[advances in chemistry] aquatic humic substances volume 219 (influence on fate and treatment of...

10 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 OF 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. An 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

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

132 AQUATIC HUMIC SUBSTANCES

In general, solutes that are relatively insoluble in water are most susceptible 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-tri-chloroethylidene) 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 water-insoluble 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 water-soluble 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 sparingly soluble organic solutes can be influenced by dissolved humic substances, 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'-tetra-chlorobiphenyl between river sediment and water are decreased substantially 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 Wu (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 particulate-bound 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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10. KILE & CHIOU Water-Solubility Enhancement 133

These studies demonstrate that interactions of many organic contaminants with naturally occurring dissolved humic substances can lead to significant 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 assessments.

Theoretical Considerations An 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" 1 ) 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 1 ) 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" 1 ) 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.

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 accomplished 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 considerably less than that of the octanol in water. Also, the pronounced sol-

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ubility enhancement of a solute such as DDT 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 + X C 0 (1)

where S * is the apparent solubility of the solute in water containing dissolved organic matter (at concentration X ) , S w is the solubility of the solute in pure water, and C 0 is the mass of solute partitioned into a unit mass of dissolved organic matter. In accordance with this model, the solute partition coefficient ( K d o m ) between dissolved organic matter and pure water can be defined as

Q d̂om — 7~ (2)

Substituting equation 2 into equation 1 gives

S* = S w ( l + XKtJ (3)

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

S* = S w ( l + X K a J (4)

where Kdoc 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 d o m (or X K d o c ) 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 d o m or K d o c ) is a function of the physical properties of the solute and the nature of the dissolved organic matter. At a given concentration of dissolved organic matter (X), the product term XKdom (or X K d o c ) should increase with decreasing water solubility for a series of solutes (or with increasing octanol-water partition coefficients) (IT). The K d o m 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 w K d o m

and an intercept equal to S w (equation 3).

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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

Δ Η = Δ Η 0 - â ï f w (5)

where Δ Η σ is the molar heat of solution in the organic phase and A H W is the molar heat of solution in water (i.e., — A H W is the molar heat of condensation from water). A nonionic organic solute with a low water solubility usually gives a high (positive) AHW because of its high incompatibility with water. Conversely, the corresponding Δ Η 0 should be much smaller because of the greater compatibility of an organic solute with the organic solvent. Both Δ Η 0 and A H W 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 ( — AHW). 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 interactions 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 Γ), which leads to a high heat of solution. The heat of solution of a solid compound in an organic phase can be expressed as

Δ Η 0 = Δ Η Γ + Δ Η ? (6)

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

AHW = àHf + Δ Η 5? (7)

where Δ Η 6 Χ represents the excess (molar) heat of solution of the supercooled liquid (29). The molar heat of fusion cancels in_a partition equilibrium (equation 5) because it is the same in both the Δ Η 0 and àHw 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 substances, in terms of the properties of solutes and the structural features of humic molecules.

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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) horizon 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 d o m ) for the pairs of dissolved humic substances and solutes were determined (according to equation 3) by plotting the apparent solubility (S*) against the concentration of the dissolved humic substance. The slope gives S w K d o m , and the intercept gives S w . The results are shown in Figures 1 through 4. Except as noted, all the solubility-enhancement experiments were conducted at p H 6.5 or below, and at a temperature of 24 ± 1 °C. The observed partition coefficients (i.e., Kdom or K d o c 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 d o m 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 o w 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 1,2,3-trichlorobenzene 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.

A comparison of the data in Figures 1 through 4 shows that the Sanhedron 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 interaction would be more effective with a large-sized macromolecule that contains regions of large nonpolar volumes. The greater enhancement for the

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Tabl

e I.

Ash

-Fre

e E

lem

enta

l C

onte

nts o

f Var

ious

Hum

ic a

nd F

ulvi

c A

cids

S

ampl

e C

H

0

Ν

S Ρ

Tot

al

Ash

Sa

nhed

ron

soil

hum

ic a

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58.0

3 3.

64

33.5

9 3.

26

0.47

0.

10

99.0

9 1.

