Competitive Sorption between 17α-Ethinyl Estradiol and Naphthalene

Steroid hormones such as 17R-ethinyl estradiol (EE2) have been frequently detected at various levels in surface waters downstream of many municipal ...
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Environ. Sci. Technol. 2005, 39, 4878-4885

Competitive Sorption between 17r-Ethinyl Estradiol and Naphthalene/Phenanthrene by Sediments Z H I Q I A N G Y U †,‡ A N D W E I L I N H U A N G * ,‡ Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China, and Department of Environmental Sciences, Cook College, Rutgers University, New Brunswick, New Jersey 08901-8551

Steroid hormones such as 17R-ethinyl estradiol (EE2) have been frequently detected at various levels in surface waters downstream of many municipal wastewater treatment facilities. Their fate, transport, and environmental risk are currently not well characterized. This study examined the competitive sorption between EE2 and two aromatic hydrocarbon compounds, phenanthrene and naphthalene, by three sediments. The sorption isotherms of phenanthrene and naphthalene were measured at 22 ( 0.5 °C using a batch technique with initial aqueous concentrations (Co) of EE2 at 0, 100, 500, and 2000 µg/L. Competitive sorption varied between EE2 and phenanthrene or naphthalene on the sediments. The linearity of the naphthalene sorption isotherm was found to increase as a function of the cosorbate EE2 concentration from 0 to 2000 µg/L. The single-point naphthalene KD value at equilibrium aqueous-phase naphthalene concentration (Ce) of 25 µg/L was reduced by 19-26% and 27-48% at Co (EE2) ) 100 and 500 µg/L, respectively. The sorption of phenanthrene at its low Ce range was similarly affected by EE2, but to a much less extent, possibly because phenanthrene is more hydrophobic than EE2. At high phenanthrene Ce, no measurable change was observed even at Co (EE2) ) 2000 µg/L. While the effect of naphthalene on EE2 sorption was insignificant, the competitive effect on the sorption of EE2 by phenanthrene was very significant at low EE2 concentrations. The measured single-point EE2 KD values decreased as much as 35% as the phenanthrene Ce increases from below 10 µg/L to slightly above 100 µg/L. This study suggests that the fate and transport of emerging pollutants such as EE2 could be affected in the presence of more hydrophobic pollutants in aquatic systems.

Introduction Estrogens including naturally occurring 17β-estradiol and estrone and synthetic 17R-ethinyl estradiol (EE2) are biologically active steroid hormones having a common cyclopenta-o-perhydrophenanthrene ring. Public concern about the environmental impact of steroid estrogens has grown over the past decade due to their potential for disrupting endocrine systems of animals and humans. Synthetic EE2 is * Corresponding author phone: (732)932-7928; fax: (732)932-8644; e-mail: [email protected]. † Chinese Academy of Sciences. ‡ Rutgers University. 4878

