Environ. Sci. Technol. 2009, 43, 7370–7375
Sorption of Buckminsterfullerene (C60) to Saturated Soils CHIA-YING CHEN AND CHAD T. JAFVERT* Purdue University, School of Civil Engineering, West Lafayette, Indiana 47907
Received April 1, 2009. Revised manuscript received July 31, 2009. Accepted August 10, 2009.
With the increasing use of C60 in many industrial and commercial sectors, it is likely that it will eventually appear in the environment; however, its environmental fate and transport is still largely unknown. The extent to which C60 partitions to soil will contribute to its environmental fate and bioavailability. Because C60 is extremely hydrophobic, in this study the distribution between soil and mixtures of ethanol (EOH) and water were measured at ethanol mole fractions ranging from XEOH ) 1.0-0.4 for two soils. By measuring Kp at XEOH ) 1.0 for a series of soils that ranged in organic carbon and clay mineral content, possible mineral contribution to the overall partition process was found for some of the soils. After correcting for any mineral contribution to sorption, the organic carbon normalized partition coefficient, Koc, at each value of XEOH was calculated from the measured Kp values. Through a classical thermodynamic relationship, the Koc values determined at XEOH ) 1.0-0.4 were extrapolated to estimate the pure water (i.e., XEOH ) 0) Koc value of 107.1 (L/kg). Accounting for any dissolved organic matter (DOM) in any pure water-soil mixtures may lower this estimate by over a factor of 2, placing this estimate in good agreement with C60’s octanol-water partition coefficient, Kow ()106.7).
Introduction Since Buckminsterfullerene (C60) was discovered in 1985 (1), the unique physicochemical properties of it and its modified derivatives are motivating factors behind industry to find new and unique applications for this class of nanomaterials (2). With the widespread use of C60, there is no doubt that C60 will eventually appear in the environment. Indeed, even without intended production, it is found in particulates emitted from coal-burning power plants (3). Although its environmental fate remains largely unknown, some potential adverse effects on health and the environment have been reported. Interpreting the results of many of these studies however is confounded by the fact that addition of C60 to the media generally occurs via addition of aqueous phase “clusters” which are nanoparticles of C60 with diameters generally in the range of 100-500 nm. Clusters, known as nC60, form a stable colloidal suspension in water, and are generally prepared by solvent exchange techniques (4, 5) or by long-term stirring of solid material in pure water over several weeks, or short-term ultrasonication in water (6, 7). C60 cluster preparations and some of its derivatives have been shown to have antibacterial or inhibitory effects to a broad * Corresponding author phone: (765) 494-2196; fax: (765) 4961107; e-mail:
[email protected]. 7370
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range of bacteria (5, 8). A study by Oberdo¨rster indicated that nC60 can cause oxidative damage and depletion of glutathione (GSH) in vivo in largemouth bass (9). Although C60 administered as aqueous clusters were found to be potentially harmful both in vitro and in vivo, several authors have suggested the toxicity is due to the method of cluster preparation (7, 10), due to solvation by the solvent used to prepare the clusters (tetrahydrofuran) or due to a solvent degradation product (γ-butyrolactone) (10). Because nC60 forms stable aqueous colloidal suspensions, information on the environmental fate and transport of C60 reported in the literature has emphasized nC60 transport, yet C60 is a molecule with a molecular weight less than that of many common chemicals of concern, including brominated flame retardants. It is quite soluble in numerous organic solvents, and its molecular aqueous solubility recently was reported at 7.96 ng/L (11). Hence, molecular C60 does exist in water, and it is through this dissolved concentration or activity that its environmental distribution will be regulated (i.e., thermodynamically controlled), including its selfassociation to form clusters or nanoparticles. In addition to aqueous solubility, the organic carbon content normalized soil-water partition coefficient, Koc, is an important parameter that regulates distribution in the aquatic environment, and through which a bulk soil-water partition coefficient, Kp, can be estimated. In the present study, we report on the phase distribution of molecular C60 between soil and ethanol-water mixtures and through a classical thermodynamic relationship, extrapolate the measured partition coefficients to the soil-water partition coefficient (Kp) that would occur in the absence of the ethanol cosolvent. The use of the water-miscible cosolvent was necessary as the aqueous solubility of C60 is lower than most instrument detection limits unless several liters of water are concentrated prior to analysis. The presence of cosolvent effectively increases the solubility of C60 to facilitate the measurements of partition coefficients and eliminate the inaccuracy from C60 lost to the vessels. Based on the thermodynamic correlation, the partition coefficient of C60 between soil organic carbon and pure water (Koc) was estimated.