19

Sanh

edro

n so

il fu

lvic

aci

de 48

.71

4.36

43

.35

2.77

0.

81

0.59

10

0.59

2.

25

Suw

anne

e R

iver

hum

ic a

cid3

54.2

2 4.

14

39.0

0 1.

21

0.82

0.

01

99.4

0 3.

18

Suw

anne

e R

iver

fulv

ic a

cide

53.7

8 4.

24

40.2

8 0.

65

0.60

0.

01

99.5

6 0.

68

Ald

rich

hum

ic a

cid,

sodi

um

99.5

6 0.

68

salt

(lot n

o. 1

204

PE

, 198

4)*

69.4

2 5.

04

39.2

9 0.

75

4.25

0.

15

118.

9 31

.0

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a-T

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m h

umic

aci

d 31

.0

(lot

no.

1591

2811

5, 19

74)*

65

.79

5.51

37

.79

0.71

3.

16

<0.

05

113.

0 32

.8

Cal

casie

u R

iver

hum

ic e

xtra

ct*

56.6

8 4.

69

35.7

2 1.

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0.64

—

98

.87

3.63

NO

TE; V

alue

s ar

e pe

rcen

ts o

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oistu

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asis.

"E

lemen

tal d

ata

are f

rom

ref.

18.

^Elem

enta

l dat

a ar

e fro

m re

f. 30

.

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Table II. Intrinsic Water Solubilities of Selected Organic Solutes

Water Solubility Temperature Compound (°C) ρ,ρ'-ΌΌΎ 5.5 x ΙΟ"3 25 2,4,5,2',5'-PCB 1.0 x io-2 24 2,4,4'-PCB 0.115 20 1,2,3-Trichlorobenzene 16.3 23 Lindane 7.80 25 NOTE: Cited in ref. 18.

Single Solutes with SSHA • ρ,ρ' - DDT (24°C) • 2,4,5,2',5' - PCB (24°C) A 2,4,4' - PCB (24°C)

_L

Binary Solutes with SSHA • 2,4,5,2',5' - PCB (25°C) Δ 2,4,4' - PCB (25°C)

_L 20 40 60 80 Concentration of SSHA(mg/L)

100

Figure 1. Dependence of the apparent water solubilities of p,p'-DDT, 2,4,5,2',5'-PCB, and 2,4,4'-PCB on the concentration of Sanhedron soil humic acid at pH 6.5. (Reproduced from ref. 18. Copyright 1986 American

Chemical Society.)

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200

3 .

.Ω J3 Ο

if) 0)

ο (/) c ω Q. α <

Concentration of SSFA (mg/L) Figure 2. Dependence of the apparent water solubilities of p,p'-DDT, 2,4,5,2',5'-PCB, and 2,4,4'-PCB on the concentration of Sanhedron soil fulvic acid at pH 6.5. (Reproduced from ref 18. Copyright 1986 American

Chemical Society.)

soil-derived humic acid thus may be attributed to its higher molecular weight and lower polar-group content (based on its lower oxygen content) relative to the other humic substances tested. Humic substances are highly poly-disperse; molecular weights from 2600 to more than 1 million have been reported for soil humic acids that were fractionated by gel chromatography and filtration through graded porosity membranes (31). Molecular weights for aquatic humic acids have been quoted in the range of 1000-10,000, while molecular weights for aquatic fulvic acids have been quoted in the range of 500-2000 (32). The proposed molecular weight of Suwannee River fulvic acid is approximately 800 (33).

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Thus, from the standpoint of molecular size and polarity, soil-derived humic acids should provide larger regions of nonpolar intramolecular environment that can more effectively promote the partition interactions with nonionic organic solutes. The enhancement effect caused by the low-molecular-weight phenylethanoic acid as dissolved organic matter is quite small on a unit weight basis of the dissolved material (Figure 6). This fact illustrates the inability of a small dissolved organic molecule to effectively promote a partitionlike interaction, regardless of its polarity. These results also indicate that such solubility enhancements are not caused by specific interactions of

200

_D Ο

if) 0)

4-» _D Ο

if) 4-» c CD CD Q. Q.

