Adsorption of Organic Vapors on Polar Surfaces
Review Articles
Adsorption of Organic Vapors on Polar Surfaces - Recent Advances Kai-Uwe Goss Ecological Chemistry and Geochemistry, University of Bayreuth, Post Box 101251, D-95440 Bayreuth, Germany
Abstract This paper summarizes recent research on the adsorption of organic vapors on surfaces. Since the low gas phase concentration range
is typical for environmental situations, this review is restricted to these adsorption coefficients. Two environmental parameters have a strong influence on the adsorption of organic vapors on polar surfaces: temperatureand relative humidity (which is the most suitable parameter for describing the influence of ambient moisture). An exponential relationship was found for the adsorption coefficientversus relative humidity and the reciprocal temperature, respectively.Comparing the heats of adsorption, two different groups of substances emerged: polar chemicals exhibited heats of sorption which were higher than their corresponding heats of condensation due to their ability to form hydrogen bonds, while for the nonpolar compounds the opposite was true. Sorption takes place on the surface of an adsorbed water film when the relative humidity exceeds the value which is necessary to form a monomolecular layer of water on the surface of the adsorbent ( _> 30 % relative humidity). Therefore, at temperature below 0 ~ a change in the adsorption behavior might be expected due to a change of properties of the adsorbed water film. However, no alterations were observed at temperatures from - 12 ~ to + 4 ~ (adsorption on quartz sand). The results were comparable to those at much higher temperatures (50 - 80 ~ A statistical approach for the prediction of the adsorption coefficients from physico-chemicalparameters of the substances (vapor pressure, polarizability, and electron-donating capability) was developed and good agreement was found with experimental results and independent data from the literature. Finally, two special cases, the adsorption on bulk water and ice, are discussed.
1
Introduction
The distribution of organic chemicals in the environment depends on their partitioning between the aqueous phase, the gas phase and different solid phases. The partition constant between the aqueous and the gas phase can be calculated from the aqueous solubility of a substance and its saturation vapor pressure. For a wide range of non-ionic organic compounds the sorptive partitioning from water to a solid is due to hydrophobic interactions that can be described by the pollutant hydrophobicity (e.g. octanol-water partition coefficient) and a measure of the hydrophobicity of the solid phase (e.g. the weight fraction of organic carbon). The third transition, the adsorption of organic vapors on surfaces, is more complex. Improving our knowledge in this field is essential for the understanding of many processes in the environment. Vapor phase sorption plays an important role in the mobility of chemicals in the gas phase of air-dry soils
34
and their volatilization from the soil surface into the atmosphere (e.g. V~a.SA~J and THIBODEAUX, 1988; SPENCER et al., 1988). The process of gas-particle partitioning affects the atmospheric transport of organic compounds as well as photolytic transformation of these compounds. The toxic risk of compounds entering the lungs also depends on the partitioning between the gas phase and particles that are inhaled (e.g. P~qKow, 1988, 1993). This paper summarizes recent research on the adsorption of organic vapors on polar surfaces.
2
Theory
Vapor adsorption is a very fast process (JAtSLMES et al., 1984). This characteristic is used, for example, in gas chromatography where high carrier gas velocities are applied. Thus, we can assume that under environmental conditions a local adsorption equilibrium exists. Adsorption processes may be hindered by certain transport phenomena (i.e. advection and diffusion to and from adsorption sites), which are not discussed in this review. The adsorption equilibrium can be described by an isotherm. At high gas phase concentrations ( _> 1 % of the saturated vapor pressure), these isotherms are typically nonlinear (POE et al., 1988; RHUE et al., 1989; SPENCER et al., 1969; PETERSON et al., 1988; THIBODEAUXet al., 1988) and are best described by an equation of the BET-type. At lower concentrations the isotherms become linear and can be simply described by an adsorption coefficient (GALE and BEEBE, 1964). Since this low concentration range is typical for environmental situations, this review is restricted to these adsorption coefficients. It must also be noted that an adsorption coefficient can only describe reversible partitioning. Irreversible chemisorption has not yet been documented in the environmental literature.
