Environ. Sci. Technol. 2010, 44, 6377–6383
Adsorption of Aromatic Compounds by Carbonaceous Adsorbents: A Comparative Study on Granular Activated Carbon, Activated Carbon Fiber, and Carbon Nanotubes SHUJUAN ZHANG, TING SHAO, H. SELCEN KOSE, AND TANJU KARANFIL* Department of Environmental Engineering and Earth Sciences, Clemson University, Anderson, South Carolina 29625
Received March 18, 2010. Revised manuscript received June 24, 2010. Accepted June 25, 2010.
Adsorption of three aromatic organic compounds (AOCs) by four types of carbonaceous adsorbents [a granular activated carbon (HD4000), an activated carbon fiber (ACF10), two singlewalled carbon nanotubes (SWNT, SWNT-HT), and a multiwalled carbon nanotube (MWNT)] with different structural characteristics but similar surface polarities was examined in aqueous solutions. Isotherm results demonstrated the importance of molecular sieving and micropore effects in the adsorption of AOCs by carbonaceous porous adsorbents. In the absence of the molecular sieving effect, a linear relationship was found between the adsorption capacities of AOCs and the surface areas of adsorbents, independent of the type of adsorbent. On the other hand, the pore volume occupancies of the adsorbents followed the order of ACF10 > HD4000 > SWNT > MWNT, indicating that the availability of adsorption site was related to the pore size distributions of the adsorbents. ACF10 and HD4000 with higher microporous volumes exhibited higher adsorption affinities to low molecular weight AOCs than SWNT and MWNT with higher mesopore and macropore volumes. Due to their larger pore sizes, SWNTs and MWNTs are expected to be more efficient in adsorption of large size molecules. Removal of surface oxygen-containing functional groups from the SWNT enhanced adsorption of AOCs.
Introduction Carbon nanotubes (CNTs), as newly emerged carbonaceous materials, have attracted extensive attention for their potential use in water treatment. Due to the hydrophobic nature, CNTs have shown strong adsorption affinities to a wide range of organic environmental contaminants (1-12). In a recent review paper (13), CNTs have been suggested as a type of superior adsorbents for removal of microorganisms, natural organic matter (NOM), and toxins from drinking water due to their large external surface area and well developed mesopores. With increasing mass production of CNTs, they might become competitive to the traditional activated carbons (ACs) in terms of cost. In the literature, CNTs have been reported having higher adsorption capacities than ACs for dioxin (1), chloroform (3, 4), vitamin B12 (7), NOM (8), bisphenol A and 17R-ethinyl * Corresponding author phone: (864)656-1005; fax: (864)656-0672; e-mail:
[email protected]. 10.1021/es100874y
2010 American Chemical Society
Published on Web 07/19/2010
estradiol (10), and tetracycline (11), but lower adsorption capacities than ACs for bromodichloromethane, dibromochloromethane, bromoform (3), creatinine (7), naphthalene (11), phenol, dichlorobenzene, and dinitrobenzene (9). Since detailed structural information of adsorbents and/or molecular size and configuration information of adsorbates have been missing in most of these papers, there was a lack of in-depth and quantitative analysis to explain the factors controlling the differences in the adsorption behaviors of CNTs and ACs. The objective of this study was to elucidate the factors controlling the adsorption behaviors of CNTs, granular activated carbons (GACs), and activated carbon fibers (ACFs) by coupling structural information of adsorbents and adsorbates with quantitative analysis of the adsorption data. The adsorption sites of the different types of adsorbents and the packing patterns of various organic chemicals on these adsorbents were examined to provide insight into the adsorption mechanisms of carbonaceous adsorbents. To the best of our knowledge, this is the first work in the literature that makes such an in-depth quantitative analysis and comparison of adsorption behaviors for different types of carbonaceous adsorbents. The correlations between the structural parameters of adsorbents and their adsorption capacities to small molecular weight organic chemicals were examined. The results were also used to assess whether CNTs are promising substitutes to the conventional GACs and ACFs.
