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11 Sorption of Chlorinated Hydrocarbons in the Water Column by Dissolved and Particulate Organic Material 1
Linda L. Henry and I. H. Suffet Environmental Studies Institute, Drexel University, Philadelphia, PA 19104 Steven L. Friant Academy of Natural Science, Philadelphia, PA 19103
Partition coefficients for dissolved and particulate organic material (K and Κ , respectively) andfiveorganic chemicals were meas ured in surface waters and leachates from soils and sediments. Kdom was measured by using changes in Daphnia magna dry-weight bioconcentration factors, and Kpom was measured by centrifugation. The results showed that sorption by dissolved organic matter can dominate the distribution of organic chemicals in the water column and affects calculation of Kpom. The linear relationship of log Kdom and log Kpom values to log Kow (octanol-water partition coefficient) was similar to the relationship found by Kenaga and Goring for soils and sediments. This resemblance indicates that organic materials in the water column, soils, and sedimentary material sorb chemicals by a similar mechanism. dom
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SORPTION O F HYDROPHOBIC ORGANIC CHEMICALS by naturally occurring
organic matter in soils and aquatic systems is an important factor in determining their distribution in the environment. Incorporation of sorption by the organic matter in soils and sedimentary material into fate and transport 1Current address: BCM Engineers, 1 Plymouth Meeting, Plymouth Meeting, PA 19462 0065-2393/89/0219-0159$06.00/0 © 1989 American Chemical Society
In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
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models has been facilitated by the commonality of the behavior of hydro phobic organic chemicals (I). Research has led to predictive relationships that correlate the partition coefficient (K ) for organic chemicals in soils and sedimentary material to the octanol-water partition coefficient ( K J and water solubility (2, 3). Sorption by dissolved organic material ( D O M , less than 0.45 μπι) and suspended particulate organic material (POM) in the water column, partic ularly for naturally occurring concentrations, has received less attention than sorption by soils and sedimentary material. The importance of sorption in teractions in the water column is illustrated by the changes in fate and transport processes for organic chemicals bound to D O M . D O M sorption interactions have been shown to retard volatilization, increase solubility, and change chemical reactivity and the rate of bioaccumulation (4-8). Studies indicate that sorption or association of contaminants by D O M may be a key component in controlling their distribution in the water column (9-12). For example, in Delaware River water, an average of 34% (n - 12) of benzo[û]pyrene was bound to less than 1 m g / L of D O M (13). Research with naturally occurring D O M has shown that sorption of certain chemicals can be correlated with D O M concentrations, but not nec essarily between different sources of D O M (9,14,15). The variability among sources suggests that K may not be as amenable for use in predictive models as K . However, further research on naturally occurring D O M is needed. Most research on K has used humic material either from a com mercial source (e.g., Aldrich humic acid) or extracted from aquatic systems (9, 11, 16), which may not truly represent D O M in its natural state. The relative importance of sorption by D O M and P O M in the water column and the effect of the D O M partition coefficient ( K ) on the cal culation of the partition coefficient for P O M ( K J have yet to be clarified. To date, has been calculated by assuming that the fraction of chemicals remaining in the water after removal of the particulates represents the freely dissolved fraction of the chemicals (two-compartment model). However, in natural waters, D O M is present and should be included as a third equilibrium compartment (Figure 1).
Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch011
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Figure 1. Three-compartment model for the distribution of organic chemicals in the water column. In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
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Sorption of Chlorinated Hydrocarbons
HENRY ET AL.
