Contaminant-mobilizing capability of fullerene nanoparticles (nC60): Effect of solvent-exchange process in nC60 formation

Authors

  • Lilin Wang,

    1. College of Environmental Science and Engineering, Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin, China
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  • John D. Fortner,

    1. Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri, USA
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  • Lei Hou,

    1. College of Environmental Science and Engineering, Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin, China
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  • Chengdong Zhang,

    1. College of Environmental Science and Engineering, Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin, China
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  • Amy T. Kan,

    1. Department of Civil and Environmental Engineering, Rice University, Houston, Texas, USA
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  • Mason B. Tomson,

    1. Department of Civil and Environmental Engineering, Rice University, Houston, Texas, USA
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  • Wei Chen

    Corresponding author
    1. College of Environmental Science and Engineering, Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin, China
    • College of Environmental Science and Engineering, Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin, China
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Abstract

Fullerene nanoparticles (nC60) in aqueous environments can significantly enhance the transport of hydrophobic organic contaminants by serving as a contaminant carrier. In the present study, the authors examine the effect of the solvent-exchange process on nC60 aggregate formation and, subsequently, on nC60's contaminant-mobilizing capability. A series of nC60 samples were prepared using a modified toluenewater solvent-exchange method through the inclusion of a secondary organic solvent in the phase transfer of molecular C60 in toluene to nC60 in water. Two groups of solvents—a water-miscible group and a non–water-miscible group—of varied polarity were selected as secondary solvents. The involvement of a secondary solvent in the phase transfer process had only small effects on the particle size and distribution, ζ potential, and mobility of the nC60 products but significantly influenced the capability of nC60 to enhance the transport of 2,2′,5,5′-polychlorinated biphenyl (PCB) in a saturated sandy soil column, regardless of whether the secondary solvent was water-miscible or non–water-miscible. The two groups of secondary solvents appear to affect the aggregation properties of nC60 in water via different mechanisms. In general, nC60 products made with a secondary water-miscible solvent have stronger capabilities to enhance PCB transport. Taken together, the results indicate that according to formation conditions and solvent constituents, nC60 will vary significantly in its interactions with organic contaminants, specifically as related to adsorption or desorption as well as transport in porous media. Environ. Toxicol. Chem. 2013;32:329–336. © 2012 SETAC

INTRODUCTION

The production and use of carbon fullerenes (C60, C70, C76, C84, and C90, among others) and corresponding derivatives have increased considerably over the past several years and are expected to increase further 1. With continued commercial handling and application, there is an increasing potential of material release (and related products) into the environment 2, 3, with unknown environmental impacts 4–6. A widely studied model fullerene material with mass production relevance 1, C60, is highly hydrophobic and virtually insoluble in water, as the theoretical solubility of C60 is only 1.3 × 10−5 µg/L 7. Interestingly, C60 can be made available in water as stable, nanoscale, colloidal suspensions (referred to as nC60 herein) through various methods, such as solvent exchange, sonication, or long-term stirring without a solvent 8–12. Nanoscale C60 can be stable in aqueous environments for prolonged periods (months to years) and has a high potential for migration through soil and aquifer materials under common environmental conditions 13–15.

In addition to being water stable and even eliciting biological response(s) 4–6, nC60 can significantly enhance the transport of highly hydrophobic organic contaminants, due to the strong adsorption of contaminants to nC60 16, 17. Such cotransport of organic contaminants could increase exposure volumes and thus potential risk. For example, under typical environmental conditions, highly hydrophobic organic contaminants, such as polychlorinated biphenyls and polycyclic aromatic hydrocarbons, sorb strongly to soil organic matter and therefore have very limited mobility in subsurface environments. In a recent study 16, we observed that trace amounts of nC60 (less than 2 mg/L) resulted in the breakthrough of 2,2′,5,5′-polychlorinated biphenyl (PCB) and phenanthrene through saturated soil columns within a few pore volumes (PVs), which vastly differed from what was expected based on conventional theories in that such significant transport enhancement of PCB or phenanthrene should not occur unless the concentrations of colloidal materials are very high (on the order of several hundred mg/L or higher) 18. Thus, it is evident that the contaminant-mobilizing capability of nC60 is far greater than those of natural colloids, such as dissolved organic matter. The surprisingly high contaminant-mobilizing capability of nC60 compared with conventional dissolved organic matter is likely attributable to surface enthalpies of interaction and the unique porous structures of nC60, which result in both enhanced adsorption affinity and desorption irreversibility 16, 19. Thus, physicochemical factors that affect the structural properties or the effective surface chemistries, or both, of nC60 might also affect nC60–contaminant interactions and consequently, nC60's capability of enhancing contaminant transport 20.