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mainly used in oral contraceptives. It is excreted in the urine as an inactive conjugated form which can be readily activated through bacterial activity in the sewage and sewage treatment plants. A recent USGS report showed that synthetic EE2 was detected in 22% of the surface water samples (1). Toxicity tests using fish as acceptors showed that EE2 has a potency 10-50-fold higher than E2 and E1 due to its longer half-life and tendency to bioconcentrate (2). It was shown that EE2 at relatively low doses could induce vitellogenin yolk precursor (3), affect reproductive behavior (4), and reduce the fertilization success or viability of embryos from exposed adults (5). It is considered as one of the most potent estrogenic compounds. Unlike conventional prioritized toxic organic pollutants (TOPs) such as chlorinated ethenes and polychlorinated biphenyls (PCBs), EE2 and other steroid hormones are not regulated by the U.S. Environmental Protection Agency (EPA), partly because little is known with respect to their chronic ecotoxicity and their fate, risk, and impact in aquatic systems (6, 7). The study presented here examines the sorptive competition between TOPs and synthetic EE2 on sediments and its potential impact on transport and ultimate fate in the environment commonly contaminated with mixed chemicals. It is known that mixtures of TOPs exhibit lower sorption capacities on soils and sediments than in their respective single solute systems, a phenomenon often called competitive sorption (8-13). Several prior studies on competitive sorption were carried out for characterizing the nonpartitioning interactions of hydrophobic organic contaminants associated with the “hard” soil organic matter (SOM) such as glassy humic acids and soot particles having low O/C atomic ratio and high specific surfaces (14-16). McGinley et al. (14) and Pignatello and Huang (17) found that sorption capacity, isotherm nonlinearity, and desorption of TOPs are affected significantly in the presence of other TOP solutes. Xing et al. (18) found that synthetic organic polymers and humic acids in their glassy states exhibit competitive sorption between atrazine and prometon or trichloroethylene. Li and Werth (9), Xia and Ball (11), and Chiou et al. (16) also reported competitive sorption phenomena and explained their observations via adsorption mechanisms associated with high surface area carbonaceous materials (HSACM). These prior studies demonstrated that competitive sorption is characteristic of the “hard” SOM domains, that the sorbate-sorbate competition is a function of the physicochemical properties of the coexisting TOPs (19, 20), and that TOPs with similar molecular structures and physicochemical properties often display greater competitive sorption than those with very different structures and properties. So far, no existing theory nor empirical approach is available for accurately assessing the magnitude of competitive sorption among TOPs, however. This study focuses on the competitive effect of EE2 on the equilibrium sorption of phenanthrene and naphthalene by sediments. The major hypothesis to be tested in this study is that EE2 can compete with TOPs such as polynuclear aromatic hydrocarbons (PAHs) for binding on soils and sediments. It is conceivable that, if present, competitive sorption can lower sorption capacities and enhance availability to living organisms for both EE2 and TOPs. The fate and transport of the organic mixtures should also be dependent upon the magnitude of the competitive sorption. The scientific questions we intended to answer are “can EE2 compete with TOPs for sorption on sediments?” and “if, yes, to what extent?” We set out a set of experiments to measure the equilibrium sorption of phenanthrene and naphthalene 10.1021/es048558k CCC: $30.25

 2005 American Chemical Society Published on Web 05/20/2005

TABLE 1. Sorbent Properties sorbent

source

EPA-15

sediment, Ohio River, IN stream sediment, Lorenzo, IL sediment, Missi. River, IL

EPA-21 EPA-26

TOCa (wt %)

BCb (%)

SSAc (m2/g)

particle sized

0.95

16

15.2

16/49/36

1.88

14

1.48

3

6.96

50/43/07

16.2

2/55/43

a Total organic carbon (22). b BC content on a TOC basis (23). c N 2 gas BET specific surface area (22). d Weight percentage of sand/silt/clay.

TABLE 2. Physicochemical Properties of the Sorbates chemical

formula

molecular weight

Swa (mg/L)

logKowb

pKac

phenanthrene naphthalene EE2c

C14H10 C10H8 C20H24O2

178.2 128.2 296.4

1.12 31.7 3.10

4.57 3.30 4.15

10.21

SW ) aqueous solubility (44, 13). Octanol-water partition coefficient (32, 44). c Deprotonation constant (33). a

b

by sediments in both the single-solute systems and the binarysolute systems with EE2 as the competing solute. The differences in the sorption behavior among the sorbentsorbate systems were explained in terms of sorbent characteristics and physicochemical properties of the sorbates. The results obtained were further discussed in the context of field measurements.