Materials and Methods Materials. Sublimed C60 (99.9%) was purchased from MER Corp. (Tucson, AZ). Ethanol, methanol, and toluene were of HPLC grade or better and all chemicals were used as received. Water was purified with a Barnstead Nanopure system (Dubuque, IA). Ethanol-Water Mixtures. To prepare C60 solutions at various ethanol-water ratios, C60 was first precipitated on the walls of glass test tubes via solvent evaporation from a toluene solution. Ethanol and water were mixed to give solutions at ethanol mole fractions (XEOH) of 0.42, 0.55, 0.64, 0.74, 0.85, and 1.0. Each solution was adding to a tube contain C60 precipitate and mixed on a horizontal shaker at low speed at 25 °C in a constant temperature room for g1 week. To prevent cluster formation and to ensure complete dissolution of the C60, the mass of C60 added to each tube was less than or equal to the amount required to reach one-half the solubility at each respective ethanol-water ratio. As a precaution, after equilibration each solution was centrifuged at 5000 rpm (i.e., 2,380 g) for 30 min in a Sorvall SA 600 rotor to remove any residual suspended particles. The supernatant C60 concentrations were determined by HPLC. Direct UV/vis analyses of all ethanol-water mixtures showed the strong absorption peaks of molecular C60 from 200-350 nm, whereas 10.1021/es900989m CCC: $40.75
2009 American Chemical Society
Published on Web 08/25/2009
TABLE 1. Physical and Chemical Properties of Soils (29) particle size (%) soil ID
CEC (meq/100 g)
sand
silt
clay
OC (%)
cm/oc
3 6 9 10 11 15 24 25
12.70 33.01 12.40 14.58 13.86 11.30 6.28 8.86
42.4 0.2 7.1 6.3 1.7 15.6 20.5 41.9
33.5 31.2 75.6 71.9 48.8 48.7 58.7 37.6
24.1 68.6 17.4 21.8 49.4 35.7 20.8 20.5
1.53 0.72 0.11 2.1 1.5 0.95 0.95 0.76
15.75 95.28 158.18 10.38 32.93 37.58 21.89 26.97
C60 aggregates (nC60) would display a red-shifted and broader less intense peak (4). Additionally, the characteristic broad absorption of nC60 between 400 and 500 nm is not observed for these solutions (5). The resulting ethanol-water stock solutions had initial C60 concentrations ranging from 0.01 to 0.9 mg/L. These solutions were further diluted with the C60free solvent mixtures at the same respective solvent mole fraction ratios, prior to adding to soil. Partition Coefficient Measurement. Table 1 lists some basic properties of the soils including the cation exchange capacity (CEC), organic carbon (OC) content, and the mass ratio of clay mineral to organic carbon (cm/oc). The distribution of C60 between soil and the ethanol-water mixtures were measured at 25 °C using two soils (3 and 15) at all the reported XEOH fractions, and for all soils at XEOH ) 0.85 and 1.0 (i.e., pure ethanol). The latter experiments were performed to investigate possible mineral contribution to sorption, as the group of soils spans a wide range in cm/oc. To construct sorption isotherms, the soil mass (g) to solution volume (mL) ratio of each ethanol-water and soil mixture was adjusted until recovery of C60 in the liquid phase was between 40 and 60%. Duplicate and sometimes triplicate soil-solution mixtures at various C60 concentrations were equilibrated in 15 mL screw-capped centrifuge tubes on an angular rotator for 72 h, as test samples equilibrated longer that 72 h showed no additional sorption to the soil phase. At 72 h, samples were centrifuging at 5000 rpm for 30 min in a Sorvall SA 600 rotor, and the concentration of C60 in each supernatant was analyzed directly by HPLC. To determine possible loss to the test tube walls, control tubes with no soil were prepared, equilibrated, centrifuged, and analyzed along with sample tubes, and showed no significant loss occurred to the glass walls. C60 Analysis. All stock ethanol-water mixtures and sample supernatants were analyzed directly by reverse-phase HPLC using a UV/vis detector set at 336 nm. Either a Discovery C-18 column (15 cm × 3 mm I. D., 5 µm particle size) with a mobile phase of methanol/toluene (60:40) or a Cosmosil 5PBB column (25 cm × 4.6 mm I.D., 5 µm particle size) with 100% toluene as the mobile phase was used. It should be noted that many HPLC detector flow-through cells are not compatible with toluene as a mobile phase solvent. Theory. Sorption equilibrium of an organic chemical is defined as the state at which the sorbed and solution phase species have equal fugacities (12-14). In the following derivation, a basic assumption is that chemical concentrations in all phases are dilute. Some parameters (e.g., activity coefficients) were interpreted accordingly. Typically, the partition coefficient, Kp, is defined by, Kp )
