Environ. Sci. Technol. 2006, 40, 1855-1861
Adsorption of Polycyclic Aromatic Hydrocarbons by Carbon Nanomaterials K U N Y A N G , †,‡ L I Z H O N G Z H U , ‡ A N D B A O S H A N X I N G * ,† Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, USA, and Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, China
Carbon nanomaterials are novel manufactured materials, having widespread potential applications. Adsorption of hydrophobic organic compounds (HOCs) by carbon nanomaterials may enhance their toxicity and affect the fate, transformation, and transport of HOCs in the environment. In this research, adsorption of naphthalene, phenanthrene, and pyrene onto six carbon nanomaterials, including fullerenes, single-walled carbon nanotubes , and multiwalled carbon nanotubes was investigated, which is the first systematic study on polycyclic aromatic hydrocarbons (PAHs) sorption by various carbon nanomaterials. All adsorption isotherms were nonlinear and were fitted well by the Polanyi-Manes model (PMM). Through both isotherm modeling and constructing “characteristic curve”, Polanyi theory was useful to describe the adsorption process of PAHs by the carbon nanomaterials. The three fitted parameters (Q0, a, and b) of PMM depended on both PAH properties and the nature of carbon nanomaterials. For different PAHs, adsorption seems to relate with their molecular size, i.e., the larger the molecular size, the lower the adsorbed volume capacity (Q0), but higher a and b values. For different carbon nanomaterials, adsorption seems to relate with their surface area, micropore volume, and the volume ratios of mesopore to micropore. Quantitative relationships between these sorbent properties and the estimated parameters of PMM were obtained. These relationships may represent a first fundamental step toward establishing empirical equations for quantitative prediction of PAH adsorption by carbon nanomaterials and possibly other forms of carbonaceous (geo-) sorbents, and for evaluating their environmental impact. In addition, high adsorption capacity of PAHs by carbon nanotubes may add to their high environmental risks once released to the environment, and result in potential alteration of PAHs fate and bioavailability in the environment.
Introduction Fullerenes, single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) are novel and interesting carbon materials. Since their first discovery (1* Corresponding author phone: (413)545-5212; fax: (413)545-3958; e-mail:
[email protected]. † University of Massachusetts. ‡ Zhejiang University. 10.1021/es052208w CCC: $33.50 Published on Web 02/16/2006
2006 American Chemical Society
3), their unique chemical and physical properties make them ideal candidates for widespread applications such as drug delivery and energy conversion. Over the past 20 years, more and more potential applications such as catalyst supports, optical devices, molecular (memory) switch, quantum computer, biomedical use, and environmental remediation have been found (4). During that period of time, numerous types of carbon nanomaterials have been produced, and new ones will be discovered and produced in the future. Because numerous and a large quantity of carbon nanomaterials will be produced and used, there is serious concern over their health and environmental risks once they are released to the environment (4-6). The primary risk of these materials comes from their toxicity. Because of the nanosize, they can enter into cells (7-9), causing damage to plants, animals, and humans. Previous researchers have explored the toxicity of carbon nanomaterials to animal and human cells (10-16). It was suggested that the toxicity of carbon nanomaterials is not only from their own harmful nature but also from the toxic substances sorbed by them (4-6). Therefore, knowledge of toxic compound adsorption by carbon nanomaterials is critical and useful for risk assessment of these nanomaterials. This knowledge is also important for understanding the effect of the nanomaterials on the fate and transport of toxic pollutants in the environments, similar to the other carbon materials such as soot and charcoal (17-19). For potential environmental applications as superior sorbents, carbon nanomaterials have been studied for removal of organic pollutants, metals, fluoride, and radionuclide 243Am(III) (20-30). Most of these studies focused on the adsorption of organic vapors and metals (20, 21, 24-28). Very few studies dealt with the interfacial interactions of organic contaminants with carbon nanomaterials in aqueous media (29-32). Peng et al. (29) examined the adsorption of 1,2-dichlorobenzene by carbon nanotubes, suggesting that they can be used as adsorbents for removal of chlorobenzenes from water. The Freundlich model was used in their study to describe the isotherms. Another study showed that MWCNTs