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Karickoff,S.W. J. Enuiron. Eng. (N.Y.) 1984,110,707-735. Lyman, W. J., Reehl, W. F., Rosenblatt, D. H., Eds. Handbook of Chemical Property Estimation Methods; McGraw-Hill Book Co.: New York, 1982; pp 4.1-4.32. Pinal, R.; Rao, P. S. C.; Lee, L. S.; Cline, P. V.; Yalkowsky, S.H. Environ. Sci. Technol., preceding article in this issue. Yalkowsky, S. H.; Roseman, T. Techniques of Solubilization of Drugs; Yalkowsky, S. H., Ed.; Marcel Dekker, Inc.: New York, 1981; pp 91-134. Yalkowsky, S.H. Project Completion Report, CR 811852-01. U S . Environmental Protection Agency, Ada, OK, 1985. Yalkowsky, S. H. Project Completion Report, CR 812581-01. U.S. Environmental Protection Agency, Ada, OK, 1986. Rubino, J. T.; Yalkowsky, S.H. Pharm. Res. 1987, 4, 220-230. Rubino, J. T.; Yalkowsky, S. H. Pharm. Res. 1987, 4 , 231-236. Morris, K. R.; Abramowitz, R.; Pinal, R.; Davis, P.; Yalkowsky, S. H. Chemosphere 1988, 17, 285-298. Rao, P. S. C.; Hornsby, A. G.; Kilcrease, D. P.; Nkedi-Kizza, P. J. Environ. Qual. 1985, 14, 376-383. Nkedi-Kizza, P.; Rao, P. S. C.; Hornsby, A. G. Enuiron. Sci. Technol. 1985,19, 975-979. Nkedi-Kizza, P.; Rao, P. S. C.; Hornsby, A. G. Enuiron. Sci. Technol. 1987,21, 1107-1111. Rao, P. S.C.; Lee, L. S.In Proceedings,24th Hanford Life Science Symposium; DOE Symposium Series 62; Battelle Pacific Northwest Labs.: Richland, WA, 1987;pp 457-471. Woodburn, K. W.; Rao, P. S. C.; Fukui, M.; Nkedi-Kizza, P. J. Contam. Hydrol. 1986, 1, 227-241.
Woodburn, K. W.; Lee, L. S.; Rao, P. S. C.; Delfino, J. J. Enuiron. Sci. Technol. 1989, 23, 407-413. Fu, J. K.; Luthy, R. G. J. Enuiron. Eng. (N.Y.) 1986, 112, 328-345. Fu, K. J.; Luthy, R. G. J.Environ. Eng. (N.Y.) 1986, 112, 346-366. Walters, R.; Guissippe-Elie, A. Environ. Sci. Technol. 1988, .~ 22, 819-825. Walters,R. W.; Ostazeski, S.A.; Guiseppi-Ellie,A. Environ. Sci. Technol. 1989, 23, 480-484. Lee, L. S.; Rao, P. S. C.; Nkedi-Kizza, P.; Delfino, J. J. Enuiron. Sci. Technol., following article in this issue. Zachara, J. A.; Ainsworth, C. C.; Schmidt, R. L.; Resch, C. T. J. Contam. Hydrol. 1988,2, 343-364. Hassett, J. J.; Banwart, W. L.; Wood, S. G.; Means, J. C. Soil Sci. SOC.Am. J. 1981, 45, 38-42. Rubino, J. T.; Berryhill, W. S.J. Pharm. Sci. 1986, 75, ~~
182-186.
Ludwig, M.; Baron, V.; Kalfus, K.; Pytela, 0.;Vecera, M. Collect. Czech. Chem. Commun. 1986, 51, 2135-2142. Munz, C.; Roberts, P. V. Environ. Sci. Technol. 1986,20, 830-836. Received for review April 17,1989. Revised manuscript received October 31,1989. Accepted December 20,1989. This work was supported, in part, by Cooperative Agreement CR-814512 with the U S . Environmental Protection Agency, Ada, OK. Approved for publication as Florida Agricultural Experiment Station Journal Series No. R-00247.