2.1
Influences on the Adsorption Coefficients
Adsorption is a surface phenomenon. At low concentrations adsorption increases linearily with the specific surface area of the adsorbent. Thus, it is useful to normalize the adsorption coefficients to the specific surface area of the adsorbent in order to make them comparable for different adsorbents. In this case, the adsorption coefficient K is defined as:
ESPR-Environ. Sci. & Pollut. Res. 1 (1) 34-37 (1994) 9 ecomed publishers, D-86899 Landsberg, Germany
Review Articles
Adsorption of Organic Vapors on Polar Surfaces
mg of substance/surface area of sorbent (cm 2)
K=
defined thermodynamic state and is easily measured in situ. To understand how vapor adsorption is influenced by relative humidity, it is necessary to distinguish three different ranges of relative humidity:
mg of substance/volume of gas phase ( c m 3)
Two environmental parameters have a strong influence on the adsorption of vapors on polar surfaces: temperature and moisture. While the mathematical relation between these parameters and the adsorption coefficients seems to be the same for all compounds and adsorbents, the coefficients describing the strength of the influence depend on the properties of the adsorbing compound and the adsorbent. 2.1.1
1. At relative humidities below that corresponding to a complete monolayer coverage of water on the adsorbent, the organic molecules can adsorb at unoccupied parts of the adsorbent. In this case, the observed adsorption coefficients are very high but the competition with the water molecules leads to a strong decline of adsorption with increasing moisture (RAo et al., 1989; CHIOU, 1985; Goss, 1992, 1993a) (-~ Pig. 1).
Influence of Moisture
. At higher relative humidities the surface will be covered by one or more molecular layers of water. For mineral surfaces this is typically the case at relative humidities above ca. 30 %, a situation which is mostly encountered in the environment. Within this humidity range (with an upper limit at 100 % rh), adsorption of organic molecules can only take place on the surface of the adsorbed water film. Recent studies with a wide range of different chemicals and mineral surfaces (quartz sand and different clay
Water molecules have a high affinity to polar surfaces and can effectively compete with organic molecules for adsorption sites, whereas other gases in the ambient air do not. Either their affinity to the surface at normal temperatures (i.e. nitrogen, oxygen, carbon dioxide) is not high enough or their concentration is too low for occupying a significant number of the adsorption sites. Relative humidity is the most suitable parameter for describing the influence of ambient moisture. It represents a well
In K (cm)
-,!
0
h c e t o n l tr! le Dlethylether 1,2-DI c h l o r o b e n z e n e I, 3-Di c h l o r o b e n z e n e p - X y I ene m-Xyl ene
•
-5
-
-6
r
-7
+
x § # x + A
n-Nonane
)r -8
9
-9
-10 0
10
20
30
40
50
60
70
80
rel.
90
100
humidity
Fig. 1: Logarithm of the adsorption coefficients K v e r s u s relative humidity for the adsorption on quartz sand at 70 ~ (Goss; 1992) recalculated with a new value o f the specific surface area (see Goss, 1993c). (Monolayer of adsorbed water at 26 % rh)
ESPR-Environ. Sci. & Pollut. Res. 1 (1) 1994
35
Adsorption of Organic Vapors on Polar Surfaces minerals) revealed an exponential decrease of adsorption coefficients with increasing relative humidity in all cases (Goss 1992i 1993a) ( ~ Fig. 1). This functionality is likely to be generally valid for all organic compounds and all mineral surfaces. The slope of the discovered exponential relationship was different for the different compounds but did not depend on temperature and was the same for the adsorption on quartz sand and Na- or Ca-kaolinite. However, it was different for bentonite (perhaps due to the influence of the electrical double layer of this clay mineral). The amount of water adsorbed below 100 % rh does not exceed 8 - 10 molecular layers. The experiments mentioned above revealed that only methanol which combines small molecular size with high water solubility, could dissolve within this thin water film. Thus, for methanol two different sorption processes (adsorption and absorption) must be considered in this humidity range. For all other compounds tested (even for ethanol) only adsorption on the surface of the water film was of importance, but not absorption in the film (Goss 1992, 1993a). 3. At relative humidities above 100 %, the third humidity range, there is an unlimited increase of the adsorbed water film. In this case, there is no further steric hindrance for dissolution of the organic molecules in the water film. Now, the overall sorption results from two additive processes: adsorption on the water surface and solution in the water film. The latter can be described by the Henry's law constant and is of higher importance with increasing water volume (ONG and DON, 1991a, b).