It has been reported that the physicochemical properties of nC60 can be greatly affected by the preparation method 8, 11, 21. Several literature reports indicate that when a solvent-exchange method is used to prepare nC60, the type of solvent used during the preparation could affect the size, stability, and surface chemistry of nC60 8, 22, 23. We hypothesize that the types of solvents and the detailed routes involved in the preparation of nC60 may have even greater effects on the aggregation and thus pore structures of nC60 and that this subsequently affects nC60–contaminant interactions. In the present study, we prepared a sequence of nC60 samples using a modified solvent-exchange method, by involving a secondary organic solvent in the phase-transfer process of C60 from toluene to water. A series of water-miscible and non–water-miscible solvents of varied polarity was selected as the secondary solvent. Column tests were conducted to examine the capabilities of the variably produced nC60 to enhance the transport of PCB through a saturated sandy porous medium. Effects of solvent types during aggregate formation on nC60's mobility and contaminant-mobilizing capability were then analyzed and compared.

MATERIALS AND METHODS

Materials

Sublimed fullerene powder (C60, >99.5%) was purchased from SES Research, and 14C-radio-labeled PCB with a specific activity of 12.2 µCi/µmol was purchased from Sigma–Aldrich. The radiolabeled PCB was diluted in methanol to obtain a stock solution of 24.8 mg/L. Tritiated water was purchased from Amersham. All organic solvents used were of chromatographic grade. Lula soil, containing 45% sand, 36% silt, and 19% clay, was collected from a ranch near Lula, Oklahoma (USA). The fractional organic carbon (fOC) value of the soil is 0.0037. Selected physicochemical properties of the soil are summarized in Supplemental Data, Table S1.

Preparation of nC60 stock suspensions

A series of nC60 stock suspensions was prepared (Table 1) using a method similar to that of Andrievsky et al. 24. One nC60 stock suspension was prepared by adding 20 ml of 1 g/L C60 solution in toluene to 200 ml of deionized (DI) water and sonicating (Vibra-Cell VCX500 sonicating probe, Sonics & Material) for 3 h. Deionized water was added every 30 min to compensate for water evaporation. Afterward, the nC60 suspension obtained was filtered with a 1-µm glass fiber filter (Thermo Fisher Scientific) and then a 0.45-µm membrane filter (Millipore) to remove the larger C60 aggregates. The final concentration of the nC60 stock suspension was approximately 15.3 mg/L. This stock suspension is referred to as the standard nC60 hereafter. In a variance of this method, additional series of nC60 stock suspensions were made using the same procedures mentioned above, except that a secondary solvent (10 ml except for two samples using methanol as the secondary solvent) was added to the 200 ml of DI water before the 20 ml of C60 solution in toluene was added. The nC60 suspensions made with a secondary solvent are referred to as secondary-solvent nC60 hereafter. Both water-miscible and non-water-miscible organic solvents with varied polarities were selected as the secondary solvents (Table 1 and Supplemental Data, Table S2). The water-miscible solvents included methanol, 2-propanol, and acetone; the non–water-miscible solvents included 1-octanol, isooctane, hexane, and cyclohexane. In the case of methanol, two additional nC60 stock suspensions were prepared by altering the amount of methanol: one using 3 ml methanol, and the other using 30 ml methanol (Table 1). The three nC60 stock suspensions made using methanol as the secondary solvent were designated as nC60/methanol(0.3:20), nC60/methanol(1:20), and nC60/methanol(3:20), with the numbers in the parentheses indicating the methanol-to-water ratios used in sample preparation. The stock solutions obtained were kept in the dark at 4°C, and concentrations and particle size distribution were checked periodically to ensure that the stock suspensions were stable during the period of the present study.

Table 1. Summary of physicochemical properties of nC60 stock suspensions
Stock suspensionConcentration of nC60 (mg/L)ζ potentiala (mV)λmax (nm)pHZaveb (nm)
  • a

    Standard deviation of five measurements.