Experimental Section Materials. A coal-based GAC (HD4000, from Norit Inc.), a phenol formaldehyde-based ACF (ACF10, from American Kynol Co.), a SWNT (outer diameter: 1-2 nm, length: 5-30 µm, purity >90%, from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences), and a MWNT (inner diameter: 3-5 nm, outer diameter: 8-15 nm, length: 10-50 µm, purity >95%, from Nanostructured & Amorphous Materials, Inc., USA) were used as received. A portion of the SWNT was heat-treated at 1173 K for 2 h under hydrogen flow to remove surface functional groups. The heat-treated SWNT was labeled as SWNT-HT. Prior to use, HD4000 was ground to particles with sizes in the range of 150-180 µm. ACF10 was used directly in strands of fiber. The selected carbons represent four typical types of carbonaceous adsorbents. Carbons were characterized with a number of techniques to obtain structural information. Nitrogen adsorption at 77 K and water vapor adsorption at 273 K were performed with a physisorption analyzer (Micromeritics ASAP 2010) to characterize the surface areas, pore volumes, and surface polarities of the adsorbents. The Brunauer-Emmett-Teller (BET) equation, t-plot method, density functional theory (DFT), and Horvath-Kawazoe (HK) model were used to calculate BET specific surface area (SBET), total pore volume (Vt), pore size distribution, and ratio of open-ended tubes (Ropen) from nitrogen adsorption isotherms. The DFT model for cylindrical pores was used for the nitrogen adsorption analysis of the CNTs and ACF10, whereas the DFT model for slit-shaped pores was adopted for the analysis of HD4000. Elemental analysis was performed with an EA1112 elemental analyzer (Thermo Electron Co.) to determine the oxygen contents of the adsorbents. Three aromatic organic compounds (AOCs) different in planarity, polarity, and hydrogen/electron-donor/acceptor ability were used as probe molecules to cover some typical adsorbate-adsorbent interactions. The AOCs included phenanthrene (PNT, 99.5+%), biphenyl (BP, 99+%), and VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Molecular Properties and Analytical Methods for the Adsorbates AOC
planarity
molecular sizea (Å × Å × Å)
dihedralb MWc densityc solubilityc (deg) (g/mol) (g/cm3) (mg/L)
PNT planar 11.7 × 8.0 × 3.4 BP nonplanar 11.8 × 6.8 × 4.7 2PP nonplanar 11.8 × 7.8 × 5.4
0 47 54
178.23 154.21 170.21
1.063 0.992 1.213
MeOH:H2Oe (v%:v%) λUVf (nm) λexf (nm) λemf (nm)
log Kowd 4.68 ( 0.17 3.98 ( 0.23 2.94 ( 0.25
1.1 6.1 700
80:20 80:20 60:40
250 248 245
293 260 290
366 315 340
a Adapted from ref 12. b Simulated with ChemBioOffice 2008 (CambridgeSoft Corporation) with the MM2 module, the dihedral is defined as the angle between the two benzene rings of the AOCs. c MW: molecular weight, these parameters are collected from material safety data sheet. d Adapted from ref 12. e Eluent for HPLC. f The maximum wavelengths for UV detection, fluorescence excitation, and emission.
TABLE 2. Structural Parameters of Adsorbents surface area distributionb
pore volume distributionb
carbon
[O] (%)
SBET (m /g)
BP for the CNTs and ACF10, whereas the order is 2PP > BP > PNT for the slitshaped HD4000. Since more than 70% of the pores in ACF10 were smaller than 1 nm and the second-widest dimension of PNT was 0.8 nm, the molecular sieving effect was expected to play a role in the adsorption (Scheme 1) and was demonstrated by the surface-area normalized isotherm of PNT on ACF10, which was much lower than those on the other four carbons (Figure 1). For the nonplanar BP and 2PP, the adsorption affinities of ACF10 (on mass basis) at high concentrations were similar or even higher than those of the other carbons, suggesting that there was no molecular sieving effect in their adsorption. These results indicate that the molecular sizes of AOCs played an important role in their adsorption. Besides, the flexibility in molecular configurations of BP and 2PP might also benefit their adsorption in some irregular-shaped pores. The VAOC/VN2 values demonstrate that complete monolayer coverage of MWNT and ACF10 was not formed by PNT (values less than 1), which could be attributed to the molecular sieving effect in PNT adsorption on ACF10 and
SCHEME 1. Schematic Adsorption Sites for AOC Adsorption on the CNTs, GAC, and ACF
the low adsorption affinity of MWNT to PNT. The adsorbed volumes of PNT on the SWNTs and HD4000 and those of BP on all carbons were higher than the monolayer nitrogen adsorption volume capacities, indicating the occurrence of multilayer adsorption and pore filling in the adsorption of AOCs by these carbons. Structural Parameters of Carbons vs Adsorbed Amounts of AOCs. A linear relationship between VAOC and VN2 was observed except for the adsorption of PNT on ACF10 VSOC ) 1.52 VN2 (r2 ) 0.974, N ) 8)
(3)
By converting the occupied volumes to the more practical parameters, KFS and SBET, a new relationship was obtained, exclusive of the case with the molecular sieving effect KFS ) 0.60SBET (r2 ) 0.965, N ) 8)
(4)
A linear relationship between qm of AOCs from the PMM simulation and SBET values of carbons was also observed by excluding the following cases: PNT on ACF10 (presence of the molecular sieving effect), BP on MWNT (large error of qm), and 2PP on MWNT (unreasonably high qm values) qm ) 0.37 SBET (r2 ) 0.702, N ) 11)
(5)
The illustration of these relationships was provided in Figure S7. A linear relationship between qm of PNT and SBET of four MWNTs and a fullerene, similar to eq 5, has been reported in the literature (5). However, this relationship could not explain the adsorption of PNT on a SWNT and was not simultaneously applicable to the other chemicals, such as pyrene and naphthalene. The different structural characteristics of the adsorbents and consequently the different packing patterns of the adsorbates might be attributable to such failures.