Experimental Section Sorbents and Sorbates. Three EPA reference sediments (EPA-15, -21, and -26) originally collected from Illinois and Indiana were used as the sorbents. They were chosen because they were characterized systematically in a prior study (21) and were used in a number of prior studies of organic contaminant sorption (21, 22). According to Accardi-Dey and Gschwend (23), these samples contained black carbon (BC) or carbonaceous particles such as soot or elemental carbon formed during combustion processes. The BC contents were 16, 14, and 3% on a TOC basis for EPA-15, -21, and -26 sediments, respectively (23). EE2, phenanthrene, and naphthalene in spectrophotometric grades were obtained from Aldrich Chemical Co., Inc., and were used as received. We chose EE2 as the representative steroid hormone because EE2-based synthetic estrogens are the second most prescribed drug in the United States, which are used for birth control and medical treatments of cancer, hormonal imbalance, osteoporosis, and other ailments (24). Meanwhile, EE2 was detected in surface waters in the U.S.A. (25), Japan (26), Canada (27), and European countries (28-31). The concentration of EE2 typically ranged from 250 µg/L was analyzed using the UV detector set at wavelength of 280 nm, whereas EE2 at lower concentrations was analyzed with the fluorescence detector set at excitation wavelength of 250 nm and emission wavelength of 312 nm. Phenanthrene and naphthalene were analyzed with both UV detector at wavelength of 250 nm for aqueous concentrations > 60 and > 600 µg/L, respectively, and the fluorescence detector set at excitation wavelength of 250 nm and emission wavelength of 364 and 332 nm for lower concentrations. External methanol phase standards of EE2 (10-3500 µg/L), phenanthrene (0.5-1000 µg/L), and naphthalene (6-20 000 µg/L) were used to establish linear calibration curves for both detectors. The mobile phase used was a mixture of HPLC-grade acetonitrile and MiliQ water at volumetric ratios of 9:1 for single solute solutions of phenanthrene and naphthalene, 2:1 for single-solute solutions of EE2, 3:2 for bisolute solutions of EE2 and phenanthrene, and 1:1 for bisolute solutions of EE2 and naphthalene. The flow rate of the HPLC system was 0.3 mL/min, and the injection volume was 20 µL. The detection limits of the fluorescence detector are 25, 3, and 1 µg/L for EE2, naphthalene, and phenanthrene, respectively, and the detection limits of UV detector are 100, 50, and 30 µg/L for EE2, naphthalene, and phenanthrene, respectively. Sorption Experiments. Equilibrium sorption experiments of single- and bisolute systems were conducted at 22 ( 0.5 °C using flame-sealed glass ampules (20 mL, Kimble) as the batch reactor systems. The sediment-solution contact time was 14 days according to the single solute sorption rate experiments reported in a prior study (13, 34). Soil-to-solution ratios were kept constant at about 1:230, 1:600, and 1:250 g/mL for phenanthrene, 1:16, 1:23, and 1:20 g/mL for naphthalene, and 1:38, 1:49, and 1:37 g/mL for EE2 on EPA-15, -21, and -26 sediment, respectively. The experimental procedures described in refs 13 and 34 were generally followed. In brief, each reactor containing a predetermined amount of a given sediment was filled with the initial aqueous solution up to the shoulder of the ampule. Each ampule was weighed when it was empty, again after the sorbent was introduced, and again after it was filled with the initial aqueous solution. All reactors were flame-sealed and continuously mixed horizontally on a rotary shaker set at 125 rpm and 22 ( 0.5 °C. At the completion of experiments, the ampules were set upright for 2 days for separating the solid suspensions from solutions. According to our preliminary test and a prior study (13), the measured sorption data using this method are statistically the same as those obtained using centrifugation at 2500 rpm for 20 min. After settling, each reactor was broken-opened, and approximately 3 mL of the supernatant was withdrawn and mixed with approximately 2 mL of HPLC-grade methanol in a 5-mL vial capped with a Teflon top. The aqueous solute concentration of each reactor was determined from the solute concentration of the supernatant-methanol mixture measured by HPLC and a dilution factor calculated based on density data for the mixture (34). The solid-phase solute concentrations were computed with eq 1 as described below. The bisolute sorption experiments were conducted using either phenanthrene or naphthalene as the primary sorbate and EE2 as the cosorbate. Each binary system consisted of VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Freundlich Isotherm Parameters and Calculated Single-Point KD Values primary solute

Co (EE2) (µg/L)

EE2

primary solute

Co (EE2) (µg/L)

Phen

0 100 500 2000 0 100 500 2000 0 100 500 2000

primary solute

Co (EE2) (µg/L)

Naph

0 100 500 2000 0 100 500 2000 0 100 500 2000

a

sorbent

n

logKF ((µg/g)/(µg/L)n)

Na

R2

) 10 µg/L

EPA-15 EPA-21 EPA-26

0.722 (0.011)b 0.805 (0.020) 0.891 (0.022)

-0.832 (0.026)b -0.815 (0.046) -1.161 (0.036)

10 10 10

0.998 0.994 0.992

78 (71c-85d) 98 (84-114) 54 (47-61)

n

logKF ((µg/g)/(µg/L)n)