  • b

    Hydrodynamic diameter of nC60 aggregates based on dynamic light scattering analysis.

nC6015.3−38.3 ± 0.23446.8165
nC60/methanol(0.3:20)25.5−47.4 ± 2.33466.8167
nC60/methanol(1:20)20.4−31.9 ± 2.03456.7157
nC60/methanol(3:20)25.2−24.1 ± 1.53456.7160
nC60/2-propanol20.4−27.8 ± 5.33466.7160
nC60/acetone15.4−45.7 ± 4.03466.8153
nC60/1-octanol16.0−30.5 ± 4.83496.8179
nC60/isooctane5.0−27.9 ± 4.93446.9179
nC60/hexane15.9−36.3 ± 4.23426.8179
nC60/cyclohexane15.6−24.3 ± 4.93516.8186

Characterization of nC60 stock suspensions

Ultraviolet (UV) absorbance spectra of the nC60 stock suspensions were obtained with a UV and visible light spectrophotometer (UV-2401, Shimadzu Scientific Instruments). The scan was performed in the wavelength range of 200 to 600 nm, and the slit width and sample interval were set at 1 and 0.2 nm, respectively. The maximum absorbance of each nC60 stock suspension was obtained. Particle size distribution and ζ potentials of the nC60 stock suspensions were measured by dynamic light scattering and electrophoretic mobility, respectively, using a ZetaPALS ζ potential analyzer (Brookhaven Instruments); the samples were sonicated for 5 min and diluted to approximately 5 mg/L before the measurements (see Supplemental Data for the detailed procedures).

The concentrations of nC60 in the stock suspensions were determined with an oxidation–toluene extraction procedure 9. Briefly, 1 volume of nC60 stock suspension was oxidized with two-fifths volume of 0.1 M Mg(ClO4)2 and extracted with 1 volume of toluene. The Mg(ClO4)2 provided consistent oxidation of nC60, allowing high extraction efficiencies of 94 to 101% 9. Concentrations of C60 were then determined by measuring UV absorbance of the extracts based on a pre-established calibration curve of C60 in toluene; the detection limit was 0.02 mg/L. Total carbon contents of the stock suspensions were determined by total organic carbon measurement using a high-sensitivity total organic carbon analyzer (Shimadzu Scientific Instruments); the detection limit was 4 µg/L. The concentration of organic solvent remaining in each stock suspension was calculated as the difference between total organic carbon concentration and nC60 concentration.

Column experiments

Column experiments were conducted using a previously developed protocol 16. Soil was dry-packed into Omnifit borosilicate glass columns (10 × 0.66 cm; Bio-Chem Valve) with 10-µm stainless-steel screens (Valco Instruments) on both ends. Each column contained approximately 3.2 g soil (dry wt), with an average length of approximately 7.0 cm. The porosity and dead volume were determined with the tritiated water–tracer test (Supplemental Data, Fig. S1). The packed columns were first flushed with 60 ml of DI water at a flow rate of 3 ml/h, and then saturated with 180 ml of 0.5 mM NaCl to stabilize the soil colloids.

To prepare the influents, aliquots of nC60 stock suspensions were diluted with the background electrolyte (0.5 mM NaCl) in amber glass vials to obtain the working nC60 suspensions, with final nC60 concentrations of approximately 5 mg/L. Then PCB stock solution in methanol was added with a microsyringe to give a total PCB concentration of approximately 11 µg/L in each working nC60 suspension. The vials were sealed with Teflon-lined screw caps and tumbled end over end at 0.001 g for 7 d; our previous study showed that adsorption equilibrium could be reached in 3 d 16. Afterward, 3 ml of the suspension in each vial was taken to measure the nC60 concentration and the total PCB concentration. Another 4 ml of the suspension was passed through a 0.02-µm Al2O3 membrane (Whatman), placed within a metal holder, to remove nC60 particles. The filtrate was collected and the concentration of PCB was analyzed (no C60 was detected in the filtrates) 16. The mass of PCB adsorbed to nC60 was calculated as the difference between the total PCB concentration in the working suspension and the PCB concentration in the filtrate.

The protocols of the column experiments are summarized in Table 2. In a typical column experiment, the influent was loaded to the soil column with a syringe pump (Harvard Apparatus). The effluent was collected at predetermined time intervals, and the concentrations of both nC60 and PCB were measured.