Influence of Solvent and Sorbent Characteristics on Distribution of Pentachlorophenol in Octanol-Water and Soil-Water Systems Linda S. L e e , * ~P. t ~Suresh ~ C. Rao,*!tszPeter Nkedi-Kizza,+and Joseph J. Delfinot
Soil Science Department, 2 169 McCarty Hall, and Environmental Engineering Sciences Department, University of Florida, Gainesville, Florida 3261 1-0151 Sorbent and solvent characteristics influencing sorption of pentachlorophenol (PCP) were investigated. Analysis of aqueous sorption data for several sorbents over a broad pH range suggested hydrophobic sorption of neutral PCP predominates a t pH 7 . At pH > 7, sorption of the pentachlorophenolate anion ( P C P ) and the formation and sorption of neutral ion pairs [e.g., metal cation (M') + (PCP-) = MPCPO] was considered. The observed sorption data were described over the entire pH range with knowledge of pH, soil organic carbon content, and PCP's pK,. Increased sorption of PCP- was observed with increasing ionic strength for batch sorption studies conducted in aqueous CaClz solutions. Octanol-water partition coefficients measured in various electrolyte solutions suggested that formation of MPCPO and the partitioning of PCP- are influenced by the cationic species. Batch sorption studies conducted in methanol-water solutions suggest the predominance of solute-solvent interactions in the sorption of both the neutral and ionized forms of PCP; sorption decreased in a log-linear manner with increasing methanol fractions.
Introduction Sorption of hydrophobic organic compounds (HOCs) by soils and sediments has been successfully predicted in many cases by application of the solvophobic theory and 'Soil Science Dept. Environmental Engineering Sciences Dept. f
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linear free energy relationships (LFER). Using LFER, many investigators have found excellent linear relationships between log K , (the sorption coefficient normalized to the fraction of organic carbon, OC, of the sorbent) and log KO,,the octanol-water partition coefficient for several HOCs (1-9). Linear relationships have also been found between log K , and hydrophobic surface area (HSA) (2, 10, II), and molecular connectivity (12,13). The different slopes and intercepts found in these regression equations are predominantly determined by the characteristics of a group of compounds (i.e., class, degree of hydrophobicity and structure), while the effects of sorbent properties other than OC appear to be minor in most cases (4,5, 7,14). These equations provide reasonable predictions of HOC distribution in diverse soil-water systems based either on LFER, as discussed above, or on experimental data obtained for only a few sorbents. The sorption of HOCs by soils and sediments is inversely proportional to their aqueous solubility (4,5,15,16). This relationship has been further extended to miscible solvent-water systems (e.g., methanol-water). For example, as the cosolvent fraction in a binary solvent increases, HOC solubility increases log-linearly (I 7, It?), while sorption decreases similarly (I2,19-24). This relationship is consistent with the solvophobic theory employed in reversed-phase liquid chromatography (RPLC) (25),which interprets the energetics of HOC retention by a nonpolar reversed-phase sorbent in a polar solvent in terms of the bulk solvent properties (23). According to the solvophobic theory, a decrease in the solution-phase polarity would
0013-936X/90/0924-0654$02.50/0
0 1990 American Chemical Society
cause the energy of cavitation for HOCs to decrease, resulting in an increase in solubility and a concomitant decrease in retention (sorption). This observed relationship suggests that solute-solvent interactions play a dominant role in the sorption of HOCs by hydrophobic sorbents. For hydrophobic, ionizable organic compounds (HIOCs), the solvophobic mechanism alone may not be sufficient for estimating soil-water distribution coefficients. The dominant sorption mechanism for HIOCs depends on the degree of their dissociation (i.e., the ratio of neutral species to the total species), which is a function of the dissociation constant and the soilsolution pH. For most soils, the soil surface exhibits a net negative charge at environmentally relevant pHs of -4-8. Therefore, for organic bases that form cationic species upon protonation, additional mechanisms that involve specific interactions (e.g., cation exchange, chemisorption, complexation) should be considered. Models for describing such processes have been used successfully for organic amines and some nitrogen heterocyclic compounds (26-30). The formation of ion-pair complexes can also play a role in the sorption of HIOCs. A specific group of HIOCs of interest are the chlorophenols, which like other organic acids form organic anions upon dissociation. For the chlorophenols, ion pairing can become important while other mechanisms applicable to organic cations may not be operative under typical environmental conditions. Ion-pair complexes can form in solution or a t the soilsolution interface. According to Bjerrum's theory of ionic association (31),ion pairs can form in the solution phase and then be transferred to the three-dimensional organic phase (e.g., octanol or organic matter) as a neutral species in the same manner as a HOC. Ion pairs can also form a t the two-dimensional soil-solution interface with the hydrophobic end of the chlorophenolate ion attracted to the lipophilic surface along with a transfer of counterions to the electrical double layer to maintain electric neutrality. This latter mechanism is similar to that which occurs with ionic surfactants (32). Data presented by Westall et al. (33) and Schellenberg et al. (34) suggest the transfer of both the phenolate anion and the neutral metal phenolate ion pairs from the bulk aqueous phase to the bulk organic phase. Pentachlorophenol (PCP) is a common groundwater contaminant. Anthropogenic inputs of PCP to the environment arise primarily from its use as a wood preservative (35). An understanding of the behavior of PCP in the environment necessitates an assessment of the processes influencing its fate and transport in soils and groundwater. PCP is a weak organic acid (pK, = 4.75), and its neutral form is strongly hydrophobic with a low aqueous solubility (11-14 pg/mL at 25 "C). PCP can partially or totally ionize under natural conditions, significantly modifying its aqueous solubility, sorption, and transport. In the present study, sorbent and solvent characteristics influencing PCP sorption were investigated. Data collected from batch experiments involving PCP sorption by clays and soils from aqueous and mixed solvents are presented. These data, along with data for PCP partitioning between octanol and water, were used to assess the various factors and processes determining PCP sorption by natural sorbents. Theory Assuming activity coefficients to be near unity, the degree of dissociation for weak organic acids, such as the chlorophenols, can be described by Ka = [A-l [H+I/ [HA1
(1)
where K, is the acid dissociation constant, and [A-1, [H+], and [HA] are the concentrations of the chlorophenolate anion, the hydrogen ion, and the neutral chlorophenol, respectively. The chlorophenols in their neutral forms exhibit increasing hydrophobicity with the addition of chloro-substituted groups and behave as HOCs. By recalling that -log (H+J= pH and -log K, = pK,, the fraction of neutral phenol present, can be defined in terms of pH and pK,: C$n = [HA]/([HA]
+ [A-1) = (1 + 10pH-pKV
(2)
PCP is essentially 100% neutral a t pH 7. The ratio (Paw) for the concentration of phenol and phenolate partitioning between the nonaqueous and aqueous phases may be defined as
Paw= ([HA],
+ [A-I,)/([HAl, + [AI,)
(3)
where the subscripts o and w refer to the organic (nonaqueous) and the aqueous phase, respectively. For sorption by soils, the distribution of the molecular form of phenol and phenolate between the sorbed and solution phases may be defined as where the subscripts s and w refer to sorbed and solution phases, respectively. The predicted K for the neutral and ionized form of a weak organic acid can then be expressed as Kn = [HAls/[HAlw
(5)
Ki = [A-ls/[A-lw
(6)
respectively. Assuming that only the neutral species is sorbed and that the organic carbon content (OC) of the sorbent predominantly determines the extent of sorption, as is the case with other HOCs, the OC-referenced sorption coefficient can be defined as Kac,p = Kac,nC$n
(7)
