Environ. Sci. Technol. 1997, 31, 3238-3243
Thermodynamic Considerations in the Sorption of Organic Contaminants by Soils and Sediments. 1. The Isosteric Heat Approach and Its Application to Model Inorganic Sorbents WEILIN HUANG AND WALTER J. WEBER, JR.* Environmental and Water Resources Engineering, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125
Isosteric heats of sorption of phenanthrene from aqueous solution were determined for a hydrophobic sorbent (graphite) and for four hydrophilic sorbents (R-Al2O3 and three mesoporous silica gels). The Clausius-Clapeyron equation was used in conjunction with temperaturecorrected aqueous-phase solute activity coefficients to compute isosteric heats from measured temperature-dependent Freundlich isotherm parameters. The results reveal that sorption of phenanthrene by graphite is exothermic, whereas its sorption by each of the other four model sorbents is primarily endothermic. This is consistent with the expected occurrence of distinctly different molecular interactions of solute and solvent molecules at hydrophobic and hydrophilic surfaces. Phenanthrene competes favorably with water for sorption on hydrophobic surfaces, but cannot compete effectively with water for sorption on hydrophilic surfaces; in the latter cases, the low level of sorption that does occur is driven by entropy gain by water molecules in bulk phase. This report on enthalpy relationships and molecular-level interpretation of observed sorption behavior for rigid solid sorbents is the first in a series of papers on the subject. Subsequent papers utilize the experimental approach and mechanistic information developed here to explore operative sorption mechanisms in the more complex realms of physically expandable and chemically more heterogeneous soil/ sediment organic matrices.
swollen domain and a hard, relatively condensed and tightly knit domain (3-14). By analogy to synthetic organic polymers, these two domains have been characterized as the respective equivalents of rubbery and glassy polymeric states of polymers (5, 7, 12-14). It has been further hypothesized that the two different types of domains, like the two different polymeric states, exhibit fundamentally different sorption behaviors. Sorption by highly amorphous SOM matrices is believed to be relatively fast, near-linear, and completely reversible, and sorption by condensed SOM domains to be relatively slow, significantly nonlinear, subject to solutesolute competition, and only partially reversible (3-14). The overall behavior of a dual-domain geosorbent would then reflect some or all of the attributes of its condensed domain. While such a dual reactive domain concept appears a feasible means for reconciling the non-partitioning-like behaviors exhibited by most soils and sediments, the specific mechanisms of sorption by the two different domains have yet to be systematically investigated and explained at the molecular level. The overall objective of the work described here and in forthcoming companion papers in this series has been to investigate mechanisms of hydrophobic organic contaminant (HOC) sorption on soils and sediments by employing a thermodynamic approach for relating macroscopic sorption measurements to molecular interactions among sorbates, sorbents, and solutions in model sorbent, soil, and sediment systems. Sorption isotherms have been measured under different temperature conditions for a range of model inorganic sorbents having various surface characteristics and natural sorbents having different amounts and types of SOM. The temperature-dependent isotherm parameters so obtained are then utilized to calculate sorption enthalpies (isosteric heats), which in turn are correlated with the chemical and physical characteristics of inorganic surfaces and SOMs associated with the model solids, soils, and sediments studied. In this first paper of the series, the thermodynamic approach is established and applied to five relatively simple systems consisting of structurally rigid model sorbents. Sorption mechanisms for these simple systems are discussed in terms of intermolecular forces operating between the two competing sorbates (an HOC solute and water) and the surfaces of the various sorbents examined. Enthalpy changes associated with sorption process may be estimated using the Clausius-Clapeyron equation
[
]
(1)
ln KD ) -∆H°/RT + constant
(2)
Qis ) R
] [
d ln aw d(1/T)
qe
≈R
d ln Ce d(1/T)
qe
or the van’t Hoff equation
Introduction It was postulated some two decades ago that sorption of weakly polar (e.g., chlorinated solvents, polynuclear aromatic hydrocarbons) and nonpolar hydrophobic organic contaminants from aqueous solutions by soils and sediments is dominated by partitioning into relatively homogeneous, lipophilic, gel-like humic matrices (e.g., refs 1 and 2). A number of more recent studies have shown, however, that such sorption often manifests behavior that is inconsistent with the partitioning hypothesis, i.e., is nonlinear, slow, and hysteretic (e.g., refs 3-9). It has been hypothesized that soil/ sediment organic matter (SOM) may comprise two distinctly different types of domainssa soft, highly amorphous and * Corresponding author: e-mail:
[email protected]; phone: 313-763-1464; fax: 313-763-2275.
