Environ. Scl. Technol. 1987, 21, 1231-1234
Grosjean, D.; Van Cauwenberghe, K.; Fitz, D. R.; Pitts, J. N., Jr. Prepr. Pap., Natl. Meet.-Am. Chem. Soc., Diu. Environ. Chem. 1978,18, 354-356. Gery, M. W.; Fox, D. L.; Jeffries, H. E.; Stockburger, L.; Weathers, W. S. Znt. J. Chem. Kinet. 1985,17,931-955. Gery, M. W.; Fox, D. L.; Kamens, R. M.; Stockburger,L. Environ. Sei. Technol. 1987, 21, 339-348. Warren, D. R.; Seinfeld, J. H. Aerosol Sci. Technol. 1984, 3, 135-153. Warren, D. R.; Seinfeld, J. H. J. Colloid Interface Sei. 1985, 105, 136-142. Crump, J. G.; Seinfeld, J. H. J. Aerosol Sci. 1981, 12, 405-415.
McMurry, P. H.; Rader, D. J. Aerosol Sei. Technol. 1985, 4, 249-268.
(32) Grosjean, D.; Fung, K. J. Air Pollut. Control Assoc. 1984, 34, 537-543. (33) Friedlander,S. K. In Smoke, Dust and Haze; Wiley: New York, 1977. (34) Okuyama, K.; Kousaka, Y.; Warren, D. R.; Flagan, R. C.; Seinfeld, J. H. Aerosol Sei. Technol. 1987, 6, 15-27. (35) Warren, D. R.; Okuyama, K.; Kousaka, Y.; Seinfeld, J. H.; Flagan, R. C. J. Colloid Interface Sci. 1987,116,563-581. (36) Stern, J. E.; Wu, J. J.; Flagan, R. C.; Seinfeld, J. H. J. Colloid Interface Sci. 1986, 110, 533-543.
Received for review March 6,1987. Revised manuscript received August 7, 1987. Accepted September 8, 1987. This work was supported by the Coordinating Research Council,Project AP-6.
A Comparison of Water Solubility Enhancements of Organic Solutes by Aquatic Humic Materials and Commercial Humic Acids Cary 1. Chiou,” Daniel E. Klle, Terry I. Brlnton, Ronald L. Malcolm, and Jerry A. Leenheer US. Geological Survey, Denver Federal Center, Denver, Colorado 80225
Patrlck MacCarthy Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 8040 1
Water solubility enhancements of 1,l-bis(p-chlorophenyl)-2,2,2-trichloroethane(p,p’-DDT), 2,4,5,2’,5’pentachlorobiphenyl (2,4,5,2’,5’-PCB), and 2,4,4’-trichlorobiphenyl (2,4,4’-PCB) by dissolved organic matter have been studied with the following samples: (1)acidic water samples from the Suwannee River, Georgia, and the Sopchoppy River, Florida; (2) a humic extract of a nearly neutral pH water from the Calcasieu River, Louisiana; (3) commercial humic acids from the Aldrich Chemical Co. and Fluka-Tridom Chemical Corp. The calculated partition coefficients on a dissolved organic carbon basis (Kdoc) for organic solutes with water samples and aquatic humic extracts from this and earlier studies indicate that the enhancement effect varies with the molecular composition of the aquatic humic materials. The Kd, values with water and aquatic humic samples are, however, far less than the observed Kdocvalues obtained with the two commercial samples, by factors of about 4-20. In view of this finding, one should be cautious in interpreting the effects of the dissolved organic matter on solubility enhancement of organic solutes on the basis of the use of commercial humic acids.
