Article pubs.acs.org/est
Efficacy of Carbonaceous Materials for Sorbing Polychlorinated Biphenyls from Aqueous Solution Bradley Beless,† Hanadi S. Rifai,*,† and Debora F. Rodrigues† †
Department of Civil and Environmental Engineering, University of Houston, Houston, Texas 77204-4003, United States S Supporting Information *
ABSTRACT: Interest in incorporating nanomaterials into water treatment technologies is steadily growing, driving the necessity to understand the interaction of these new materials with specific water contaminants. In the present study, five different carbonaceous materials: activated carbon (AC), charcoal (BC), carbon nanotubes (CNT), graphene (GE), and graphene oxide (GO) were investigated as sorbent materials for 11 polychlorinated biphenyl (PCB) congeners in aqueous concentrations in the pg-μg/L range. Sorbent-water distribution coefficients (Ks) calculated in aqueous concentrations of ng/L show that AC is superior to GE, GO, CNT, and BC for the 11 PCB congeners investigated by an average of 1.1, 1.1, 1.3, and 2.5 orders of magnitude, respectively. Additionally, maximum capacity and sorption affinity parameters from the Langmuir, Freundlich, and Polanyi−Dubinin− Manes (PDM) models show a similar result. Interestingly, however, the effect of molecular planarity has greater impact on PCB sorption to the nanomaterials, such that the planar congeners form stronger bonds with CNT, GE, and GO compared to AC and BC. This work demonstrated superior PCB sorption by AC as compared with the nanomaterials examined such that substantial post production modifications would be necessary for the nanomaterials to out-perform AC.
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INTRODUCTION Polychlorinated biphenyls (PCBs), a class of hydrophobic organic compounds (HOCs) known to be ubiquitous in the environment, are harmful to both natural ecosystems and humans in regions of elevated concentrations.1 In recent years, there has been an increasing effort to understand and quantify the potential use of carbonaceous materials (CMs) as sorbents for the remediation of HOCs from water and sediments.2 The recent production of CMs at the nanoscale, such as carbon nanotubes (CNTs) and graphene (GE) has sparked new interest into investigating the potential of finding a sorbent material superior to the traditionally used activated carbon (AC). The strong affinity of PCBs to AC has been demonstrated in a number of studies that reported sorbent-water distribution coefficients (Ks) and Freundlich coefficient (KF) values up to 3.5 orders of magnitude greater than the octanol−water partitioning coefficients (Kow) for specific PCB congeners.3−6 Research has also shown that various types of black carbon (BC) perform well as a sorbent material. Jonker and Koelmans4 measured log Ks values of many types of BC (diameter fractions BC. To account for the large differences in SSA of the five sorbents, the Ks values were normalized by each sorbent respective SSA and plotted in SI Figure S1. As expected, the five sorbent materials demonstrate similar affinities to PCBs on a per unit area basis. Interestingly, GE and CNT show very comparable sorption per unit area, whereas GO clearly has higher sorption affinity, especially for the more hydrophobic PCB congeners. The PCBs have a similar affinity to the three nanomaterials compared to AC on a per unit area basis, giving rise to the notion that if the surface area of the nanomaterials were to increase to similar values as AC, they would perform as well, or in the case of GO, even superior to AC. As discussed in the Methods section, the log Ks values were calculated for the sorption of the 11 PCBs to the five sorbents by averaging the samples with Cw values in the 0.5−50 ng/L range; the range being 1−2 orders of magnitude greater than Cw ranges used in other similar studies.4,20 The potential disagreement in findings created by differing Cw ranges was quantified by comparing the log Ks values reported in Jonker and Koelmans4 for the sorption of a peat based AC with the 6 PCB congeners similarly used in this study. The