<

150h

100 F

15

10i

2,4,4' - PCB (25°C)

2,4,5,2',5' - PCB (25°C)

ρ,ρ' - DDT (24°C)

o « 1 I I I I 0 20 40 60 80 100

Concentration of SRHA (mg/L)

Figure 3. 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 River humic acid at pH 4.1 to 5.5. (Reproduced from ref 18. Copyright 1986 Amer

ican Chemical Society.)

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10

σι 5 ε

• · I I

Lindane (25°C)

I I

•

I

W

I

η Ο CO ω j5 ο CO c CO α α <

20 40 60 80

Concentration of SRFA(mg/L) 100

Figure 4. Dependence of the apparent water solubilities of ρ,ρ'-DDT, 2,4,5,2',5'-PCB, 2,4,4'-PCB, and lindane on the concentration of Suwannee

River fulvic acid at pH 3.9 to 5.7. (Adapted from ref 18.)

solutes with polar groups of the dissolved humic molecules. If they were caused by specific interactions, then phenylethanoic acid would most likely be as effective as humic substances for promoting the solubility of D D T .

Given that the molecular sizes of dissolved organic substances are sufficiently large (compared to the solute molecules), their efficiencies in enhancing solute solubility by partition effect would be a function of their polarities, configurations, and conformations. This point is illustrated by the

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Table III. Solute Octanol-Water Partition Coefficients (log Ο and

Compound logK*/ log Kdnn/hg KdJ

Compound logK*/ SSHAC SSFAd SRHAe SRFAf

ρ,ρ'-ΌΌΎ 6.36 4.82/5.06 4.27/4.58 4.12/4.39 4.13/4.40 2,4,5,2',5'-PCB 6.11 4.63/4.87 3.81/4.12 3.80/4.07 3.83/4.10 2,4,4'-PCB 5.62 4.16/4.40 3.58/3.89 3.27/3.54 3.30/3.57 l,2,3-Trichlorobenzeneg 4.14 -2.8/3.0 -2.0/2.3 -1.7/2.0 -1.7/2.0 Lindane8 3.70 -2.5/2.7 -1.5/1.8 -1.2/1.5 -1.2/1.5

"Values from ref. 18. feData from ref. 18, pH <6.5. CSSHA, Sanhedron soil humic acid. dSSFA, Sanhedron soil fulvic acid. eSRHA, Suwannee River humic acid. ^RFA, Suwannee River fulvic acid. approximate log Kdon,/log Kdoc values for 1,2,3-trichlorobenzene and lindane are obtained by linear extrapolations of the plot of log Kdom/log Kdoc versus the log K o w of ρ,ρ'-ΌΌΎ, 2,4,5,2',5'-PCB, and 2,4,4'-PCB.

25

— 20 C D

Ε

> IS 5 15 ο

(f)

Φ 4-»

_3 Ο

( / ) 4-* C φ CD Q. Q.

<

10

-4-

1,2,3 - Trichlorobenzene (25°C)

Lindane <24°C)

JL 20 40 60 80

Concentration of SSHA (mg/L) 100

Figure 5. Dependence of the apparent water solubilities of lindane and 1,2,3-trichlorobenzene on the concentration of Sanhedron soil humic acid at pH 6.5.

(Reproduced from ref. 18. Copyright 1986 American Chemical Society.)

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comparable enhancements caused by the Suwannee River fulvic acid and the Suwannee River humic acid (Figures 3 and 4), even though differences in their molecular weights are probably large. The correspondence of the enhancement effects by these two acids is consistent with their comparable carbon-oxygen ratios, as indicated by elemental analyses. Thus, although molecular size is important, polarity, configuration, and conformation appear to be the controlling factors for high-molecular-weight dissolved humic substances. The much larger enhancement effect observed for the Sanhedron soil humic acid relative to the enhancement for aquatic humic substances probably results more from its fewer polar groups per unit weight and certain molecular configurations than from its larger molecular size.