Introduction One important consequence of dissolved organic matter (DOM) in natural waters is the fact that this DOM can significantly enhance the apparent water solubility of some otherwise extremely insoluble organic solutes. As a result, there has been a great deal of interest in both the mechanism that enhances water solubility of the solute and the magnitude of such a solubility enhancement caused by various types of DOM in order to determine potential effects of the organic solutes in different aquatic environments (1-11). In an earlier study ( l l ) we , considered the mechanism for the water solubility enhancement of nonionic organic solutes by DOM of soil and aquatic origins. Such enhancement effects were effectively explained in terms of a partitionlike interaction of solutes with dissolved high molecular weight humic materials on the basis of the properties of the solutes and humic materials. The ob-
served solubility enhancement of the solute by DOM can be expressed by Sw* = s,(l -k XKdom)
(1)
or alternatively on a dissolved organic carbon (DOC) basis by (2) s w * = sw(l + xKdoc) in which S*, and S, are the apparent water solubility in the organic matter solution and the solubility in pure water, respectively. X expresses the concentration of DOM or DOC (in grams per milliliter of water), Kdom,the partition coefficient between DOM and water, and KdOc,the partition coefficient based on DOC. The difference in values of Kdom(or &oc) for a solute with different types of fractionated humic materials has been explained in terms of the polarity, molecular size, and molecular configuration of the humic species (11). While the effect of molecular configuration (structure) is subject to more detailed investigations, the polarity of the dissolved high molecular weight humic materials (based on elemental data analysis) gives a reasonable estimate of the relative enhancing effects among humic extracts. Conceivably, the compositions and structures of humic materials in different aquatic systems can be significantly different because of such environmental factors as water pH, biological processes, and the presence of other chemical species that affect the concentration (solubility) of humic materials. In more acidic streams or rivers, there appears to be a tendency for the humic material to contain a larger percentage of oxygen compared to samples from a neutral or basic water. For instance, the oxygen content of the humic and fulvic acid extracts from the Suwannee River (pH 4.8) in Georgia is about 40%, whereas the oxygen content of the combined humic-fulvic extract from the Calcasieu River (pH 6.5) in Louisiana is only about 36 %. A decrease in oxygen content of the humic materials from acidic to neutral water can also be accompanied by an increase in carbon content (e.g., about 54% carbon for the Suwannee River extracts versus 57% carbon for the Calcasieu River extract). Elemental data for aquatic humic
Not subject to U S . Copyright. Published 1987 by the American Chemical Society
Environ. Sci. Technoi., Vol. 21, No. 12, 1987
1231
Table I. Ash-Free Elemental Contents of Commercial Humic Acids and Aquatic Humic Extracts from the Calcasieu River and the Suwannee River (in Percent on Moisture-Free Basis) sample
C
H
0
N
S
P
total
ash
Aldrich humic acid, sodium salt (lot 1204PE, 1984) Fluka-Tridom humic acid (lot 159128115, 1974) Sanhedron soil humic acid” Calcasieu River humic extract Suwannee River humic acid” Suwannee River fulvic acid“
69.42 65.79 58.03 56.68 54.22 53.78
5.04 5.51 3.64 4.69 4.14 4.24
39.39 37.79 33.59 35.72 39.00 40.28
0.75 0.71 3.26 1.14 1.21 0.65
4.25 3.16 0.47 0.64 0.82 0.60
0.15 0.05 0.10
118.9 113.0 99.09 98.87 99.40 99.56
31.0 32.8 1.19 3.63 3.18 0.68
0.01 0.01
‘Elemental data from ref 11.
