Environ. Sci. Technol. 1989, 23, 407-413
Comparison of Sorption Energetics for Hydrophobic Organic Chemicals by Synthetic and Natural Sorbents from MethanoVWater Solvent Mixtures Kent B. Woodburn,*it*t Linda S.
P. Suresh C. Rao,ti5 and Joseph J. Deifinot
University of Florida, Gainesvilie, Florida 3261 1 Reversed-phase liquid chromatography (RPLC) was used to investigate the thermodynamics and mechanisms of hydrophobic organic chemical (HOC) retention from methanol/water solvent mixtures. The enthalpy-entropy compensation model was used to infer that the hydrophobic sorptive mechanisms were different for polycyclic aromatic hydrocarbons (PAHs) and monohalobenzenes compared with alkylbenzenes in methanol/water RPLC systems. The estimated compensation temperatures (P) for the two retention mechanisms were essentially independent of the organic cosolvent content (3540% methanol by volume) and RPLC chain length (C-2-C-8). Sorption of four PAHs by Webster surface soil from 30/70 methanol/water solution at three temperatures was measured by the batch equilibration technique. A /3 value of 506 K was calculated by using these data and was within the range of /3 values estimated from the RPLC data (471-762 K). This suggests that the sorptive mechanisms were similar for PAH retention by the RPLC sorbents and the Webster soil from the binary mixed solvent.
Introduction Recent advances in reversed-phase liquid chromatography (RPLC) suggest that this technique may be applicable as a model for soil systems, given an appropriate selection of the sorbent/solvent system. The evidence supporting the use of RPLC stationary phases as models of soils may be found in the recent chromatographic literature. Veith et al. (1) demonstrated the relationship between a chemical's corrected retention time (k? on an octadecylsilane (C-18) RPLC column and its octanol/water partition coefficient (KO& Excellent correlations exist between K,, and K,, the carbon-normalized soil sorption coefficient (2,3). Correlations between measured K , and (2-18 retention time for selected organic solutes have also been reported ( 4 , 5 ) . McCall et al. (6) proposed a solute mobility classification scheme based on a compound's RPLC retention time. These authors observed a significant linear correlation between leaching distance in soil columns and RPLC retention time. Although sorption plays a critical role in attenuating the transport and transformations of hydrophobic organic chemicals (HOCs) in soils and sediments, data characterizing the thermodynamics of the sorption process(es) are sparse. A better understanding of mechanisms and thermodynamics of sorption should improve our ability to predict HOC behavior in soils and groundwater. Chromatographic techniques are well suited to thermodynamic studies, and this approach may be applicable for examining liquid-phase sorption reactions in soils and chromatographic media. In this paper, we will briefly outline the use of RPLC as a model for solute retention and transport in aqueous and mixed solvents. The influence of miscible organic cosolvents on the sorption of HOCs by soils and 'Present address: Residue/Environmental/MetabolismResearch, Dow Chemical Co., Bldg. 9001, Midland, MI 48640. Environmental Engineering Science Dept. 8 Soil Science Dept.
*
0013-936X/89/0923-0407$01.50/0
RPLC sorbents has been the focus of several recent studies (7-11). The emphasis of the present work was on the thermodynamicsof HOC sorption from mixed solvents and the manner in which the energetics of the sorption process may be studied and evaluated by using the entropy-enthalpy compensation model. While this model has been applied extensively in chromatography, it has not been used to analyze data for HOC sorption by soils.