C*s φ*l Vlγ*l ) ) C*l φ*s Vsγ*s
(1)
where the subscripts s and l denote the sorbed and liquid phases, respectively, and the asterisk signifies mutual
saturation of the phases. C*s and C*l denote sorbed and liquid phase concentrations, respectively, where C*l conventionally is referenced to aqueous or liquid phase volume and C*s to the dry mass of the soil or sediment. Therefore, Kp generally is reported with units of L/kg. φ*s and φ*l are fugacity coefficients, which are equal to the product of the corresponding activity coefficient (γ*) and reference state fugacity. Vl and Vs are the molar volumes of liquid and sorbent phase, respectively; hence the ratio Vl/Vs is simply a unit adjustment factor converting the mole fraction-based partition coefficient *l ) γ*/γ *) (Kp’ ) X */X s l s to the mass fraction-based partition coefficient, as the density of water ≈ 1.0 kg/L and the concentrations are sufficiently dilute. For both phases, a convenient reference state is the true liquid, or for solids the hypothetical subcooled liquid state of the compound for which the chemical activity is 1. Since the “liquid” chemical at its solubility in water is in equilibrium with this reference state, the corresponding aqueous activity coefficient is defined by γl )
1 1 ) Xl,sat S lV l
(2)
where Xl,sat and Sl are the solubility or hypothetical subcooled liquid solubility of the compound in mole fraction and mol/L units, respectively. For solutions that are under-saturated (X < Xsat), and because we assume low chemical concentrations in both phases, γ values are assumed constant at any chemical concentration for any combination of sorbent-liquid system (i.e., Henry’s constant domain). Combining eqs 1 and 2 yields Kp )
γ*l 1 · SlVsγ*s γl
(3)
or: log Kp ) -log Sl - log Vs - log γ*s + log(γ*/γ l l)
(4)
For the case of ambient temperature solids like C60, a crystal energy term that accounts for the energy required to melt the compound can be used to convert the ambient temperature solubility of the solid to the subcooled liquid solubility (15): Sl ) Sl,c ·
ln
(
)
fL fc
(5)
(
)
∆hfus Tt ∆cp Tt ∆cp Tt fL ) -1 -1 + ln RTt T R T R T fc
(6)
where Sl,c is the aqueous solubility of the crystalline material; f L and f c are the fugacities of the hypothetical liquid state and pure crystalline state, respectively; ∆hfus and ∆Sfus are the enthalpy of fusion and entropy of fusion, respectively; Tt is the triple-point temperature (K), and cp is the heat capacity of the solid phase. Since, the first term on the righthand side is the dominant term and the triple-point temperature is usually close to the normal melting temperature in most cases, eq 6 generally is simplified to ln
(
)
(
∆hfus Tm ∆Sfus Tm fL ) -1 ) -1 c RTm T R T f
)
(7)
ln f L /f c in eq 7 is generally referred to as the ‘crystal energy term’ or Ec and was estimated to be 6.24 for C60 (16). Note however that here this term is defined as a positive (+) number to decrease any ambiguity regarding its sign. Equation 4 therefore can be expressed as: VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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log Kp ) -log Sl,c -
Ec - log Vs - log γ*s + log(γ*/γ l l) 2.3026 (8)
In this study, partition coefficients were determined with mixtures containing significant ethanol. Previous work indicates that ethanol does not form solvates with C60 (16), suggesting constant Ec as a function of mole fraction ethanol. The term Vs can be viewed as the average molar volume of the sorbent phase and constant, however for dilute solutions it is strictly a remnant of the mole fraction unit convention and may be considered an intercept-correction term. This is especially true when the sorbent phase is a polymer of equivocal molecular weight; and as we will show, calculations performed with weight fraction units are much less awkward (15). For example, if the sorbent phase is the organic carbon in the soil, Vs is the average molar volume of the organic carbon, γ*s is the activity coefficient of C60 in the organic carbon phase, and