The importance of molecular configuration is illustrated by the observation that poly(acrylic acid) is unable to promote enhancement of solute solubility (Figure 7), although its molecular weight (ranging from 2000 to 90,000; data for M W 90,000 are not shown) is sufficiently large, and that its composition (C = 50%, Ο = 44.4%, Η = 5.6%) is comparable to that of some humic substances. The ineffectiveness of poly(acrylic acid) in promoting the solubility enhancement of a solute appears to be related to its non-branched structure and regular attachment of the polar carboxyl groups to the carbon chain, which do not permit the formation of a sizable nonpolar environment required for solute partitioning. Similarly, poly(ethylene glycol), with molecular weights ranging from 200 to 3400, exhibited no noticeable solubility enhancement of D D T at ambient p H values (our unpublished results).

The effect of p H on solute-solubility enhancement, while probably not very significant in most natural aquatic environments, nevertheless provides useful information on the mechanism of the interaction between solute and dissolved humic substance. This effect can be explained in terms of the influence of p H on the structure and conformation of the humic macromolecule. At an alkaline p H , the hydration of the dissociated acidic groups and the concomitant repulsion between ionized functional groups would lead to more extended molecular conformations and thus decrease the intensity of the nonpolar environment. The data presented in Figure 8 for D D T and two PCBs with Suwannee River fulvic acid show a substantial decrease of solute-solubility enhancement at a p H of 8.5, relative to that reported earlier by Chiou et al. (18) at p H values of 4.0-6.5. The calculated Kdom values at p H 8.5 for D D T , 2,4,5,2' ,5 '-PCB, and 2,4,4'-PCB are 31%, 61%, and 35%, respectively, of the corresponding Kdom values obtained at the lower p H . A similar decline in the Kdom of solutes with the Sanhedron soil humic acid was noted at p H 8.5, in which the observed Kdom values for D D T , 2,4,5,2 ' ,5 / -PCB, and 2,4,4'-PCB were decreased by 54%, 61%, and 42%, respectively (Figure 9 and Table IV). Carter and Suffet (4) reported a similar decrease in the binding constant of D D T with dissolved humic acid when the p H of the solution was increased from 6.0 to 9.2. Thus, the effect of p H

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on nonionic organic solute-solubility enhancement appears to be more significant in the alkaline pH range.

The effectiveness of certain dissolved humic macromolecules as a microscopic organic medium for the partitioning of relatively nonpolar organic solutes leads one to speculate that humic molecules contain sizable regions that are predominantly nonpolar. Stream fulvic acids with relatively high oxygen contents would have a more limited distribution of such nonpolar regions. This point is substantiated by nuclear magnetic resonance (NMR) studies that show a higher degree of aromaticity for soil humic and fulvic acids than for stream humic substances (34). Molecules such as stream fulvic acids would be likely to assume more extended (linear) conformations because of the hydrophilicity of the polar functional groups and the mutual repulsion of the ionized groups. Presumably, the presence of large regions

Figure 6. Effect of phenylethanoic acid at ambient pH values (pH 3.7 to 4.1) on the water solubilities of ρ,ρ'-DDT, lindane, and 1,2,3-tnchlorobenzene.

(Reproduced from ref. 18. Copyright 1986 American Chemical Society.)

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of relatively nonpolar moieties (such as branched aliphatic and aromatic structures) in humic molecules would more effectively promote partition interactions of humic substances with (nonpolar) organic solutes by London forces.

Solubility enhancement of various nonionic organic solutes by dissolved organic matter is significant only for solutes whose intrinsic water solubilities are at least 2 orders of magnitude lower than concentrations of dissolved organic matter (35). This dependence of the enhancement effect on solute solubility is characteristic of a partition equilibrium. Therefore, at a given

20

15 150

ι I ι

} }

1

τ Τ • I I

1,2,3 - Trichlorobenzene (24°C) 1 1 1

ϊ ϊ

15h

R- σ> ιοί

5*

t

f

J L

-A \ |L-2,4,4' - PCB (24°C)

2,4,5,2',5' - PCB (24°C)

— ί — ρ,ρ' - DDT (23°C)

_J I 20 40 60 80

Concentration of Polyacrylic Acid (mg/L)

* — i

100

Figure 7. Effect of poly(acrylic acid) (MW 2000) at ambient pH values (pH 4.1 to 4.7) on the water solubilities of ρ,ρ'-DDT, 2,4,5,2',5'-PCB, 2,4,4'-PCB, and 1,2,3-trichlorobenzene. (Reproduced from ref 18. Copyright 1986 Amer

ican Chemical Society.)