Theory Review of Sorption Thermodynamics. The fundamental expression associated with equilibrium sorption on chromatographic supports is k'= @K (1) where k' is the chromatographic retention factor, iP is the volume phase ratio of the stationary and mobile phases, and K is the thermodynamic equilibrium binding constant. The chromatographic retention factor is calculated as k ' = [(t,- to)/to], where t, and toare the retention times for the solute under study and an unretained compound, respectively. The equilibrium binding constant may be related to AHo and AS" by In K = -AG"/RT = -AHo/RT + A S o / R (2) Substituting eq 2 into eq 1 yields In k' = -AHo/RT A S o / R + In iP
+
(3)
Equation 3 is a frequently used expression for relating solute retention (k? to the enthalpy and entropy changes involved in the sorptive process. For many hydrophobic compounds in RPLC systems, the enthalpy term dominates the entropy term in the overall free energy change (12,13). However, for many of the ubiquitous and carcinogenic PAHs, entropic processes may control RPLC sorption (14). It is noteworthy that enthalpy-entropy effects have not been extensively researched for HOC sorption by soils from aqueous solutions and mixed solvents. Mills and Biggar (15)measured the AHo for sorption of aqueous hexachlorocyclohexaneonto organic and inorganic surfaces, while Wauchope et al. (16) determined AH" and ASofor aqueous naphthalene sorbing onto a sandy loam soil. If the heat capacity change upon the binding of the solute to the stationary phase is zero and the phase ratio (a) is independent of temperature, then a plot of In k' versus T1(K-l) is linear, according to eq 3. With such a diagram (termed a van't Hoff plot), the AH" and ASo values may be calculated from the slope and the intercept, respectively, of the regression line. Departure from linearity can occur if the heat capacities of the solute in sorbed and solution phases are different. Generally, most van't Hoff plots of RPLC data are linear and allow easy determination of AH" (and AS" if iP is known). Typical values of AH" for hydrophobic solutes range from -2 to -12 kcal/mol on RPLC and pyrocarbon LC supports (12,13). Enthalpy-Entropy Compensation Effects. In many physicochemical interactions that are governed by the
0 1989 American Chemical Society
Environ. Sci. Technol., Voi. 23, No. 4, 1989 407
4
Table I. List of Hydrophobic Compounds Used in Chromatographic Studies and Their Respective Hydrocarbonaceous Surface Area (HSA) Values
PAH compounds
compound benzene naphthalene biphenyl phenanthrene
2
3
4
5
6
7
8
same basic mechanism, the overall free energy change is proportional to the change in enthalpy (17, 18). This is indeed the case in RPLC, as In k'(a measure of AGO) is linearly related to the enthalpy of binding (12,13). From eq 3, one would expect the slope of In k 'versus -AHo to be approximately 1/RT. In actual practice, however, the increase in the natural logarithm of the retention factor with the enthalpy is much less than expected. In Figure 1,the slope &95% confidence limits) of the In k'versus -AHo regression line is 3.4 X (f4.8 X mol/cal for several PAHs. This value is significantly less than the 1/RT value at 298 K, i.e., 1.69 X mol/cal. This difference is attributed to changes in the binding enthalpy which are accompanied by corresponding changes in the binding entropy (13, 19). The changes in the binding entropy may be due to structural modifications of the sorbate molecule or changes in solvent entropy (18,20). This effect is termed enthalpy-entropy compensation and has been observed on RPLC supports (13, 19-22) and pyrocarbon LC columns (12). The enthalpy-entropy compensation model was used in the present work to interpret the energetics of HOC retention on synthetic and natural sorbents. Compensation effects may be conveniently expressed by the relationship
AH" = PASo + AG,"
(4)
where AG,' denotes the change in free energy of sorption at the temperature 0,and ,d is a proportionality constant termed the "compensation temperature". Comparison of ,d values obtained from thermodynamic data can be used to investigate whether the intrinsic mechanism of solute retention for one chromatographic system is similar to that found on another system (13). Substituting eq 2 into eq 4, and rearranging, yields AGT" = AH"(1 - T/P)
+ T(AG,/p)
(5)
where AGTo is the standard free energy change at temperature T. Equation 3 may now be rewritten as In kT' = -(AHo/R)(l/T - 1/p) - (AG,"/R@) + In CP (6) where kT' is the solute retention factor at temperature T. According to eq 6, a plot of In kT' versus (AHOIR) for various solutes yields a straight line when compensation occurs, i.e., when the solute/sorbent binding mechanism is similar for all solutes. The compensation temperature, p, may be evaluated from -the slope of the regression line. If measured values obtained for various solvent/sorbent 408
Environ. Sci. Technol., Vol. 23, No. 4, 1989
compound
A. Polycyclic Aromatics" 110 anthracene 156 pyrene 182 fluoranthene 198 chrysene
toluene ethylbenzene n-propylbenzene n-butylbenzene n-hexylbenzene
B. Alkylbenzenes" 127 o-xylene 145 p-xylene 163 m-diethylbenzene 181 1,2,4-trimethylbenzene 217
fluorobenzene chlorobenzene
C. Halobenzenes" 114 bromobenzene 127 iodobenzene
-Ai(Kcal/mol)
Flgure 1. Ln k'at 298 K versus -AHo (kcaVmol) for PAH retention in RPLC. Data taken from Chmielowiec and Sawatsky (74).