eq 8 defines log Koc. Additionally, we will show that for nonhydrogen bonding neutral chemicals, values of γ*s for “dissolution” into soil organic carbon increase only by about an order of magnitude over an increase in aqueous solubility of about 8 orders of magnitude. For Kp (or Koc) values measured with the same solute and soil with decreasing ethanol volume fraction, some change in γ*s may result, however, these changes are likely small and proportional to Sl. With increasing dissolved organic matter (DOM) content and increasing solute hydrophobicity, the ratio γ*/γ l l decreases due to greater interactions of solute molecules with DOM. At the ethanol-water ratios employed in this study, any DOM effect will be minimal, and so, γ* ≈ γl, resulting in good l correlation between log Kp and log Sl (or log Sl,c) values calculated at different XEOH values. In a previous study, the solubility of C60 was measured in ethanol-water mixtures at ethanol volume fractions from 0.5 to 1.0, and an estimate of the pure aqueous solubility was given by ref 11. Using Wohl’s equation, described in detail elsewhere (11, 16), to model the activity coefficient of C60 as a function of mole fraction composition, the reported solubilities were used to train Wohl’s model by adjusting the size parameter for water (qwater ) 0.75) until accurate predictions results, as shown by the line in Figure 1. The resulting model was used to calculate the solubilities of C60 at each value of XEOH used in this study.
FIGURE 1. C60 solubility as a function of mole fraction ethanol (XEOH) in ethanol-water mixtures with measured values from reference (11) displayed as circles and Wohl’s model, with qwater ) 0.75, displayed as a line. q is “the effective volume” of the pure or hypothetical pure liquid of each respective component.
Results and Discussions Sorption isotherms of C60 distributing between EOH-water mixtures and two different soils are shown in Figure 2. Kp values, equal to the slopes of the isotherms, sharply increase with decreasing XEOH from 1.0 to 0.4, from Kp ) 0.9-78.2, and 2.4-129.4 (L/kg) for Soil 3 and 15, respectively. Similar trends in Kp values of other hydrophobic compounds in watermiscible cosolvents mixtures have been reported in the literature (17, 18). Although partitioning to clay minerals and other soil constituents may be important in some cases, it is widely recognized that at low swelling clay mineral to organic carbon ratios, sorption to soil organic matter (as measured by organic carbon content) dominates the overall phase distribution process (13), resulting in a nearly constant value for the organic carbon content normalized partition coefficient, Koc, for any nonhydrogen bonding compound, Koc )
Kp foc
(9)
where foc is the mass fraction of organic carbon in the soil. Values of Koc for all sorption isotherms shown in Figure 2 are reported in Table 2 and show that Koc values for Soil 15 were higher than those for Soil 3 at each respective XEOH. As a 7372
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FIGURE 2. Sorption isotherms of C60 after 72 h of equilibration for (a) Soil 3 and (b) Soil 15 at different values of XEOH. result, sorption isotherms were measured for the six additional soils listed in Table 1 at XEOH ) 1 and 0.85 to test for potential contribution to sorption by the swelling clay mineral content of Soil 15. Among these soils, the ratio of the mass fraction of swelling clays in the soil, fcm (g/g) to mass fraction of organic carbon in the soil (foc) ranges from 10 to 158, with Soil 15 having fcm/foc ) 37.6. The observed sorption partition coefficient Kp,obs can thus be expressed as a sum of its component contributions (13), Kp,obs ) Km fcm fa,cm + Koc foc fa,oc
(10)
where Km is the partition coefficient for solute sorption to the clay minerals (L/kg), and fa,cm and fa,oc are the active or available fraction (0 < fa < 1) of each respective phase. These
TABLE 2. Hypothetical Subcooled Liquid Solubilities (Sl) of C60 and observed Organic Carbon-Normalized Partition Coefficients (Koc,obs) for Two Soils log Koc,obs XEOH (mol/mol) 1 0.85 0.74 0.64 0.55 0.42 a
V
a EOH
(L/L)
1 0.95 0.90 0.85 0.80 0.70
log Sl (mol/L)
Soil 3
Soil 15
-2.91 -3.20 -3.50 -3.81 -4.11 -4.71
1.79 1.97 2.42 2.60 3.13 3.71
2.40 2.85 3.09 3.38 3.57 4.13
VEOH is the ethanol volume fraction.