Table 2. Experimental protocols and breakthrough results of column experiments
Exp. No.Column propertiesInfluent propertiesEffluent propertiesf
ρba (g/cm3)θbvc (m/d)C_PCB (µg/L)Type of nC60C_nC60 (mg/L)Residual solventd (%)Adsorbed masse (%)pHC/C0_nC60 (%)C/C0_PCB (%)
  • a

    Bulk density of soil column.

  • b

    Porosity of soil column.

  • c

    Linear velocity.

  • d

    Amount of residual solvent in the influent (wt/wt).

  • e

    Percentage of PCB adsorbed to nC60 in the influent.

  • f

    Average values of last three data points of respective breakthrough curves.

    PCB = polychlorinated biphenyl.

11.410.461011.3nC605.9826.890.9 ± 0.621.8 ± 0.4
2a1.370.481011.1nC60/methanol(0.3:20)5.23.5E-02896.984.6 ± 0.248.8 ± 0.3
2b1.380.471010.3nC60/methanol(1:20)4.41.1E-01876.889.2 ± 0.663.4 ± 1.2
2c1.340.491010.5nC60/methanol(3:20)4.85.6E-01826.792.6 ± 0.832.8 ± 0.7
2d1.360.481010.3nC60/2-propanol5.17.9E-02856.780.5 ± 0.834.0 ± 0.5
2e1.350.481011.4nC60/acetone4.44.2E-02886.888.6 ± 2.949.6 ± 0.9
3a1.360.489.910.4nC60/1-octanol6.11.3E-02886.988.9 ± 0.614.3 ± 1.1
3b1.320.499.911.9nC60/isooctane5.01.9E-03916.897.8 ± 0.340.4 ± 1.2
3c1.350.481011.1nC60/hexane5.24.5E-04946.892.6 ± 0.477.2 ± 0.5
3d1.330.499.910.2nC60/cyclohexane5.48.0E-05926.984.8 ± 0.845.1 ± 1.5

Analytical methods

Concentrations of C60 in toluene or in electrolyte solution were analyzed with a HACH DR/4000 UV and visible light spectrophotometer (HACH) 16. Quality control experiments indicated that the presence of small amounts of residual solvent in the solution had a negligible effect on C60 measurement. A Beckman LS 6500 scintillation counter (Beckman Coulter) was used to analyze PCB (see Supplemental Data for detailed procedures), and the detection limit was approximately 0.05 µg/L. Our previous study showed that adsorption to nC60 had no quenching effect on the radioactive reading of PCB 16. Gas chromatography–mass spectrometry analysis was used to verify the concentration of 14 C-radiolabeled PCB. No decay was observed.

Statistical analysis

Statistical analysis was performed using Statistical Product and Service Solutions (SPSS) 16.0. The equality of means was analyzed by independent-samples t test with a 95% confidence interval. The bivariate correlation and correlation coefficient were determined with Pearson's correlation analysis. The correlation between two sets of data points was analyzed by the SPSS General Linear Model procedure.

RESULTS AND DISCUSSION

Characterization of nC60 stock suspensions

Selected physicochemical properties of the nC60 stock suspensions are summarized in Table 1. Adding a secondary organic solvent had noticeable effects on the ζ potential of the nC60 aggregates: all the samples, except nC60/hexane, have a ζ potential value statistically different from that of the standard nC60 (p = 0.000–0.022). The ζ potential of most secondary-solvent nC60 stock suspensions is less negative than that of the standard nC60, except for nC60/acetone and nC60/methanol(0.3:20); however, no clear trend is apparent. The involvement of a secondary organic solvent had no significant effect on the maximum UV absorbance (λmax; Table 1). The UV and visible light spectra of different nC60 stock suspensions (Supplemental Data, Fig. S2) also show that no noticeable differences can be seen among the spectra of different nC60 stock suspensions. In Figure 1, the particle size distribution profiles of different nC60 stock suspensions are compared. It appears that the involvement of a secondary solvent did not result in very significant changes in the particle size and distribution of the nC60 products. For most of the secondary-solvent nC60 stock suspensions, the primary peak shifted little compared with that of the standard nC60, and nC60/1-octanol is the only product showing a considerably broader primary peak. The primary changes were associated with the intensity of the primary peaks.

