Journal of Environmental Quality 31:275-280 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Organic Compounds in the Environment
Combined Effect of Natural Organic Matter and Surfactants on the Apparent Solubility of Polycyclic Aromatic Hydrocarbons
Hyun-Hee Cho,
Jaeyoung Choi,
Mark N. Goltz and
Jae-Woo Park*
National Subsurface Environmental Research Laboratory (NSERL), Ewha Womans Univ., 11-1 Daehyon-dong, Seodaemun-gu, Seoul 120-750, South Korea
* Corresponding author (jaepark{at}ewha.ac.kr)
Received for publication March 8, 2001.
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ABSTRACT
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Both natural organic matter (NOM) and surfactants are known to enhance the apparent aqueous solubility of hydrophobic organic contaminants (HOCs) in aqueous systems. In this study, the combined effect of NOM and surfactants on enhancing the solubility of HOCs was investigated, since both may occur and affect the fate and transport of HOCs in natural aqueous environments. Experimental results indicated that the apparent solubility of naphthalene, phenanthrene, and pyrene in NOM and anionic surfactant solution was lower than their solubility in NOM solution alone. However, the apparent solubility of an HOC in NOM and nonionic surfactant solution is almost the same as the sum of the HOC's solubility in NOM solution plus its solubility in nonionic surfactant solution. The observation that apparent aqueous solubility of HOCs in NOM and anionic surfactant solution is decreased is probably due to the fact that the cations that are released when the anionic surfactant dissociates may form ion pairs with acidic or phenolic groups associated with the NOM. This serves to increase the size of hydration of these groups, thereby decreasing the effective size of the nonpolar moieties associated with the NOM, and thus decreasing hydrophobic partitioning of the HOCs into the NOM. The results presented here will help us to understand the effect of NOM and surfactants on the fate and transport of HOCs in aquatic systems.
Abbreviations: CMC, critical micelle concentration • HOC, hydrophobic organic contaminants • NOM, natural organic matter • PAH, polycyclic aromatic hydrocarbon • SDS, sodium dodecyl sulfate • TOC, total organic carbon
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INTRODUCTION
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NATURAL ORGANIC MATTER (NOM) is ubiquitous in natural aqueous environments, and the fate and transport of hydrophobic organic compounds (HOCs), such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, chlorinated pesticides, in the aqueous phase can be significantly affected by the presence of NOM (Hunchak-Kariouk et al., 1997; Ravichandran et al., 1998; Raber et al., 1998).
Polycyclic aromatic hydrocarbons (PAHs) are composed of two or more fused aromatic rings. Some PAHs have been determined to be animal carcinogens and suspected human carcinogens (Li et al., 2001). They are usually formed by natural or anthropogenic pyrolysis of organic matter, and may be produced during forest fires, fossil fuel use, and chemical manufacturing (Borneff and Kunte, 1983; Guha et al., 1998).
The extent of HOC binding to humic substances varies, depending upon humic properties such as aromaticity, molecular weight, dispersivity, and aliphatic composition (Chin et al., 1994; Chin and Aiken, 1997; Jones and Tiller, 1999). For example, enhanced sorption of PAHs to humic substances with higher aromatic fractions has been demonstrated (Kile and Chiou, 1989a; Chiou et al., 1998). Molecular interaction of humic substances is dependent on temperature, pH, ionic strength, and the types of ions in solution, as well as other physiochemical parameters (Schlautman and Morgan, 1993).
The partitioning of HOCs into dissolved humic substances parallels the partitioning behavior of HOCs into aqueous phase synthetic surfactants, whose capacity to solubilize HOCs increases significantly above the critical micelle concentration (CMC) where the surfactant monomers associate to form micelles (Kile and Chiou, 1989b; Rosen, 1989; Park and Jaffe, 1993, 1995; Park and Boyd, 1999). As the central portion of the micelle is hydrophobic, the aqueous solubility of poorly water-soluble compounds such as HOCs will be enhanced greatly at surfactant concentrations that exceed the CMC.
By far the largest class of surfactants in general use today are anionic surfactants, which constitute approximately 70 to 75% of total surfactant consumption (Myers, 1988). Anionic and nonionic surfactants are mostly used as detergents that are subsequently found in wastewater from households and may flow into aquatic systems such as rivers or lakes, especially in countries like Korea where not all wastewater undergoes secondary treatment (Shiau et al., 1994). These surfactants in aquatic systems may serve to cause deterioration of water quality. In addition, some surfactants themselves are suspected endocrine disruptors (Shang et al., 1999). The average concentration of anionic surfactants in household discharges is 6 to 7 mg/L in Korea (Korea Institute of Construction Technology, 1991). Although this concentration range is much lower than the CMCs, solubility enhancement of some HOCs has been observed at these sub-CMC concentrations (Kile and Chiou, 1989a).
