15 Effect of Dissolved Organic Matter on Extraction Efficiencies Organochlorine Compounds from Niagara River Water

Caryl L. Fish, Mark S. Driscoll, and John P. Hassett

Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, NY 13210

Simon Litten

New York State Department of Environmental Conservation, 50 Wolf Road, Albany, NY 12233

Two techniques for the extraction of a series of organochlorine com­pounds (chlorinated benzenes, polychlorinated biphenyls, DDT, DDE, and mirex) from centrifuged Niagara River water were com­pared. The more hydrophobic compounds were extracted more effi­ciently by a digestion technique than by conventional hexane extraction. Plots of the relative recovery (R = undigested/digested) versus log of the octanol-water coefficient (K o w) show R decreasing exponentially with log Kow. This decrease suggests that the diges­tion-extraction recovers both the dissolved fraction and the fraction bound to organic matter, although conventional solvent extraction does not recover the bound fraction efficiently.

H UMIC ACIDS, FULVIC ACIDS, AND DISSOLVED ORGANIC MATTER, which are found in all natural waters, are very complex compounds of both aquatic and terrestrial origin. Humic substances are polyelectrolytes containing both aromatic and aliphatic carbon with phenolic, alcoholic, carbonyl, acidic, and amino functional groups.

0065-2393/89/0219-0223$06.00/0 © 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.

224 AQUATIC HUMIC SUBSTANCES

The binding of hydrophobic organic compounds to humic substances has been studied by several investigators. Mirex (l,la,2,2,3,3a,4,-5,5,5a, 5b, 6-dodecachlorooctahydro-1,3,4-metheno- lfi-cyclobuta[ cd] penta-lene) (I), 2,2',5,5'-tetrachlorobiphenyl (2, 3), D D T (l,r-(2,2,2-trichloro-ethylidene) bis[4-chlorobenzene]) (4), and cholesterol (5) bind to humic substances. This binding has been shown to affect gas exchange (2, 3), bioavailability (6), particle adsorption (7), photolysis (8), and hydrolysis (9) reactions. To date, very few investigators have looked at how this binding affects the extraction efficiencies of hydrophobic compounds from water. Hassett and Anderson (5) found that the solvent extraction efficiency of cholesterol from water was reduced in the presence of dissolved organic matter (DOM). Similarly, Carlberg and Martinsen (JO) showed that both solvent and X A D - 2 extraction efficiencies of alkanes, polycyclic aromatic hydrocarbons (PAH), chlorinated hydrocarbons, and phthalates were less than 100% in the presence of humic substances. The extraction efficiencies of these compounds decreased as the equilibrium time of the binding was increased from 4 to 60 days. Therefore, the equilibrium time between spiking and extraction is very important.

In this study, the extraction efficiencies of 23 organochlorine compounds from Niagara River water were determined. The compounds were chloro-benzenes, polychlorinated biphenyl (PCB) congeners, D D T , D D E , mirex, and photomirex. The extraction efficiencies were determined by two meth­ods: conventional l iquid-l iquid extraction with hexane and digestion to break down dissolved organic matter combined with hexane extraction.

Experimental Details

Reagents. All extractions were done with pesticide-grade hexane (Baker Chemical), and digestion-extractions used chromic acid cleaning solution (90-96% sulfuric acid, 1% Cr0 3 ). The acetone and petroleum ether (30-60 °C boiling range) used for cleaning were glass-distilled. All stock solutions were made with pesticide-grade benzene (Fisher Scientific).

Standard Compounds. Table I lists the 23 compounds used in this study in order of increasing elution time from the gas chromatograph. The chlorinated ben­zenes were obtained from Aldrich Chemical Co.; mirex was donated by the United States Environmental Protection Agency; photomirex was donated by Environment Canada. All other compounds were obtained from Ultra Scientific Inc.

Accurate amounts of each standard compound were prepared by weighing on a microbalance, with the exception of 1,2,4-trichlorobenzene, which was measured volumetrically. Each standard was dissolved in benzene and diluted to 10 m L in an individual volumetric flask. Mixed standards were prepared by measuring appropriate volumes of the stock solutions into a 10-mL volumetric flask and diluting to the mark with hexane. One mixed standard contained mirex and photomirex; the other con­tained the remaining 21 compounds.