Figure 1.

(A–C) Intensity-weighted particle size distribution of different nC60 samples. [Color figure can be seen in the online version of this article, available at wileyonlinelibrary.com.]

Effect of secondary solvent on mobility of nC60

The mobility of different nC60 samples is compared in Figure 2 and Table 2. High breakthroughs of nC60 (80.5–98.1%) were observed for all nC60 samples. For the standard nC60, significant breakthrough occurred at one to two PVs, quickly reached approximately 90% after six to seven PVs, and then remained nearly constant. Most secondary-solvent nC60 samples (with the exception of nC60/isooctane) exhibited a certain degree of inhibited transport, especially in the first few PVs—for several secondary-solvent nC60 samples, relatively constant breakthrough (i.e., the plateau of the breakthrough curve) was not reached until approximately 10 to 15 PVs. This initially inhibited transport of nC60 seems to be particularly significant for the nC60 samples prepared using a secondary non–water-miscible solvent (Fig. 2C). Interestingly, no clear trend is observed between the percentage of breakthrough at the end of the column experiments (the nC60_C/C0 values in Table 2) and the ζ potential for the test nC60 samples (Supplemental Data, Fig. S3). Similarly, the percentage breakthrough is not closely related to the hydrodynamic diameters (Zave) of the nC60 samples (Supplemental Data, Fig. S3). The lack of strong correlation between the mobility of nC60 samples and the surface chemistry and size of nC60 might be due to the relatively high linear velocity (10 m/d) used in the column experiments, which is consistent with the findings of Lecoanet et al. 25. Additionally, such a lack of correlation might also have occurred because the differences in size and ζ potential are quite small among the nC60 samples in the present study. Overall, the experimental results indicate that the involvement of a secondary solvent during nC60 preparation did not result in significant differences in the mobility of the nC60 products.

Figure 2.

(A–C) Comparison of breakthrough curves and contaminant-mobilizing capabilities of different nC60 samples. PCB = polychlorinated biphenyl; PV = pore volume.

Enhanced PCB transport by different nC60 samples

In Table 2 and Figure 2 the enhanced breakthrough of PCB by different nC60 samples are compared. In contrast to the relatively small differences in the mobility of nC60 (as shown in Fig. 2), significant differences in nC60's capability to enhance PCB transport can be observed among different nC60 samples. Figure 2A shows that when a water-miscible solvent is added as the secondary solvent during the preparation of nC60, the contaminant-mobilizing capability of nC60 is enhanced considerably. With the standard nC60 (Experiment 1), breakthrough of PCB reached approximately 20% after three PVs and remained at a near constant value afterward; after 20 PVs the breakthrough of PCB was 21.8%. However, with nC60/2-propanol, nC60/actone, and nC60/methanol(1:20), PCB breakthrough was approximately 27, 46, and 55%, respectively, after four or five PVs and was 34.0, 49.6, and 63.4% at the end of the column tests. Most of the nC60 samples prepared with a secondary non–water-miscible solvent also exhibited greater contaminant-mobilizing capabilities compared with the standard nC60. At the end of the column experiments, the breakthrough of PCB was 40.4% with nC60/isooctane, 45.1% with nC60/cyclohexane, and 77.2% with nC60/hexane. One exception is nC60/1-octanol, which resulted in lower breakthrough of PCB than the standard nC60; for this experiment (Experiment 3a), the final breakthrough of PCB was 14.3%. Interestingly, the three methanol-mediated nC60 samples also exhibited different contaminant-mobilizing capabilities, even though these three samples have very similar mobility (Fig. 2B). The highest contaminant-mobilizing capability is with nC60/methanol(1:20), for which 1 ml methanol was added to every 20 ml DI water during sample preparation. Both increased amount of methanol (3 ml methanol per 20 ml DI water, as in nC60/methanol(3:20)) and decreased amount of methanol (0.3 ml methanol per 20 ml DI water, as in nC60/methanol(0.3:20)) resulted in weakened contaminant-mobilizing capability compared with that of nC60/methanol(1:20). The experimental results discussed above indicate that the capability of nC60 to enhance the transport of PCB is significantly affected by the involvement of a secondary organic solvent during sample preparation, regardless of whether the secondary solvent is water-miscible or non–water-miscible.