HSA, A2
nitrobenzene
HSA, A' 202 213 218
241 147 150 180 161
133 142
D. Substituted Benzenesb 86
"HSA values taken from Hermann (51),Valvani et al. (55),Yalkowsky and Valvani (57), and Yalkowsky et al. (56). bHSA value computed by Belfort (59) using a modified Hermann (51) model.
systems are comparable, one may infer that the major mechanisms of the sorptive process are similar for those systems (13, 19,22). According to Melander et al. (13),two aspects of entropy-enthalpy compensation analysis are important. First, it has been shown that calculating ,B according to eq 5 or 6 (i.e., a plot of AGO or In kT' versus AHo)rather than using eq 4 (Le., plotting ASo versus AH")minimizes statistical artifacts, thus permitting inference as to the chemical causality for the observed compensation. Second, in order to maximize the accuracy of this evaluation, the reference temperature chosen for estimating p must be near the harmonic mean of the temperatures used to calculate AH". Both these recommendations were adopted in our study. Knox and Vasvari (23) examined the retention of various substituted benzenes by RPLC supports and reported enthalpy-entropy compensation effects in a 40/60 (v/v) methanol/water eluent. Compensation effects have been investigated (13)for buffered and ionized aromatic acids on C-18 material in 100% aqueous and in acetonitrile/ water systems (up to 30% acetonitrile) and for alkylbenzenes on C-8 and C-18 phases in various methanol/ water mixtures (22). Compensation temperatures in the range of 500-700 K were calculated for each of these RPLC systems (13,22). Melander et al. (13)hypothesized that the mechanism of hydrophobic sorption is essentially the same, regardless of the nature and concentration of the organic solvent present and the chemical nature of the sorbate molecules. Further support for this conclusion comes from the data of Kikta and Grushka (24). Compensation temperatures of 593 and 512 K, computed from their data for alkylphenone retention on two different types of nonylsilica stationary phases in 50/50 (v/v) methanol/water eluent, are in the same range as those discussed above. It is interesting to note that, in chromatographic systems employing polar stationary phases and nonpolar eluents (i.e., normal-phase chromatography), the calculated compensation temperature, 140 K, was markedly lower than those obtained in RPLC (23). The lower p value suggests that the retention mechanism in normal-phase chromatography is apparently different from that operating in RPLC.
Selection of Hydrophobic Solutes. The HOCs chosen for this study (Table I) were compounds of general environmental concern. Chemicals that were distinctly different in their molecular conformations were studied to examine the effect of solute structure on retention thermodynamics. One of the classes of HOCs studied was the polycyclic aromatic hydrocarbons (PAHs), which have been the focus of numerous studies concerning their fate and sorption in the aqueous environment (25-28). Members of this class of ubiquitous environmental pollutants have been found widely distributed in air (29, 30), rainwater (31), freshwater (32), waste water (33), and sediments and soils (34,35). The environmental sources of PAHs include petroleum spills and byproducts of energy production (32) and the combustion of organic material (36,37);a number of the PAHs are potent carcinogens (38). RPLC is one of the techniques generally used in the analysis of environmental samples containing PAHs. Due to their extensive aromaticity and lack of substituent groups, the PAHs are generally rigid, planar molecules with little or no internal degrees of movement. Mathematical relationships between the molecular structure of PAHs and their RPLC retention factors have also been examined (39-42). Another class of HOCs studied was the substituted benzenes. In particular, the alkylbenzenes were selected for study since members of this class of compounds have been the subject of considerable thermodynamic and RPLC retention research (43-45) and have been discovered in landfill leachate plumes (46). The alkylbenzenes are also of interest by way of structural contrast when compared to the rigid, planar PAHs. The alkyl-chain portion of a molecule in the solid state has been observed to exist in a fully extended form (47). The extended form is also a stable conformation in the liquid state, but it may not predominate because its statistical weight is small compared with the sum of other possible conformers (48). When placed in aqueous solution, the C1-CI n-alkanes are suggested to exist predominantly in an extended conformation (49),while the C6 and large aliphatic hydrocarbons are proposed to exist in folded or coiled conformations (49-51). It is these coiled and folded conformations that present the smallest amount of hydrophobic surface area for contact with water molecules. The space inside a coiled or folded alkyl chain may consist of a nonsolvated, empty interior volume stabilized by intramolecular interactions (49,50), or the hydrocarbon chains may