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Here Qis is the isosteric heat of sorption (kJ/mol), a measure of the enthalpy change involved in the transfer of solute from the reference state to the sorbed state at a constant solidphase concentration; aw is the aqueous-phase solute activity; ∆H° is the enthalpy change (kJ/mol); R is the universal gas constant (8.314 × 10-3 kJ mol-1 K-1); Ce and qe are the equilibrium aqueous-phase and solid-phase solute concentration, respectively; T is temperature in Kelvin; and KD () qe/Ce) is a single-point or linear sorption distribution coefficient. Previous studies have generally concluded that enthalpy changes associated with the sorption of weakly polar and nonpolar HOCs by soils and sediments are small and frequently negative (i.e., exothermic), suggesting that sorption
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1997 American Chemical Society
is dominated by weak solute-sorbent interactions (i.e., physisorption) and driven primarily by hydrophobic interactions (i.e., entropic effects) (e.g., refs 1, 5, and 15-27). Reported sorption enthalpies, however, are essentially not comparable from one study to another, largely because thermodynamic approaches, reference states, sorbate-sorbent systems, or experimental procedures employed by previous investigators for measuring temperature-dependent sorption equilibria are different (23). For example, eq 1 can be applied to sorbate-sorbent systems exhibiting either linear or nonlinear sorption, whereas eq 2 is appropriate only for those exhibiting linear sorption. Many previous studies assumed that aw was approximately equal to Ce over the T ranges examined (i.e., assuming application of a Henry’s law or infinitely dilute solution convention in which the aqueousphase activity coefficient, γw, of a solute is 1.0). An isosteric heat calculated using the term on the right-hand side of the second equality in eq 1 is positive (i.e., endothermic sorption) only if Ce decreases as 1/T decreases at a given qe. When aw rather than Ce is considered, however, the criterion for endothermic sorption is that the product of Ce and γw decreases as 1/T decreases. Thus, if Ce at a given qe value increases only slightly as 1/T decreases while the value of γw decreases significantly, apparent additional endothermic circumstances are created. In other words, isosteric heats calculated using the approximation aw ≈ Ce would tend to be more exothermic than actual values. Similarly, if the value of γw is temperature dependent and given the fact that the solid-phase solute activity coefficient (γs) is not rigorously defined (21), eq 2 would not be an appropriate approach for calculating enthalpy changes of sorption because KD is not equal to the ratio of solute activities in the solid and aqueous phases. Different experimental procedures may also have significant effects on enthalpy calculations. For example, many studies have utilized sorption data collected over solidsolution contact times ranging from a few hours to a few days; these may not be sufficient times for attainment of equilibrium. The use of nonequilibrium sorption data will inevitably result in erroneous estimation of enthalpy changes. As a result, enthalpies or isosteric heats reported in many previous studies are most likely inaccurate in numerical value and, in many cases, incorrect in sign (i.e., exo- or endothermic). They thus provide little insight to the nature of sorption mechanism(s). Young and Weber (5) and Spurlock (23) recently employed rigorous thermodynamic approaches that considered isotherm nonlinearity in sorption enthalpy calculations for analysis of enthalpy relationships in several sorbent-sorbatesolution systems. Their results revealed that isosteric heats of sorption change from significantly exothermic to less exothermic or even to endothermic as the equilibrium solidphase concentration, qe, increases, depending on the characteristics of the sorbent and sorbate examined. Spurlock (23) related the more exothermic heats observed at low qe values to thermodynamically more favored site-specific interactions between soil humic matter and polar solutes (phenylureas).
Experimental Section Sorbents and Their Characterization. The five model solids used include graphite (R-Al2O3) and three mesoporous silica gels (Gel-150, -100, and -40). Graphite was chosen because of its nonporosity, its relatively homogeneous surfaces and associated sorption energies, and its hydrophobic surfaces that should adsorb HOCs from aqueous solution strongly. This sample was obtained from Alfa Chemical Company, purified by soaking sequentially for 1 day each in toluene, hexane, and methanol, and then rinsed six times with methanol to remove any extractable organic carbons. The washed sample was dried at 105 °C and stored in a bottle for subsequent characterization (i.e., N2-BET surface area mea-