had potential applications for trihalomethane removal from drinking water, and both Langmuir and Freundlich models fitted the isotherms well (30). Cheng et al. (31, 32) investigated the adsorption-desorption of naphthalene and 1,2-dichlorobenzene by different aggregates of fullerene (C60). They used the linear model to describe the isotherms of large C60 aggregates and thin C60 films, and the Freundlich model was used to describe the isotherms of small C60 aggregates. In the above four studies, however, isotherm models were used without comparing them with other common models, and only a narrow range of equilibrium concentrations ( MWCNTs . fullerene. These nanotubes also had much higher adsorption affinity (over 2 orders of magnitudes) than natural soils/sediments (18). High PAHs adsorption capacity of CNTs indicates their potential environmental risks and significant effect on the fate of PAHs once released to the environment. Though Freundlich coefficients (Kf) cannot be directly compared due to different units, SWCNT had the highest Kf value for phenanthrene (Kf ) 102.52 ) 331 (mg/ g)/(mg/L)1/n, listed in Table SI), close to activated carbon, 273 (mg/g)/(mg/L)1/n (33), implying it may be used as a potential sorbent for PAHs removal from wastewater and drinking water if it was designed properly and had low production cost. VOL. 40, NO. 6, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Isotherms Modeling. The pronounced nonlinearity of all the isotherms is apparent when compared with the dashed lines of linear isotherms (Figure 1). Therefore, nonlinear isotherm models (Table 3), i.e., Freundlich (FM), Langmuir (LM), Brunauer-Emmett-Teller (BET), Dual-mode (DMM), Dual-Langmuir (DLM), and Polanyi-Manes (PMM) models were tested to fit experimental data. The fits of PMM to the adsorption data of naphthalene, phenanthrene and pyrene on MWCNT15 and that of phenanthrene on fullerene, SWCNT, and MWCNTs are displayed in Figure 1A and 1B as examples. The fits of other models are shown in Figures S3 and S4. Respective fitting parameters are given in Table SI. The PMM had good fit for all the isotherms (Figure 1), supported by the lowest MWSE values among all tested models except for the isotherm of phenanthrene on fullerene for which FM had the lowest MWSE value (Table SI). Significant deviation of model estimation from the experimental data was observed for LM at high concentrations and for BET, DMM, and DLM at relatively low concentrations (Figures S3 and S4). Inadequate fits were also reflected by the high MWSE (Table SI). For FM, the goodness of fit varied with both the tested chemicals and carbon nano-sorbents. Good fits of FM were obtained for naphthalene on MWCNT15 and phenanthrene by SWCNT and fullerene, while it failed for other solute-adsorbent systems (Table SI, Figure S3A and Figure S4A). Since FM is a special form of PMM (b ) 1) (38), it was not surprising that FM was applicable for a few cases. Overall, the model fitting results indicate that the adsorption process of PAHs on these carbon nanomaterials (i) would be neither monolayer formation on a homogeneous surface (i.e., LM) nor simple multilayer formation (i.e., BET), (ii) may not be a combination of partition and Langmuir-type adsorption domains (i.e., DMM), and (iii) could not be limited by two types of adsorption sites (i.e., DLM). PMM seems not only applicable for pore filling, but also applicable for flat surfaces (38), supported by the fact that both FM and PMM had good fits for a few cases. It is often suggested that good model fits may result from over-parametrization (17). However, the PMM with three fitting parameters had lower MWSE values than DLM with four parameters for all isotherms. Furthermore, FM with only two parameters had the lowest MWSE values than all the others with same or more parameters for the phenanthrene isotherm by fullerene. These data indicate that the effect of over-parametrization could be evaluated by the MWSE calculation. From MWSE evaluation (Table SI), PMM generally had the best fit. We will discuss if Polanyi theory could mechanistically capture the adsorption process of carbon nanomaterials below. Characteristic Curve. The Polanyi theory was originally used to describe gas adsorption by activated carbon and later was applied to adsorption from aqueous solution. The theory has three basic assumptions (39). The first is that there exists an adsorption space on the sorbent surface. For any molecule in this space, the magnitude of adsorption potential varies within the adsorption space, depending on its proximity to solid surface atoms. Adsorption potential, ) RTln(Cs/Ce), is defined as the energy that is required to remove the molecule from its location in the adsorption space to a point outside the attractive force field of the solid surface; where R is the universal gas constant, T is the absolute temperature, Cs is the water solubility, and Ce is the equilibrium solution phase concentration. The second is that the adsorption potential is independent of temperature. The last is that the adsorbed liquid has properties similar to the corresponding bulk liquid. A direct consequence of these assumptions is that, for a given sorbent, a plot of sorbed volume (qv) against equilibrium adsorption potential () should yield a curve that is tem1858