2.1.2
Influence of Temperature
The functional relation between the adsorption coefficients and the absolute temperature T is described by the following equation: InK = ____AH~ 1 / T + const. R where R is the gas constant and AH s the heat of adsorption. AHs depends on temperature, but to such a small extent that it may be regarded as constant at environmental temperatures (GALE and BEEBE, 1964; Goss, 1993b). Recent work on the heats of adsorption of organic vapors on mineral surfaces covered with an adsorbed water film revealed the following results (Goss, 1992, 1993a, b): It was found that AH s depends neither on relative humidity nor on the type of the tested mineral surface but only on the type of compound. Comparing the heats of adsorption, two different groups of compounds emerged: Nonpolar chemicals exhibited heats of adsorption that were less negative than the heats of condensation, whereas compounds containing an oxygen atom or other electrondonating groups had heats of adsorption more negative than the corresponding heats of condensation. These increased binding forces of polar compounds were explained by hydrogen bonds between the hydrogens of the water surface and the electron-donating compounds. HARTKOPF and
36
Review Articles KARGER(1973) had found the same phenomenon for the adsorption on a bulk water surface. Such increased binding forces do not only influence the temperature dependence but also the absolute extent of adsorption. For example, naphthalene and acetone showed similar adsorption on a quartz surface which would not have been expected on the basis of their vapor pressures differing by 3 orders of magnitude (Goss, 1992). As all these adsorption processes take place on the surface of an adsorbed water film, a change in the adsorption behavior might be expected at temperatures below 0 ~ due to a change of properties of the adsorbed water film. However, no alterations were observed at temperatures from - 12 ~ to + 4 ~ (adsorption on quartz sand). The results were comparable to those at much higher temperatures ( 5 0 - 8 0 ~ (Goss, 1993b). 2.2
Predicting Adsorption Coefficients
The adsorption of organic vapors on polar surfaces described above may be summarized by the following equation that is valid when the surface is covered at least with one monolayer of water (relative humidities ca. > 30 %): AHs InK = A - - - 1 / T + Crh R A statistical approach to predict the parameters A, AHs and C from physico-chemical properties of the substances was successful (Goss, 1993c). Parameters describing the polarizability of the compounds correlated with C, while A was best described with the aid of the vapor pressure and a parameter for the electron-donation capability of the compounds. The adsorption of all tested compounds on quartz and kaolinite could be predicted with good accuracy. Comparison with independent data from literature also gave good results indicating that the predictive model could be applied to substances other than those used for its development. The model, however, could not be used to predict the adsorption on bentonite as in this case there was an additional influence of the adsorbent on parameter C, which is not yet understood. It is not clear to what extent these results dealing with soil minerals can be applied to the gas-particle partitioning in the atmosphere. Some previous results indicate that particles of urban origin have a high hydrophobic soot fraction (PANKOW, 1991; GOTHAM and BIDLEMAN, 1992). In this case there will be no adsorbed water film on the particles and the results presented here cannot be used. On the other hand, PANKOW et al. (1993) suggest the same exponential relationship between gas-particle partitioning and relative humidity for urban field data that has been described for mineral surfaces (Goss 1992, 1993a). 2.3
Adsorption on the Surface of Bulk Water and Ice
An interesting special case of the adsorption on polar surfaces is the adsorption on the surface of bulk water and ice. The amount of pollutants transported with fog depends on the adsorption on the high specific surface area of the water droplets (Goss, 1993d). In polar and subpolar regions, adESPR-Environ. Sci. & Pollut. Res. 1 (1) 1994
Review Articles
s o r p t i o n on ice and snow is expected to have a considerable i m p a c t on the distribution o f p o l l u t a n t s (WANIA, 1993). It m a y be expected that the surface of an adsorbed water film at 100 % rh corresponds to a b u l k w a t e r surface. Indeed, g o o d agreement was found between a d s o r p t i o n coefficients predicted for 100 % relative h u m i d i t y with the predictive model mentioned above and those m e a s u r e d by HARTKOPF and KARGEP, (1973) for the same substances on a b u l k water surface (Goss, 1993c). A d s o r p t i o n experiments on the surface of a b u l k liquid are difficult to carry out because absorption as an additional process c a n n o t be excluded and the d e t e r m i n a t i o n of the surface area is difficult. Thus, it is helpful to have a predictive model that can supply these data. T h e results of a d s o r p t i o n experiments with n o n p o l a r s u b s t a n c e s on ice are similar to those expected for the adsorption on the surface o f subcooled w a t e r (Goss, 1993b). F o r p o l a r c o m p o u n d s this was the same w h e n a d s o r p t i o n was studied on flesh ice surfaces resulting from a fracture. H o w ever, there was an aging process of the flesh ice surface - p r o b a b l y a rearrangement o f w a t e r molecules in the surface layer - resulting in a decrease o f the binding forces o f the a d s o r b e d p o l a r substances, whereas no impact was observed on the a d s o r p t i o n o f the n o n p o l a r c o m p o u n d s . T h e hydrogen b o n d s responsible for the increased binding forces o f electron-donating c o m p o u n d s on the surface of adsorbed w a t e r films and bulk w a t e r seemed to d i s a p p e a r during the aging process of the ice surface.
