Binding of Nonpolar Pollutants to Dissolved Organic Carbon

9 Binding of Nonpolar Pollutants to Dissolved Organic Carbon Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch009

Environmental Fate Modeling 1

Gail Caron and I. H. Suffet Environmental Studies Institute, Drexel University, Philadelphia, PA 19104

Nonpolar compounds associate with organic carbon in the environment. The interaction between pollutants and dissolved organic carbon in natural waters is not as well defined as that between pollutants and sedimentary organic matter. The limitations of experimental techniques and extraction and concentration procedures are partially responsible for the incomplete description of pollutant-DOC (dissolved organic carbon) interactions. Despite the lack of complete understanding of the phenomenon, the association of nonpolar compounds with natural DOC can exert a significant influence on their environmental partitioning. Mathematical models of environmental behavior should include dissolved organic carbon in both overlying and sedimentary interstitial waters as compartments for equilibrium partitioning.

NUMEROUS PHYSICAL, CHEMICAL, AND BIOLOGICAL PROCESSES act upon organic chemicals that are released into the environment. The interaction of these factors determines the ultimate environmental fate of pollutant compounds, as well as the hazard they pose to living organisms. To assess the risk associated with a released chemical, it is necessary to understand how the compound will behave in the environment. In view of the large 1Current address: U.S. Environmental Protection Agency, Region 3, 841 Chestnut Street, Philadelphia, PA 19107 0065-2393/89/0219-0117$06.00/0 © 1989 American Chemical Society

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

Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch009

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and ever-increasing number of organic chemicals being produced, experi mental study of individual compounds is an impossible task. Considerable effort is currently being directed toward developing math ematical models to accurately predict the environmental distribution of or ganic chemicals. Simple compartmental models such as the quantitative water, air, and sediment interactive (QWASI) fugaeity model of Mackay et al. (I) and the chemical equilibrium partitioning and compartmentalization (CEPAC) model of McCall et al. (2) predict the environmental distribution of pollutants from physical-chemical properties of the compound that de termine its affinity for various media. More complex models add the con sideration of transformation reactions and transport processes. The various environmental transport processes are poorly understood, especially for compounds associated with dissolved humic materials in the environment. We have a new approach to the modeling of hydrophobic organic pollutant behavior in the aquatic environment, in which dissolved humic materials play an important role.

Binding of Nonpohr Organic Compounds to Sedimentary Organic Carbon The association of nonpolar organic pollutants with soils and sediments has been studied extensively and identified as a major process affecting the environmental fate and distribution of these compounds. The binding of nonpolar organic compounds to sedimentary organic carbon is important background information related to the association of these compounds to dissolved humic materials. The distribution of hydrophobic organic compounds between aquatic sediments and the overlying water column has typically been viewed as a surface adsorption phenomenon and, as such, has been studied with batch sorption isotherm techniques. Adsorption isotherms of nonpolar organic compounds on a number of soils and sediments are linear over a wide range of equilibrium solute concentrations (3-5). This behavior can be expressed as

C d se

=

&p

X

(1)

where C a and are sorbed and dissolved concentrations of a compound, respectively; and K is the distribution, or partition, coefficient describing the ratio of the equilibrium concentration of a compound in the sediment to its equilibrium concentration in the water. A number of studies have shown that the binding of nonpolar organic compounds to natural sediments is highly correlated with the organic carbon content of the solid material. Because of the important influence of organic se

p


9.

CARON & SUFFET

119

Binding of Nonpolar Pollutants to DOC

carbon, sediment-water distribution coefficients are often normalized to the organic-carbon fraction of the sediment (fj) by the expression Kœ = -f-

(2)

Joe

For a given compound, the magnitude of is relatively constant among sediments (6, 7). K values, therefore, provide good predictions of sorptive behavior. The value of constants for describing the distribution of organic compounds between sedimentary organic carbon and water is further en hanced by the fact that values can be closely correlated with a chemical's octanol-water partition coefficient ( K ) and water solubility (3, 6, 8). values that have not been experimentally determined thus may be estimated from measured K values for the same compound. Lambert (9) and Chiou et al. (3, 4) have proposed that the association between nonpolar compounds and the organic carbon fraction of sediments, soils, and natural waters is better described as a liquid-liquid partitioning phenomenon than as a surface adsorption process. A n organic-matter par titioning process is supported by a number of observations, including


