Environ. Sci. Technol. 2005, 39, 381-382
Comment on “Modeling Maximum Adsorption Capacities of Soot and Soot-like Materials for PAHs and PCBs” Van Noort et al. (1) derive maximum surface area normalized adsorption capacities (Qmax*) for the sorption of PAHs and PCBs to soot and carbon materials. They use the Langmuir isotherm to extrapolate Qmax* at the solubility limit from solidwater distribution coefficients (Ks) measured in the picogram per liter to nanogram per liter range (2) with estimated Langmuir affinities for adsorption (b) at a carbonaceous surface (3). The purpose of this comment is to provide a comparison with estimated surface area normalized monolayer adsorption capacities (Qmono*) and with empirical sorption data from the literature. The Langmuir isotherm has been derived for monolayer adsorption on a homogeneous smooth surface (4). For a monolayer, the theoretical surface area normalized monolayer capacity (Qmono*) can be calculated from the average surface covered by a sorbate molecule. Taking half of the molecular surface area for the contact area between a PAH molecule and the surface, the theoretical Qmono* for naphthalene, phenanthrene, fluoranthene, benz[a]anthracene, and benzo[k]fluoranthene were estimated and compiled in Table 1. We used a molecular surface area of 155.8, 198.0, and 218.0 Å2 for naphthalene, phenanthrene, and fluoranthene (5) and estimated the molecular surface area of benz[a]anthracene and benzo[k]fluoranthene from the mo-
TABLE 1. Comparison of Estimated Maximum Surface Area Normalized Adsorption Capacity (Qmax*) of a Purely Carbonaceous Surface Based on eq 4 in Ref 1 with the Theoretical Maximum Surface Area Normalized Monolayer Capacity (Qmono*)
PAH compd
molecular surface area (5) (Å2)
Qmax* according to ref 1 (mol/m2)
Qmono* (mol/m2)a
naphthalene phenanthrene fluoranthene benz[a]anthracene benzo[k]fluoranthene
156 198 218 268b 295b
7.6E-4 7.9E-5 2.6E-5 6.3E-6 2.0E-6
2.1E-6 1.7E-6 1.5E-6 1.2E-6 1.1E-6
a Estimated from the molecular surface area of PAHs. b Estimated from the molecular surface area of naphthalene with the CA factors published in ref 1.
lecular surface area of naphthalene and the relative contact area factor (CA) published by Van Noort et al. (1). PAHs are not perfectly flat molecules, and Qmono* is somewhat underestimated by using half of the molecular surface for the contact area. At the same time, we somewhat overestimate Qmono* by neglecting uncovered surface between the PAH molecules of a monolayer. The Qmax* estimated according to eq 4 in Van Noort et al. (1) for a purely carbonaceous surface (foc ) 1) are much higher than our estimated monolayer capacity Qmono*, as can be seen in Table 1. Apparently, eq 4 in Van Noort et al. (1) predicts more than an adsorbed monolayer, which is not compatible with the Langmuir adsorption model. It is noteworthy that Langmuir himself excluded porous bodies from his theory of monolayer adsorption when he wrote: “With charcoal, on the other hand, there is no definite surface which can be covered by a layer one molecule deep” (4). Empirically determined sorption isotherms of hydrophobic chemicals on carbon materials are often better described by the Freundlich isotherm (6). The Freundlich isotherm can be perceived as the sum of numerous Langmuir-type isotherms, each describing a number of similar sorption sites (7). The soot or carbon material-water distribution coefficients (Ks) measured by Jonker and Koelmans (2) at extremely low aqueous sorbate concentrations could thus be interpreted as the product of the sorption affinity (bs) and the sorption capacity (Qmax,s) of the strongest sorption sites. This sorption affinity (bs) of the strongest sites would then have to be significantly larger than estimated by Van Noort (3) to result in a Qmax,s*, which is equal or smaller than the monolayer capacity Qmono*. Van Noort et al. (1) mention that eq 4 significantly overpredicts the empirically determined maximum sorption capacities for PAHs on F400 activated carbon reported by Walters and Luthy (8). In Table 2, we compare the estimated maximum sorption capacity (Qmax*) with the empirically determined maximum volume of phenanthrene adsorbed to various other carbon materials (6). For this comparison, we estimated the volume occupied by each sorbed phenanthrene molecule from the density of phenanthrene solid, which is 1.02 (g/cm3) (6). Apparently, eq 4 overestimates the maximum sorption capacity of microporous carbons such as activated carbon, charcoal, and lignite coke and underestimates the sorption capacity of polymeric carbons such as coals. As reported by Kleineidam et al. (6), the maximum volume of phenanthrene sorbed to carbon materials is reasonably well-correlated with the total meso- and micropore volume (Table 2), which suggests that pore-filling is
