t a given concentration of dissolved organic matter (X), the product term XK (or X K ) should increase with decreasing water solubility for a series of solutes (or with increasing octanol-water partition coefficients) (IT). The K can be experimentally determined by measuring the apparent solute solubility over a range of dissolved organic matter concentrations. A plot of S* versus X should yield a straight line with a slope equal to S K and an intercept equal to S (equation 3). doc

d o m

d o c

d o m

dom

d o c

d o c

d o m

w

w


d o m

136


The assumed partitionlike interaction of nonionic solutes with dissolved organic matter can be independently evaluated by comparing the observed enthalpy of interaction with that found in a typical solvent-water mixture (24). The enthalpy change (ΔΗ), for the transfer of a solute from water to the solvent phase is described as ΔΗ = Δ Η - âïf


0

(5)

w

where Δ Η is the molar heat of solution in the organic phase and A H is the molar heat of solution in water (i.e., — A H is the molar heat of con densation from water). A nonionic organic solute with a low water solubility usually gives a high (positive) AH because of its high incompatibility with water. Conversely, the corresponding Δ Η should be much smaller because of the greater compatibility of an organic solute with the organic solvent. Both Δ Η and A H are relatively independent of solute concentrations. Thus, the molar heat of partitioning (ΔΗ) would be relatively small and independent of solute concentrations, and less exothermic (negative) than the heat of condensation from water ( — AH ). This characteristic is in contrast to the heat of adsorption, which should be much more exothermic (deriving from solute condensation from solution and additional exothermic interac tions with adsorbent) in order to compensate for the decrease in entropy. The difference in the equilibrium heat between adsorption and partition processes becomes more pronounced for a high-melting-point solid solute because of its large heat of fusion (ΔΗ ), which leads to a high heat of solution. The heat of solution of a solid compound in an organic phase can be expressed as σ

W

W

W

0

0

W

W

Γ

ΔΗ

0

(6)

= ΔΗ + ΔΗ ? Γ

and the heat of solution of a solid compound in water can be expressed as AH

W

= àHf + Δ Η 5?

(7)

6 Χ

where Δ Η represents the excess (molar) heat of solution of the supercooled liquid (29). The molar heat of fusion cancels in_a partition equilibrium (equa tion 5) because it is the same in both the Δ Η and àH terms, whereas in adsorption equilibria it adds to the overall heat of adsorption. The foregoing account of the mechanistic differences of partition and adsorption equilibria serves as a useful basis for evaluating the cause of the water-solubility enhancement of organic solutes by dissolved humic sub stances, in terms of the properties of solutes and the structural features of humic molecules. 0

w


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Influences of Molecular Properties Humic substances isolated from different sources have significantly different effects on solubility enhancement; this fact has been well documented (4, 18). These results have been effectively explained by Chiou et al. (18, 30) in terms of a partitionlike interaction between solute and dissolved humic substances. The magnitude of the solubility enhancement of a nonionic solute is related to its intrinsic solute solubility in water and to the molecular size, structure, and polarity of the humic macromolecule. In the study of Chiou et al. (18, 30), highly purified soil and aquatic humic samples were used to minimize complications resulting from mineral constituents in the sample. Soil-derived humic and fulvic acids were isolated from the surface (Al) ho rizon of Sanhedron soil obtained from the Mattole River Valley in northern California, and stream-derived humic and fulvic acids were isolated from the Suwannee River, Georgia. These samples had ash contents less than 4%. Elemental analyses for these humic samples are given in Table I. Intrinsic water solubilities of a group of investigated organic compounds are given in Table II. Partition coefficients ( K ) for the pairs of dissolved humic substances and solutes were determined (according to equation 3) by plotting the ap parent solubility (S*) against the concentration of the dissolved humic sub stance. The slope gives S K , and the intercept gives S . The results are shown in Figures 1 through 4. Except as noted, all the solubility-enhance ment experiments were conducted at p H 6.5 or below, and at a temperature of 24 ± 1 °C. The observed partition coefficients (i.e., K or K values) for various solute-dissolved humic-substance systems are given in Table III. These data show that: (1) a high degree of linearity exists between apparent water solubility and concentration of dissolved organic matter for all solutes tested; (2) the observed solubility enhancements of solutes with a given dissolved organic matter (i.e., the K values) are inversely related to the solubilities of the solutes in water ( S J , in the order of D D T > 2,4,5,2',5'-PCB > 2,4,4'-PCB, or in a linear relation to their log K values (Table III), indicating similarity in the nature of the equilibria between the two systems; and (3) no competitive interference is found for individual solutes in the binary-solute system (Figure 1). Conversely, no discernible enhancement is noted for the relatively water-soluble lindane and 1,2,3trichlorobenzene throughout the concentration range of Sanhedron soil humic acid (Figure 5). These findings are consistent with a partitionlike interaction of solutes with dissolved organic matter. dom

w

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w

dom

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d o m

o w

A comparison of the data in Figures 1 through 4 shows that the San hedron soil humic acid was most effective in enhancing solute solubility. It was about 4 times as effective as the soil-derived fulvic acid, and 5 to 7 times as effective as the stream-derived humic and fulvic acids. A partition inter action would be more effective with a large-sized macromolecule that con tains regions of large nonpolar volumes. The greater enhancement for the



e

3

NOTE; Values are percents on moisture-free basis. "Elemental data arefromref. 18. ^Elemental data arefromref. 30.