comparison indicated that the log Ks values obtained for AC in this study were an average of 0.60 ± 0.40 log units lower than those of Jonker and Koelmans.4 Although many factors could contribute to this difference: particle size, feed stock type, and type of activation process, to name a few, the difference between the two studies closely resembles the quantity of log Ks decrease discussed in Jantunen et al.21 due to the acknowledged difference in Cw range. The comparable findings between the present study and Jonker and Koelmans4 lend confirmation to the continuity between the use of PDMS and POM as a solidphase microextraction material. In contrast, a comparison of the charcoal used in Jonker and Koelmans4 shows a significant difference (average log Ks values 1.64 ± 0.65 lower for the 6 similar congeners, p < 0.05), which is likely explained by the difference in SSAs (the present BC had a SSA value 9 times lower than that used by Jonker and Koelmans). The stair step pattern observed in Figure 1 is evidence of the effect of molecular planarity on sorption and clearly differs from a smooth linear increase that would be expected of purely hydrophobic sorption. To further illustrate this observation, the planarity effect was quantified by accounting for the change in log Kow between nonortho/mono-ortho and mono-ortho/ diortho groups (Δlog Ks − Δlog Kow). These results agree with past works,4,20 with values ranging from 0.05−0.6 for the AC and BC materials. Interestingly, the quantified effect of planarity on the three nanomaterials was significantly greater than that of AC and BC (p < 0.05). GO demonstrated the greatest effect, followed by CNT and last GE, with values ranging from 0.07−2.13, 0.22−1.06, and 0.04−0.78, respectively. The elevated effect of planarity for the nanomaterials is evidence of π−π bonding that is a reported mechanism for CNTs adsorption to HOCs.9,11,12 Past works have reported a decline in planarity effect with increasing chlorination, demonstrating that the effect eventually became negligible by the hexa-homologue group.4,20,21 In the present study, although diminished, there was still evidence of increased sorption due to increased planarity in the hexahomologue group. One possible explanation is that the affinity between the planar congeners and the graphene surface outweighs the deterrence due to steric hindrance. Although it might be assumed that the planarity effect increases between
The hydrophobic CNT formed flocs within the same size distribution range as the AC and BC particles. Although GO is known to be hydrophilic, the pre-existing dry aggregates were not dispersed by mechanical mixing, leading to a size distribution that was greater than the other four sorbent materials (percentiles shown in SI Table S3). The median particle diameters of AC, BC, CNT, GE, and GO were 50, 54, 39, 92, and 208 μm, respectively. Although the GO can be dispersed by ultrasonication,26 this procedure was not conducted for the CNT and GE and was not performed for GO to maintain consistency. While the presence of the nanomaterials as flocs instead of homogeneous dispersion diminishes the surface area available for interaction with the sorbate, the addition of surfactants to achieve dispersion could cause undesired competition with the sorbates or generate other unknown or synergistic effects.27 Ultimately, the use of flocculated CNT, GE, and GO support the objective of comparing the nanomaterials to AC and BC and avoid the added complexity of introducing competitive sorbates. Distribution Coefficients and the Planarity Effect. Figure 1 illustrates the resulting log Ks values for the 11 PCB congeners studied. The five materials investigated have log Ks values ranging from 4 to 10, with standard deviations averaging 0.15 log units (numerical values are in Table S4 of the SI). It is evident from the calculated Ks values that the 11 PCB congeners had the greatest affinity toward AC compared to BC and the nanomaterials. The average separation of log Ks values between the AC and BC materials remained constant at approximately 2.50 log units (standard deviation of 0.23). This highly correlated difference between AC and BC (R2 = 0.974) indicates that the mechanisms of sorption are similar, the main difference being the amount of surface area available. The three nanomaterials have log Ks values that are lower than AC for the smaller and less hydrophobic PCB congeners, but converge toward similar log Ks values as molecular size and hydrophobicity increase. This phenomenon is in accordance with earlier findings that demonstrate attenuated sorption with increasing molecular size due to steric hindrance toward the micropores in AC and BC,6,28 but not to the graphene surface of the nanomaterials. On the basis of the log Ks results presented in Figure 1, the general progression of sorption affinity is as follows: AC > GE
Figure 1. Calculated sorbent−water distribution coefficients (Ks) for the five sorbent materials. The error bars represent the standard deviation of the average. The dashed lines are for visual purposes only. The PCB congeners are arranged such that each homologue group has the most coplanar (diortho substituted) congener on the left, moving to the most planar (nonortho substituted) congener on the right. 10375
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and appeared to be closer to the saturation concentration range as opposed to the higher chlorinated homologue groups. As the sorbent material becomes increasingly saturated by PCB molecules, it is possible that the more hydrophobic congeners were preferentially sorbing and causing the lesser hydrophobic congeners to have a more nonlinear isotherm graph. Kah et al.30 studied the sorption of PAHs to CNT over a wide Cw range and found that there was little to no competitive effects for low equilibrium concentrations (Cw values approximately less than 0.1 μg/L), whereas Yang et al.31 reported significant hindrance due to the presence of additional solutes at high concentrations. Sorption competition between the 11 PCB congeners was quantified by calculating the percent of sorbent surface area covered by PCBs (the area of analyte on the sorbent was calculated by summing the total molecular surface area of the 11 PCB congeners and dividing it by the total sorbent surface area). The maximum percent coverage of the sorbent by PCBs was 77, 36, 56, and 30% for BC, CNT, GE, and GO respectively. Jantunen et al.21 concluded that surface coverage areas of >35% lead to competitive effects on coal; consequently, the high percent theoretical surface coverage of the sorbent materials by PCBs in the present study likely led to competitive effects at the higher concentration isotherm batches. The existence of sorbate competition at high concentrations is representative of real systems and is important to consider when utilizing the isotherm graphs and resulting parameters. A detailed list of the generated isotherm parameters can be found in Table S6 of the SI. The generated Langmuir and PDM maximum sorption capacities, QLmax and Qpmax, respectively, both show that AC on average had a maximum sorption capacity approximately 1 order of magnitude greater than the four other sorbent materials. Both QLmax and QPmax show that GE had a slightly higher maximum sorption capacity for PCBs than CNT. The QLmax values for GO aligned closely with that of CNT and GE (0.3−16 × 106 μg/kg), whereas the QPmax values for GO were smaller than both CNT and GE and more so in the range of BC (0.04−5 × 106 μg/kg). With the exception of GE, the generated Qmax values for Langmuir and PDM were not significantly different (p > 0.05) and had largely overlapping standard errors. However, the Langmuir model produced smaller relative standard errors than PDM (average SE = 18% and 29% respectively), likely due to the presence of a third model parameter in the PDM equation. The Langmuir affinity constant (b) values generated had an increasing trend in correlation with molecular hydrophobicity (average R2 = 0.82), ranging from values of 0.1 to 930 for the five sorbent materials. The b values also showed distinct trends associated with the planarity effect, similar to the log