Acknowledgements
Special thanks are due to M. HINKELand J. KLASMEIERfor critical revision of the manuscript. This work was supported by the Deutsche Forschungsgemeinschaft.
3
References
CHIOU, C.T. (1985): Soil Sorption of Organic Vapors and Effects of Humidity on Sorptive Mechanism and Capacity. Environ. Sci. Technol. 19, 1196-1200 COTHAM, W.E.; T.F. BIDLEMAN(1992): Laboratory Investigations of the Partitioning of Organochlorine Compounds Between the Gas Phase and Atmospheric Aerosols on Glass Fiber Filters. Environ. Sci. Technol. 26, 469 - 478 GALE,R.L.; R.A. BEEBE(1964): Determination of Heats of Adsorption on Carbon Blacks and Bone Mineral by Chromatography Using the Eluted Pulse Technique. J. Phys. Chem. 68, 555- 567 GLOTrELTY, D.E.; J.N. SEmER; L.A. LtLJEDAHL(1987): Pesticides in Fog. Nature 325, 602- 605 GLOTFELTY,D.E.; M.S. MAIEWSRI;J.N. SEmER(1990): Distribution of Several Organophosphorus Insecticides and Their Oxygen Analogues in a Foggy Atmosphere. Environ. Sci. Technol. 24, 3 5 3 - 357
ESPR-Environ. Sci. & Pollut. Res. 1 (1) 1994
Adsorption of Organic Vapors on Polar Surfaces Goss, K.-U. (1992): Effects of Temperature and Relative Humidity on the Sorption of Organic Vapors on Sand. Environ. Sci. Technol. 26, 2287- 2294 Goss, K.-U. (1993a): Effects of Temperature and Relative Humidity on the Sorption of Organic Vapors on Clay Minerals. Environ. Sci. Technol. 27, 2127- 2132 Goss, K.-U. (1993b): Sorption of Organic Vapors on Ice and Quartz Sand at Temperatures Below 0 ~ Environ. Sci. Technol., in press Goss, K.-U. (1993c): Adsorption of Organic Vapors on Polar Surfaces: Development of a Predictive Model. Environ. Sci. Technol., submitted Goss, K.-U. (1993d): Predicting the Enrichment of Organic Compounds in Fog Caused by Adsorption on the Water Surface. Atmos. Environ., submitted HARTKOPF,A.; B.L. KARGER(1973): Study of the Interfacial Properties of Water by Gas Chromatography. Accounts Chem. Res. 6, 209 - 216 JAULMES,A.; C. VIDAL-MADJAR;A. LADURELLI;G. GUIOCHON(1984): Study of Peak Profiles in Nonlinear Gas Chromatography. 1. Derivation of a Theoretical model. J. Phys. Chem. 88, 5 3 7 9 - 5385 ONG, S.K.; L.W. LION(1991 a ): Effects of Soil Properties and Moisture on the Sorption of Trichloroethylene Vapor. Water Res. 25, 29 - 36 ONG, S.K.; L.W. LION(1991b): Mechanism for Trichloroethylene Vapor Sorption onto Soil Minerals. J. Environ. Qual. 20, 180 - 188 PANKOW,J.F. (1988): The Calculated Effects of Non-Exchangeable Material on the Gas-Particle Distributions of Organic Compounds. Atmos. Environ. 22, 1405- 1409 PANKOW,J.F. (1991 ): Common Y-Intercept and Single Compound Regressions of Gas-Particle Partitioning Data VS 1/T. Atmos. Environ. 25A, 2229- 2239 PANKOW,J.F.; J.M.E. STOREY;H. YAMASAKI(1993): Effects of Relative Humidity on Gas/Particle Partitioning of Semi-Volatile Organic Compounds. Environ. Sci. Technol. 27, 2220- 2226 PETERSON, M.S.; L.W. LION; C.A. SHOEMAKER(1988): Influence of Vapor-Phase Sorption and Diffusion on the Fate of Trichloroethylene in an Unsaturated Aquifer System. Environ. Sci. Technol. 22, 571-578 POE, S.H.; K.T. VALSARAJ;L.J. THIBODEAUX;C. SPRINGER(1988): Equilibrium Vapor Phase Adsorption of Volatile Organic Chemicals on Dry Soils. J. Hazardous Mater. 