e

Table I. Ash-Free Elemental Contents of Various Humic and Fulvic Acids Sample C H 0 Ν S Ρ Sanhedron soil humic acid" 58.03 3.64 33.59 3.26 0.47 0.10 Sanhedron soil fulvic acid 48.71 4.36 43.35 2.77 0.81 0.59 Suwannee River humic acid 54.22 4.14 39.00 1.21 0.82 0.01 Suwannee River fulvic acid 53.78 4.24 40.28 0.60 0.65 0.01 Aldrich humic acid, sodium salt (lot no. 1204 PE, 1984)* 69.42 5.04 39.29 0.75 4.25 0.15 Fluka-Tridom humic acid (lot no. 159128115, 1974)* 65.79 5.51 37.79 0.71 3.16

100 h

Β

15

2,4.4'-PCB (25°C)

2,4,5,2',5'-PCB(25°C)

10'

Ρ,Ρ'-DDT (25°C)

20

40

60

80

100

Concentration of SRFA(mg/L)at pH8.5 Figure 8. Dependence of the apparent water solubilities of ρ,ρ'-DDT, 2,4,5,2' ,5'-PCB, and 2,4,4'-PCB on the concentration of Suwannee Riverfulvic acid at pH 8,5.

concentration of dissolved organic matter, the effect on apparent water sol ubility of highly insoluble solutes may be rather pronounced, but the effects would be less significant for highly water-soluble solutes. As a result, ap parent water solubilities of compounds such as D D T , some PCBs, and higher alkanes are sensitive to low levels of dissolved humic substances. Their related partition constants (such as soil sorption coefficients and bioconcen tration factors) in aqueous environments would be concomitantly decreased when the solubility enhancement effect is not taken into account. Similarly, the decreased volatility and hydrolysis rates, as noted for some sparingly soluble solutes in water containing dissolved humic sub-

American Chemical Society Library 1155 16th St, N.W. In Aquatic Humic Substances; Suffet, I., et al.; Washington» D.C* Society: 20036 Washington, DC, 1988. Advances in Chemistry; American Chemical


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0

20

40

60

80

Concentration of SSHA ( mg/L)at pH 8.5

100

Figure 9. Dependence of the apparent water solubilities of ρ,ρ'-DDT, 2,4,5,2',5'-PCB, and 2,4,4'-PCB (nght ordinate) on the concentration of San hedron soil humic acid at pH 8.5.

Table IV. Comparison of log K Values for Selected Organic Solutes 10g Kan, log Κdom > pH < 6.5 pH = 8.5 Compound SRFA SSHA SRFA SSHA ρ,ρ'-ΌΌΎ 4.13 4.63 3.62 4.55 2,4,5,2',5'-PCB 3.83 4.63 3.61 4.41 2,4,4'-PCB 3.30 4.16 2.84 3.78 dom

e

b

"SRFA, Suwannee River fulvic acid. SSHA, Sanhedron soil humic acid. fc


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stances, may be considered a result of the use of the (higher) apparent solute concentration in manipulating the data. Compounds that are more water soluble, such as toluene, 1,2,3-trichlorobenzene, and lindane, would not be sensitive to low levels of dissolved humic substances.


Effect of Temperature It is instructive to substantiate the type of interactions between dissolved humic substances and organic solutes in terms of the equilibrium enthalpy for the solutes. The enthalpy associated with the transfer of an organic solute from water to an organic phase in a partition equilibrium (ΔΗ) is usually less exothermic than the heat of condensation from water, —àH (i.e., the reverse heat of solution). Consequently, the equilibrium partition coefficient of the solute would be less sensitive to temperature than the equilibrium adsorption coefficient. U p to this time very little data has been available on the effect of temperature on the interaction of organic solutes with dissolved organic matter, from which the enthalpy value can be determined. Carter and Suffet (36) provided the only published data describing the temperature effect on the binding of D D T to dissolved aquatic- and sediment-derived humic acid. These studies showed a nearly twofold increase in the binding constant of D D T when the temperature was decreased from 25 to 9 °C. The molar enthalpy value, Δ Η , for the binding of D D T with dissolved humic acid can be calculated by the van't Hoff equation w