Na

R2

) 2 µg/L

EPA-15

0.711 (0.012) 0.697 (0.015) 0.724 (0.024) 0.775 (0.020) 0.708 (0.016) 0.713 (0.011) 0.712 (0.027) 0.750 (0.021) 0.786 (0.013) 0.780 (0.022) 0.763 (0.007) 0.783 (0.014)

-0.141 (0.019) -0.139 (0.024) -0.176 (0.039) -0.308 (0.031) 0.303 (0.026) 0.285 (0.018) 0.290 (0.042) 0.183 (0.033) -0.142 (0.033) -0.145 (0.036) -0.122 (0.012) -0.198 (0.023)

10 10 10 10 10 9 9 10 10 10 10 10

0.998 0.996 0.990 0.994 0.995 0.998 0.991 0.993 0.998 0.993 0.999 0.997

592 (562-623) 593 (551-628) 551 (495-613) 421 (387-458) 1641 (1529-1762) 1580 (1504-1659) 1597 (1423-1792) 1282 (1171-1403) 622 (571-677) 615 (557-678) 610 (590-662) 545 (512-581)

EPA-26

304 (281-329) 302 (265-324) 292 (248-343) 251 (220-286) 838 (752-933) 816 (757-879) 823 (689-983) 721 ((627-828) 380 (339-426) 370 (319-430) 364 (354-390) 331 (301-364)

) 200 µg/L 156 (127-167) 154 (134-175) 154 (124-192) 149 (125-178) 428 (370-494) 421 (381-465) 424 (334-589) 405 (336-489) 232 (201-268) 223 (183-273) 205 (202-229) 201 (177-228)

n

logKF ((µg/g)/(µg/L)n)

Na

R2

) 25 µg/L

KD (L/kg) @ Ce ) 250 µg/L

) 2500 µg/L

EPA-15

0.679 (0.009) 0.775 (0.022) 0.834 (0.026) 0.843 (0.020) 0.695 (0.017) 0.776 (0.018) 0.799 (0.010) 0.801 (0.011) 0.792 (0.021) 0.873 (0.011) 0.906 (0.022) 0.914 (0.017)

-1.092 (0.026) -1.359 (0.063) -1.592 (0.072) -1.614 (0.055) -0.584 (0.045) -0.788 (0.050) -0.864 (0.028) -0.874 (0.031) -1.425 (0.041) -1.632 (0.030) -1.777 (0.062) -1.813 (0.049)

10 10 10 10 10 9 10 10 10 9 9 10

0.998 0.994 0.991 0.995 0.995 0.995 0.998 0.998 0.994 0.999 0.995 0.997

29 (26-31) 21 (18-26) 15 (12-19) 15 (12-18) 98 (83-114) 79 (67-94) 72 (65-79) 70 (63-78) 19 (16-23) 16 (14-17) 12 (10-15) 12 (10-14)

14 (12-15) 13 (10-16) 10 (9-16) 10 (8-13) 48 (40-59) 47 (38-59) 45 (40-51) 45 (39-51) 12 (10-15) 12 (10-13) 10 (8-13) 10 (8-12)

7 (7-6) 8 (5-10) 7 (5-10) 7 (5-9) 24 (19-30) 28 (22-36) 28 (25-33) 28 (24-33) 7 (6-10) 9 (7-10) 8 (6-11) 8 (6-10)

EPA-21

EPA-26

b

Standard deviation. c Calculated based on n+σ and log KF+σ.

three sets of reactors that corresponded to three levels of initial EE2 concentrations of 100, 500, and 2000 µg/L, which are well below the EE2 solubility of 3100 µg/L (Table 1). Each set had the same background solution and a fixed initial aqueous concentration of the cosorbate. Each set had 10 different levels of initial aqueous concentrations of the primary solute, yielding an isotherm of the primary solute with a fixed initial EE2 concentration. Meanwhile, all reactors contained a fixed mass of the sorbent and a known volume of initial aqueous solutions. The details of the experimental procedure were very similar to those described above for single-solute sorption experiments. To evaluate potential biodegradation of EE2 in the three EE2-sorbent systems, we conducted preliminary tests following the procedure described in ref 13. No measurable EE2 degradation was observed in the systems that were run for 18 days, suggesting that NaN3 at 100 mg/L could inhibit the growth of EE2 degraders in the tested systems. For both single- and bisolute systems, control experiments were conducted using reactors prepared similarly but contained no sorbent for assessing loss of solutes to the reactor components during sorption tests. Results of triplicate reactors showed that the average solution phase concentrations of each solute were within 98-102% of the respective initial concentration of the same solution analyzed similarly. Hence, no correction was made during reduction of the sorption data. The equilibrium solid-phase concentrations of the sorbates, qe, were quantified based on a mass balance of each solute between the two phases with the following equation 9

22 (19-25) 40 (31-51) 33 (26-41)

sorbent

Number of observations.