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FIGURE 2. Characteristic curves of phenanthrene adsorption on MWCNT15 at 24 °C, 40 °C and 55 °C. perature-invariant and determined by the structure of the sorbent (39). This curve was called as “characteristic curve”. Therefore, characteristic curves can be employed to examine whether the Polanyi theory mechanistically captures the adsorption process of PAHs by carbon nano-sorbents. Characteristic curves of phenanthrene adsorption on MWCNT15 at 25 °C, 40 °C, and 55 °C are shown in Figure 2 as an example. They all fell onto a single curve, indicating mechanistic usefulness of Polanyi theory to describe PAHs adsorption by these carbon nanomaterials. Correlation Curve. Correlation curve is described as a curve obtained from plotting qv against (/Vs), where Vs is the molar volume of adsorbates (39). This curve has been employed to evaluate the usefulness of Polanyi theory and PMM for describing adsorption (17, 18). Based on two more assumptions (39), in addition to the three original ones of Polanyi theory, correlation curves for different adsorbates on a given sorbent would yield a single curve. One assumption is that all adsorption space of a given adsorbent is accessible for all adsorbates, i.e., that there is no molecular sieving effect. Under this assumption, characteristic curves of all adsorbates on an adsorbent will have a limiting adsorption volume at zero adsorption potential based on the Polanyi theory. The second assumption is that no other properties of adsorbates except for the molar volume will affect the adsorption. Figure 3 shows the correlation curves of naphthalene, phenanthrene, and pyrene on MWCNT15. It is clear that the correlation curves of the three PAHs deviated substantially from one another. Separation of correlation curves for different adsorbates has been observed widely for sorbents such as charcoal (17), natural soil (18), and activated carbons (39). Manes et al. (39) introduced a factor (γ) to adjust the adsorbate molar volume to a common limiting adsorption volume, so that plots of qv vs (/γVs) were made to collapse to a single correlation curve for different adsorbates on activated carbon. Crittenden et al. (40) introduced another factor, normalizing factor, F, to replace the molar volume, so that a plot of qv vs (/F) would produce a single correlation curve. The normalizing factor F is calculated from linear solvation energy relationship (LSER) parameters including the intrinsic molar volume, the polarity/polarizability, and the hydrogen-bonding accepting and donating ability. However, there are still some cases in which no single correlation curve could be obtained if only the molar volume was adjusted/replaced (39, 40), which is still not well understood. Similarly, in this study, a single correlation curve could not be obtained by adjusting only adsorbate molar volume (Figure
FIGURE 4. Plot of surface areas (multi-point BET), micropore volumes and mesopore volumes of carbon nanomaterials against the estimated adsorbed capacities (Q0) from PMM for phenanthrene. FIGURE 3. Correlation curves of nanphthalene, phenanthrene, and pyrene adsorption on MWCNT15. 3). Therefore, molar volume alone as abscissa scaling factor did not seem to be appropriate for the correlation curve, as reported by Sander et al. (17). It may be the time to reconsider whether two assumptions of the correlation curve are universally applicable. Though molar volume (Vs) alone is not adequate to obtain a single correlation curve for various adsorbates on a given adsorbent as mentioned above, PMM was good and valid to describe sorption data of individual isotherms (Table SI, Figures 1 and 2) because Vs was used as a constant in the regression analysis. Furthermore, correlation curves cannot be used as the only criterion to test whether the Polanyi theory is applicable for describing the adsorption process because both PMM and Polanyi theory are independent of the two assumptions of the correlation curve. Therefore, we tried here to use the three estimated parameters of PMM to correlate adsorption behavior with the properties of adsorbates and adsorbents. Relations of Adsorption with Adsorbate Properties. The first parameter of PMM is adsorbed capacity. The adsorbed mass capacities (Q0) (Table SI) of naphthalene, phenanthrene, and pyrene on MWCNT15 were 101.87 