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200

150

> 100 h

Β 15

10'

2,4.4'-PCB (25°C)

2,4,5,2',5'-PCB(25°C)

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 River fulvic

acid at pH 8,5.

concentration of dissolved organic matter, the effect on apparent water solubility of highly insoluble solutes may be rather pronounced, but the effects would be less significant for highly water-soluble solutes. As a result, apparent 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 bioconcentration 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. Washington» D.C* 20036

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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 Kdom Values for Selected Organic Solutes

Compound

log Κ pH <

dom > 6.5

10g Kan, pH = 8.5

Compound SRFAe SSHAb 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 "SRFA, Suwannee River fulvic acid. fcSSHA, Sanhedron soil humic acid.

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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, —àHw (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. Up 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

where Δ Η expresses the molar heat of equilibrium, R is the gas constant, K2 is the binding constant (i.e., K d o c ) at temperature T2 (K), and Kj is the binding constant at Tl (K). The observed K r (at 282 K) and K2 (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 Kx and K2 values of D D T with the sediment-derived humic acid are 718,000 and 444,000, respectively. The calculated ΔΗ value is approximately -20.9 kj mol" 1 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 W ) , as determined from its water solubility at 10 and 25 °C, is approximately 39 kj mol" 1

(our unpublished results). As expected, this value is greater than the heat of fusion for D D T , which is about 25 kj mol" 1 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 W value. Apparently, the interaction between D D T and dissolved humic substances is not mechanistically consistent with surface adsorption or other specific interaction

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(such as complexation) because the molar heat for these processes should be greater than the — Δ Η w , 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.

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 dissolved humic substances are largely controlled by local environmental conditions (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 d o c ) , 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) presented 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 (DDT > 2,4,5,2',5'-PCB > 2,4,4'-PCB). Solubility en-

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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 d o c ) are again determined from the slope (which gives S w K d o c ) and the intercept (which gives S J of the plot. K d o c is used in these studies (rather than Kdom) because of the high ash content of the commercial humic materials. The calculated Kdoc values for various solute-humic-substance pairs are summarized in Table V.

The substantially higher Kdoc of the Calcasieu humic-fulvic extract relative to the Kdoc for the Suwannee River humic and fulvic acids is in accord with the higher polarity (inferred by the carbon-oxygen ratio) of the humic

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120

120 H

100 H

5 80 D Ο

( Λ

Φ I 60 ο

</>

c α> (0 α α <

40 Η

20 Η

2,4,5,2',5'- PCB

Ο Aldrich Humic Acid

• Fluka-Tridom A Humic Acid

/ Δ Calcasieu River / Humic Extract

ο/ • Suwannee River Water

• Sopchoppy River Water Suwannee Humic-Fulvic Acid

• Ο ρ / Ό ο ρ

10 20 30 40 50 60


Figure 11. Dependence of the apparent water solubility of 2,4,5,2',5'-PCB on the dissolved-organic-carbon concentration of selected humic materials and natural waters. (Reproduced from ref. 30. Copyright 1987 American

Chemical Society.)

substances derived from the more acidic water of the Suwannee River. Similarly, the slightly smaller solubility-enhancement effect with the Calcasieu humic extract, in comparison to the soil-derived humic acid, is in agreement with the greater oxygen and smaller carbon contents of the sample. The Kdoc values of solutes with natural whole-water samples (Sopchoppy River and Suwannee River) are identical to those with the humic and fulvic acids from the Suwannee water, a result suggesting that the humic- and fulvic-acid fractions, which are usually the predominant dissolved organic matter components in natural waters, are primarily responsible for the observed solubility enhancement. On the basis of these results, it may be concluded that differences in water-solubility enhancements by aquatic

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450

400 H

* 350

Ô 300 ^

250 H

200

150

100

2,4,4'-PCB

• Fluka-Tridom Humic Acid

Δ Calcasieu River Humic Extract

• Suwannee River Water

• Sopchoppy River Water Suwannee Humic-Fulvic Acid

Δ ^ -• /

Ψ /

10 20 30 40 50


60

Figure 12. Dependence of the apparent water solubility of 2,4,4'-PCB on the dissolved-organic-carbon concentration of selected humic materials and natural waters. (Reproduced from ref 30. Copyright 1987 American

Chemical Society.)