be separated by one or more layers of solvent molecules (51). A RPLC retention study encompassing a number of straight-chained and branched alkylbenzenes, as well as the rigid PAHs, may therefore provide useful information for determining the importance of solute conformation upon the mechanisms and energetics of hydrophobic sorption. Nitrobenzene and the four monohalobenzenes were also chosen for study because of their structural simplicity and concern over their environmental fate (52). A listing of the solutes used in the retention experiments and their respective hydrocarbonaceous surface area (HSA) values is shown in Table I. Except for nitrobenzene, it was assumed that the total molecular surface area (TSA) was equivalent to HSA. The HSA value for nitrobenzene was calculated by a modification of the surface area model proposed by Hermann (51). Materials and Methods Reagents. The chemicals used in the RPLC and soil studies were generally of reagent- or analytical-grade quality. The PAHs (98+%) and n-butyl- and n-hexylbenzenes (99+ % ) were obtained from the Aldrich Chem-
ical Co.; the biphenyl (reagent grade) was obtained from Fisher Scientific and the benzene (99+%) from Mallinckrodt, Inc. The remaining alkylbenzenes and substituted benzenes (reagent grade) were obtained from Eastman Kodak, Inc. The HPLC-grade solvents (water, acetonitrile, and methanol) used in the RPLC and soil experiments were obtained from Fisher Scientific Co. RPLC Studies. The isocratic elution of the HOCs through packed columns of the RPLC sorbents was performed with a modular liquid chromatography system consisting of two Gilson Model 302 metering pumps, a Gilson 1.5 mL analytical mixer, and two Gilson Model 802 manometric modules interfaced with an Apple IIe microcomputer-controlled gradient management system. The absorbance of the column effluent was monitored at 254 nm by a Waters 450 variable-wavelengthUV detector, with the chromatograms recorded on a Hewlett-Packard 3390A reporting integrator. In the RPLC retention studies, injections of the HOCs onto the RPLC supports were made by using a Rheodyne 7161 switching valve with a 20-pL injection loop. All RPLC sorbate mixtures were 5-500 mg/L in 100% methanol. The flow rate for RPLC thermodynamic retention experiments was set at 1.0 mL/min and verified to be within 5% of this value. All RPLC experiments were performed at least in triplicate. All RPLC column packings were purchased from Analytichem International Inc. (Harbor City, CA). The precolumn packing material was 40-pm Sepralyte unbonded silica gel, and the reversed-phase stationary phases consisted of porous, irregularly shaped, 10-pm-diameter silica gel particles chemically bonded with the following trichloroalkylsilanes: (2-2, C-4, and C-8. These stationary phases were slurry-packed into 5 cm X 4.6 mm (id.) X 6.35 mm (0.d.) stainless steel HPLC columns, equipped with 2-pm frits at each end. The unbonded silica gel was drypacked into a similar column for use as a saturator column (precolumn); however, the outlet end of this column contained a 0.5-pm frit to prevent fines from clogging the injection valve and downstream tubing. Both the precolumn and the analytical HPLC column were themostated by circulating water jackets to maintain constant temperature. The precolumn was placed before the injection valve to assist in bringing the mobile phase to the desired temperature while also saturating the solvent mobile phase with dissolved silica. The former effect is desired to avoid differential temperature bands in the analytical column, while the latter minimized the loss of silica support from the downstream analytical column. The circulating water bath was a Brinkman Model RC20, capable of operating over a temperature range of -15 to 100 "C, with an accuracy of f0.2 "C. The temperature of the water bath was monitored with a Bailey Model BAT-8 digital thermocouple thermometer. This instrument had a stated range of 0-100 "C and a maximum sensor error of f O . l OC at 100 "C. The RPLC retention studies were routinely performed at temperatures of 288, 298,308, and 318 K for a single isocratic solvent mixture. A Shandon HPLC packing pump was used to slurrypack the RPLC sorbent materials into the HPLC columns. Approximately 2 mL of the selected sorbent material was slurried in 20 mL of chloroform (reagent grade, Fisher Scientific Co.). This slurry was shaken and placed in an ultrasonic bath for 20 min to ensure adequate distribution of the 10-pm material. The sorbent/chloroform slurry was then poured into the packing reservoir and packed into the column at 6000 psi by using a succession of increasingly viscous solvent mixtures: 50/50 (v/v) chloroform/methanol, 100% methanol, and 50/50 methanol/water. The Environ. Sci. Technol., Vol. 23, No. 4. 1989
409
Table 11. Physical and Chemical Properties of Webster Soil
mech anal. sand silt clay P" OC,b % CEC," mequiv/100 g major clay minerals
2
38 21 41 5.8 2.23 37.2 smectites
"Measured in a 1/1 paste of soil and 0.01 M CaC1,. carbon content. Cation-exchange capacity.