Ks values discussed above. According to the b values generated, PCBs had greater affinity for sorption with AC than BC; however, this difference was not statistically significant (p > 0.05) and likely caused by greater ability of PCBs to interact with the surface of AC as compared to BC. Interestingly, in contrast with the maximum sorption capacity, the b values show that GE, CNT, and GO have higher affinities than AC for planar PCB congeners. This difference can be explained by the aforementioned π−π bonds between the planar congeners and the flat graphene surfaces, resulting in greater PCB affinity toward the nanomaterials as compared to AC and BC. The fitting parameters Z and d of the PDM model represent the free energy of adsorption and distribution of adsorption energies, respectively. Both of these variables are associated
the diortho/mono-ortho and yet still more for the monoortho/nonortho congeners, there was no consistent evidence supporting this hypothesis between all five sorbents. (The only exception was for the quadra-homologue group, in which all five sorbents demonstrated a stronger planarity effect between mono-ortho/nonortho group than diortho/mono-ortho group.) Overall, the confirmation of a strong increase in sorption with increase in planarity is beneficial for the practical application of the nanomaterials, knowing that the most toxic, dioxin like-PCB congeners are also the most planar.29 Sorption Isotherms. The three models used in the current study generally agree with Jantunen et al.;21 the PDM and Langmuir equations on average had higher R2 values than the Freundlich. The average R2 values for the 11 PCB congeners were virtually identical ( 0.05, for every model parameter except Z), which eventually diminishes for the higher chlorinated congeners. One explanation for the observed difference could simply be the structural shape of GE versus CNT. Earlier findings have demonstrated that CNTs with small diameter have attenuated sorption,9,10 which could explain this small difference between the tubular CNT and the flat GE materials. With this said, the ability to control the number of concentric walls, the outer diameter, and the resulting SSA is becoming increasingly more accurate, therefore creating potential for increasing the sorption of PCBs to CNTs. Interestingly, GO generally performed at the same efficiency as CNT and GE despite the large percentage of oxygen functional groups. It is widely accepted that the presence of oxygen functional groups inhibits the sorption of nonpolar sorbates from aqueous solution,14,27,41 which would imply that GO, having a similar physical structure as GE but with substantially greater amounts of oxygen functional groups, would have lesser capability for PCB sorption than GE. However, Gotovac et al.10 observed the increased sorption of tetracene, a nonpolar molecule, to single walled CNTs in toluene solution by charge transfer interaction catalyzed by the functional group on the graphene surface, which could explain the favorable results of GO in this study. More research is needed to understand the difference in sorption between GE and GO. For the purpose of PCB sorption from water, the difference in sorption between the three nanomaterials is sufficiently small such that the ease of synthesis and manipulation presently outweigh the differences in PCB sorption efficiencies as a design criterion. Overall, the nanomaterials investigated in this study are inferior as sorbents compared to AC, yet show promise as sorbents even in their unaltered state. More research is needed to create specialized functional groups and increase their available surface area in easily applied methods before they can be widely viewed as viable and/or superior alternative to AC for use in HOC sorption from water.