Table V. Comparison of log Kdœ Values log Kdœ

Dissolved Organic Carbon p,p'-DDT 2,4,5,2',5'-PCB 2,4,4'-PCB Aldrich humic acid sodium salt 5.56 5.41 — Fluka-Tridom humic acid 5.56 5.41 4.84 Calcasieu River humic extract 4.93 4.81 4.24 Suwannee River water sample 4.39 4.09 3.53 Sopchoppy River water sample 4.39 4.01 3.57 SOURCE: Data from ref. 30.

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humic substances are largely related to the composition of the humic molecules.

Commercial Humic Acids as Models Because the solubility enhancement of a solute depends on the type and composition of the dissolved organic matter, it is pertinent to evaluate the appropriateness of some commercial humic acids as models for aquatic humic substances. This evaluation is particularly important because certain conclusions reached in previous studies (4, 5, 8, 9, 14-16) were based on experiments that used such commercial materials as models for dissolved humic substances.

Malcolm and MacCarthy (34) gave a critical evaluation of the use of these materials as analogs of naturally occurring humic substances, based on analyses of elemental composition, IR spectra, and 1 3 C - N M R data. Some compositional and structural features of the commercial humic acids were shown to be significantly different from those of the humic and fulvic acids isolated from natural soil and freshwater sources. Such analyses suggest that the commercial humic samples are not representative of normal aquatic or soil humic substances. One of the important findings in that study was the low carboxyl and carbohydrate contents of the commercial (Aldrich and Fluka-Tridom) humic samples, which would presumably make the molecule more effective in enhancing the water solubility of sparingly soluble organic solutes. Solubility enhancement data obtained with these two commercial humic acids and several humic extracts of stream origin (Figures 10-12) confirm this expectation. Solutions of the commercial humic acids in salt form were filtered through a 0.45-μπι silver membrane filter to remove suspended particulate matter. Results of the solubility studies were expressed in terms of dissolved organic carbon because of the high ash contents of the commercial humic acids. The calculated partition coefficients (Kdoc) are presented in Table V. These results show a markedly greater enhancement effect with both Aldrich and Fluka-Tridom samples, compared to both soil-derived and stream-derived humic and fulvic acids.

The fact that these two commercial samples exhibit virtually identical enhancement effects supports the earlier contention by Malcolm and MacCarthy (34) that the samples are essentially identical in nature (and may have come from the same source). As shown in Table V, the Kdoc values obtained with the commercial humic acids are about 3 times as large as those for the soil-derived humic acid (Table III), approximately 4 times as large as those of the Calcasieu River humic extract, and about 20 times larger than the values for two acidic water samples and the Suwannee River humic and fulvic acids. Similar results with Aldrich humic acid have been reported by others. For example, Carter and Suffet (4) gave a log Kdoc of 5.61-5.74 for DDT, and Hassett and Milicic (8) gave a log of 4.86 for 2,5,2',5'-PCB.

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The stronger enhancement effect shown by these commercial materials may be attributed to their lower polarity and possibly to their higher molecular weights, as well as to some unique molecular structures. The reported high oxygen contents of commercial humic acids appear to contradict the observed large solubility-enhancement effects. However, it has been suggested that the oxygen content for high-ash commercial samples is subject to substantial error, presumably because the oxygen associated with ash is also counted into the elemental oxygen content (34). In support of this speculation, a purified ( H + form) low-ash Aldrich humic acid shows an elemental content of C = 63.31%, H = 5.94%, Ο = 25.05%, Ν = 0.51%, S = 3.36%, and Ρ < 0.05% on an ash-free and moisture-free basis, when the ash content is decreased to 4.46% (34).