Adsorption properties versus contaminant-mobilizing capabilities

In the present study, PCB was preloaded to nC60 before the column tests by allowing adsorption equilibrium of PCB to nC60. Thus, for each of the influents in Table 2, PCB molecules were distributed in equilibrium between the adsorbed phase and the dissolved phase. In a previous study 16, we have demonstrated that under experimental conditions such as those in Table 2, all the PCB in the effluent has to be coeluted with nC60, and freely dissolved PCB is completely detained by the porous media due to the strong sorption of PCB to soil organic matter. Note that some of the influents contained residual solvent, with mass fraction from 0.000080 to 0.56% (Table 2). Theoretically, residual solvent in the solution—it is also possible that a fraction of the residual solvent is entrapped within nC60 aggregates—could enhance the apparent solubility of PCB and therefore inhibit the sorption to soil organic matter, and consequently could enhance the breakthrough of PCB; however, this effect was found to be negligible under the experimental conditions of the present study (see Supplemental Data for detailed calculation and analysis). Furthermore, in Figure 3 and Supplemental Data, Figure S4, The PCB breakthrough is compared with nC60 breakthrough, and the strong correlations further indicate that PCB breakthrough occurred via coelution with nC60.

Figure 3.

Comparison of nC60 breakthrough and polychlorinated biphenyl (PCB) breakthrough for selected column experiments. PV = pore volume.

Because only those PCB molecules adsorbed to nC60 can break through the soil column, it is reasonable to assume that the larger the fraction of PCB is in the adsorbed phase, the greater the PCB breakthrough should be. However, this is not necessarily true, as seen when the nC60 samples prepared with secondary water-miscible solvents (Experiments 2a–2e) are compared with the nC60 samples with non–water-miscible solvents (Experiments 3a–3d). In Figure 4, the percentage of PCB breakthrough is plotted as a function of the mass fraction of PCB in the adsorbed phase in the influent. The PCB breakthrough is normalized with the nC60 breakthrough to eliminate the bias from the small differences in nC60 breakthrough. Two interesting observations can be made from this figure. First, for both the nC60 samples made with a secondary water-miscible solvent and the nC60 samples made with a secondary non–water-miscible solvent, there is a relatively strong correlation between the PCB breakthrough per unit nC60 breakthrough and the mass fraction of adsorbed PCB in the influent (p = 0.091 for the water-miscible group, and p = 0.018 for the non–water-miscible group). Second, the nC60 samples made with a secondary water-miscible solvent and the nC60 samples made with a secondary non–water-miscible solvent exhibited different correlations. Specifically, the group of nC60 samples made with a secondary water-miscible solvent seems to have a stronger effect on PCB breakthrough. For example, for nC60/1-octanol, 88% PCB in the influent was in the adsorbed phase, and the percentage of PCB breakthrough per unit nC60 breakthrough was less than 20%; for nC60/methanol(1:20), however, a similar percentage of PCB in the influent was in the adsorbed phase (87%), but the percentage of PCB breakthrough was much greater (71%). In our previous study, we found that the capability of nC60 to enhance the transport of PCB is not only due to the strong adsorption affinity of PCB to nC60 but, more importantly, to the resistant desorption—including both slow desorption kinetics and thermodynamically irreversible adsorption—of PCB molecules adsorbed to nC60 16. Thus, one possible explanation for the difference between the nC60 samples made with a secondary water-miscible solvent and the nC60 samples made with a secondary non–water-miscible solvent, as shown in Figure 4, is that the former is more effective in leading to resistant desorption of adsorbed PCB.

Figure 4.

Comparison between polychlorinated biphenyl (PCB) breakthrough and mass fraction of PCB in the adsorbed phase in the influent.

Mechanistic aspects

It has been proposed that the resistant desorption of hydrophobic organic contaminants from the aggregates of C60 is due to the adsorption of organic molecules within the micropores of the C60 aggregates 12, 26–28. Thus, it might be reasonable to speculate that the two groups of nC60 samples involved in the present study are of different microporous structures, resulting from the differences in aggregate formation between these two groups during the preparation of nC60 samples. The solvent-exchange method used to prepare nC60 involves the phase transfer of C60 molecules from a favorable environment (toluene) to a highly unfavorable environment (water), and several studies have shown that the property of the solvent–water interface is crucial for the physicochemical properties of the final nC60 product 11, 13, 29. The properties of the toluene–water interface are likely affected by the presence of a secondary solvent; in particular, when the secondary solvent is miscible with (or highly soluble in) both toluene and water (Supplemental Data, Table S2), the surface tension of the toluene–water interface can be lowered considerably.