where Δ Η expresses the molar heat of equilibrium, R is the gas constant, K is the binding constant (i.e., K ) at temperature T (K), and K j is the binding constant at T (K). The observed K (at 282 K) and K (at 298 K) values for D D T bound to a freshwater humic acid are about 222,000 and 145,000, respectively, and the corresponding K and K values of D D T with the sediment-derived humic acid are 718,000 and 444,000, respectively. The calculated Δ Η value is approximately -20.9 kj mol" for D D T with both humic acids used in this study. _ By comparison, the heat of solution of D D T in water (AH ), as deter mined from its water solubility at 10 and 25 °C, is approximately 39 kj mol" (our unpublished results). As expected, this value is greater than the heat of fusion for D D T , which is about 25 kj mol" as reported by Plato and Glasgow (37). The observed binding of D D T to dissolved humic acids in water is therefore much less exothermic than the — A H value. Apparently, the interaction between D D T and dissolved humic substances is not me chanistically consistent with surface adsorption or other specific interaction 2

d o c

l

2

r

2

x

2

1

W

1

1

W


150


(such as complexation) because the molar heat for these processes should be greater than the — Δ Η , unless the effects are extremely weak. The observed heat effect is largely consistent with a partition interaction of D D T with dissolved humic acids according to equations 5-7. w


Source of Dissolved Humic Substances Substantial variations can occur in the solubility enhancement of a nonionic solute by dissolved humic substances from different sources because of the effects of polarity, molecular size, and configuration of the organic matter on partition interactions. Probably, the structure and composition of dis solved humic substances are largely controlled by local environmental con ditions (such as p H and climate) and by biological processes specific to the aquatic system in which they occur. Thus, humic substances from different surface water systems may display some distinct molecular characteristics. For example, humic substances derived from more acidic surface water sources appear to contain a higher percentage of oxygen (i.e., more polar groups) compared to samples from neutral waters. This point is illustrated by a comparison of the humic and fulvic acids derived from the Suwannee River, Georgia (pH 4.8), with the corresponding fractions from the Calcasieu River, Louisiana (pH 6.5). The humic and fulvic acids from the Suwannee River have oxygen contents of about 40% by weight, and the combined humic and fulvic acids from the Calcasieu River have oxygen contents of only 36%. Similarly, elemental data for aquatic humic samples from other near-neutral-pH rivers, such as the Ohio River (34) and the Missouri River (R. L . Malcolm, U . S . Geological Survey, unpublished data) give oxygen contents of about 34-36%, which is comparable to that for the Calcasieu River sample. Elemental data for various humic substances (Table I) and partition coefficients ( K ) , obtained for D D T and other solutes with humic extracts from the Suwannee and Calcasieu Rivers, substantiate the difference in composition between these stream humic substances. Chiou et al. (30) pre sented data elucidating the relation between the enhancement by these various humic fractions and their sources; results of these studies are shown in Figures 10-12 and in Table V. A plot of the apparent water solubility of D D T versus the dissolved organic carbon concentration of the Calcasieu River humic extract (a near-neutral river), the Suwannee River humic and fulvic acids (an acidic source), and whole-water samples from the Suwannee River, Georgia and the Sopchoppy River, Florida, all at p H 6.5, is presented in Figure 10. Similar plots for 2,4,5,2',5'-PCB and 2,4,4'-PCB are shown in Figures 11 and 12, respectively. The apparent solute solubility is again linear with respect to the dissolved organic carbon content, as predicted by equation 4, and the order of solute enhancements follows the same sequence as noted before ( D D T > 2,4,5,2',5'-PCB > 2,4,4'-PCB). Solubility endoc



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Concentration of Dissolved Organic Carbon(mg/L)

Figure 10. Dependence of the apparent water solubility of ρ,ρ'-DDT on the dissolved-organic-carbon concentration of selected humic materials and natural waters. {Reproduced from ref. 30. Copyright 1987 American Chemical Society.) hancement data for two commercial humic acid samples are also shown in these figures. The organic carbon-based partition coefficients (K ) are again determined from the slope (which gives S K ) and the intercept (which gives S J of the plot. K is used in these studies (rather than K ) because of the high ash content of the commercial humic materials. The calculated K values for various solute-humic-substance pairs are summarized in Table V. The substantially higher K of the Calcasieu humic-fulvic extract rel ative to the K for the Suwannee River humic and fulvic acids is in accord with the higher polarity (inferred by the carbon-oxygen ratio) of the humic doc

w

d o c

d o c

dom

doc

doc

doc


152

AQUATIC HUMIC SUBSTANCES 120 2,4,5,2',5'- PCB Ο Aldrich Humic Acid

120 H

• A 100 H


/ 5

/

ο/

80

D Ο (Λ

Φ I ο

Δ Calcasieu River Humic Extract •

Suwannee River Water

•

Sopchoppy River Water Suwannee Humic-

60

Fulvic Acid

•

c α> (0 α α

Water-Solubility Enhancement of Nonionic Organic Contaminants

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