FIGURE 4. Correlation between log Koc and the subcooled liquid solubility of C60 (∆ Soil 15; 0 Soil 3, --- model line, 2 predicted Koc at the subcooled liquid solubility in pure water).
FIGURE 3. Observed Koc values as a function cm/oc ratio at XEOH ) 1 for all soils reported in Table 1. latter terms account for the fact that each phase may shield or block available sorption sites on the other phase. In particular, sufficient soil organic carbon may coat some of the clay minerals preventing sorption of the chemical to a significant fraction of the minerals (13). As a result, the differences between true Koc values that account for sorption only to OC and observed values, Koc.obs, calculated from measured Kp values assuming sorption occurs only to OC, generally become significant at higher fcm/foc ratios, Koc,obs ) Km · fa,cm · (fcm /foc) + Koc
(11)
All values of the apparent Koc determined with eq 9 for all the soils at XEOH ) 1 and 0.85 are shown in Figure 3 and Supporting Information (SI) Figure S1, respectively, as a function of fcm/foc. Especially at XEOH ) 1, results are quite similar to those found by Karickhoff et al. (13) who found an upper limit of fcm/foc ) 30, below which Koc,obs ) Koc for two compounds with Koc values e104. In our case, a threshold of fcm/foc ≈ 20 is observed above which the Koc,obs values appear to be scattering, also similar to that reported by Karickhoff et al. Hence, the higher ratio of fcm/foc ()37.58) for Soil 15 suggests the first term on the right-hand-side of eq 11 is significant for this soil and accounts for the higher observed carbon normalized partition coefficients reported in Table 2, whereas partitioning to Soil 3 is dominated by sorption to organic carbon. Therefore, for Soil 3, eq 4 can be normalized to the organic carbon content of the soil,
the Flory-Higgins polymer model to account for this change in log γ*oc with the resulting change proportional to the logarithm of the molar volume of each compound. In our case, the size of the solute molecule (C60) is constant, and so unless the activity of ethanol in the organic carbon phase significantly effects the solute-sorbent interactions, the value of log γ*oc should remain fairly constant and independent of XEOH. As discussed above, Voc can be considered an intercept correction factor. For the values reported in Table 2, regression of log Koc,obs versus log Sl (eq 12) results in (log Koc.obs ) -1.10 × log Sl - 1.487) for Soil 3; and (log Koc,obs ) -0.92 × log Sl - 0.171) for Soil 15. Because the slopes of both regressions are ∼ -1 as expected, recalculating the intercepts with the slopes set to -1 results in: (log Koc,obs ) -log Sl 1.101) for Soil 3; and (log Koc,obs ) -log Sl - 0.467) for Soil 15. Again, the 0.63 log unit shift in the intercept for Soil 15 is attributable to sorption to clay minerals. After adjusting the log Koc,obs values for Soil 15 by this constant shift ()0.63), the regression of log Koc versus log Sl (with intercept ) -1.101) for both soils is shown in Figure 4, were Sl are the subcooled liquid solubilities of C60 calculated at each experimental value of XEOH (11). Because the aqueous solubility of C60 has been reported at 7.96 ng/L (1.11 × 10-11 M) (11), the regression equation can be used to extrapolate to XEOH ) 0, where the partition coefficient is for the chemical distribution between soil and pure water, and is equal to (Koc )) 107.1 (L water/kg OC). Because the extreme hydrophobicity of C60 makes the direct experimental determination of Koc quite challenging, our value calculated by extrapolation can be compared to Koc values of other compounds through its octanol-water partition coefficient, Kow, and estimated aqueous solubility. Water solubilities and Kow and Koc values reported in the literature for 28 other compounds are given in the SI. To simplify comparisons, subcooled liquid solubilities of ambient temperature solids are used in all subsequent calculations and figures. Additionally, since Voc is an ambiguous term for soil organic carbon, and because chemical concentrations in both phases are sufficiently dilute, activity coefficients can be conveniently defined on a mass-fraction basis, where the water phase activity coefficient γ*l,m equals the reciprocal mass fraction based solubility, Sl,m (g/g) (15, 20), γ*l,m )