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The chemicals remaining in the water after removal of particulate ma terial include a fraction bound to the D O M and a freely dissolved fraction. A more correct method of estimating would be to subtract the fraction of chemicals bound to D O M from the total chemicals remaining in the water (three-compartment model) (17, 18). Depending on the significance of DOM-pollutant interactions, current methods may be underestimat ing K . To date, experimental measurements of K have not been included in Kpon, studies, although their importance is recognized. The presence of D O M has been used to explain decreases in K^ as the concentration of particulate matter increases (17, 18). The effect of organic matter concen tration has also been observed for K (9). The phenomena are controversial for both K and K . The observed decrease in K has been attributed, in some cases, to the presence of D O M in the test water. When an estimated value for K was included in the calculation of Kpo , the decrease in K ^ was not seen (17, 18). In both of these studies, K was incorporated with literature values rather than experimentally determined values on the same samples (synoptic). The importance of K i n the determination of has yet to be demonstrated experimentally, and synoptic measures of K and K are considered an important component of the study design for the research presented here. The objective of this research was to evaluate the predictability of K and for five chlorinated hydrocarbons by using different components of the organic matter and the effect of K on measurements in natural waters. The tests, designed to simulate conditions i n the water column, included measures of sorption by D O M and suspended P O M from a surface water and leachates of soils and sedimentary material.
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Experimental Methods Study Compounds. Five chemicals—lindane, dieldrin, ρ,ρ'-DDT [1,1-dichloro-2,2-bis(p-chlorophenyl)ethane], and two PCB isomers (2,4,4'-trichlorobiphenyl and 2,3,4,5,6-pentachorobiphenyl) (Ultra Chemical Co.)—were chosen to represent a range of sorption potentials (log K 3 . 3 5 - 6 . 7 ) . Greater than 9 9 % purity was verified with gas chromatography. Actual concentrations of the chemicals in the test solutions varied, but all were less than half of the solubility limits. Lindane (log Kow 3 . 3 5 ) was not expected to sorb to either DOM or POM at the experimental levels and was included to monitor for experimental error. o
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Experimental Design. Each experiment measured K p « and K d in surface waters and leachatesfromsediment or soil. Natural waters were collectedfromsix sources: a large river (the Delaware River, which runs between Pennsylvania and New Jersey), two major tributaries (Schuylkill and Cooper Rivers), and three small ponds in southern New Jersey (mesotrophic Lincoln Lake, eutrophic Harris Lake, and highly dystrophic Pakim Pond). m
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In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
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The waters were collected in 5-gal polyethylene containers and used within 48 h. Surface soils and sedimentary materials were also collected from each of the rivers or ponds. Leachates were prepared by mixing the soil or sediment (1:20 wet weight) with the surface water for 18 h (19). After allowing at least 5-6 h settling time, the supernatant liquid was siphoned off and used as the experimental water. Kpom was measured in the surface water or leachate with all three compartments present:freelydissolved, DOM, and POM. Kdom was measured in the same water after the particulate material had been removed by centrifugation. Characterization of D O M and P O M . The effect of source and the relative ability of certain factors to predict Kdom (total DOM, humic acids, and DOM >1000 daltons) and K p o (total suspended solids and POM) was evaluated. Total suspended solids (TSS) was determined as mg/L dry weight (evaporated at 103 °C for 24 h) removed by centrifugation at 5000 rpm for 1 h; the value for POM was estimated as half the ash-free dry weight of the TSS as milligrams of carbon per liter (20). Total DOM, humic acid content, and DOM >1000 daltons were measured on 0.45-μπιfilteredwater as dissolved organic carbon by a carbon analyzer (Dohrmann DC-80). Humic acid content was defined as the amount of dissolved carbon removed by a second 0.45-μπιfiltration24 h after acidifying the sample to pH 2. DOM >1000 daltons was measured as the amount of carbon that did not pass through a prewashed dialysis membrane (Spectrapor 6) after dialyzing natural water against distilled water. m
Sorption to D O M . Kdom was measured by using bioconcentration factors (BCF) for Daphnia magna, with and without DOM present in the test water. Chemicals sorbed to DOM are not accumulated by amphipods and daphnids, and changes in BCF have been used to measure thefractionof chemical bound to the DOM (16, 21, 22). Kdom was measured in all water types after the particulate matter was removed. Thefractionbound to the DOM was measured as the decrease in BCF found in Daphnia magna juveniles exposed to the study chemicals in the natural water with DOM (POM removed) relative to BCF in control water without DOM. The study chemicals were added in methanol stock solutions (