with the sorption strength between the PCB congeners and the sorbent materials, and therefore yield similar results compared to the Langmuir affinity constant discussed above. The relative standard errors for the Z and d fitting parameters were lower than the Langmuir affinity constant, averaging 8% and 18%, respectively, compared to 20% for b. The Z values generated were within the range of 6−22 kJ/mol consistent with values reported for HOCs in water,20,32 with the exception of BC and GO, having values as low as 2.5 kJ/mol and as high as 25 kJ/ mol, respectively. Likewise, the d values generated were within the range of 1−5.5 as commonly reported,20,22,32 with the exception of PCB-169 sorption to CNT reaching near 10 and GO sorption having d values as high as 11. Freundlich style sorption can be implied if the d value approaches unity,33 however, the d values for the five sorbent materials tested were consistently greater than 1, which indicates that the sorption site energies were not distributed exponentially away from the sorbent surface but rather more closely resembled single site energy distribution. This conclusion is in agreement with the comparison between the R2 values for the three isotherms discussed above. Both the Z and d values demonstrated similar trends over the 11 PCB congeners used; for AC and BC, the values decreased inversely with increasing chlorination and showed little planarity effect, yet for the nanomaterials, Z and d show no overall positive or negative trend with congener chlorination yet large dependency on molecular planarity. In agreement with the Langmuir affinity constant, the Z and d values indicated that the nanomaterials showed a consistent and substantial increase in bond energy for the planar congeners with increasing superiority over AC as the congeners increase in chlorination. Among the three nanomaterials, GO demonstrated the largest bond energy toward the 11 congeners, followed by GE and CNT. Despite the minimal goodness of fit compared to the Langmuir and PDM models, the Freundlich model generated the lowest relative standard errors, with the Freundlich constant (KF) and the Freundlich exponent (n), having 16% and 7% average standard error, respectively. The KF values compared closely to the Ks values discussed above, producing the same hierarchy of sorbent materials; however, the empirical quantities of KF were systematically lower than the Ks values by an average of 0.65 log units. This reduction in quantity is intuitive due to the inclusion of the higher Cw range in the isotherm that is not considered in the Ks calculations. The n values for each of the sorbent materials trend upward with increasing chlorination toward the value of 1, with BC reaching unity in the hexa-chlorobiphenyl homologue group whereas AC and GE plateau around 0.6. Differing from the b, Z, and d fitting parameters, the n values did not illustrate a relationship with congener planarity. This difference agrees with previous works, in that the strength of sorption for PCBs to the sorbent materials is not due to occlusion of planar congeners in pore spaces, but rather due to their increased ability for surface interaction.4,5,21 This explanation supports the present findings that AC has a higher maximum sorption capacity primarily due to its greater surface area instead of its narrow pore structure, whereas the nanomaterials create stronger bonds with the planar congeners because of the interaction of PCBs with the smooth graphene surface. Effectiveness of the Sorbent Materials. Currently, the three nanomaterials, in their natural flocculated state and without the presence of specialized functional groups, have calculated Ks values in similar ranges to the charcoal used in 10377
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(14) Yang, K.; Xing, B. Adsorption of organic compounds by carbon nanomaterials in aqueous phase: Polanyi theory and its application. Chem. Rev. 2010, 110 (10), 5989−6008. (15) Hummers, W. S.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80 (6), 1339−1339. (16) Jonker, M. T. O.; Koelmans, A. A. Polyoxymethylene solid phase extraction as a partitioning method for hydrophobic organic chemicals in sediment and soot. Environ. Sci. Technol. 