surement, ≈2.66 m2/g) and adsorption experiments. The R-Al2O3 and three mesoporous silica gels employed were utilized earlier as sorbents in studies characterizing the rates and equilibria of phenanthrene sorption (28). These four solids were cleaned pursuant to standard procedures (29), and each of the cleaned samples were then characterized with respect to nine-point N2-BET specific surface areas and pore size distributions. Pore size distributions for the three silica gel samples were reported in ref 28. Graphite and R-Al2O3 have been shown to have no measurable porosity. Sorbate and Background Solutions. Phenanthrene, a polynuclear aromatic hydrocarbon (PAH) having a molecular weight of 178.2, was obtained in spectrophotometric grade (>98%) from Aldrich Chemical Company. This solute was selected as a probe HOC because it is relatively nonpolar and because it has been employed by us in a number of earlier sorption rate and equilibrium studies (5-11). A primary stock, standard solutions, and background aqueous solutions were prepared following procedures described previously (6, 28). The background solution contained 0.01 M NaNO3, and its pH was adjusted to ∼7.0 by dilute HCl or NaOH. Except for experiments run at temperatures lower than 25 °C, initial aqueous phenanthrene solutions were also prepared employing procedures developed and described previously (6, 28). Initial solutions for the 0), solventmotivated forces (i.e., hydrophobic interactions) will add to the surface-motivated sorption. In other words, sorption capacity or surface excess should be the sum of the contributions from both sorbent surface-motivated sorption and solvent-motivated sorption. When sorption is endothermic, it is possible that the surface excess of a solute can still be positive if there is a solvent-motivated sorption greater than the negative sorbent surface-motivated sorption. It is known that HOC molecules in aqueous solution are surrounded by water molecules that are more ordered than those existing in pure water to form so-called cage structures. When HOC solutes leave the aqueous phase, the cage structures break up and water molecules gain entropy. This entropy gain is directly proportional to the size of the HOC molecule involved. Assuming an average molecular diameter of 1.0 nm for phenanthrene, the free energy change attributable to this entropy gain is about -20 kJ/mol at ambient conditions (44). The surfaces of Al2O3 and SiO2 are hydrophilic and thus favor interactions with water molecules. Measured heats of immersion for Al2O3 and SiO2 in water and hexane have been shown to be -400 to -600 and about -100 erg/cm2, respectively (40), suggesting that interaction energies between water and hydrophilic surfaces are higher than those between hexane and hydrophilic surfaces. Water is thus the “wetting phase” and hexane the “non-wetting” phase in a three-phase (hexane-water-Al2O3/SiO2) system. It is expected that sorptions of hexane from aqueous solutions by Al2O3 and SiO2 are endothermic, in which the surface excess of hexane would be negative if the entropy change were neglected. Similarly, absolute values of heats of immersion for Al2O3 and SiO2 in hypothetical supercool phenanthrene liquid should be smaller than those in water. Consequently, it should be expected that sorptions of phenanthrene from aqueous solutions by Al2O3 and SiO2 should also be endothermic, i.e., phenanthrene sorption would not be predicted if the entropy change were neglected. It is therefore apparent that sorption of HOCs such as phenanthrene from aqueous solution to hydrophilic surfaces is endothermic and driven by solvent-motivated forces (i.e., hydrophobic interactions ≈ -20 kJ/mol for phenanthrene). These sorbed solute molecules would replace weakly physisorbed water molecules near hydrophilic surfaces, a vicinal
water region in which surface potential is near zero (40). Within this region, sorbed solute molecules are not very sensitive to surface potential and therefore should be influenced at a lesser degree by heterogeneities in hydrophilic surface energies. In this and previous studies (28), it has been shown that (i) isosteric heats for phenanthrene sorption on the four inorganic sorbents (R-Al2O3 and three silica gels) are near-zero or slightly endothermic (Table 1); (ii) such isosteric heats change slightly as solute surface loading increases; and (iii) different external solid surfaces (R-Al2O3, kaolinite, and silica) that presumably have highly different heterogeneous surface energies exhibit roughly similar surface area-based sorption capacities for phenanthrene (28). The differences in the calculated Qis values among the four hydrophilic solids may come from the differences of physicochemical properties of the surfaces. Sorption of HOCs by the hydrophobic surfaces of graphite are fundamentally different. Measured heats of immersion of graphon (graphitized carbon black) in water and hexane are -32 and -103 erg/cm2, respectively (40), suggesting that interaction energies between HOC molecules and graphite surfaces are higher than those between water molecules and graphite surfaces, i.e., EHOC/Surface > EWater/Surface. In this case, HOC liquids will be the wetting phase in three-phase (HOC liquid-water-graphite) systems. It is expected that under aqueous solution conditions HOC solutes can compete effectively with water molecules for “sites” on graphite surfaces, exhibiting strong exothermic adsorption. This is consistent with our observations of phenanthrene uptake from aqueous solution by graphite.
Acknowledgments We thank Hong Yu, Henry Harris, Robert Hunter, and Mayra Portalatin for their assistance in the experimental phases of the work. We also acknowledge three unanimous reviewers for their constructive comments and suggestions. This research was funded by the U.S. Environmental Protection Agency, Office of Research and Development, in part by Grant R-819605 to the Great Lakes and Mid-Atlantic Center (GLMAC) for Hazardous Substance Research and in part by Cooperative Agreement CR818213-01-0 with the Risk Reduction Engineering Laboratory, Cincinnati, OH. Partial funding of the research activities of GLMAC is also provided by the State of Michigan Department of Environmental Quality.
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Received for review March 13, 1997. Revised manuscript received July 18, 1997. Accepted July 24, 1997.X ES970230M X
Abstract published in Advance ACS Abstracts, September 15, 1997.
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