samples from other nearly neutral pH rivers such as the Ohio River (12) and Missouri River (R. L. Malcolm, U.S. Geological Survey, unpublished results) indicate oxygen contents of about 34-36%, similar to that found for the Calcasieu River sample. With this consideration, it is of interest to investigate the enhancing effects on organic solute solubility by aquatic humic materials with different molecular compositions to establish the range of the effect appropriate for these humic materials. In addition to quantifying the differences in solubility enhancement by various types of aquatic humic materials, it is also of interest to compare the enhancing effects by aquatic humic materials and some commercial “humic acids”,as the latter have been frequently used as analogues of aquatic humic materials in similar studies (a comprehensive review of earlier work with commercial humic acids is given in ref 12). As pointed out by Malcolm and MacCarthy (12), the commercial humic acids appear to have very different molecular characteristics compared to humic substances derived from soils and natural waters, based on analyses of elemental composition, and 13CNMR and infrared spectroscopic data. These analyses suggest that the commercial samples cannot be treated as analogues of normal soil and aquatic humic substances. One of the unique characteristics of commercial humic acids is their low carbohydrate and carboxyl contents that would make the samples far less hydrophilic (note that some commercial humic acids are distributed in salt form to render adequate aqueous solubility). Therefore, such commercial humic acids may be expected to be far more effective in enhancing solute solubility relative to common aquatic humic materials on account of their higher content of nonpolar molecular moieties. As a supplement to our earlier work on the solubility enhancement of organic solutes by humic and fulvic acids from an acidic (Suwannee River) water and from a soil ( I I ) , we here investigated the enhancement effects by a humic extract from a nearly neutral pH water (Calcasieu River water), acidic water samples from the Suwannee River, Georgia, and the Sopchoppy River, Florida, and two commercial humic acids (Aldrich and Fluka-Tridom). The organic solutes p,p’-DDT, 2,4,5,2’,5’-PCB, and 2,4,4’-PCB used in this study were the same as used in our previous study (11)because of the sensitivity of their apparent water solubilities to dissolved humic materials. Comparison is made between solubility enhancement effects (&% values) of these organic solutes for an evaluation of the suitability of the commercial humic acids as substitutes for aquatic humic materials in this context. Experimental Section Test organic solutes (p,p’-DDT, 2,4,5,2’,5’-PCB, and 2,4,4’-PCB) and procedures for sample equilibration and determination of solute solubility enhancement by DOM were the same as described earlier (11). Sample equilibration times ranged from 24 to 48 h. The pH of the solution was set at 6.5. 1232
Environ. Sci. Technol., Vol. 21, No. 12, 1987
Six liters of acidic water (initial pH 4.8) was collected from the Suwannee River near Fargo, GA, in June 1986, and 6 L of acidic water (initial pH 4.2) was collected from the Sopchoppy River near Sopchoppy, FL, in August 1986. The humic extract (humic plus fulvic acid) from the Calcasieu River, sampled 3 miles west of Oberlin, LA, in May 1986, was isolated by passing 1500 L of water (initial pH 6.5) through two 10-L columns of XAD-8 resin connected in series; water entering the second column was acidified to pH 2 by the addition of HC1. The humic extract was recovered from the second column by first rinsing HC1 from the column with 1bed volume of distilled water and eluting the extract with 4 L of acetonitrile followed by 4 L of water. Water and acetonitrile were removed from the eluted extract by a combination of vacuum evaporation and freeze-drying. Aldrich humic acid (sodium salt) and Fluka-Tridom humic acid were purchased from the Aldrich Chemical Co. and Fluka Chemical Corp., respectively. The water samples from the Suwannee and Sopchoppy Rivers were filtered (with 0.45-pm silver filters) to remove particulates, and the pH was adjusted to 6.5. The DOC concentration was then determined. Solutions containing the Calcasieu River humic extract, Aldrich humic acid, and Fluka-Tridom humic acid, at pH 6.5, were prepared according to the established procedures (11). Except for the solution of the Calcasieu River humic extract, which was clear, all solutions were filtered to remove suspended particulates. Because of the large ash content of the commercial humic acids, the concentration of the filtrates was expressed in terms of DOC determined by a Coulometrics total organic carbon analyzer. The DOC concentration for the Suwannee water sample was 37 mg/L and that for the Sopchoppy water sample was 44 mg/L. Data for elemental analysis of the Calcasieu River humic extract and the commercial humic acids are presented in Table I. Results and Discussion Plots of the apparent water solubility of p,p’-DDT versus the DOC concentrations of Aldrich humic acid (sodium salt), Fluka-Tridom humic acid, Calcasieu River humic extract, and Suwannee water and Sopchoppy water, all at pH 6.5 and at 24 f 1 “C, are presented in Figure 1. Included for comparison in Figure 1 is the DDT solubility enhancement caused by Suwannee River humic or fulvic acid from the earlier work ( I I ) , where the two acid fractions were found to have virtually the same enhancement effects. Similar plots for 2,4,5,2’,5’-PCB and 2,4,4’-PCB are shown in Figures 2 and 3, respectively. For all solutes with all humic samples, the apparent solubility is linearly related to the DOC concentration of the humic samples, as predicated by eq 2. From the slope of the plot that gives s & d w and the intercept that gives s,, the respective KdOc value for each solute with a given type of DOM is calculated. Solubilities of the solutes in pure water (S,) are virtually the same as reported earlier (11). The calculated
70
/
p, p’-DDT
450
DOC
0 Fluka.Tridom
Acid
.