2001, 35 (18), 3742−3748. (17) Jonker, M. T. O. Absorption of polycyclic aromatic hydrocarbons to cellulose. Chemosphere 2008, 70 (5), 778−782. (18) Lu, X.; Skwarski, A.; Drake, B.; Reible, D. D. Predicting bioavailability of PAHs and PCBs with porewater concentrations measured by solid-phase microextraction fibers. Environ. Toxicol. Chem. 2011, 30 (5), 1109−1116. (19) Cornelissen, G.; Breedveld, G. D.; Kalaitzidis, S.; Christanis, K.; Kibsgaard, A.; Oen, A. M. P. Strong sorption of native PAHs to pyrogenic and unburned carbonaceous geosorbents in sediments. Environ. Sci. Technol. 2006, 40 (4), 1197−1203. (20) Koelmans, A. A.; Meulman, B.; Meijer, T.; Jonker, M. T. O. Attenuation of polychlorinated biphenyl sorption to charcoal by humic acids. Environ. Sci. Technol. 2009, 43 (3), 736−742. (21) Jantunen, A. P. K.; Koelmans, A. A.; Jonker, M. T. O. Modeling polychlorinated biphenyl sorption isotherms for soot and coal. Environ. Pollut. 2010, 158 (8), 2672−2678. (22) Allen-King, R. M.; Grathwohl, P.; Ball, W. P. New modeling paradigms for the sorption of hydrophobic organic chemicals to heterogeneous carbonaceous matter in soils, sediments, and rocks. Adv. Water Resour. 2002, 25 (8–12), 985−1016. (23) Pikaar, I.; Koelmans, A. A.; van Noort, P. C. M. Sorption of organic compounds to activated carbons. Evaluation of isotherm models. Chemosphere 2006, 65 (11), 2343−2351. (24) Ng, J. C. Y.; Cheung, W. H.; McKay, G. Equilibrium studies of the sorption of Cu(II) ions onto chitosan. J. Colloid Interface Sci. 2002, 255 (1), 64−74. (25) Yaneva, Z. L.; Koumanova, B. K.; Georgieva, N. V. Linear and nonlinear regression methods for equilibrium modelling of pnitrophenol biosorption by Rhizopus oryzae: Comparison of error analysis criteria. J. Chem. 2012, 2013. (26) Paredes, J. I.; Villar-Rodil, S.; Martínez-Alonso, A.; Tascón, J. M. D. Graphene oxide dispersions in organic solvents. Langmuir 2008, 24 (19), 10560−10564. (27) Pan, B.; Xing, B. Adsorption mechanisms of organic chemicals on carbon nanotubes. Environ. Sci. Technol. 2008, 42 (24), 9005−9013. (28) Cornelissen, G.; Elmquist, M.; Groth, I.; Gustafsson, O. Effect of sorbate planarity on environmental black carbon sorption. Environ. Sci. Technol. 2004, 38 (13), 3574−3580. (29) Ahlborg, U. G.; Becking, G. C.; Birnbaum, L. S.; Brouwer, A.; Derks, H.; Feeley, M.; Golor, G.; Hanberg, A.; Larsen, J. C.; Liem, A. K. D.; Safe, S. H.; Schlatter, C.; Waern, F.; Younes, M.; Yrjänheikki, E. Toxic equivalency factors for dioxin-like PCBs: Report on WHOECEH and IPCS consultation, December 1993. Chemosphere 1994, 28 (6), 1049−1067. (30) Kah, M.; Zhang, X.; Jonker, M. T. O.; Hofmann, T. Measuring and modeling adsorption of PAHs to carbon nanotubes over a six order of magnitude wide concentration range. Environ. Sci. Technol. 2011, 45 (14), 6011−6017. (31) Yang, K.; Wang, X.; Zhu, L.; Xing, B. Competitive sorption of pyrene, phenanthrene, and naphthalene on multiwalled carbon nanotubes. Environ. Sci. Technol. 2006, 40 (18), 5804−5810. (32) Kleineidam, S.; Schüth, C.; Grathwohl, P. Solubility-normalized combined adsorption-partitioning sorption isotherms for organic pollutants. Environ. Sci. Technol. 2002, 36 (21), 4689−4697. (33) Condon, J. B. Equivalency of the Dubinin−Polanyi equations and the QM based sorption isotherm equation. A. Mathematical derivation. Microporous Mesoporous Mater. 2000, 38 (2−3), 359−376. (34) Carrillo-Carrion, C.; Lucena, R.; Cardenas, S.; Valcarcel, M. Surfactant-coated carbon nanotubes as pseudophases in liquid−liquid extraction. Analyst 2007, 132 (6), 551−559.
ASSOCIATED CONTENT
S Supporting Information *
List of PCB physical and chemical properties, experimental design criteria, sorbent particle size distribution percentiles, detailed results of Ks values, plot of Ks values normalized by the sorbent SSA, model fit R2 values, detailed results of the isotherm parameters, and plots of the isotherm models for the 11 PCB congeners and five sorbent materials. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 713-743-4271; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded by the NSF GK-12 program, the Texas Commission on Environmental Quality, and the U.S. Environmental Protection Agency. Their funding is gratefully acknowledged.