4880

) 1000 µg/L

41(37-46) 62 (51-76) 42 (35-50)

KD (L/kg) @ Ce ) 20 µg/L

sorbent

EPA-21

KD (L/kg) @ Ce ) 100 µg/L

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 13, 2005

d

Calculated based on n-σ and log KF-σ.

qe )

Voaq(Co - Ce) Wsorbent

(1)

where Wsorbent and Voaq are the mass (g) of sorbent and the volume (L) of aqueous solution in a given reactor; Co and Ce are the initial and final residual aqueous-phase solute concentrations expressed in µg/L; and qe is in µg/g. The volume of aqueous solution was calculated from its weight assuming a density of 1.0 g/mL. Sorption Isotherm Model. The isotherm data measured for single-solute systems and for the primary solutes of the bisolute systems were fit to the Freundlich model with the following form

log qe ) log KF + n log Ce

(2)

where KF (µg1-n g-1 Ln) and n are the Freundlich sorption coefficient and the isotherm linearity index, respectively. The sorption isotherm parameters and their standard deviations were obtained by fitting the logarithmically transformed data to eq 2 using a linear regression procedure and SYSTAT software (Ver 10.0, SPSS, Chicago, IL).

Results and Discussion Sorption Isotherm Data. Table 3 lists the isotherm parameters obtained for the single solute systems and for the primary solutes of the bisolute systems. The standard errors of the isotherm parameters and the number of observations involved in each isotherm are also included in Table 3. For

FIGURE 1. Single-solute sorption isotherms measured for the three sediments. Phen and Naph represent phenanthrene and naphthalene, respectively. better comparison of sorption equilibria among different systems, the single-point distribution coefficients (KD ) qe/ Ce) with units of L/kg at three different levels of Ce were calculated from the Freundlich parameters listed in Table 3. The upper and lower limits of KD values were calculated by increasing or decreasing the isotherm parameters by one standard deviation listed in Table 3. The single-solute sorption isotherms measured for the three solutes, and three sediments are shown in Figure 1. The sorption isotherms of phenanthrene and naphthalene in the presence of the cosorbate EE2 at the initial concentrations of 100, 500, and 2000 µg/L, respectively, are shown in Figure 2. Sorption Equilibria of Single Solute Systems. Table 3 lists the isotherm parameters and their standard deviations obtained for the single solute systems. The isotherm data are also presented in Figure 1 for the nine sorbent-sorbate systems. The data listed in Table 3 and presented in Figure 1 show that all the sorption isotherms of single-solute systems can be adequately fitted with the Freundlich model with the R2 values > 0.99. All sorption isotherms are nonlinear with n values ranging from 0.679 to 0.891. Among the three