As shown in eqs 4 and 5, the correlation between qm and SBET was obviously lower than that between KFS and SBET, and the KFS values were larger than the qm values obtained from the PMM simulation. In some cases, the KFS may overestimate the adsorption capacities. However, the good linear relationship between KFS and SBET indicates that in the absence of the molecular sieving effect, the adsorption capacities of AOCs by the carbonaceous adsorbents are controlled and can be estimated by the surface areas of these carbons, independent of their types. Since these relationships were obtained from small sample sizes, further tests with additional adsorbates and adsorbents are warranted to examine their applicability and limitations for practical applications. Environmental Implications. Adsorption of AOCs on porous carbonaceous adsorbents can be subjected to three effects: micropore, molecular sieving, and hydrophobic effects. The results obtained in this study showed that on a mass basis, adsorption capacities of the GAC and ACF were higher than those of the SWNT and MWNT in the absence of the molecular sieving effect. This was due to the higher microporosities of the GAC and ACF than those of the SWNT and MWNT. Considering the fact that commercial ACs designed for the removal of small molecular weight organic contaminants from water have typically higher surface areas and microporosities than CNTs, the results obtained in this study indicate that from the adsorption capacity point of view, CNTs are not more advantageous than ACs for the removal of hydrophobic organic contaminants from water in the absence of any molecular sieving effect. On the other hand, in the presence of the molecular sieving effect, the capacity of the SWNT was comparable to that of the GAC and higher than that of the ACF on a mass basis. This was due to the hybrid characteristics of the SWNT bundles: a considerable portion of the nanospaces trapped in the aggregates of SWNTs is within micropore range and the presence of a large quantity of VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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manuscript has not been subjected to the peer and policy review of the agency and therefore does not necessarily reflect its views.
Supporting Information Available Nitrogen and water vapor adsorption isotherms of carbons in Figure S1, nonlinear fit of experimental isotherms with four isotherm models in Figures S2-S4, linear vs nonlinear FM fit in Figure S5, adsorption isotherms of AOCs on carbons in Figure S6, structural parameters of carbons vs adsorbed amounts of AOCs in Figure S7, isotherm model information in Table S1, and nonlinear fits of adsorption isotherms in Table S2. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited
FIGURE 2. Adsorption site energy distribution curves of AOCs on the CNTs, GAC, and ACF.
mesoporous spaces (more than 50% in terms of surface area) and the large external surface (which does not subject to the molecular sieving effect). Therefore, it appears that if adsorption of a molecule is subject to the molecular sieving effect, SWNTs may exhibit comparable or better adsorption capacities than ACs. As for MWNTs, they usually exhibited lowest adsorption capacities on a mass basis because the inner cavities of MWNTs were not completely filled by AOCs and the structure of MWNT bundles were mainly mesoporous and macroporous. Since these observations are based on equilibrium conditions, additional analyses on adsorption kinetics are also needed to fully assess the adsorption behaviors of CNTs and ACs.
Acknowledgments This work was partly supported by a research grant from the National Science Foundation (CBET 0730694). However, the 6382
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