log Koc ) -log Sl - log Voc - log γ*oc + log(γ*/γ l l) (12) where variation in log Sl dominates any change in log Koc (13). As we will show for a series of compounds, log γ*oc slightly decreases with increasing log Sl. Chiou et al. (19) have invoked
1 Sl,m
(13)
and Koc,m )
∗ C oc,m ∗ C l,m
)
∗ γl,m
(14)
∗ γoc,m
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or with conventional units on Kow after correcting for the density of octanol, R ) 0.690 and 0.428, respectively, providing support for the notion that Koc ≈ 0.5 · Kow for moderately hydrophobic compounds. The value of Koc for C60 (107.1 L/kg) determined by extrapolating the line in Figure 4 to C60’s water solubility value does not account for DOM effects in the absence of the ethanol cosolvent. Solubility enhancement by DOM has been addressed by many authors and is often quantified by considering it a separate phase to which partitioning occurs (22, 23), Koc ) Koc,purewater(1 + KDOC[DOM])-1
FIGURE 5. Comparison of chemical activity coefficients in water (0), octanol (O), and organic carbon (∆) phases regressed against log Kow and (actual or subcooled) liquid solubility. The solid symbols are values for C60 with all other data reported in SI Table S1 for the other 28 compounds. where concentrations in organic carbon and water (C*oc,m and C*w,m, respectively) have units of g/g. With this unit convention, the corresponding equation to eq 12 is ∗ log Koc,m ) -log Sl,m - log γoc,m
(15)
Since the density of water at 25 °C (Fw ) 0.998 g/mL) is near unity, Koc,m ) Koc.; however, conventional volume-based Kow values (M/M) are converted to Kow,m by multiplying by (Fw/ Fo), where the density of octanol, Fo is 0.824 g/mL, and where Kow,m )
∗ γw,m ∗ γo,m
(16)
Since log Kow values are generally compared (not Kow values), this ≈20% correction on Fo results in log Kow,m ≈log Kow. With eqs 13, 15, and 16, the unambiguous relationship between activity coefficients for a chemical in water, soil organic carbon, and octanol can be examined. Figure 5 shows the magnitude of these values regressed against log Kow,m (g/g) values for those chemicals listed in the SI, and for C60 * * determined in this study (γoc,m ) and our previous study (γo,m , * * * ) (11). Figure 5 also shows log γo,m and log γoc,m regressed γw,m against log Sl,m, for which, log γ*o,m ) -0.107log Sl,m + 0.104
(17)
log γ*oc,m ) -0.211log Sl,m + 0.037
(18)
Regressed directly or by combining eqs 15 and 18, the Koc,m - Sl,m relationship for these data is defined, log Koc,m ) -0.79log Sl,m - 0.037
(19)
and because Fw ≈ 1.0 g/mL, Sl,m with units of g/mL produces Koc with conventional units of L/kg OC (i.e., Koc ) Koc,m). Previously, Karickhoff has suggested the relationship Koc ≈ R · Kow where the proportionality constant is R ) 0.63 (21) or R ) 0.411 (12). Taking the ratio of eq 17 to 18, where Rm ) (γ*o,m/γ*oc,m) ) (Koc,m/Kow,m), and solving at log Sl,m ) -3 and -5 (1 mg/L and 10 µg/L), results in Rm ) 0.569 and 0.352, 7374
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(20)
where the Koc is the partition coefficient measured in water with a specific DOM concentration (kg/ L), and KDOC is the partition coefficient between DOC and pure water, and Koc,pure water is the Koc value in the absence of DOM. DOM in most natural waters is usually below 60 mg/L (24), and log KDOC values range between 4 and 6 for hexa- to octa-chlorodibenzop-dioxins (25) and is between 4 and 5 for p,p’-DDT (22). Thus, a maximum correction on our value of Koc for C60 can be C60 ) 6.2 to 7.1 in natural estimated at ≈ 0.9 log units, (log KOC waters), placing it in good agreement with C60’s reported log Kow of 6.67 (11). Compared to many compounds of environmental concern, such as p,p’-DDT (26), PCBs (19), PAHs (21), and many pesticides (27), the organic carbon normalized partition coefficient of C60 is extremely high. Our estimated Koc value for C60 is of a similar magnitude to those of hexa- through octa-chlorinated dibenzo-p-dioxins and dibenzofurans, which also have extremely low water solubilities (0.4-36.1 ng/L for hexa- to octa- chlorodibenzo-p-dioxins) (28).
Acknowledgments The financial support by the National Science Foundation under award EEC-0404006 is acknowledged. We thank Dr. Changhe Xiao for technical assistance.
Supporting Information Available Additional information on data in Figures 3 and 5 is provided. This material is available free of charge via the Internet at http://pubs.acs.org.
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