19, 1 7 - 3 2 RAO, P.S.C.; R.A. OGWADA;R.D. RHUE (1989): Adsorption of Volatile Organic Compounds on Anhydrous and Hydrated Sorbents: Equilibrium Adsorption and Energetics. Chemosphere 18, 2177 - 2191 RHUE, R.D.; K.D. PENNELL;P.S.C. RAO;W.H. REVE (1989): Competetive Adsorption of Alkylbenzene and Water Vapors on Predominandy Mineral Surfaces. Chemosphere 18, 1971 - 1986 SPENCER,W.F.; M.M. CLLATH;W.J. FARMER(1969): Vapor Density of Soil-Applied Dieldrin as Related to Soil-Water Content, Temperature, and Dieldrin Concentration. Soil Sci. Soc. Amer. Proc. 33, 509-511
SPENCER,W.F.; M.M. CLIATH;W.A. JURY; L.-Z. ZHANG(1988): Volatilization of Organic Chemicals from Soil as Related to Their Henry's Law Constants. J. Environ. Qual. 17, 504-509 THIBODEAUX,L.J.; K.T. VALSARAJ;C. SPV,rNGER; G. HILDEBRAND ( 1988): Mathematical Models for Predicting Chemical Vapor Emissions from Landfills. J. Hazardous Mater. 19, 101-118 VALSARAJ,K.T.; L.J. THIBODEAUX(1988): Equilibrium Adsorption of Chemical Vapors onto Surface Soils, Landfills and Landfarms - A Review. J. Hazardous Mater. 19, 7 9 - 99 WANIA,F.; D. MACKAY(1993): Global Fractionation and Condensation of Low Volatility Organichlorine Compounds in Polar Regions. Ambio 22, 10 - 18
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Review Articles
Adsorption of Organic Vapors on Polar Surfaces - Recent Advances Kai-Uwe Goss Ecological Chemistry and Geochemistry, University of Bayreuth, Post Box 101251, D-95440 Bayreuth, Germany
Abstract This paper summarizes recent research on the adsorption of organic vapors on surfaces. Since the low gas phase concentration range
is typical for environmental situations, this review is restricted to these adsorption coefficients. Two environmental parameters have a strong influence on the adsorption of organic vapors on polar surfaces: temperatureand relative humidity (which is the most suitable parameter for describing the influence of ambient moisture). An exponential relationship was found for the adsorption coefficientversus relative humidity and the reciprocal temperature, respectively.Comparing the heats of adsorption, two different groups of substances emerged: polar chemicals exhibited heats of sorption which were higher than their corresponding heats of condensation due to their ability to form hydrogen bonds, while for the nonpolar compounds the opposite was true. Sorption takes place on the surface of an adsorbed water film when the relative humidity exceeds the value which is necessary to form a monomolecular layer of water on the surface of the adsorbent ( _> 30 % relative humidity). Therefore, at temperature below 0 ~ a change in the adsorption behavior might be expected due to a change of properties of the adsorbed water film. However, no alterations were observed at temperatures from - 12 ~ to + 4 ~ (adsorption on quartz sand). The results were comparable to those at much higher temperatures (50 - 80 ~ A statistical approach for the prediction of the adsorption coefficients from physico-chemicalparameters of the substances (vapor pressure, polarizability, and electron-donating capability) was developed and good agreement was found with experimental results and independent data from the literature. Finally, two special cases, the adsorption on bulk water and ice, are discussed.