mg/g, 101.62 mg/g and 101.63 mg/g, respectively. Their adsorbed volume capacities calculated from their mass capacities and respective solidphase density were about 0.074 cm3/g, 0.040 cm3/g, and 0.033 cm3/g, respectively, yielding an order: naphthalene > phenanthrene > pyrene. This order is negatively related to molecular size (MV): 126.9 Å3 for naphthalene, 169.5 Å3 for phenanthrene, and 186.0 Å3 for pyrene (Table 1). One reason for this order is that the available adsorption space of adsorbent varied for different adsorbates because of their molecular sizes. Molecules cannot have access to micropores with pore size smaller than their size. Therefore, the larger the adsorbate molecular size, the lower the adsorbent adsorption space available. Another reason is that adsorbed phase maybe different from the corresponding bulk phase (17, 37). Adsorption of PAHs onto carbon nanomaterials is removing PAHs from water to the adsorbed phase. Given the monolayer adsorption volume capacity of N2 on MWCNT15 (0.061 cm3/g, calculated from the adsorbent multi-point BET surface area, 174 m2/g, and the size of nitrogen molecule, 0.354 nm), complete monolayer coverage did not form for phenanthrene and pyrene; their coverage rates were about 75 and 55%, respectively. Under such low coverage, it is
neither likely that solute liquification would occur to a significant degree to form condensates having bulk liquid properties nor that solute would form crystallites having bulk solid properties (17). Consequently, expansion of molar volume may occur from solid phase to adsorbed phase as that from solid phase to liquid phase. From solid to liquid phase, the expansion ratio for naphthalene, phenanthrene, and pyrene, calculated from the critical molar volume divided by molar volume (Table 1), are 3.19, 3.32, and 3.89, respectively. Therefore, expansion for large molecules should be greater than small molecules and the density of large molecular adsorbates would decrease more than that of small ones when the phase changes from solid phase to adsorbed phase, which result in the adsorbed volume capacities (calculated from solid-phase density) for large molecular adsorbate being lower than small molecular adsorbate, and its actual adsorbed volume. The second parameter for PMM is b (Table SI). For activated carbons, b has been suggested to relate with the adsorption potential energy, which depends on the adsorbent nature rather than chemical properties (40, 41). However, values of b for naphthalene, phenanthrene, and pyrene on MWCNT15 were 0.968, 2.16, and 2.49, respectively (Table SI), which varied clearly with chemical properties in this case and giving an order: naphthalene < phenanthrene < pyrene. This order seems to be in agreement with the variation of logKow or MV (Table 1). The last fitting parameter for PMM is a (Table SI). Values of a for naphthalene, phenanthrene and pyrene on MWCNT15 were -1.31 × 10-2, -4.11 × 10-5, and -1.35 × 10-5, respectively, having the same order with parameter b: naphthalene < phenanthrene < pyrene. Based on the above analysis, we may conclude that adsorption of different adsorbates is related with the molecular size. Since only three chemicals were investigated in this study, more chemicals need to be examined. Relations of Adsorption with Adsorbent Properties. The three estimated parameters of PMM were employed to examine the effect of adsorbent properties on adsorption. The estimated adsorbed capacities (Q0, mg/g) by PMM for phenanthrene on carbon nanomaterials increased with increasing surface area (multi-point BET), micropore volume, and mesopore volume (Figure 4). A linear relationship between Q0 and surface areas (multi-point BET) or micropore volumes of these carbon nanomaterials except for VOL. 40, NO. 6, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SWCNT is obtained as follows,
Q0 ) 0.248Asurf
understanding of these relationships,
(r2 ) 0.999, n ) 5)
log(-a) ) -0.235 Rmeso/micro - 1.18
and
(r2 ) 0.885, n ) 5)
and 0
-3
Q ) 1.04 × 10 Vmicro
2
(r ) 0.997, n ) 5)
b ) 0.112 Rmeso/micro + 0.656
(r2 ) 0.858, n ) 5)
2
where, Asurf is the surface area (multi-point BET), m /g, and Vmicro is the micropore volume, cm3/g. Given the density of phenanthrene (1.063 g/cm3) and the size of nitrogen molecule (0.354 nm), one can obtain the relationships between the adsorbed volume capacity (Q0v, cm3/g) and monolayer adsorption volume capacity of N2 (VN, cm3/g) or micropore volumes, i.e.,
Q0v ) 0.658 VN
(r2 ) 0.999, n ) 5)
and
Q0v ) 0.976 Vmicro
(r2 ) 0.997, n ) 5)
