Chapter 18
Enhancement of the Water Solubility of Organic Pollutants Such as Pyrene by Dissolved Organic Matter 1,2
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Howard H. Patterson , Bruce MacDonald , Feng Fang , and Christopher Cronan Downloaded by UNIV OF NEW SOUTH WALES on August 24, 2015 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch018
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Department of Chemistry and Department of Environmental Science and Ecology, University of Maine, Orono, ME 04469
Many factors determine thefateand transport of an organic pollutant in the environment but water solubility is certainly one of the most important. Among the environmental factors that alter the solubility of a molecule is naturally occurring dissolved organic carbon (DOC). We have hypothesized that the DOCfromdifferent sources within a watershed have different binding affinitiesforpollutants such as pyrene. This could lead to different rates of transport or bioavailability within the watershed. DOC samples have been isolated from a stream, adjacent wetland and nearby wooded upland sites. A fluorescence quenching method was developed to quantify the binding coefficient of the pollutants with the dissolved organic carbon. From these results a model has been constructed to determine the sites with the greatest potential to modify pollutant contamination in the environment. Many factors determine the fete and transport of an organic pollutant in the environment but water solubility is certainly one of the most important physical properties of a molecule. While the solubility of a compound in pure water is an important guide to its overall fete, predicting the pollutant's solubility in the environment - a stream, an agriculturalfield,or an aquifer - is fer more involved. Among the environmental factors that alter the solubility of a molecule are naturally occurring humus or organic matter. Typically, organic matter ranges in size from partially decayedfragmentsof plant material to particulate matter, colloids, and simple water soluble molecules. The water soluble organic carbon found in the soil is analogous to the dissolved organic matter found in an aquatic or marine environment. These twofractionswill be the focus here and the more general term dissolved organic matter, DOC, will be used to refer to both sources of organic matter. Researchers have applied a variety of techniques and probe molecules to DOC to assess the binding affinity of DOC to the probe. Carter and Suffet (1) sealed humic acid solutions in dialysis bags and then allowed the bags to equilibrate in solutions of radiolabled DDT. A DOC concentration of approximately 10 mg/L bound about 40% 0097-6156/96/0651-0288$15.00/0 © 1996 American Chemical Society
In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF NEW SOUTH WALES on August 24, 2015 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch018
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of the total DDT. Chiou et. al. (2) employed a hexane solvent extraction method to determine the quantity offreepesticide in the presence of DOC. They found the higher molecular weight DOC with lower oxygen content (therefore less polar) increased the apparent solubility of the probe molecule. Wang et. al. (3) observed the binding of the polar herbicide atrazine with a fulvic acid. The bound and unbound atrazine were separated by ultrafiltration, followed by liquid chromatography to quantify the free atrazine concentration. Here also it was found that most of the binding capacity was due to the high molecular weight fraction. In another approach Gauthier et. al. (3) developed a technique based on fluorescence quenching to determine the equilibrium binding constants of polycyclic aromatic hydrocarbons (PAH) with DOC. PAH are efficientfluorophores,nonionic, and are usually considered to be insoluble in water. These last two properties thermodynamically drive the PAHfromthe aqueous phase to the less polar DOC. Also, PAHs are important examples of hydrophobic organic contaminants in the environment because these properties cause them to accumulate in the lipid deposits of higher organisms. Herbert, Bertsch et al. (4) conducted quenching studies utilizing DOCfromsoils with pyrene for a probe. The DOC was divided into molecular weightfractionsby ultrafiltration. Their results indicated that the more hydrophobic, high molecular weightfractionwas responsible for the major portion of the probe binding. Backus and Geschwend (5) studied the quenching of perylene by organic matter and calculated that the organic matter they use could potentially double the total amount of contaminant transported. The previous literature studies referenced above document the potential for DOC to affect thefeteof an organic contaminant in the environment. In our research we have hypothesized that the DOCfromdifferent sources within a watershed have different binding affinities for pyrene. This could lead to different rates of pyrene transport or bioavailability within the watershed. DOC was isolatedfroma stream, an adjacent wetland, and nearly wooded upland sites. Afluorescencequenching method was utilized to detennine the binding coefficient of pyrene with the different DOC samples. Experimental