A
50
Humic Acid
400
FlukaTridom Humic Acid e
DOC -
2,4,4‘- PCB
0 AldrichHumic
A
. n
Calcasieu River Humic Extract
0
0 Suwannee River
350
Water
21
n -1
-
0 SuwanneeRiver Water Sopchoppy River Water ---Suwannee HumicFulvic Acid
300
--a
Sopchoppy River Water
/
s
$
Calcasieu River Humic Extract
v)
--- Suwannee HumicFulvic Acid
0)
2
250
-
C
2 n 4
0 1 0
I
I
10
20
I
30
I
I
40
50
60
Concentration of Dissolved Organic Carbon(mg/L)
Flgure 1. Dependence of the apparent water solubility of p ,p’-DDT on the dissolved organic carbon concentration of selected humic materials and natural waters. 120 2,4,5,2’,5’-PCB
DOC -
0 AldrichHumic
120
Acid
200-
0
10
20
30
Table 11. Comparison of log KdooValues of p ,p’-DDT, 2,4,5,2’,5’-PCB, and 2,4,4’-PCB with Dissolved Organic Carbon of Soil and Aquatic Origins and Commercial Humic Acids
Humic Acid
n
100
A Calcasieu River
”z -
L
-a
dissolved organic carbon
DDT
PCB
PCB
Aldrich humic acid sodium salt Fluka-Tridom humic acid Calcasieu River humic extract Sanhedron soil humic acid” Suwannee River humic acid” Suwannee River fulvic acid” Suwannee River water sample Sopchoppy River water sample
5.56 5.56 4.93 5.06 4.39 4.40 4.39 4.39
5.41 5.41 4.81 4.87 4.07 4.10 4.09 4.01
4.84 4.24 4.40 3.54 3.57 3.53 3.57
Water
Water --.Suwannee Humii Fulvic Acid
60
v)
+
E n
log Kdoo 2,4,5,2’,5’- 2,4,4’-
Humic Extract
ISopchoppy Rivei
D
2
p,p’-
0 Suwannee River 80
v) 0
I
50
Flgure 3. Dependence of the apparent water solubility of 2,4,4’-PCB on the dissolved organic carbon concentration of selected humic materials and natural waters.
0 Fluka.Tridom $ m
40
Concentrat ion of Dissolved Organic Carbon (mg/L)
40
a
“ Data from ref 11. 20
0
I
10
I
20
I
30
I
40
I
50
Concentration of Dissolved Organic Carbon(mg/L)
Dependence of the apparent water solubility of 2,4,5,2‘,5’-PCB on the dissolved organic carbon concentration of selected humlc materials and natural waters.
Flgure 2.