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15. FISH ET AL. Organochlorine Compounds from Niagara River Water 225

Table I. Organochlorine Compounds Used in This Study Rehtive Mass

No. Compound Name log K J Recoveryb Spiked0 (ng) 1 1,2,4-Trichlorobenzene 3.98 1.008 145.4 2 1,2,3-Trichlorobenzene 4.04 1.001 76.6 3 1,2,3,4-Tetrachlorobenzene 4.55 0.926 26.6 4 2-Chlorobiphenyl 4.5 0.577 984.0 5 Pentachlorobenzene 5.03 0.960 39.3 6 4-Chlorobiphenyl 4.61 0.671 2720.0 7 2,4-Dichlorobiphenyl 5.15 0.947 249.0 8 Hexachlorobenzene 5.47 0.924 41.2 9 4,4'-Diehlorobiphenyl 5.36 0.890 675.0

10 2,4,4'-Triehlorobiphenyl 5.74 0.935 103.9 11 2,2',5,5'-Tetrachlorobiphenyl 6.26 0.938 107.0 12 2,2',3,4-Tetraehlorobiphenyl 6.11 0.916 49.1 13 o,p'-DDE — 0.879 30.5 14 2,2 ', 4,5,5 ' -Pentachlorobiphenyl 6.85 0.896 99.1 15 p,p'-DDE — 0.792 34.7 16 3,3',4,4'-Tetrachlorobiphenyl 5.62 0.707 243.0 17 2,3' ,4,4' ,5-Pentachlorobiphenyl 7.12 0.793 95.6 18 ο,ρ'-ΌΌΊ — 0.902 12.9 19 2,2',4,4',5,5'-Hexachlorobiphenyl 7.75 0.830 117.0 20 p,p'-DDT 6.36 0.830 139.0 21 Photomirex — 0.760 57.8 22 2,2',3,4,4',5,5'-Heptachlorobiphenyl 7.20 0.686 96.6 23 Mirex 6.89 0.640 63.4 aData are from ref. 13. T̂he relative recovery is the undigested fraction recovered per digested fraction recovered.

Into approximately 1 L of water.

Sampling. Samples were collected biweekly from January 22 to April 30, 1986, from the outlets of continuous-flow centrifuges at sampling stations established by Environment Canada on the Niagara River. These were located at Fort Erie and Niagara-on-the-Lake, Ontario. Seven sets of samples were taken. Duplicate samples for direct solvent extraction were collected in 1-L glass Wheaton bottles with alu-minum-foil-lined screw caps. Bottles were weighed on a triple-beam balance before and after sampling to determine the amount of water collected. Duplicate samples for digestion-extraction were collected in 2-L round-bottom flasks with standard taper (24/40) glass stoppers. The flasks were filled to an approximate 1200-mL mark, and the exact amount was determined by weighing as just described.

Solvent Extraction. Samples in the Wheaton bottles were spiked with 5 μL· of the mirex-photomirex stock solution and 10 u,L of the other combined standard solution. These samples were shaken for 24 h at 25 °C on a shaker table. To extract each spiked sample, the contents of the sample bottle was poured into a 2-L separatory funnel, the bottle was rinsed with 70 mL of hexane, and the hexane was poured into the separatory funnel. The funnel was shaken vigorously for 2 min to allow the phases to separate, and the hexane layer was removed. This process was repeated twice more, including the rinsing of the sample bottle, so that the water was extracted a total of three times. The combined hexane layers were passed through a column of 8 g of anhydrous sodium sulfate to remove traces of water and evaporated to 5-10 mL in a concentration apparatus (Kuderna-Danish).

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226 AQUATIC HUMIC SUBSTANCES

Digestion-Extraction. The round-bottom flasks were spiked with 5 of the mirex-photomirex standard solution and 10 μι . of the other combined standard solution. The round-bottom flasks were shaken for 24 h on a shaker table at 25 °C. Each sample was treated by addition of 5 mL of chromic acid and 200 mL of hexane. After each flask was fitted with a reflux condenser and placed in a heating mantle, the hexane was refluxed for 2 h. The samples refluxed smoothly without boiling chips or stirring. After it cooled, each sample was transferred to a 2-L separatory funnel and shaken vigorously. Once the phases had separated, the hexane layer was re­moved, passed through a column containing 8 g of anhydrous sodium sulfate, and evaporated to 5-10 mL in a concentration apparatus (Kuderna-Danish).

Gas Chromatography. Extracts were analyzed with a gas chromatograph (Var-ian 3400) with a splitless capillary column injector, auto injector, a 60-m x 0.25-mm (i.d.) SPB-1 (0.25-μπι film thickness) fused silica capillary column (Supelco Inc.) and a Ni-63 electron-capture detector. Data were collected by a data aquisition system (Keithley DAS, Series 500) connected to a microcomputer (Leading Edge, Model D) and stored on floppy disks. Data aquisition was controlled with Labtech Notebook software. The output from the gas chromatograph was also sent to a strip chart recorder. Prior to injection, 100 μL· of 2,2\3,3\5,5\6,6'-octachlorobiphenyl was added to the extracts as an internal standard. The column temperature was held at 60 °C for 5 min, raised to 270 °C at a rate of 10 °C/min, and held at 270 °C for 30 min. Compounds were quantified by comparing peak heights to a standard curve and making volume corrections based on the internal standard response.