Sample Collection and Workup. Samples were collectedfroma watershed in the Penobscot Experimental Forest, Bradley, Maine. Organic horizon material was collectedfroma deciduous and coniferous she. Approximately 12 L of water was takenfromthe Blackman stream. Finally, a wetland sample was takenfroma sedge marsh bordering the stream. Holes 20 cm wide were dug which promptlyfilledwith sediment laden water which was collected for analysis. The samples were worked up as in Figure 1. The Blackman Stream sample was clean enough for immediatefiltration.Gelman A/E glassfiberfilters,47 mm diameter, were used with vacuumfiltrationfollowedby Rainin 0.45 μπι, 47 mm diameter Nylon66filters.The sedge marsh sample required centrifugation at 8,000 rpm for 20 minutes to remove the coarse organic particulates. The supernatant was then passed through two stacked glassfiberfiltersfollowedby 0.45 μπι filtration.
In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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Sample
Preparation
Methods Column e x t r a c t i o n
Centrifugation, 8,000 rpm, 20 min
A/E Glass f i b e r filtration
Samples
Deciduous and Coniferous S o i l s
Sedge Marsh
Blackman Stream
0.45 micron nylon membrane filtration
C a t i o n exchange
Ultrafiltration Figure 1 : Summary of the steps used in the isolation of DOC from the two upland samples (Deciduous and Coniferous), a wetland (sedge marsh) and an aquatic site (Blackman Stream).
In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF NEW SOUTH WALES on August 24, 2015 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch018
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The coniferous and deciduous samples were first packed in columns and then leached with water. Two columns were set up for each sample. Between 550 to 800 grams of organic horizon material were packed to a net height of 100 cm in each column and then the columns werefilledwith enough distilled deionized water (1.7 to 2.0 L) to cover the samples. Columns were equilibrated for 5 h, after which they were drained at a rate of less than 5 ml/min. The slow drainage rate was to limit the colloidal material in the leachate. Leachates were dark brown in color with a combined final volume for each sample of 2 to 2.5 L. After all samples had passed through the 0.45 μπιfiltration,subsamples were analyzed for metals by ICP spectroscopy. Samples were passed through cation exchange columns to remove metals. It should be pointed out that in the experiments reported herein, we have attempted to limit the colloidal material. We recognize that the potential impact of colloidal material on these studies is an important area of research to pursue. Pyrene Solution Preparation. Pyrene was purchased from Aldrich Chemical Company and used directly. A phosphate buffer, pH 6 upon dilution was used (6). For the fluorescence quenching experiments an aqueous pyrene stock solution was preparedfroma 2.05 χ 10" solution of pyrene in methanol. 1.5 ml of the methanolic pyrene solution was mixed with 498.5 ml of pH 6 buffer water to give a concentration of 6.15 χ 10"^ M aqueous pyrene. The pyrene fluorescence intensity tended to be more stable if the solution were allowed to mix 1 hour before use. 4
Fluorescence Quenching Experiments. The binding affinity of pyrene with DOC samples was measured by fluorescence quenching using a Perkin Elmer MPF-4 spectrafluorimeter. The excitation wavelength was 334.5 nm with the emission maximum at 374 nm. For these experiments a series of DOC stock solutions at 7 different concentrations was preparedfromeach DOC source. An aliquot was placed in a cuvette and the absorbance and DOC fluorescence measured. Next, an aliquot of aqueous pyrene was added to the cuvette for the quenching experiment and the fluorescence intensity recorded at three and four minutes after mixing. If the two measurements differed a third measurement was made and the two closer measurements retained. We have found that thefluorescenceintensity did not change to an appreciable amount (