œ

ow

o w

1. linear sorption isotherms to near aqueous saturation concen trations of nonpolar organic substances, with no evidence of isotherm curvature at the higher concentration range; iso therm curvature at higher concentrations is predicted by ad sorption theories; 2. small temperature effects on solute sorption; 3. absence of competition in experiments using binary solute systems; and 4. data covering seven orders of magnitude in which sedi ment-water partition coefficients were inversely proportional to aqueous solubility and well correlated to octanol-water partition coefficients. The actual physical mechanism of the reaction between nonpolar organic compounds and natural organic matter is still a matter of controversy. The terms sorption and partitioning will, therefore, be used loosely in this chapter. A number of workers have attempted to describe the association be tween nonpolar organic compounds and humic material on a molecular level. Schnitzer and Khan (10) proposed that the humic polymer consists of an aromatic core to which peptides, carbohydrates, metals, and phenols are attached. This proposed structure is an open network, and it has been sug-



120

AQUATC I HUMIC SUBSTANCES

gested that organic molecules are trapped inside the spaces of the humic structure. Freeman and Cheung (JJ) picture humic material as highly branched polymer chains that form a three-dimensional randomly oriented network. Interconnections between the chains prevent the network from dissolving in liquids. Instead, liquids may be absorbed, and absorption is typically accompanied by swelling of the network to form a gel. Freeman and Cheung suggested that humic substances bind organic chemicals by a process of incorporation into the humic gel structure, and that the binding of hydro phobic compounds is controlled by the relative affinity of the compound for the aqueous and gel phases. At present, it is not known which of the proposed structures best de scribes the molecular configuration of naturally occurring humic material. Further research is necessary in this area. Relatively recent evidence indicates that dissolved organic matter in natural waters can, like sedimentary organic carbon, "sorb" or bind nonpolar organic compounds. Dissolved organic carbon is composed largely of dis solved humic material. The binding of nonpolar organic chemicals with dis solved organic carbon (DOC) can be described by an equilibrium distribution coefficient, K , where doc

Cdoc — Kdoc

X

C

a q

(3)

where is the concentration of the chemical associated with the D O C at equilibrium. Dissolved organic carbon in natural waters must be considered as a separate environmental compartment in a model of pollutant behavior. Math ematical models developed to date have not included the nonpolar-organicpollutant-DOC interaction. Where D O C concentrations are high, this in teraction can exert an important influence on the environmental behavior of nonpolar organic materials, especially those with a strong tendency to bind to dissolved humic substances. Systems that contain naturally high levels of D O C include bogs, swamps, and interstitial waters of soils and sediments. Interstitial water (porewater) is formed by the entrapment of water during sedimentation, which isolates it from the overlying water. Porewater is considered to be in equilibrium with the sedimentary solid phase and separate from the over lying water column, or bulk water (12, 13). Dissolved organic carbon con centrations in sedimentary porewater can exceed 100 m g / L , whereas overlying surface waters typically contain less than 5 m g / L of D O C (14). For modeling purposes, a kinetic boundary can be hypothesized at the sediment-water interface, as illustrated in Figure 1. The hypothesized boundary would describe conditions in lakes, reservoirs, and slow-moving streams, where the rates of dispersion and difiusion between the sediment and water column are orders of magnitude slower than those within the


9.

CARON & SUFFET

121 Binding of Nonpolar Pollutants to DOC

WATER COLUMN


C

aq

=0

τ

Sediment Column

ι i


ppump

τ

Reservoir Figure 3. Sediment column apparatus usedfor equilibration of aqueous phases with sedimentary particulate material.

Summary Dissolved organic carbon in natural waters interacts with and influences the environmental behavior of nonpolar organic compounds. A number of meth ods have been developed to study and quantify this interaction. At present, no universally applicable technique has been defined. Further research is necessary to develop new methods that will overcome the experimental difficulties encountered with existing procedures. The extent of binding of nonpolar organic compounds to D O C is a function of the octanol-water partition coefficient and aqueous solubility of the compound. Present data indicate that the magnitude of binding is a function of the humic material as well. It is not known whether there are indeed differences in the binding ability of D O C from different sources or whether the discrepancies result from alteration of the structure of natural organic matter during sampling, isolation, and extraction. A n experimental design using D O C as found in the environment (i.e., with no fractionation, extraction, or chemical alteration) was used to inves tigate the importance of DOC-pollutant interactions in the aquatic envi ronment. Results of this research indicate that D O C in the interstitial water of natural sediments can significantly affect the behavior of hydrophobic compounds exhibiting high K values. Environmental models should ino w


9. CARON & SUFFET

Binding of Nonpolar Pollutants to DOC

129

elude this interaction i n the prediction of the ultimate fate and transport of nonpolar pollutants.