As previously stated, both NOMs and surfactants can increase the solubility of HOCs in the aqueous phase. When untreated wastewater is introduced into aquatic systems, NOM and surfactant both may be present, and their presence can significantly influence the fate and transport of HOCs by affecting HOC solubility. Hence, the objective of this paper was to study how the concurrent presence of NOM and anionic surfactants affects the solubility of HOCs in water.
The initial hypothesis was that two anionic and amphiphilic compounds such as NOM and anionic surfactant with similar molecular structures might exert a synergistic effect to further enhance the solubility of HOCs. A nonionic surfactant, Triton X-100, was also studied for comparision. The results from this study will help us to understand the effect of amphiphiles on the mobilization and bioavailability of HOCs in surface water (Tiehm et al., 1997; Lamoureux and Brownwell, 1999) as well as in ground water, where surfactants are intentionally applied for remediation of contaminated ground water and/or soils (Neupane and Park, 1999, 2000).
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MATERIALS AND METHODS
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Materials
The NOM chosen for this study, which had been originally extracted from the Suwannee River (USA), was obtained from the International Humic Substance Society (IHSS, St. Paul, MN). Reported elemental compositions of Suwannee River NOM on a dry basis are C = 48.80%, H = 3.90%, O = 39.7%, N = 1.02%, S = 0.60%, P = 0.02%, and ash = 7.00% (IHSS). The anionic and nonionic surfactants used were sodium dodecyl sulfate (SDS) and Triton X-100, respectively, purchased from Sigma Co. (St. Louis, MO).
Polycyclic aromatic hydrocarbons (PAHs) used in this study (naphthalene, phenanthrene, and pyrene) were purchased from Aldrich (Milwaukee, WI), and were 99, 98, and 98% pure, respectively. They were used without any further purification. Table 1 lists the characteristics of the three PAHs.
Solubility Measurement of Polycyclic Aromatic Hydrocarbons in Natural Organic Matter–Surfactant Solution
All solutions were prepared with deionized and distilled water in 40-mL glass vials capped with Teflon septa. After solutions of surfactant, NOM, and surfactant plus NOM were prepared, excess amounts (more than aqueous solubility) of individual PAHs were added to the solutions. These solutions were mixed for 72 h at 250 rpm and 25 ± 1°C on a shaker table and were filtered using a 0.1-µm inorganic membrane filter (Anotop 10 Plus; Whatman, Maidstone, England) mounted on a 10-mL syringe, in order to remove undissolved HOCs. The concentration of HOCs in NOM and/or surfactant solution was measured with high performance liquid chromatography (HPLC) (Model no. 515; Waters, St. Milford, MT), equipped with the UV detector (Waters 2487 dual 
absorbance detector). The analyses were conducted at a wavelength of 254 nm with a flow rate of 1.8 mL/min, mobile phase of 80% acetonitrile (optima grade; Fisher Scientific, Pittsburgh, PA) and 20% water, and a 3.9- x 300-mm µ-bondapak C18 reverse phase column (Waters). The PAHs and the amphiphiles (NOM and surfactants) were well separated in the HPLC analysis at 254 nm. The retention time of NOM was approximately 1 min, while those of the PAHs were approximately 2.5 to 4 min (Fig. 1)
. The peaks of SDS and Triton X-100 at the concentrations in this research were not detectable in the chromatogram. Total organic carbon (TOC) content was determined using a TOC analyzer (Model 5000A; Shimadzu, Kyoto, Japan). All experiments were duplicated.
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Fig. 1. High performance liquid chromatography (HPLC) chromatograms of naphthalene (10 mg/L), surfactants (10 mg/L), and natural organic matter (NOM, 10 mg/L) at 254 nm. (a) Naphthalene in water, (b) naphthalene in sodium dodecyl sulfate (SDS) and NOM solution, and (c) naphthalene in Triton X-100 and NOM solution.
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RESULTS AND DISCUSSION
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Kinetic studies were conducted to determine how long it takes for HOC solubilization into NOM and SDS solution to reach equilibrium. Using naphthalene as the HOC, it was seen that after a significant increase in dissolved concentration in the first 24 h, napthalene concentration in NOM and SDS solution did not change after 3 d. Therefore, in all subsequent experiments, the samples were equilibrated for 72 h.