log Kdoevalues for the three solutes with two commercial humic acids, the Calcasieu River humic extract, and two acidic water samples are given in Table 11. In addition to the linearity of the apparent solubility versus DOC plot, relative solubility enhancements of the three test solutes (Le., the Kdocvalues) follow the order of DDT > 2,4,5,2’,5’-PCB > 2,4,4’-PCB for each aquatic humic sample. This order is the same as reported in the earlier study with other DOM samples (11) and is consistent with the proposed partition interaction between solute and DOM. The data in Figures 1-3, as summarized in Table 11,indicate that the enhancement effect on a DOC basis is markedly greater with Aldrich and Fluka-Tridom commercial humic acids than with the Calcasieu River humic extract and two (acidic) riverwatersamples. The two commercial samples show virtually identical enhancement .effects. On a DOC basis, the commercial
samples are about 4 times as effective as the Calcasieu River humic extract and 20 times as effective as the two acidic water samples on the basis of the Kdocvalues of the three test solutes. The solubility enhancement effects of individual humic samples appear to be closely correlated with the polarity of the materials (using elemental data as approximate indices), suggesting that differences in molecular sizes of these humic materials are not as much a critical factor as the polarity in affecting the partition interaction with organic solutes, a point discussed in more detail earlier (11). Large enhancement effects of the commercial humic acids and their high oxygen contents appear to be inconsistent with this analysis. However, it has been pointed out (12) that the oxygen contents of high-ash commercial humic acids are subject to gross errors (presumably because the oxygen associated with ash is also counted in the elemental “oxygen”). For example, a low-ash Aldrich humic acid (purified to H+form) obtained by extraction with methyl isobutyl ketone (12) has an elemental content of C (65.31%), H (5.94%), 0 (25.05%),N (0.51%), S (3.36%), and P (< 0.05%) on an ash-free, moisture-free basis when the ash content is reduced to 4.46%. Malcolm and MacCarthy (12) also indicated that the Fluka-Tridom humic acid is likely in the sodium salt form as is the Aldrich sample and possibly comes from the same source as the Environ. Sci. Technol., Vol. 21, No. 12, 1987
1233
Aldrich sample. The comparable elemental data and solubility enhancement effects, as noted here, would also indicate that the two commercial samples are practically identical in nature. By this account, it seems evident that the strong solubility enhancement caused by Aldrich and Fluka-Tridom humic acids is strongly related to their high-carbon and low-oxygen contents, factors that greatly promote a partitionlike interaction with the nonionic organic solutes. The smaller enhancement effect with the Calcasieu humic extract is in accord with an increased oxygen content and a decreased carbon content of the sample in comparison with the commercial humic acids. The log Kdm values derived with the Calcasieu humic extract are slightly smaller than the values for Sanhedron soil humic acid reported earlier (II), which has a slightly lower oxygen content. The results indicate that the enhancement effect caused by an aquatic humic material is very sensitive to the composition of the material. For instance, the Calcasieu humic extract is about 4-5 times as effective as the Suwannee River humic or fulvic acid, while their carbon and oxygen contents differ by less than 3% and 5%, respectively. The enhancement effects of the Suwannee River and Sopchoppy River waters are very small and virtually the same as those of the humic-fulvic acid extracts from the Suwannee water. This suggests that the two acidic waters likely have comparable organic matter compositions and that the net contribution to solubility enhancement by other non-humic-fulvic fractions of DOC in these waters is not significantly different from that by the humic-fulvic acid fractions. It would also appear that the isolation and fractionation steps involved in obtaining the humic and fulvic acid fractions of the Suwannee water (13) do not alter the samples in such a way that their abilities to enhance solute solubility differ considerably from those of the original, unfractionated, whole water sample. However, a more thorough investigation of the various (operationally defined) DOC fractions will be required to give a better account of this point. In a similar solubility enhancement study with Aldrich humic acid, Carter and Suffet ( 4 ) reported log Kd, values of 