Envlron. Scl. Technol., Vol. 23, No. 4, 1989
/
3
Rc
b 2 5
bOrganic
first two solvent mixtures were run for 1-2 min, with the methanol/water mixture then packing the column for 10-15 min. Following the packing procedure, the column was removed and installed on the Gilson RPLC system. Batch Sorption Studies. A bulk sample of the Webster sandy clay loam (surface soil, 0-30 cm) was collected in Iowa from a profile classified as a Typic Haplaquoll. The physical and chemical properties of this soil sample are listed in Table 11. The Webster soil has been used extensively in earlier studies of sorption and leaching of hydrophobic pesticides (8,53). This soil was chosen for study since it represents a highly carbonaceous, hydrophobic surface soil. The high organic carbon content (2.23%) allowed an evaluation of hydrophobic sorption on a natural sorbent. The Webster soil was air-dried and passed through a 2-mm sieve to remove stones and root fragments prior to use. Sorption of four PAHs (phenanthrene, anthracene, fluoranthene, and pyrene) by the Webster soil from 30/70 (v/v) methanol/water was measured at three temperatures (287, 296.5, and 308 K). Batch equilibration techniques were employed, where soil samples were shaken a t the desired temperature with PAH solutions for a 3-day period. At each temperature, sorption isotherms were determined by measuring solute uptake at four concentrations in triplicate for each PAH. To 35-mL Kimax glass centrifuge tubes, different volumes of stock methanol solutions containing all four PAHs were added, and the solvent was evaporated leaving a thin coating of PAH precipitate on the tube walls. Total mass of each PAH added to the tubes in this manner was varied in order to achieve equilibrium solution-phase concentrations that did not exceed 3,1,0.3, and 0.1 pg/L, respectively, for phenanthrene, fluoranthene, anthracene, and pyrene. These values are well below the solubility limits for these compounds in 30/70 methanol/water over the temperature range investigated (54-57). To each tube, 3 g (air-dry basis) of soil and 35 mL of 30170 methanol/water were added, leaving no headspace. The centrifuge tubes were then capped with Teflon-lined screw caps and shaken for 3 days at the desired temperature. The tubes were kept in a Thelco Model 32MR constant temperature chamber for studies at 287 K and in a Blue M water bath for studies at 308 K. For sorption at 296.5 K, the samples were shaken on the laboratory bench; the temperature variations, as monitored with a Bailey Model BAT-8 digital thermocouple thermometer, were within 1 K. At the end of the equilibration period, the tubes were centrifuged at 1000 RCF in a Sorvall Model RC5C super centrifuge for 25 min; the centrifuge temperature was matched with that at which the soil samples were equilibrated. Aliquots of clear supernatent solutions were then transferred to 4.5-mL autosampler vials and the PAH concentrations assayed by RPLC techniques. In selected cases, the soil plug was removed from the bottom of the centrifuge tubes and extracted twice with 100% acetone. 410
/
1
0 I / 1
I
2
3
id/^) ( K ) Flgure 2. Ln k'at 298 K versus -AHo/R for retention of PAHs on C8 material in 80/40 methanoi/water. -(AHOX
PAH concentrations in these acetone extracts were also analyzed by RPLC. The RPLC system used for PAH analyses consisted of a LDC/Milton Roy CM4000 ternary solvent pump with a 50/ 10/40 (v/v/v) acetonitrile/methanol/water mobile phase a t a flow rate of 1.5 mL/min and an Alltech C-18 column (4.6 mm i.d. X 25 cm) connected in series to a Waters 490 programmable UV detector (240 nm) and a Gilson Model 121 fluorometer (350 and 420 nm, excitation and emission filters, respectively). Sample aliquots (15-75 pL) were injected with a Waters Intelligent Sampler Processor (WISP) Model 710B. Peak areas were monitored by Hewlett-Packard reporting integrators (HP 3390A and H P 3392) and converted to concentrations by using standard calibration curves. In selected cases, the amount of PAH sorbed was measured in two ways: (i) directly by extraction of the soil and (ii) indirectly by calculating the difference between the total mass added and that in solution at equilibrium. These two determinations of the amount of PAH sorbed differed by less than 10% in all cases, indicating that PAH sorption onto the container walls was not significant for our systems. All data reported here for PAH sorption by the Webster soil are from the measurements calculated by difference. The sorption coefficients, Kp (mL/g), at each temperature for each PAH were estimated by fitting the sorption data to a linear isotherm: S = KpC, where S and C are sorbed (pg/g) and solution (pg/mL) concentrations at equilibrium. In all cases, the coefficients of determination (P) for the linear fits exceeded 0.95. For each PAH, the Kp values measured at three temperatures were used to determine AHo values from the slopes of the van't Hoff plots (eq 2), which were used to estimate the compensation temperature, /3 (eq 6).