Acknowledgment


We thank R. Lee Lippincott for help in the development of the equilibrium model presented in this chapter and for providing computer graphics.

References 1. Mackay, D.; Paterson, S.; Joy, M. In Fate of Chemicals in the Environment; Swann, R. L.; Eschenroeder, Α., Eds.; ACS Symposium Series 225; American Chemical Society: Washington, DC, 1983; pp 175-196. 2. McCall, P. J.; Swann, R. L.; Laskowski, D. A. In Fate of Chemicals in the Environment; Swann, R. L.; Eschenroeder, Α., Eds.; ACS Symposium Series 225; American Chemical Society: Washington, DC, 1983; pp 105-123. 3. Chiou, C. T.; Peters, L. J.; Freed, V. H. Science (Washington, DC) 1979, 206, 831. 4. Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983, 17, 227-231. 5. Karickhoff, S. W. Chemosphere 1981, 10, 833-846. 6. Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241-248. 7. Means, J. C.; Wood, S. G.; Hassett, J. J.; Banwart, W. L. Environ. Sci. Technol. 1980, 14, 1524-1528. 8. Perdue, Ε. M. In Aquatic and Terrestrial Humic Materials; Christman, R. F.; Gjessing, E. T., Eds.; Ann Arbor Science: Ann Arbor, 1983; pp 441-460. 9. Lambert, S. M. J. Agric. Food Chem. 1967, 15, 572-576. 10. Schnitzer, M.; Khan, S. U. Humic Substances in the Environment; Marcel Dekker: New York, 1972. 11. Freeman, D. H.; Cheung, L. S. Science (Washington, DC) 1981, 214, 790-792. 12. Glass, G. E.; Poldoski, J. E. Verh. Int. Ver. Limnol. 1975, 19, 405-420. 13. Batley, G. E.; Giles, M. S. Water Res. 1979, 13, 879-886. 14. Thurman, Ε. M. Organic Geochemistry of Natural Waters; Kluwer Academic: Hingham, MA, 1985. 15. Carter, C. W. Ph.D. Thesis, Drexel University, 1982. 16. Brownawell, B. J. Ph.D. Thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institute, 1986. 17. Hassett, J. P.; Anderson, M. A. Environ. Sci. Technol. 1979, 13, 1526-1529. 18. Means, J. C.; Wijayaratne, R. Science (Washington, DC) 1982, 215, 968-970. 19. Landrum, P. F.; Nihart, S. R.; Eadie, B. J.; Gardner, W. S. Environ. Sci. Technol. 1984, 18, 187-192. 20. Carter, C. W.; Suffet, I. H. Environ. Sci. Technol. 1982, 16, 735-740. 21. 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. 22. Whitehouse, B. G. Estuarine Coastal Shelf Sci. 1985, 20, 393-402. 23. Diachenko, G. W. Ph.D. Thesis, University of Maryland, 1981. 24. Hassett, J. P.; Milicic, E. Environ. Sci. Technol. 1985, 19, 638-643. 25. Yin, C.; Hassett, J. P. Environ. Sci. Technol. 1986, 20, 1213-1217. 26. May, W. E.; Wasik, S. E.; Freeman, D. H. Anal. Chem. 1978, 50, 175-179. 27. Garbarini, D. R.; Lion, L. W. Environ. Sci. Technol. 1985, 19, 1122-1128.


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28. Chiou, C. T.; Malcolm, R.L.;Brinton, T. I.; Kile, D . E. Environ. Sci. Technol. 1986, 20, 502-508. 29. Krom, M. D . ; Scholkovitz, E. R. Geochim. Cosmochim. Acta 1977, 41, 1565-1573. 30. Eadie, B.J.;Landrum, P.F.;Faust, W. Chemosphere 1982, 11, 847-858. 31. Brownawell, B. J.; Farrington, J. W. In Marine and Estuarine Geochemistry; Sigleo, A. C.; Hattori, Α., Eds.; Lewis Publishers: Chelsea, MI, 1985; pp 97-120. 32. Brownawell, B. J.; Farrington, J. W. Geochim. Cosmochim. Acta 1986, 50, 157-169. RECEIVED for review November 5, 1987. ACCEPTED for publication July 19, 1988.


Binding of Nonpolar Pollutants to Dissolved Organic Carbon

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