The apparent solubility of naphthalene in SDS solution was higher than the solubility of naphthalene in distilled water, and the apparent solubility of naphthalene in SDS and NOM solution was lower than the solubility of naphthalene in NOM solution alone (paired t test, t = 27.9, degrees of freedom [df] = 1, p value = 0.0114) (Fig. 2)
. The same trend was found using phenanthrene (paired t test, t = 191.4, df = 1, p value = 0.00166) and pyrene (paired t test, t = 97.9, df = 1, p value = 0.00325) as the HOC. In all cases, the enhancement of pyrene solubility due to the presence of SDS and/or NOM was much higher than the solubility enhancement observed for naphthalene or phenanthrene.
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Fig. 2. Polycyclic aromatic hydrocarbon (PAH) solubility in sodium dodecyl sulfate (SDS, 10 mg/L) and Suwannee River natural organic matter (NOM, 10 mg/L) solution; C is the increased solubility in SDS and NOM solution and C0 is the solubility in water.
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Typically, the presence of NOM would be expected to only slightly enhance the solubility of nonpolar organics. Chiou et al. (1986)(1987) noted that the NOM–water partition coefficient for nonpolar organics (Knom) is typically less than 5% of the octanol–water partition coefficient (Kow). Based on the solubility enhancement measured in this study, however, we calculate that Knom would be similar in magnitude to Kow. The measured C/C0 (where C is the solubility in SDS and NOM solution and C0 is the solubility in water) values for PAHs in this study were considerably higher, that is, the calculated partition coefficient of NOM (Knom) for PAHs would be similar in magnitude to Kow (the octanol–water partition coefficient), while NOM enhances the solubility of nonpolar solutes with Knom less than 5% of Kow (Chiou et al., 1986, 1987). Apparently, the NOM used in this study has a relatively high aromatic content, which greatly enhances the partition interaction with highly aromatic PAHs.
The apparent solubility of the PAHs increased due to the presence of Triton X-100 and NOM individually (Fig. 3)
, and the apparent solubility of the PAHs in the mixed Triton X-100 and NOM solution was almost the same as the sum of the PAH's solubility in NOM solution plus its solubility in Triton X-100 solution. As shown in Table 2, for all three PAHs, the increase in solubility (C/C0 - 1) due to the concurrent presence of both NOM and Triton X-100 is within 5% of the increase in solubility that is calculated by summing the increases in solubility due to the individual presence of NOM and Triton X-100.
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Fig. 3. Polycyclic aromatic hydrocarbon (PAH) solubility in Triton X-100 (10 mg/L) and Suwannee River natural organic matter (NOM, 10 mg/L) solution.
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Table 2. Solubility of polycyclic aromatic hydrocarbon (PAHs) in natural organic matter (NOM), Triton X-100, and Triton X-100 + NOM solutions (relative to the aqueous solubility).
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In order to ensure that the above results are applicable over a range of surfactant and NOM concentrations, studies were conducted measuring pyrene solubility at various NOM and SDS concentrations (Fig. 4)
. At all NOM and SDS concentrations studied (5 to 20 mg/L), the apparent solubility of pyrene in the mixed solution of NOM and SDS was lower than in the NOM solution.
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Fig. 4. Pyrene solubility in Suwannee River natural organic matter (NOM) and sodium dodecyl sulfate (SDS) solution at various NOM and SDS concentrations.
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In order to investigate if the decrease in PAH solubility in NOM solution with the addition of SDS was due to any physical or chemical change of NOM and SDS, such as precipitation, TOC measurement was performed on the solutions of NOM, surfactant, and NOM plus surfactant (Table 3). The TOC value of NOM plus SDS solution was 10.40 mg/L, while the sum of TOC values of NOM and SDS solution was 10.41 mg/L. Since the difference between the two values is within 0.1%, this result implies that NOM and SDS do not seem to undergo any phase-change reaction.
The interaction between NOM and HOCs can be dependent on pH, but the measured pH values of the prepared solutions were constant (i.e., between 6.5 and 6.7), so the effect of pH was assumed to be minimal.
It is possible to explain the above observations based on the structure of humic molecules and the release of dissolved cations when anionic surfactants are dissociated in water. Humic molecules contain many aromatic rings with carboxylic (-COOH) and phenolic (-OH) groups, sugars, and peptides (MacCarthy et al., 1990). As PAH molecules approach humic substances, some molecules bind to the aromatic moiety of the humic substances and some fractions partition into the hydrophobic part of the humic substances. When NOM and SDS are dissolved in water, functional groups of the humic substances, such as the carboxylic and phenolic groups, are dehydrogenated, and SDS is ionized into sodium ion and anionic surfactant (Sparks, 1995; Lin et al., 2001). The formation of ion pairs between the sodium cation and the negatively charged groups increases the hydration sphere of the acidic or phenolic groups, which decreases the effective size of the hydrophobic moiety of the humic molecule. Analogously, Chiou (1990) found that clays are often ineffective in removing neutral organic compounds from water as the hydration of inorganic cations creates a hydrophilic environment at the clay mineral surfaces and interlayers.