5.61-5.74 for DDT in a solution with an ionic strength of 0.01, and Hassett and Milicic (14) reported a log &oc value of 4.86 for 2,5,2’,5’-PCB. The magnitudes of the reported log Kdmvalues are comparable to the present data obtained with Aldrich or Fluka-Tridom humic acid. On the other hand, Carter and Suffet (4) reported the log Kdoc of DDT with Pakim Pond water (pH 4.0-4.5) as 4.84 and that with Boonton Reservoir water (pH unspecified) as 4.83, with both water samples from New Jersey. These log Kdm values are about the same as the log Kdocof DDT with the Calcasieu River humic extract. Similarly, Poirrier et al. (2)found that a colloidal material (containing about 68% iron) from a surface water of a highly colored acidic stream in southeastern Louisiana concentrated DDT in water by a factor of 15 800. If the organic matter content of this colloid is assumed to be about 30% and its carbon content is 56% of the total organic matter, the calculated log Kdmfor DDT would be 5.0, which is also in reasonable agreement with the value for DDT with the Calcasieu humic extract. The significant difference in the log Kdoc values for DDT in the study of Carter and Suffet ( 4 ) with the acidic Pakim Pond water and the corresponding values with the acidic Suwannee and Sopchoppy River waters from this study suggests that the (ash-free) elemental composition of DOM is influenced not only by pH but also by other environmental factors. A more detailed account of the factors affecting the composition and structure of 1234
Environ. Sci. Technol., Vol. 21, No. 12, 1987
aquatic humic materials is needed to gain a better understanding of their abilities to enhance the solubility of organic solutes. In consideration of the data presented, it may be concluded that the difference in water solubility enhancement of sparingly soluble organic solutes by dissolved aquatic humic materials is closely related to the composition of the DOM. The effect of molecular sizes of these humic materials on solubility enhancement appears to be secondary, as discussed earlier (11),presumably because their molecular weights are sufficiently large. Although a significant variation in Kdocis found for a solute with different DOM samples on the basis of results from this and other studies, the range of the Kdocvalues so determined is, however, much less than the magnitude found with the commercial samples. The magnitude predicted by using a commercial sample can be greater by a factor of 4-20 than the actual effect of the natural DOM, as revealed by the data in this study. It appears that a more reliable and prudent approach to study the enhancement effect of DOM (or DOC) would be to relate the effect with the composition and structure of DOM, in spite of the fact that such information may not be readily available. In any event, one needs to be cautious in interpreting the environmental effects as related to solubility enhancement of organic solutes by DOM, based on the use of commercial humic samples as a substitute for humic materials in natural surface water.
Acknowledgments We thank Steve H. Jones, U S . Geological Survey at Tifton, GA, and Leslie L. Batts and George A. Irvin, U S . Geological Survey at Tallahassee, FL, for collecting the Suwannee River water sample and Sopchoppy River water sample, respectively. Literature Cited (1) Wershaw, R. L.; Burcar, P. J.; Goldberg, M. C. Enuiron. Sei. Technol. 1969,3, 271. (2) Poirrier, M. A,; Bordelon, B. R.; Laseter, J. L. Environ. Sei. Technol. 1972,6 , 1033. (3) Boehm, P. D.; Quinn, J. G. Geochim. Cosmochim. Acta 1973,37,2459. (4) Carter, C. W.; Suffet, I. H. Environ. Sei. Technol. 1982,16, 735. (5) Hassett, J. P.; Anderson, M. A. Water Res. 1982,16,681.
(6) 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. (7) Landrum, P. F.; Nihart, S. R.; Eadie, B. J.; Garner, W. S. Environ. Sei. Technol. 1984,18, 187. (8) OConnor,-D. J.; Connolly, J. B. Water Res. 1982,14,1517. (9) Chiou, C. T.; Porter, P. E.; Shoup, T. D. Enuiron. Sei. Technol. 1984,18, 297. (10) Gschwend, P. M.; Wu, S.-C. Environ. Sei. Technol. 1985, 19,90. (11) Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Environ. Sci. Technol. 1986,20, 502. (12) Malcolm, R. L.; MacCarthy, P. Environ. Sei. Technol. 1986, 20,904. (13) Thurman, E. M.; Malcolm, R. L. Environ. Sei. Technol. 1981,15, 463. (14) Hassett, J. P.; Milicic, E. Enuiron. Sei. Technol. 1985,19, 638. Received for review December 9, 1986. Revised manuscript received August 6,1987.Accepted September 8,1987. The use of trade or product names in this article is f o r identification purposes only and does not constitute endorsement by the U S . Geological Survey.