1
Introduction
The distribution of organic chemicals in the environment depends on their partitioning between the aqueous phase, the gas phase and different solid phases. The partition constant between the aqueous and the gas phase can be calculated from the aqueous solubility of a substance and its saturation vapor pressure. For a wide range of non-ionic organic compounds the sorptive partitioning from water to a solid is due to hydrophobic interactions that can be described by the pollutant hydrophobicity (e.g. octanol-water partition coefficient) and a measure of the hydrophobicity of the solid phase (e.g. the weight fraction of organic carbon). The third transition, the adsorption of organic vapors on surfaces, is more complex. Improving our knowledge in this field is essential for the understanding of many processes in the environment. Vapor phase sorption plays an important role in the mobility of chemicals in the gas phase of air-dry soils
34
and their volatilization from the soil surface into the atmosphere (e.g. V~a.SA~J and THIBODEAUX, 1988; SPENCER et al., 1988). The process of gas-particle partitioning affects the atmospheric transport of organic compounds as well as photolytic transformation of these compounds. The toxic risk of compounds entering the lungs also depends on the partitioning between the gas phase and particles that are inhaled (e.g. P~qKow, 1988, 1993). This paper summarizes recent research on the adsorption of organic vapors on polar surfaces.
2
Theory
Vapor adsorption is a very fast process (JAtSLMES et al., 1984). This characteristic is used, for example, in gas chromatography where high carrier gas velocities are applied. Thus, we can assume that under environmental conditions a local adsorption equilibrium exists. Adsorption processes may be hindered by certain transport phenomena (i.e. advection and diffusion to and from adsorption sites), which are not discussed in this review. The adsorption equilibrium can be described by an isotherm. At high gas phase concentrations ( _> 1 % of the saturated vapor pressure), these isotherms are typically nonlinear (POE et al., 1988; RHUE et al., 1989; SPENCER et al., 1969; PETERSON et al., 1988; THIBODEAUXet al., 1988) and are best described by an equation of the BET-type. At lower concentrations the isotherms become linear and can be simply described by an adsorption coefficient (GALE and BEEBE, 1964). Since this low concentration range is typical for environmental situations, this review is restricted to these adsorption coefficients. It must also be noted that an adsorption coefficient can only describe reversible partitioning. Irreversible chemisorption has not yet been documented in the environmental literature.
2.1
Influences on the Adsorption Coefficients
Adsorption is a surface phenomenon. At low concentrations adsorption increases linearily with the specific surface area of the adsorbent. Thus, it is useful to normalize the adsorption coefficients to the specific surface area of the adsorbent in order to make them comparable for different adsorbents. In this case, the adsorption coefficient K is defined as:
ESPR-Environ. Sci. & Pollut. Res. 1 (1) 34-37 (1994) 9 ecomed publishers, D-86899 Landsberg, Germany
Review Articles
Adsorption of Organic Vapors on Polar Surfaces
mg of substance/surface area of sorbent (cm 2)
K=
defined thermodynamic state and is easily measured in situ. To understand how vapor adsorption is influenced by relative humidity, it is necessary to distinguish three different ranges of relative humidity:
mg of substance/volume of gas phase ( c m 3)
Two environmental parameters have a strong influence on the adsorption of vapors on polar surfaces: temperature and moisture. While the mathematical relation between these parameters and the adsorption coefficients seems to be the same for all compounds and adsorbents, the coefficients describing the strength of the influence depend on the properties of the adsorbing compound and the adsorbent. 2.1.1
1. At relative humidities below that corresponding to a complete monolayer coverage of water on the adsorbent, the organic molecules can adsorb at unoccupied parts of the adsorbent. In this case, the observed adsorption coefficients are very high but the competition with the water molecules leads to a strong decline of adsorption with increasing moisture (RAo et al., 1989; CHIOU, 1985; Goss, 1992, 1993a) (-~ Pig. 1).