sorbates, EE2 exhibits relatively more linear sorption isotherms for a given sorbent. Among the sorbents examined, the sorption isotherms measured for EPA-26 are the most linear with n values of 0.786, 0.792, and 0.891 for phenanthrene, naphthalene, and EE2, respectively. As shown in Table 1, this sample has the lowest black carbon content among the three sediments, suggesting that the isotherm nonlinearity may be greatly influenced in the presence of black carbon as indicated in several recent studies (23, 35-37). At a fixed level of Ce, the KD values listed in the table for the three sorbates decrease as a function of Ce. For example, at Ce ) 2 µg/L, the phenanthrene KD values of EPA-15, -21, and -26 sediment scatter in a wider range of 5.92 × 102-1.61 × 103 L/kg, whereas the KD values at Ce ) 200 µg/L are two to four times lower (1.56 × 102-4.28 × 102) for the same sorbent-sorbate system. The decrease of KD as a function of Ce is primarily due to the nonlinearity of sorption isotherms (16, 22). At lower concentrations, the sorbing molecules preferentially interact with sorption “sites” having greater affinities or site energies, exhibiting greater sorption distribution coefficients. These sorption sites are more likely associated with the particulate black carbon particles characterized by relatively lower O/C atomic ratios, larger sorption capacity, and greater isotherm nonlinearity than extractable humic materials (36). Effects of EE2 on Naphthalene and Phenanthrene Sorption Equilibria. The sorptive competition between EE2 and PAHs is highly dependent upon the sorbate and the EE2 concentrations. According to Figure 1 and Table 3, the naphthalene sorption isotherms measured for the three sediments are variously affected in the presence of EE2. It is obvious that the measured naphthalene isotherm becomes more linear as the Co of EE2 increases. As shown in Table 3, the n value of the naphthalene isotherms measured for EPA-15 increases as the Co of EE2 increases from 0 to 2000 µg/L. The difference the two n values (0.834 and 0.843) measured at the higher EE2 Co levels is statistically insignificant at one standard deviation, suggesting that the competitive effect does not increase further as the EE2 concentration increases beyond 500 µg/L. The n values of the naphthalene isotherms measured for the other two sediments exhibits similar trend as a function of the EE2 concentration. The parameter KF of the naphthalene sorption isotherms is lowered in the presence of EE2. The log KF value of the naphthalene isotherms measured for EPA-15 decreases from -1.092 to -1.614 as the Co of EE2 increases from 0 to 2000 µg/L. The difference between the two log KF values measured at the higher EE2 Co levels is statistically insignificant at one standard deviation (Table 3). Similar trend can be found for the log KF value of the naphthalene isotherms measured for EPA-21 and -26. The competitive effect of EE2 on the capacity of the naphthalene sorption isotherms is even better reflected by the single-point KD values calculated at a given Ce level. According to Table 3, the naphthalene KD value calculated at a fixed naphthalene Ce for a given sorbent decreases as a function of the Co of EE2. At a fixed EE2 Co level, the overall competitive effect of EE2 on the naphthalene sorption decreases as Ce of naphthalene increases. The observed competitive sorption is statistically significant at one standard deviation level (Table 3). In the high Ce of naphthalene, the presence of EE2 appears to yield increased sorption capacity for naphthalene, which is contrary to the observations in the low Ce range. As shown in Table 3, the naphthalene KD values calculated at naphthalene Ce ) 2500 µg/L for the three binary solute systems of each sorbent are 5.8-18.9% greater than those calculated for their respective single solute system. This indicates that the competitive effect of EE2 disappears and that a synergetic effect may take place at very high concentration levels of naphthalene. VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Bisolute sorption isotherms measured for the three sediments. Phen and Naph represent phenanthrene and naphthalene, respectively. Contrary to naphthalene, the sorption of phenanthrene is affected by EE2 only when the phenanthrene concentrations are low and the cosorbate EE2 concentrations are very high (Figure 2). According to Table 3, the n values are 0.775 and 0.750 for the phenanthrene isotherms measured at Co (EE2) ) 2000 µg/L for EPA-15 and -21, respectively. The n values are statistically greater than those measured for their respective single solute systems. The phenanthrene KD values calculated at its Ce ) 2 µg/L for EPA-15, -21, and -26 decreased as Co (EE2) increased from 0 to 2000 µg/L. The corresponding reductions in the KD values of EPA-15, -21, and -26 are 28.9, 21.9, and 12.4%, respectively, at EE2 Co ) 2000 µg/L and EE2 Ce ) 1430-1670 µg/L. Table 3 indicates that the competitive effect observed at Co (EE2) ) 2000 µg/L decreases or becomes statistically insignificant as phenanthrene concentration increases. For example, at Ce of phenanthrene ) 200 µg/L, the calculated KD values for EPA-15, -21, and -26 are 1.49 × 102, 4.05 × 102, and 2.01 × 102, respectively, whereas the KD values of the respective single solute systems are 1.56 × 102, 4.28 × 102, and 2.32 × 102. 4882