Results and Discussion Enthalpy-Entropy Compensation Effects in RPLC. RPLC retention data for the HOCs listed in Table I were examined for enthalpy-entropy compensation effects by using eq 6. The RPLC systems under study were the C-2, C-4, and C-8 sorbents with a wide variety of methanol/ water mobile phases. A plot of In kT' versus AHO/Rfor solute retention on the C-8 support in a 60/40 (v/v) methanol/water eluent is presented in Figure 2. The solutes included in Figure 2 are the PAHs, alkylbenzenes, and halobenzenes listed in Table I. Solute retention data at the reference temperature of 298 K were chosen to evaluate the /3 values; however, the use of other reference temperatures produced similar results. Nitrobenzene ex-
Table 111. Comparison of Compensation Temperatures (8) Collected on Three RPLC Supports in a Methanol/Water Mobile-Phase System
RPLC phase c-2
c-4
C-8
av std dev
compensation temp a, K PAHs, benzene, O(Me0H)" and halobenzenes alkylbenzenes 0.60 0.50 0.40 0.38 0.75 0.70 0.60 0.50 0.40 0.80 0.70 0.60 0.50
762 (611, 1010)* 554 (538, 572) 521 (509, 533) 517 (501, 533) 541 (488, 606) 699 (675, 724) 563 (542, 586) 532 (515, 549) 516 (493, 541) 471 (451, 491) 504 (481, 531) 535 (516, 557) 547 (527, 569) 559 81
820 (637, 1148) 657 (603, 721) 695 (516, 1061) 725 (630, 855) 696 (642, 760) 886 (782, 1023) 650 (782, 1023) 745 (652, 868) 780 (689, 900) 623 (573, 682) 774 (657, 941) 783 (706, 880) 1034 (980, 1210) 759
Table IV. Temperature Dependence of PAH Sorption on Webster Soil from 30/70 Methanol/Water"
PAH
sorption coeff K,,mL/g 287 K 296.5 K 308 K
24.4 (0.8) 17.3 (0.7) phenan29.8 (0.9) threne 39.7 (1.3) 29.4 (1.2) anthracene 53.1 (1.6) fluoranthene 105.9 (2.9) 77.7 (2.4) 52.6 (1.9) pyrene 1221.3 (24.8) 561.1 (10.8) 411.0 (14.4)
-AHo,
kcal/mol 4.55 (0.52) 5.42 (0.14) 5.85 (0.18) 9.02 (2.57)
Standard error values are shown in parentheses. 8
Webster Sol1 3070 Methano1:Water
Fluoranthene
111
Anthracene
Volume fraction methanol in methanol/water mobile phase. bThe 95% confidence limits are shown in parentheses. (I
hibited anomalous thermodynamic behavior compared to the other solutes and was excluded from the compensation studies. This behavior is believed due to preferential polar interactions of the nitro moiety with free silanol groups on the RPLC surface (58). From the plot of In kT' versus -AHo/R(Figure 2), the system compensation temperature, @, was calculated from the slope of the linear regression line (eq 6). The PAHs and halobenzenes exhibited markedly different compensation effects compared to the alkylbenzenes, and separate linear regression lines were developed. In Figure 2, the calculated @ values (f95% confidence intervals) were 535 (516,557) K for the PAHs and halobenzenes and 783 (706, 880) K for the alkylbenzenes. Similar differences between PAHs and alkylbenzenes were observed for each sorbent/solvent combination and the system compensation temperatures were calculated accordingly (Table 111). The consistent difference in compensation temperatures between the PAHs and alkylbenzenes for each solvent/ sorbent combination suggests that these two chemical groups differ somewhat in their RPLC retention mechanisms in methanol/water solvent systems. Although data are presented here for only four monohalobenzenes, other halobenzenes may be expected to behave in a similar manner as these rigid, planar molecules. The observed distinction between PAHs/halobenzenes and alkylbenzenes may be related to their conformational differences or because of the manner in which the alkyl chain(s) of the alkylbenzenes may or may not preferentially interact with the nonpolar stationary phase. Note that the greatest deviation in Figure 2 from the regression line for PAHs is for the long-chained alkylbenzenes (n-butyl, n-hexyl, etc.). For the systems we investigated, the compensation temperatures for both groups appear to be affected minimally by the methanol content of the mobile phase or the chain length of the stationary phase (Table 111). These results are consistent with those observed by Melander et al. (13). Enthalpy-Entropy Compensation Effects on Webster Soil. A fundamental premise of the RPLC experiments was to study the applicability of RPLC as a model for HOC sorption in soil/water/mixed-solvent systems. For this purpose, batch experiments were conducted to measure the sorption of four PAHs on Webster soil from 30/70 (v/v) methanol/water mixture to examine enthal-
* i 1
3.2
3.3
3.4
3.5
Reclprocal Temperature, 1000/T (K)
Figure 3. Van't Hoff plots for temperature dependence of PAH sorption on Webster soil from 30/70methanollwater. 7 Webster Sol1
- 3 4 7 0 Methanol-Watrr
8
-
.y
In $ 5 N
-
$4 -I
3
2
2
I
I
3
4
- ( A Hex 10' /R)
5
( K)
Figure 4. Ln K, at 296.5 K versus -AHo/R for sorption of PAHs on Webster sol1 from 30170 methanol/water.