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REFERENCES
(1) Dobson, S.; van Esch, G. J. Polychlorinated Biphenyls and Terphenyls: Environmental Health Criteria 140; World Health Organization: Geneva, Switzerland, 1993. (2) Koelmans, A. A.; Jonker, M. T. O.; Cornelissen, G.; Bucheli, T. D.; Van Noort, P. C. M.; Gustafsson, O. Black carbon: The reverse of its dark side. Chemosphere 2006, 63 (3), 365−377. (3) Hawker, D. W.; Connell, D. W. Octanol−water partition coefficients of polychlorinated biphenyl congeners. Environ. Sci. Technol. 1988, 22 (4), 382−387. (4) Jonker, M. T. O.; Koelmans, A. A. Sorption of polycyclic aromatic hydrocarbons and polychlorinated biphenyls to soot and soot-like materials in the aqueous environment: Mechanistic considerations. Environ. Sci. Technol. 2002, 36 (17), 3725−3734. (5) McDonough, K. M.; Fairey, J. L.; Lowry, G. V. Adsorption of polychlorinated biphenyls to activated carbon: Equilibrium isotherms and a preliminary assessment of the effect of dissolved organic matter and biofilm loadings. Water Res. 2008, 42 (3), 575−584. (6) Amstaetter, K.; Eek, E.; Cornelissen, G. Sorption of PAHs and PCBs to activated carbon: Coal versus biomass-based quality. Chemosphere 2012, 87 (5), 573−578. (7) Long, R. Q.; Yang, R. T. Carbon nanotubes as superior sorbent for dioxin removal. J. Am. Chem. Soc. 2001, 123 (9), 2058−2059. (8) Gotovac, S.; Hattori, Y.; Noguchi, D.; Miyamoto, J.; Kanamaru, M.; Utsumi, S.; Kanoh, H.; Kaneko, K. Phenanthrene adsorption from solution on single wall carbon nanotubes. J. Phys. Chem. B 2006, 110 (33), 16219−16224. (9) Gotovac, S.; Honda, H.; Hattori, Y.; Takahashi, K.; Kanoh, H.; Kaneko, K. Effect of nanoscale curvature of single-walled carbon nanotubes on adsorption of polycyclic aromatic hydrocarbons. Nano Lett. 2007, 7 (3), 583−587. (10) Gotovac, S.; Yang, C.-M.; Hattori, Y.; Takahashi, K.; Kanoh, H.; Kaneko, K. Adsorption of polyaromatic hydrocarbons on single wall carbon nanotubes of different functionalities and diameters. J. Colloid Interface Sci. 2007, 314 (1), 18−24. (11) Tournus, F.; Charlier, J. C. Ab initio study of benzene adsorption on carbon nanotubes. Phys. Rev. B 2005, 71, 165421. (12) Chen, W.; Duan, L.; Zhu, D. Adsorption of polar and nonpolar organic chemicals to carbon nanotubes. Environ. Sci. Technol. 2007, 41 (24), 8295−8300. (13) Shao, D.; Sheng, G.; Chen, C.; Wang, X.; Nagatsu, M. Removal of polychlorinated biphenyls from aqueous solutions using Bcyclodextrin grafted multiwalled carbon nanotubes. Chemosphere 2010, 79 (7), 679−685. 10378
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Environmental Science & Technology
Article
(35) Shao, D.; Hu, J.; Jiang, Z.; Wang, X. Removal of 4,4′dichlorinated biphenyl from aqueous solution using methyl methacrylate grafted multiwalled carbon nanotubes. Chemosphere 2011, 82 (5), 751−758. (36) Shao, D.; Hu, J.; Chen, C.; Sheng, G.; Ren, X.; Wang, X. Polyaniline multiwalled carbon nanotube magnetic composite prepared by plasma-induced graft technique and its application for removal of aniline and phenol. J. Phys. Chem. C 2010, 114 (49), 21524−21530. (37) Zhao, G.; Jiang, L.; He, Y.; Li, J.; Dong, H.; Wang, X.; Hu, W. Sulfonated graphene for persistent aromatic pollutant management. Adv. Mater. 2011, 23 (34), 3959−3963. (38) Kang, S.; Herzberg, M.; Rodrigues, D. F.; Elimelech, M. Antibacterial effects of carbon nanotubes: size does matter! Langmuir 2008, 24 (13), 6409−6413. (39) Mejias Carpio, I. E.; Santos, C. M.; Wei, X.; Rodrigues, D. F. Toxicity of a polymer−graphene oxide composite against bacterial planktonic cells, biofilms, and mammalian cells. Nanoscale 2012, 4 (15), 4746−4756. (40) Musico, Y. L. F.; Santos, C. M.; Dalida, M. L. P.; Rodrigues, D. F. Improved removal of lead(II) from water using a polymer-based graphene oxide nanocomposite. J. Mater. Chem. A 2013, 1, 3789− 3796. (41) Cho, H.-H.; Smith, B. A.; Wnuk, J. D.; Fairbrother, D. H.; Ball, W. P. Influence of surface oxides on the adsorption of naphthalene onto multiwalled carbon nanotubes. Environ. Sci. Technol. 2008, 42 (8), 2899−2905.
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