While the exact mechanism(s) through which nC60 aggregates is formed during the phase transfer remains unknown, it is generally assumed that C60 molecules first form small and crystalline primary aggregates and then the primary aggregates further form secondary aggregates 8, 10, 30–32. It is possible that the size and shape of the primary and secondary aggregates, as well as the packing or arrangement of both monomer C60 within primary aggregates and primary aggregates within secondary aggregates—and, consequently, the pore structures of the final nC60 product—can be significantly affected by the presence of a secondary solvent. In the present study, the secondary solvent was added to DI water before adding the C60 solution in toluene. For water-miscible secondary solvents, a fraction would partition into the toluene phase—the mass fraction in toluene ranges from 72% for methanol to 97% for 2-propanol, based on equilibrium calculations (Table 3) 33. Thus, a secondary water-miscible solvent could possibly affect C60 aggregation in two ways. First, it decreases the hydrophilicity of the aqueous phase due to the cosolvent effects, thus decreasing the solvation energy and fugacity coefficient for the large, highly hydrophobic C60 molecules. Second, the partition of the water-miscible solvent into toluene can increase the fugacity coefficient of C60 molecules in toluene, as C60 is essentially insoluble in the three water-miscible solvents used (Supplemental Data, Table S2). Both of these effects tend to facilitate the transfer of C60 molecules from toluene to water. For a non–water-miscible secondary solvent, however, essentially 100% of a solvent should partition into the toluene phase (Table 3), and thus, the primary effect is possibly the increase of the fugacity coefficient of C60 molecules in the organic phase (C60 is much less soluble in these secondary solvents than in toluene [Supplemental Data, Table S2].) The exact effects of a secondary solvent on C60 aggregation are likely dependent on the relative polarity of the solvents and on the nature of solvent–C60 interactions, which taken together are complex.

Table 3. Estimated mass fraction of secondary solvent in toluene and aqueous phases during nC60preparationa
SampleSecondary solventMass fraction in toluene phaseMass fraction in aqueous phase
TypeVolumeb (ml)
  • a

    Calculated with Universal Quasi-Chemical Functional Group Activity Coefficients computer codes 33.

  • b

    Volume of secondary solvent added to 20 ml toluene and 200 ml deionized water.

nC60/methanol(0.3:20)methanol37.48E-012.52E-01
nC60/methanol(1:20)methanol107.21E-012.79E-01
nC60/methanol(3:20)methanol306.45E-013.55E-01
nC60/2-propanol2-propanol109.69E-013.09E-02
nC60/acetoneacetone109.19E-018.11E-02
nC60/1-octanol1-octanol101.00E + 009.07E-05
nC60/isooctaneisooctane101.00E + 002.12E-06
nC60/hexanehexane101.00E + 002.92E-05
nC60/cyclohexanecyclohexane101.00E + 006.82E-05

To date, little is known about the exact mechanisms by which organic solvents affect the physicochemical structures of nC60 aggregates 8, 11, and further studies (in particular, direct microscopic evidence showing the packing or aggregation properties) are necessary to fully understand the complex process of C60 phase transfer via aggregate formation. Nevertheless, findings in the present study indicate that the type and amount of organic solvents involved in the preparation of nC60 products can significantly affect the capability of nC60 to affect contaminant fate and transport. Thus, it is deduced that nC60 from different sources, or formed in the environment under different aquatic conditions, will have different contaminant-mobilizing capabilities. This is an important aspect to be considered both in the risk assessment and for the applications of fullerene nanoparticles and other types of engineered carbonaceous nanoparticles.

SUPPLEMENTAL DATA

Figures S1–S4; Tables S1 and S2. (2.1 MB DOC)

Acknowledgements

This project was supported by the National Natural Science Foundation of China (grants 21177063, 20977050, and 21150110140), the Tianjin Municipal Science and Technology Commission (grants 09JCYBJC26900 and 10SYSYJC27200), the China–U.S. Center for Environmental Remediation and Sustainable Development, the Brine Chemistry Consortium, and the Advanced Energy Consortium.

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