py-entropy compensation effects. The measured K values are summarized in Table IV. Also shown are t i e AHo values estimated from the van't Hoff plots (Figure 3); the linearity of the plots in Figure 3 suggests that heat capacity effects were negligible, at least over the temperature range studied. The AHo and Kpvalues at 296.5 K were used to generate the plot shown in Figure 4. The compensation temperature (@)calculated from the slope of this plot (eq 6) was 506 K (with a 95% confidence interval of 422-590 K). This value is within the range of the @ values calculated for PAH retention by the RPLC sorbents; the average fi value for retention of PAHs and halobenzenes by RPLC supports from methanol/water was 559 K (Table 111). These findings indicate that the dominant mechanism for sorption of PAHs by the Webster soil from methanol/water may be similar to the mechanisms for retention of PAHs by RPLC sorbents. It is noteworthy that Opperhuizen et al. (60) have recently reported a value of 714 K for octaEnviron. Sci. Technol., Vol. 23, No. 4, 1989 411
nol/water partitioning of chlorobenzenes; HOC behavior in the octanol/water system has been successfully related to sorption by soils and sediments (2, 3). The similarity in sorption energetics may possibly be related to (i) the structure of the polymeric RPLC sorbents and soil organic carbon, (ii) the conformation of the solution- and sorbed-phase solutes, and (iii) dominance of the “solvophobic”mechanism (61)in both systems. Although data presented here are for sorption of only four PAHs by one soil, this study does present experimental evidence of similarity in solute retention mechanism operating in the RPLC and soil systems. This conclusion needs to be verified by collecting additional thermodynamic data for sorption of HOCs on several soils. Such data obtained by using soils with a wide range in organic carbon contents will permit evaluation of the compensation model for sorbents with differing hydrophobicities.
Acknowledgments We are grateful to A. G. Hornsby and P. Nkedi-Kizza for helpful comments and criticisms and to J. Dorsey for assitance and guidance in the RPLC studies. Registry No. C-2, 115-21-9; C-4, 7521-80-4; (2-8, 5283-66-9; MeOH, 67-56-1; benzene, 71-43-2; naphthalene, 91-20-3; biphenyl, 92-52-4; phenanthrene, 85-01-8; anthracene, 120-12-7; pyrene, 129-00-0; fluoranthene, 206-44-0; chrysene, 218-01-9; toluene, 108-88-3; ethylbenzene, 100-41-4; n-propylbenzene, 103-65-1; n-butylbenzene, 104-51-8;n-hexylbenzene, 1077-16-3; o-xylene, 95-47-6; p-xylene, 106-42-3; m-diethylbenzene, 141-93-5; 1,2,4trimethylbenzene, 95-63-6; fluorobenzene, 462-06-6; chlorobenzene, 108-90-7; bromobenzene, 108-86-1; iodobenzene, 591-50-4; nitrobenzene, 98-95-3.
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Receiued for review July 27,1987. Reuised manuscript receiued June 13, 1988. Accepted October 27, 1988. This study was funded, in part, by US.EPA Cooperatiue Agreement No. CR811144. Approved for publication as Florida Agricultural Experiment Station Journal Series No. 9012.