TABLE 2. Comparison of Monolayer (Qmono*) and Estimated (Qmax*) Adsorption Capacities and Empirical Maximum Volume Adsorbed (Vmax) for Phenanthrene and Various Carbon Materials estimated according to ref 1
monolayer sorbent
meso-/micropore vola (6) (cm3/kg)
BET-SA (6) (m2/g)
foc (6) (g/g)
Qmono* (mol/m2)
Vmaxc (cm3/kg)
empirical (6) Vmaxb (cm3/kg)
Qmax* (mol/m2)
Vmaxc (cm3/kg)
activated carbon (F100) lignite coke charcoal high-volatile bituminous coal sub-bituminous coal
441 218 112 11 2
790 306 210 3.45 0.56
0.89 0.88 0.82 0.72 0.55
1.7 E-6 1.7 E-6 1.7 E-6 1.7 E-6 1.7 E-6
235 91 63 1.0 0.17
415 89 86 17 3.8
3.4 E-5 3.1 E-5 2.0 E-5 9.2 E-6 2.5 E-6
4700 1700 730 5.5 0.24
a The micropore volume is from top to bottom: 283, 73, 64, 0.022 (cm3/kg), and not detectable. b Kleineidam et al. (6) derive V max from the solubility normalized sorption equilibrium data based on the Polanyi-Dubinin-Manes (PDM) model superposed with a linear isotherm. The values are obtained from isotherms normalized by the solid solubility. c Calculated for comparison using a volume of 175 cm3/mol of phenanthrene.
10.1021/es048579e CCC: $30.25 Published on Web 12/03/2004
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the relevant sorption process near the solubility limit of phenanthrene. For coals, absorption into the polymeric matrix might also be relevant at high solute concentrations (6). These two processes cannot be anticipated from solidwater partitioning coefficients measured at extremely low aqueous concentrations, and they are not considered in the Langmuir model. As an aside, it is also noteworthy that the pore volume and BET-surface area determined by N2-adsorption might underestimate the actual pore volume and BET-surface area for coals and similar polymeric sorbents. In typical adsorption experiments at very low temperatures, N2 does not entirely fill the available cavities within coals due to “activated diffusion in narrow micropores” (9). This might result in artifacts, when the sorption capacity of these sorbents is normalized by BET-surface area. In conclusion, we believe that the Langmuir isotherm cannot be used to extrapolate maximum sorption capacities (Qmax*) near the aqueous sorbate saturation limit from distribution coefficient measured at extremely low aqueous sorbate concentrations. Adsorption sites in carbon materials are not uniform and other processes such as, multilayer adsorption, condensation in capillary pores, and absorption into the polymeric matrix may be relevant near the solubility limit.
Literature Cited (1) Van Noort, P. C. M.; Jonker, M. T. O.; Koelmans, A. A. Modeling maximum adsorption capacities of soot and soot-like materials for PAHs and PCBs. Environ. Sci. Technol. 2004, 38, 33053309. (2) Jonker, M. T. O.; Koelmans, A. A. Sorption of polycyclic aromatic hydrocarbons and polychlorinated biphenyls to soot and soot-
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(4) (5)
(6) (7) (8) (9)
like materials in the aqueous environment: mechanistic considerations. Environ. Sci. Technol. 2002, 36, 4107-4113. Van Noort, P. C. M. A thermodynamic-based estimation model for adsorption of organic compounds by carbonaceous materials in environmental sorbents. Environ. Toxicol. Chem. 2003, 22, 1179-1188. Langmuir, I. The constitution and fundamental properties of solids and liquids. J. Am. Chem. Soc. 1916, 38, 2221-2295. Yalkowsky, S. H.; Valvani, S. C. Solubilities and partitioning 2. Relationships between aqueous solubilities, partitioning coefficients, and molecular surface areas of rigid aromatic hydrocarbons. J. Chem. Eng. Data 1979, 24, 127-129. Kleineidam, S.; Schuth, C.; Grathwohl, P. Solubility-normalized combined adsorption-partitioning sorption isotherms for organic pollutants. Environ. Sci. Technol. 2002, 36, 4689-4697. Grathwohl, P. Diffusion in Natural Porous Media: Contaminant Transport, Sorption/Desorption and Dissolution Kinetics; Kluwer: Boston, 1998. Walters, R. W.; Luthy, R. G. Equilibrium adsorption of polycyclic aromatic hydrocarbons from water onto activated carbon. Environ. Sci. Technol. 1984, 18, 395-403. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982.
David Werner* School of Civil Engineering and Geosciences University of Newcastle upon Tyne Newcastle upon Tyne NE1 7RU, U.K.
Hrissi K. Karapanagioti Marine Sciences Department University of the Aegean Mytilene 81100, Greece ES048579E