On the basis of the preceding solubility-enhancement data, the commercial samples cannot be treated as analogs of normal water and soil humic substances. Their use may lead to an improper estimation of the behavior of organic solutes in natural waters.

Summary

An evaluation of the interaction mechanism between dissolved humic substances and nonionic organic contaminants is based on the intrinsic properties of humic macromolecules and solutes. This interaction, as manifested in apparent water-solubility enhancements of relatively water-insoluble organic solutes, can best be described by a partition interaction. The assumed partition mechanism effectively explains the solubility-enhancement data and other modifications of solute behavior, which influence the transport and fate of pollutants in natural aquatic systems. The interaction characteristics for nonionic organic solutes with dissolved humic substances are similar to those found with bulk soil organic matter, a result indicating that a common partition mechanism is involved for both systems. However, the effectiveness of the partition interaction with dissolved humic substances appears to be more limited in magnitude than that for the bulk soil organic matter because of the relatively smaller organic environment and the normally more polar nature of the dissolved organic matter.

At a given concentration of dissolved humic substances, the observed solubility enhancement is significantly greater for highly water-insoluble compounds, such as D D T and 2,4,5,2' ,5'-PCB. The effect decreases with increasing water solubility of the solute. In general, for a solute to show a significant solubility enhancement, the concentration of the dissolved organic matter has to be more than 2 orders of magnitude greater than the intrinsic water solubility of the solute. Critical properties of the dissolved organic matter for enhancing the solubility of the solute are polarity, molecular size, configuration, and conformation. The composition of the humic molecule

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(and therefore the enhancement effect) is related to the source and environment from which the humic substance originated. The soil-derived humic acid is generally more effective in enhancing solute solubility than stream-derived humic substances.

Commercial humic acids enhance the solubility of nonionic organic compounds to a significantly greater extent than soil-derived and freshwater humic substances. These commercial materials are on the order of 3 times more effective than a soil-derived humic acid, and from 4 to 20 times more effective than stream-derived humic samples. The greater enhancement effects of the commercial humic acids appear to be related to their low polar-group contents, and possibly to other molecular properties such as molecular sizes and structures. Use of some commercial humic samples as substitutes for naturally occurring humic substances should be regarded with caution, as their use can lead to gross overestimation of the impact of dissolved organic matter on the solubility of nonionic compounds.

Many observed modifications of the behavior of sparingly soluble organic solutes in the presence of dissolved organic matter (such as hydrolysis, bioaccumulation factor, and soil-water sorption coefficient) appear to be mainly consequences of the increase in the solute's apparent water solubility.

References 1. Wershaw, R. L.; Burcar, P. J.; Goldberg, M. C. Environ. Sci. Technol. 1969,

3, 271-273. 2. Poirrier, Μ. Α.; Bordelon, B. R.; Laseter, J. L. Environ. Sci. Technol. 1972, 6,

1033-1035. 3. Boehm, P. D.; Quinn, J. G. Geochim. Cosmochim. Acta 1973, 33, 2459-2477. 4. Carter, C. W.; Suffet, I. H. Environ. Sci. Technol. 1982, 16, 735-740. 5. Landrum, P. F.; Nihart, S. R.; Eadie, B. J.; Gardner, W. S. Environ. Sci.

Technol. 1984, 18, 187-192. 6. Hassett, J. P.; Anderson, M. A. Environ. Sci. Technol. 1979, 13, 1526-1629. 7. Griffin, R. Α.; Chian, E. S. K. Attenuation of water-soluble polychlorinated

biphenyl by earth materials; U.S. Environmental Protection Agency: Cincinnati, 1980; EPA-600/2-80-028.

8. Hassett, J. P.; Milicic, E. Environ. Sci. Technol. 1985, 19, 638-643. 9. Perdue, E. M.; Wolfe, N. L. Environ. Sci. Technol. 1982, 16, 847-852.