To demonstrate that the hydrating effect of the sodium ions dissociated from SDS decreases the sorption of PAHs, 3.47 x 10-5 M of NaCl solution, which is the same molar concentration of sodium as was added by SDS in the previous experiment, was mixed with NOM solution. Note from Fig. 5
that the solubility decrease due to the addition of NaCl is nearly identical, for all three PAHs, to the solubility decrease due to addition of an equivalent amount of SDS.
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Fig. 5. Polycyclic aromatic hydrocarbon (PAH) solubility in sodium dodecyl sulfate (SDS, 10 mg/L) and Suwannee River natural organic matter (NOM, 10 mg/L) solution with NaCl (3.47 x 10-5 M).
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To show that ion pairings have an effect on the solubility of PAHs and increase the hydration of the functional groups of humic substances, the same concentration of CaCl2 as SDS (3.47 x 10-5 M) was added to NOM solution. As seen in Fig. 6
, the solubility decrease due to the addition of CaCl2 is greater than the solubility decrease due to the addition of SDS, for all three PAHs. Comparing with Fig. 5, we observe that the solubility decrease due to the addition of CaCl2 is also greater than the decrease due to the addition of NaCl. This appears to be due to the fact that Ca2+ hydrates much more strongly than Na+, thereby serving to further decrease PAH solubility (McBride, 1994).
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Fig. 6. Polycyclic aromatic hydrocarbon (PAH) solubility in sodium dodecyl sulfate (SDS, 10 mg/L) and Suwannee River natural organic matter (NOM, 10 mg/L) solution with CaCl2 (3.47 x 10-5 M).
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To further investigate the effect of cations on PAH solubility, NOM and PAHs were mixed and equilibrated for 3 d. Subsequently, SDS was added and mixed for another 3 d. As shown in Fig. 7
, for all three PAHs, the subsequent addition of SDS led to increased solubility of the PAH, and in fact, total PAH solubility could be calculated by summing the solubilities of PAH in NOM solution and PAH in SDS solution. This is in contrast to the experiments where NOM and SDS were added simultaneously, and PAH solubility was observed to be less than the solubility in NOM alone. The solubility of the three PAHs resulting from the sequential addition of NOM and SDS was statistically higher than the solubility due to the simultaneous addition of the two amphiphiles (naphthalene: paired t test, t = 53.7, df = 1, p value = 0.00592; phenanthrene: paired t test, t = 387.9, df = 1, p value = 0.000821; pyrene: paired t test, t = 63.0, df = 1, p value = 0.00505). This result can be due to the inhibition of PAH sorption to NOM by sodium ions.
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Fig. 7. Differences in polycyclic aromatic hydrocarbon (PAH) solubility in sodium dodecyl sulfate (SDS, 10 mg/L) and natural organic matter (NOM, 10 mg/L) solution when SDS is added simultaneosly with the NOM and subsequent to the NOM.
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Previous studies support the hypothesis that the presence of counterions results in a decrease in the apparent solubility of HOCs. Schlautman and Morgan (1993) showed that binding of PAHs to Suwannee River humic material decreased with increasing ionic strength at fixed pH. Traina et al. (1989) indicated that the concentration and type of counterions bound to dissolved organic matter chemically and physically affected the distribution of HOCs. However, little work has been done looking at the sorptive amphiphiles addressed in this study, which are typically found in wastewater. It is noteworthy that the effect of cations on these amphiphiles was so large that the solubility enhancement of PAHs due to the presence of a surfactant at typical concentrations was more than offset by the decrease in sorption to NOM due to the cation effect. Results from the current study will be useful in predicting the fate and transport of HOCs, such as agrochemicals, in aquatic systems where wastewater is not properly treated and discharged, so that surfactants may be present. Additionally, the results have relevance to ground water remediation studies, where surfactant may be added to mobilize contaminant or increase contaminant bioavailability. This study appears to indicate that depending on conditions, addition of an anionic surfactant may, in fact, reduce contaminant mobility and bioavailability.
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ACKNOWLEDGMENTS
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This research was supported by National Research Laboratory Program of the Korean Ministry of Science and Technology of South Korea. The authors are grateful to Dr. Cary Chiou at the USGS for helpful advice.
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ABSTRACT
INTRODUCTION