Results and Discussion

The 23 compounds used in this study are listed in Table I, with amounts spiked into the Niagara River water samples, log K o w , and average relative recoveries [(R) = fraction recovered by solvent extraction per fraction re­covered by digestion-extraction]. Unspiked samples were also analyzed, and the native concentrations were not significant relative to the spiked concen­trations. Preliminary experiments indicated that all 23 compounds were recovered efficiently from distilled water by both the solvent extraction and the digestion-extraction methods. Relative recoveries in Table I demonstrate that some of the test compounds were recovered more efficiently from N i ­agara River water by the digestion-extraction method than by the solvent extraction method (relative recoveries less than 1). In addition, hexane-water emulsions did not form in the digested samples, but frequently caused phase separation problems in the undigested samples. Therefore, the diges­tion-extraction method is a superior technique for extraction of the test compounds from the Niagara River water. However, use of chromic acid digestion may not always be the method of choice for the extraction of natural waters. The oxidation of some compounds of interest is a possible problem with this method. Problems may also arise from interfering compounds formed as a result of the partial digestion of the dissolved organic matter.

Relative recovery also decreased with increasing log K o w (Figure 1). This result may appear counterintuitive but can be explained if some fraction of a compound bound by D O M in Lake Erie water (dissolved organic carbon

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228 AQUATIC HUMIC SUBSTANCES

(DOC) is 2.1-7.0 mg/L) is recovered more efficiently by digestion-extraction than by solvent extraction. This effect could occur as a result of partial or complete destruction of the D O M by reaction with chromic acid, which would release DOM-bound compounds and make them more available for solvent extraction. Compounds with higher K w values tend to be bound to a greater extent by D O M (11). These results can be explained quantitatively from

CT = Cb + Cf (1)

where C T is the total concentration of the organochlorine compound in water, Cj, is the bound concentration, and Cf is the free concentration. The binding constant, K d o c , is defined as

^* cf C / t D o e ] K }

where C ^ is the concentration of the bound compound in the dissolved organic carbon and [DOC] is the concentration of the dissolved organic carbon. Combining equations 1 and 2 yields

£ = [ D O C ] * ^ + 1 (3)

Kdoc is related to K o w (12) by an equation that takes the form of

log Kdoc = a log KoW + h (4)

Equation 4 can be written as

Kdoc = 10e b g ^ h (5)

So, from equations 3 and 5,

Cj = [DOC]10 a los "™+b + 1 (6)

Assuming that the digestion-extraction method extracts both bound and free compounds (C r ) , and that solvent extraction recovers only the free compounds (Cy), then R = Cf/CT, and therefore

R = [[DOCJIO* I o* + I ] " 1 (7)

The line in Figure 1 is the nonlinear least squares fit of equation 7 to the experimental points. Compounds 4 (2-chlorobiphenyl), 6 (4-chlorobi-

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15. FISH ET AL. Organochlorine Compounds from Niagara River Water 229

phenyl), and 16 (3,3 ', 4,4 '-tetrachlorobipheny 1) appear to be outliers and are not included in the regression. The use of an average D O C concentration of 4.0 mg /L for Lake Erie water yields a and b values of 0.29 and 2.76, respectively.

As can be seen from Table I and Figure 1, the ratios for compounds 4, 6, 16, and 23 (mirex) are substantially lower than the line fitted to equation 7. The low ratios indicate that they are less efficiently extracted by solvent extraction than predicted by the correlation with log K o w . Such deviations might occur if the driving force binding these compounds to organic matter included both hydrophobic interactions (as reflected by dependence on K o w ) and specific electronic interactions of some compounds with sites in the D O M .

Conclusion A chromic acid digestion technique yields enhanced performance compared to conventional hexane solvent extraction for recovery of organochlorine compounds from Niagara River water. The effect is especially pronounced for highly hydrophobic compounds. These results may be due to digestion of natural organic matter, which otherwise binds hydrophobic compounds and inhibits their solvent extraction.

References 1. Yin, C.; Hassett, J. P. Environ. Sci. Technol. 1986, 20, 1213. 2. Hassett, J. P.; Milicic, E. Environ. Sci. Technol. 1985, 19, 638. 3. Jota, M. M.S. Thesis, State University of New York, College of Environmental

Science and Forestry, 1983. 4. Carter, C. W.; Suffet, I. H. Environ. Sci. Technol. 1982, 16, 735. 5. Hassett, J. P.; Anderson, M. A. Environ. Sci. Technol. 1979, 13, 1526. 6. Boehm, P. D.; Quinn, J. G. Estaurine Coastal Mar. Sci. 1976, 4, 93. 7. Hassett, J. P.; Anderson, M. A. Water Res. 1982, 16, 681. 8. Zepp, R. G. Chemosphere 1981, 10, 109. 9. Perdue, Ε. M.; Wolfe, N. L. Environ. Sci. Technol. 1982, 16, 847.

10. Carlberg, G. E.; Martinsen, K. Sci. Total Environ. 1982, 25, 245. 11. Carter, C. W. Ph.D. Thesis, Drexel University, 1984. 12. Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241. 13. Rappaport, R. Α.; Eisenreich, S. J. Environ. Sci. Technol. 1984, 18, 163-170.

RECEIVED for review July 24, 1987. ACCEPTED for publication February 12, 1988.

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