These relationships indicate that the adsorption capacities of phenanthrene by these carbon nanomaterials were related with surface area or micropore volume. However, the adsorbed volume capacity of phenanthrene on SWCNT deviated from these relationships. The adsorbed volume capacity of phenanthrene on SWCNT was about 0.249 cm3/ g, while the monolayer adsorption volume capacity of N2 was only 0.192 cm3/g and micropore volume was only 0.13 cm3/g for SWCNT. The adsorbed volume capacity of phenanthrene on SWCNT was over both the monolayer adsorption volume capacity of N2 and micropore volume of SWCNT. Similarly, the adsorbed volume capacity of naphthalene on MWCNT15 (0.074 cm3/g) was also greater than both the monolayer adsorption volume capacity of N2 (0.061 cm3/g) and micropore volume (0.044 cm3/g) of MWCNT15. Therefore, additional adsorption such as multilayer adsorption and mesopore filling should be considered, or the surface area and micropore volume were underestimated by the N2 method at 77K (42). Another empirical relationship between Q0v and VN or Vmicro was observed for CNTs including SWCNT (Figure S5), though not well understood. The Langmuir equation could be used to fit these relationships and the regression equations were as follows:
1/Q0v ) 0.00151 × 1/VN -0.0021
(r2 ) 0.992, n ) 5)
and
1/Q0v ) 0.00104 × 1/Vmicro -0.0022
(r2 ) 0.988, n ) 5)
Because Asurf and Vmicro values of fullerene were taken from Ismail et al. (36), whose sample is not the same with this study, they did not follow these relationships. However, smaller Asurf value (0.07-0.17m2/g) for fullerene was reported by Cheng et al. (31). If this small Asurf value was considered, resulting a lower VN value, fullerene would follow the relationship between Q0v and VN. Estimated parameter b of PMM (Table SI) increased from 1.0 to a plateau value of about 2.0 with the structures of carbon nanomaterials: fullerene, SWCNT, MWCNT8, MWCNT15, MWCNT30, and MWCNT 50. The estimated parameter a of PMM had the same trend with parameter b (Table SI), which increased from -1.82 × 10-2 to -4.11 × 10-5. Furthermore, log(-a) and b for carbon nanotubes were linearly related to the ratios of mesopore volume to micropore volume (Rmeso/micro ) Vmeso/Vmicro) (Figure S6). The regression equations are shown below, though we still do not have full 1860
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All the above equations may be viewed as a first step to establish empirical relationships for quantitative prediction of PAH adsorption by carbon nanomaterials, and then to evaluate their environmental impact. Since PMM is adequate to describe the adsorption isotherms of organic chemicals by activated carbon and charcoal (17, 43), similar relationships could be obtained for these carbonaceous sorbents. These relationships would be useful for estimating the parameters of PMM from carbonaceous sorbent surface area, micropore volume and Rmeso/micro. Furthermore, one can also estimate the parameters of PMM from PAH properties such as MV, if quantitative relationships between parameters of PMM and PAH properties can be obtained, which requires further research involved with more PAH compounds. Then, PAHs adsorption by carbon nanomaterials and possibly other forms of carbonaceous (geo-) sorbents could be evaluated from the parameters of PMM. Two possible types of adsorption sites had been proposed for carbonaceous (geo-) sorbents (35): (i) adsorption on external surface, and (ii) adsorption in nanopores inside the sorbents. Though adsorption of PAHs by carbon nanomaterials in this study was described well by Polanyi theory, it is still early to conclude the exact adsorption sites and mechanism because: (i) Polanyi theory is not only applicable for pore filling but also applicable for flat surfaces (38); and (ii) PMM estimated adsorption parameters for carbon nanomaterials were correlated with both surface areas and micropore volume as presented by the above equations, due to the positive relationship between the pore volumes and surface areas of these nanomaterials (Table 2). Further experiments focused on competitive sorption, desorption, and microscopic observations may help to advance the understanding of adsorption sites and mechanisms.
Acknowledgments This work was supported by the Massachusetts Agricultural Experiment Station (MAS 00090 and MAS 8532).
Supporting Information Available Figure S1 shows typical TEM images of fullerene and carbon nanotube aggregates. Figure S2 shows the solubility normalized adsorption isotherms of pyrene, phenanthrene, and naphthalene. Figures S3 and S4 display the fitting lines of Freundlich (FM), Langmuir (LM), Brunauer-Emmett-Teller (BET), Dual-mode (DMM), and Dual-Langmuir (DLM) models to the adsorption data. Figures S5 and S6 present empirical relationships between phenanthrene adsorption parameters of PMM and carbon nanotube properties. Table SI lists the model fitting results of isotherms. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review November 3, 2005. Revised manuscript received January 20, 2006. Accepted January 24, 2006. ES052208W
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