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Effects of Phenanthrene and Naphthalene on EE2 Sorption. The same sets of bisolute sorption data were recalculated to reflect the effects of phenanthrene and naphthalene on the single-point KD values of EE2. The results are selectively presented in Figure 3 (a-d). Inspection of these figures indicates that the sorption of EE2 is affected in the presence of phenanthrene and such an effect decreases as the EE2 concentration increases. As shown in Figure 3a, the KD values measured for EE2 at Co (EE2) ) 100 µg/L decrease from 1.13 × 102 to 0.97 × 102 and to 0.77 × 102 L/Kg for EPA-21 as the phenanthrene Ce increases from 6.5 to 45.3 and to 194.8 µg/L, respectively. The EE2 KD values measured at the same Co for the other two sediments also decrease by 20.1-22.9% and 30.0-35.6% as the phenanthrene Ce increases from 6.5 to 45.3 and to 195 µg/L, respectively. At Co (EE2) ) 500 µg/L (Figure 3b), the EE2 KD values measured for EPA 15, -21, and -26 decrease from 0.52 × 102, 0.96 × 102, and 0.54 × 102 L/Kg at Ce (phenanthrene) ) 6.3 µg/L to 0.33 × 102, 0.51 × 102, and to 0.40 × 102 L/Kg at Ce (phenanthrene) ) 136 µg/L, respectively. As shown in Figure 3c, the EE2 KD

FIGURE 3. The changes of single-point distribution coefficients (KD) of EE2 as a function of Ce of phenanthrene (a-c) and naphthalene (d). values measured at Co (EE2) ) 2000 µg/L for the three sediments have a smaller change as the phenanthrene Ce increases, indicating that phenanthrene has a weaker effect on the sorption of EE2. The lower KD values at the higher Co concentration result solely from the nonlinearity of the EE2 sorption isotherms. Unlike phenanthrene, naphthalene appears not to affect the sorption of EE2 by the three sediments. Figure 3d presents the EE2 KD values measured for the three sediments at the lowest Co (EE2) (100 µg/L) tested in this study, the level at which the competitive sorption, if present, would be the most prominent. The data shown in this figure indicate that the EE2 KD value for each sediment is constant over the examined range of phenanthrene Ce, suggesting that naphthalene does not compete with EE2 for sorption. Possible Mechanisms for Competitive vs Synergetic Effects. The observed competitive sorption between EE2 and naphthalene/phenanthrene is generally consistent with the competitive sorption phenomena observed for bisolute systems consisting of prioritized TOPs (10, 11, 14, 16, 1820). It is apparent from this study that the sorption isotherm of the primary sorbate becomes more linear as the cosorbate concentration increases and that the competitive effect is the greatest when the primary sorbate (naphthalene or EE2) concentration is low and the competing sorbate (EE2 or phenanthrene) concentration is high. This competitive sorption can be generally explained via a conceptual dualmode sorption model predicated on a hypothesis that SOM consists of two types of domains: a hard carbon SOM domain and a soft carbon SOM domain (38, 39). Sorption to the soft carbon domains may follow a linear partitioning process without capacity limitation and hence having no competition among different sorbates. Conversely, sorption in the hard carbon domains may be dominated by nonlinear adsorption process on their external and internal surfaces. Because of its capacity limiting nature, the hard carbon SOM domains exhibit strong competitive sorption among different sorbates.