Influence of Moisture
. At higher relative humidities the surface will be covered by one or more molecular layers of water. For mineral surfaces this is typically the case at relative humidities above ca. 30 %, a situation which is mostly encountered in the environment. Within this humidity range (with an upper limit at 100 % rh), adsorption of organic molecules can only take place on the surface of the adsorbed water film. Recent studies with a wide range of different chemicals and mineral surfaces (quartz sand and different clay
Water molecules have a high affinity to polar surfaces and can effectively compete with organic molecules for adsorption sites, whereas other gases in the ambient air do not. Either their affinity to the surface at normal temperatures (i.e. nitrogen, oxygen, carbon dioxide) is not high enough or their concentration is too low for occupying a significant number of the adsorption sites. Relative humidity is the most suitable parameter for describing the influence of ambient moisture. It represents a well
In K (cm)
-,!
0
h c e t o n l tr! le Dlethylether 1,2-DI c h l o r o b e n z e n e I, 3-Di c h l o r o b e n z e n e p - X y I ene m-Xyl ene
•
-5
-
-6
r
-7
+
x § # x + A
n-Nonane
)r -8
9
-9
-10 0
10
20
30
40
50
60
70
80
rel.
90
100
humidity
Fig. 1: Logarithm of the adsorption coefficients K v e r s u s relative humidity for the adsorption on quartz sand at 70 ~ (Goss; 1992) recalculated with a new value o f the specific surface area (see Goss, 1993c). (Monolayer of adsorbed water at 26 % rh)
ESPR-Environ. Sci. & Pollut. Res. 1 (1) 1994
35
Adsorption of Organic Vapors on Polar Surfaces minerals) revealed an exponential decrease of adsorption coefficients with increasing relative humidity in all cases (Goss 1992i 1993a) ( ~ Fig. 1). This functionality is likely to be generally valid for all organic compounds and all mineral surfaces. The slope of the discovered exponential relationship was different for the different compounds but did not depend on temperature and was the same for the adsorption on quartz sand and Na- or Ca-kaolinite. However, it was different for bentonite (perhaps due to the influence of the electrical double layer of this clay mineral). The amount of water adsorbed below 100 % rh does not exceed 8 - 10 molecular layers. The experiments mentioned above revealed that only methanol which combines small molecular size with high water solubility, could dissolve within this thin water film. Thus, for methanol two different sorption processes (adsorption and absorption) must be considered in this humidity range. For all other compounds tested (even for ethanol) only adsorption on the surface of the water film was of importance, but not absorption in the film (Goss 1992, 1993a). 3. At relative humidities above 100 %, the third humidity range, there is an unlimited increase of the adsorbed water film. In this case, there is no further steric hindrance for dissolution of the organic molecules in the water film. Now, the overall sorption results from two additive processes: adsorption on the water surface and solution in the water film. The latter can be described by the Henry's law constant and is of higher importance with increasing water volume (ONG and DON, 1991a, b).