10. Hassett, J. P.; Anderson, M. A. Water Res. 1982, 16, 681-686. 11. Caron, G.; Suffet, I. H.; Belton, T. Chemosphere 1985, 14, 993-1000. 12. Gschwend, P. M.; Wu, S.-C. Environ. Sci. Technol. 1985, 19, 90-96. 13. Leversee, G. J.; Landrum, P. F.; Giesy, J. P.; Fannin, T. Can. J. Fish. Aquat.

Sci. 1983, 40(Suppl. 2), 63-69. 14. McCarthy, J. F. Environ. Contam. Toxicol. 1983, 12, 559-568. 15. McCarthy, J. F.; Jimenez, B. D. Environ. Sci. Technol. 1985, 19, 1072-1076. 16. Haas, C. M.; Kaplan, Β. M. Environ. Sci. Technol. 1985, 19, 643-645. 17. Chiou, C. T.; Schmedding, D. W.; Manes, M. Environ. Sci. Technol. 1982, 17,

227-231. 18. Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Environ. Sci. Technol.

1986, 20, 502-508.

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19. McBain, M. E. L.; Hutchinson, E. Solubilization and Related Phenomena; Academic Press: New York, 1955.

20. Elworthy, P. E.; Florence, A. T.; McFarlane, C. B. Solubilization by Surface Active Agents; Chapman and Hall: London, 1968.

21. Tanford, C. The Hydrophobic Effect; Wiley: New York, 1973. 22. Nagarajan, R.; Ruckenstein, E. In Surfactants in Solution; Mittal, K. L.; Lind

man, B., Eds.; Plenum: New York, 1984; Vol. 2, pp 923-947. 23. Moroi, Y.; Sata, K.; Noma, H.; Matuura, R. In Surfactants in Solution; Mittal,

K. L.; Lindman, B., Eds.; Plenum: New York, 1984; Vol. 2, pp 963-979. 24. Chiou, C. T.; Peters, L. J.; Freed, V. H. Science (Washington, DC) 1979, 206,

831-832. 25. Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983,

17, 227-231. 26. Chiou, C. T.; Shoup, T. D.; Porter, P. E. Org. Geochem. 1985, 8, 9-14. 27. Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241-248. 28. Means, J. C.; Wood, S. G.; Hassett, J. J.; Banwart, W. L. Environ. Sci. Technol.

1980, 14, 1524-1528. 29. Chiou, C. T. In Hazard Assessment of Chemicals: Current Developments; Sax

ena, J.; Fisher, F., Eds.; Academic Press: New York, 1981; pp 117-153. 30. Chiou, C. T.; Kile, D. E.; Brinton, T. I.; Malcolm, R. L.; Leenheer, J. Α.;

MacCarthy, P. Environ. Sci. Technol. 1987, 21, 1231-1234. 31. Cameron, R. S.; Thornton, Β. K.; Swift, R. S.; Posner, A. M. J. Soil Sci. 1972,

23, 394-408. 32. Thurman, E. M.; Wershaw, R. L.; Malcolm, R. L.; Pinckney, D. Org. Geochem.

1982, 4, 27-35. 33. Aiken, G. R.; Brown, P.; Noyes, T.; Pinckney, D. In Humic Substances in the

Suwannee River, Florida and Georgia: Interactions, Properties, and Proposed Structures; Averett, R. C., Ed.; U.S. Geological Survey: Reston, VA, in review; Water Supply Paper 1507-B.

34. Malcolm, R. L.; MacCarthy, P. Environ. Sci. Technol. 1986, 20, 904-911. 35. Chiou, C. T.; Porter, P. E.; Shoup, T. D. Environ. Sci. Technol. 1984, 18,

295-297. 36. Carter, C. W.; Suffet, I. H. In Fate of Chemicals in the Environment; Swann,

R. L.; Eschenroeder, A., Eds.; ACS Symposium Series 225; American Chemical Society: Washington, DC, 1983; pp 215-229.

37. Plato, C.; Glasgow, R. Anal. Chem. 1969, 41, 330-336.

RECEIVED for review July 24, 1987. ACCEPTED for publication December 21, 1987.

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[advances in chemistry] aquatic humic substances volume 219 (influence on fate and treatment of...

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