As shown in Table 1, the three sediment samples contain black carbon, which, by definition, is a major hard carbon SOM that exhibits nonlinear and competitive sorption for a range of TOPs (39, 40). Kerogen and glassy humic acid have been shown to exhibit varied nonlinear and competitive sorption behavior for TOPs (20, 41-43). When associated with sediments, the hard carbon SOM domains tend to dominate the sorption in relatively low Ce ranges (34, 36). As Ce increases, the soft carbon SOM domains become dominant in the overall sorption by sediments. This is consistent with our observation that the competitive sorption is predominant at lower Ce of naphthalene. While BC may be a major SOM component responsible for the observed competitive sorption, the sorptive properties measured for the sediments cannot be correlated quantitatively with their BC content. The percentage reduction of naphthalene KD values at Ce ) 25 µg/L is on the order of EPA-15 > EPA-26 > EPA-21 for each of the three different Co(EE2) levels, whereas the BC content on a TOC basis is on the order of EPA-15 > EPA-21 > EPA-26 (Table 1). A similar trend can be found for the EE2 effect on phenanthrene sorption. Apparently, the difference in the observed competitive sorption among the three sediments may result from several factors such as the difference in the physicochemical property of BC materials associated with different sediments, varied contributions of sorption from non-BC fractions (e.g., kerogen and humic acid), and the partial accessibility of BC due to its encapsulation within inorganic and organic matrixes. The enhanced sorption capacity at the high naphthalene Ce level is also consistent with a prior study by Weber et al. (20), who reported the synergetic effect in the high concentration ranges for two bisolute systems, trichloroethenephenanthrene and 1,4-dichlorobenzene-phenanthrene. Their interpretation is that the swelling of SOM matrixes in the presence of massive concentrations of cosorbate has an effect similar to organic matrix swelling at relatively high temperVOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ature. In both cases, energy input (respectively chemical and thermal in character) is increased, leading to increased disordering of SOM structure and hence enhanced capacity of SOM matrix for accommodating more sorbate molecules. The difference of the competitive effect observed between phenanthrene and naphthalene suggests that the competitive sorption highly depends on both the relative sorption affinity of the coexisting solutes and the concentration of the cosorbate. In general, the solute with greater sorption affinity for a given sorbent can be strongly competitive against the solute having lower sorption affinity. As mentioned above, the hydrophobicity and hence the sorptive affinity of the three solutes examined in this study is on the order of phenanthrene > EE2 > naphthalene. In the EE2-naphthalene binary system, the cosorbate EE2 is more hydrophobic and less soluble than naphthalene. It can strongly compete for sorption sites against naphthalene even at low EE2 concentrations, hence exhibiting a very significant competitive effect of EE2 on naphthalene sorption (Table 3) but little or no effect of naphthalene on EE2 sorption (Figure 3d). Conversely, in the EE2-phenanthrene binary system, the cosorbate EE2 is less hydrophobic than phenanthrene, and the competitive effect of EE2 is measurable only at much higher EE2 concentrations (Table 3). However, phenanthrene can compete with EE2 as expected and hence can lower the KD values for EE2 (Figure 3a). It should be pointed out that the competitive effects observed between EE2 and phenanthrene or naphthalene are different from those observed between the chemicals having similar structure but very different hydrophobicities. Xiao (12) found that, for a bisolute system, naphthalene can also compete with phenanthrene, the more hydrophobic sorbate, for the sorption by soils. As the naphthalene Ce/SW ratio increases from 0 to 0.2, the n value of the phenanthrene sorption isotherm increases from 0.73 to 0.94. This is quite different from our finding that only the sorbate with greater hydrophobicity exhibits a competitive effect on the sorption of the coexisting sorbate with lower hydrophobicity. It is likely that the competitive sorption between EE2 and the two PAHs examined in our study is weaker than that between PAHs due to the fact that EE2 has different molecular structure and polarity than PAHs. Environmental Implications. The competitive sorption observed in this laboratory study suggests that EE2 could influence the fate and transport of organic pollutants with log KOW values < 4 and that its fate and transport could be affected similarly by coexisting organic pollutants with log KOW values > 4. As mixed pollutants are often found in contaminated surface aquatic and groundwater systems, the competitive effects of coexisting organic pollutants on the sorption and other related fate and transport processes of individual chemicals should not be ignored. Our study indicates that the competitive effect on a target pollutant (EE2) is prominent only if the aqueous phase concentrations of the background pollutants (e.g., phenanthrene) exceed 10 µg/L. For individual pollutants, this concentration level could be found only in heavily polluted environments such as hazardous waste sites. However, in moderately polluted aquatic systems such as the downstream of the effluent discharge point of a wastewater treatment facility, the total concentration of the pollutant mixtures may be at the µg/L levels or higher. The collective effect of multiple classes of organic pollutants on individual chemicals may not be ignored even though the concentration of each pollutant is below the µg/L level. Modeling the fate and transport of individual organic pollutants should therefore consider the competitive sorption by the background pollutants present in aquatic systems. 4884

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Acknowledgments This study was funded by the National Science Foundation under Grant No. 0404487 and by CSREES/USDA under Grant 2001-35107-11129. Partial funding was also provided by the Chinese Natural Science Foundation through the International Young Investigator Program (40128002) to W.H. and (40102009) to Z.Y.

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Received for review September 15, 2004. Revised manuscript received April 13, 2005. Accepted April 22, 2005. ES048558K

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