2.1.2
Influence of Temperature
The functional relation between the adsorption coefficients and the absolute temperature T is described by the following equation: InK = ____AH~ 1 / T + const. R where R is the gas constant and AH s the heat of adsorption. AHs depends on temperature, but to such a small extent that it may be regarded as constant at environmental temperatures (GALE and BEEBE, 1964; Goss, 1993b). Recent work on the heats of adsorption of organic vapors on mineral surfaces covered with an adsorbed water film revealed the following results (Goss, 1992, 1993a, b): It was found that AH s depends neither on relative humidity nor on the type of the tested mineral surface but only on the type of compound. Comparing the heats of adsorption, two different groups of compounds emerged: Nonpolar chemicals exhibited heats of adsorption that were less negative than the heats of condensation, whereas compounds containing an oxygen atom or other electrondonating groups had heats of adsorption more negative than the corresponding heats of condensation. These increased binding forces of polar compounds were explained by hydrogen bonds between the hydrogens of the water surface and the electron-donating compounds. HARTKOPF and
36
Review Articles KARGER(1973) had found the same phenomenon for the adsorption on a bulk water surface. Such increased binding forces do not only influence the temperature dependence but also the absolute extent of adsorption. For example, naphthalene and acetone showed similar adsorption on a quartz surface which would not have been expected on the basis of their vapor pressures differing by 3 orders of magnitude (Goss, 1992). As all these adsorption processes take place on the surface of an adsorbed water film, a change in the adsorption behavior might be expected at temperatures below 0 ~ due to a change of properties of the adsorbed water film. However, no alterations were observed at temperatures from - 12 ~ to + 4 ~ (adsorption on quartz sand). The results were comparable to those at much higher temperatures ( 5 0 - 8 0 ~ (Goss, 1993b). 2.2
Predicting Adsorption Coefficients
The adsorption of organic vapors on polar surfaces described above may be summarized by the following equation that is valid when the surface is covered at least with one monolayer of water (relative humidities ca. > 30 %): AHs InK = A - - - 1 / T + Crh R A statistical approach to predict the parameters A, AHs and C from physico-chemical properties of the substances was successful (Goss, 1993c). Parameters describing the polarizability of the compounds correlated with C, while A was best described with the aid of the vapor pressure and a parameter for the electron-donation capability of the compounds. The adsorption of all tested compounds on quartz and kaolinite could be predicted with good accuracy. Comparison with independent data from literature also gave good results indicating that the predictive model could be applied to substances other than those used for its development. The model, however, could not be used to predict the adsorption on bentonite as in this case there was an additional influence of the adsorbent on parameter C, which is not yet understood. It is not clear to what extent these results dealing with soil minerals can be applied to the gas-particle partitioning in the atmosphere. Some previous results indicate that particles of urban origin have a high hydrophobic soot fraction (PANKOW, 1991; GOTHAM and BIDLEMAN, 1992). In this case there will be no adsorbed water film on the particles and the results presented here cannot be used. On the other hand, PANKOW et al. (1993) suggest the same exponential relationship between gas-particle partitioning and relative humidity for urban field data that has been described for mineral surfaces (Goss 1992, 1993a). 2.3
Adsorption on the Surface of Bulk Water and Ice
An interesting special case of the adsorption on polar surfaces is the adsorption on the surface of bulk water and ice. The amount of pollutants transported with fog depends on the adsorption on the high specific surface area of the water droplets (Goss, 1993d). In polar and subpolar regions, adESPR-Environ. Sci. & Pollut. Res. 1 (1) 1994
Review Articles
s o r p t i o n on ice and snow is expected to have a considerable i m p a c t on the distribution o f p o l l u t a n t s (WANIA, 1993). It m a y be expected that the surface of an adsorbed water film at 100 % rh corresponds to a b u l k w a t e r surface. Indeed, g o o d agreement was found between a d s o r p t i o n coefficients predicted for 100 % relative h u m i d i t y with the predictive model mentioned above and those m e a s u r e d by HARTKOPF and KARGEP, (1973) for the same substances on a b u l k water surface (Goss, 1993c). A d s o r p t i o n experiments on the surface of a b u l k liquid are difficult to carry out because absorption as an additional process c a n n o t be excluded and the d e t e r m i n a t i o n of the surface area is difficult. Thus, it is helpful to have a predictive model that can supply these data. T h e results of a d s o r p t i o n experiments with n o n p o l a r s u b s t a n c e s on ice are similar to those expected for the adsorption on the surface o f subcooled w a t e r (Goss, 1993b). F o r p o l a r c o m p o u n d s this was the same w h e n a d s o r p t i o n was studied on flesh ice surfaces resulting from a fracture. H o w ever, there was an aging process of the flesh ice surface - p r o b a b l y a rearrangement o f w a t e r molecules in the surface layer - resulting in a decrease o f the binding forces o f the a d s o r b e d p o l a r substances, whereas no impact was observed on the a d s o r p t i o n o f the n o n p o l a r c o m p o u n d s . T h e hydrogen b o n d s responsible for the increased binding forces o f electron-donating c o m p o u n d s on the surface of adsorbed w a t e r films and bulk w a t e r seemed to d i s a p p e a r during the aging process of the ice surface.
Acknowledgements
Special thanks are due to M. HINKELand J. KLASMEIERfor critical revision of the manuscript. This work was supported by the Deutsche Forschungsgemeinschaft.
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