Mathematical Modeling of the Chemistry and Physics of Aerosols in Plumes A. Belle Hudischewskyj"
Systems Applications, Inc., 101 Lucas Valley Road, San Rafael, California 94903 Christian Seigneur
ENSR Consulting and Engineering, 1320 Harbor Bay Parkway, Alameda, California 94501 This paper describes the formulation and evaluation of a mathematical model that calculates the gas-phase (e.g., SO2,NO,, 0,) and aerosol-phase (sulfate, nitrate, PM-10) pollutant concentrations in a plume that undergoes transport, dispersion, and dry deposition in the atmosphere. A full treatment of the aerosol size distribution and chemical composition is provided. The model is evaluated with data collected from the plumes of three power plants (Navajo in Arizona, Mohave in Nevada, and Labadie in Missouri) and one smelter (San Manuel in Arizona). These data allow evaluation of the performance of the aerosol model under various plume and background conditions. Comparison between predicted and measured values of the aerosol size distributions, sulfate aerosol concentrations, and scattering coefficients is quite satisfactory, although the model tends to underpredict the latter two. Results indicate that this model is a useful tool for understanding the dynamics and chemistry that govern aerosol concentrations and for analyzing the impact of pollutant emission sources on regulated aerosol levels (e.g., PM-lo), atmospheric visibility, and acidic species (sulfate and nitrate) levels.
Introduction The concentration, size distribution, and chemical composition of atmospheric aerosols play a pivotal role in air quality issues, such as regulated aerosol concentrations (ambient concentrations of particulate matter under 10 pm in diameter, i.e., PM-lo), acid deposition (dry deposition and droplet scavenging of sulfate and nitrate aerosol species), and visibility degradation due to light scattering and absorption by aerosols. For example, removal of aerosols by dry and wet processes is strongly dependent on the aerosol sizes. Scattering of light is also a strong function of the aerosol size distribution. The chemical composition of the aerosol affects its hygroscopic behavior, and the amount of water in the aerosol phase directly affects the aerosol size and associated phenomena (deposition, scattering). Therefore, it is essential to be able to predict the size distribution, concentration, and chemical composition of atmospheric aerosols in a rigorous and scientificallysound manner. Then, mathematical modeling of atmospheric aerosols will become a key component of many air quality models. Mathematical modeling of the formation of atmospheric aerosols has been the subject of several research efforts over the past recent years, and major studies may be summarized as follows. 0013-936X/89/0923-0413$01.50/0
Eltgroth and Hobbs (1) developed a model that treated plume dispersion, gas-phase chemistry, H2S04condensation, and aerosol dynamics. The aerosol size distribution was approximated by 3 log-normal distributions. Nitrate and ammonium aerosols were not treated. The evolution of the aerosol size distribution was evaluated with power plant plume data. Tsang and Brock ( 2 , 3 )developed an aerosol dynamics model based on an accurate solution of the coagulation, condensation, and evaporation processes. Atmospheric dispersion was also considered in one application. This model presently simulates only monocomponent aerosols and does not provide a size resolution of the aerosol chemical composition. Bassett and co-workers ( 4 ) used a sectional representation of the aerosol size distribution to simulate the evolution of a bicomponent aerosol (primary and sulfate aerosols) in a plume. The model treated the condensation of H2S04and the oxidation of SO2 to sulfate in liquidcoated aerosols. The model only applied to high humidities and did not include any treatment of nitrate aerosols. Seigneur and co-workers (5, 6 ) also used the sectional representation of the aerosol size distribution to simulate primary, ammonium sulfate, and ammonium nitrate aerosols. The model included aerosol dynamics, gas-phase chemistry, gas-to-particle conversion, and atmospheric dispersion. Model calculations were compared with aerosol data from power plant plumes. The aerosol treatment, however, was limited to dry aerosols. Russell and co-workers (7) included the treatment of ammonium nitrate aerosol formation in a trajectory model with a coupled treatment of gas-phase chemistry. The model results were later compared with aerosol chemistry data obtained in the Los Angeles air basin (8, 9). The aerosol size distribution was not treated. Recently, Pilinis and co-workers ( I 0 , I I ) incorporated a detailed treatment of the thermodynamics and dynamics of sulfate, nitrate, ammonium, sodium, and chloride aerosols into a trajectory model with a coupled treatment of gas-phase chemistry. The aerosol size distribution was represented by a sectional distribution. This multicomponent aerosol model was also evaluated with Los Angeles data. However, these data pertained solely to the aerosol chemical composition and did not include any information on the aerosol size distribution. In this paper we present the formulation of a mathematical model that describes the evolution of the concentration, size distribution, chemical composition, and
0 1989 American Chemical Society
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