where Koc,n= (KJOC), and the subscript p refers to the predicted value. If there is a transfer of the ionized species to the organic phase, the predicted sorption would be underestimated by eq 7. For most environmental scenarios ion exchange of the anion is not expected, as discussed previously. However, sorption of the ionized species may occur upon formation of a neutral ion pair, or the hydrophobic part of the organic anion may sorb to a hydrophobic surface, with its polar end oriented toward the more polar aqueous phase. In either of these cases, solute-solvent interactions will dominate the sorption of the ionized species, as they do with the neutral species. Therefore, assuming that sorbent OC will also determine the extent of sorption for the ionized species as well as the neutral species, the overall sorption of a weak organic acid should then be described by (8) L , p = Kac,nC$n+ Kac,i(l - C$n) where KWi= (KJOC). The sorption predicted for a weak organic acid in its ionized form would be less than that for the neutral form because of the difference in their hydrophobicities. Thus, KW,i< Km,n. Equation 8 may better describe sorption of a weak organic acid than eq 7, because sorption of both the neutral and the ionized forms are accounted for, while allowing Environ. Sci. Technol., Vol. 24, No. 5, 1990
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Table I. Physical Characteristics of the Sorbents Used in This Study soil Eustis (1) Eustis (2) Webster (1) Webster (2) Webster (3)
sand
Darticle size. wt '70 silt clay
96.4 91.6 29.6 38.0 30.7
1.8 4.2 40.9 21.2 42.3
1.8 4.2 29.5 40.8 27.0 >99%
54.0 88.6
31.4 9.4
14.6 2.0
SAz-1
Pahokee muck Kidman Lincoln
oc 0.39 0.78 3.41 2.23 3.70 0.204 57.5 0.61 0.22
the magnitude of the individual sorption coefficients Koc,n and Koc,ito be different. If the anionic species does not contribute significantly to the overall sorption process (i.e., if KO,, >> KO,,$, eq 8 reduces to eq 7 . Materials and Methods
Sorbents. The sorbents used in this study were as follows: two different samples of Eustis fine sand (Psammentic Paleudult) from Florida; three different samples of Webster silty clay loam (Typic Haplaquoll) from Iowa, Lincoln sand (Typic Ustivfluvent) from Oklahoma, Kidman loamy sand (Calcic Haploxeroll) from Utah, CaMontmorillonite clay (SAz-1) from Arizona received from the Clay Mineral Society (Dept. of Geology, University of Missouri), and a Pahokee muck from Florida received air-dried from the Agricultural Research Center in Belle Glade, FL. The Eustis, Webster, Lincoln, and Kidman soils were air-dried and passed through a 2-mm sieve prior to use. These sorbents represent a wide range in soil physical and chemical properties (Table I). In one series of experiments, subsamples of the Eustis and Webster soils, SAz-1 clay and Pahokee muck were further treated to be homoionic with Ca2+at pH values of 4.0 and 8.4. To achieve a homoionic state, sorbents were suspended in 50-mL polycarbonate centrifuge tubes with 1.0 N calcium acetate adjusted to the desired pH with acetic acid or sodium hydroxide, centrifuged (Sorvall RC5C centrifuge at 12000 RCF for 20 min), and decanted. This procedure was repeated four times, followed by three washes with 1.0 N CaCl,. Two final washes were done with 0.01 N CaCl,, the solution matrix to be used in the batch experiments. Sorbents were then air-dried and gently ground. The SAz-1 clay was oven-dried to facilitate grinding. Chemicals. The [14C]pentachlorophenol(PCP) used was uniformly ring-labeled (specific activity, 12 mCi/mmol = 444 MBq/mmol), purchased from Pathfinders Laboratories, and had a radiopurity of 99.6% and a chemical purity of >98%. The purity of PCP was verified by reversed-phase liquid chromatographic techniques employing an Alltech C-18 column with a methanol-water mobile phase acidified to pH = 2.0 with HC1, and a Waters Model 450 programmable UV detector (254 nm). Initial PCP concentrations were within the 0.02-5.0 pg/mL range for the studies presented. Calcium acetate, acetic acid, sodium hydroxide, sodium chloride, potassium hydroxide, potassium chloride, calcium chloride, calcium hydroxide, calcium perchlorate, and hydrochloric acid used for the various soil treatments, electrolyte matrices, and pH adjustments were of reagent-grade purity, purchased from Fisher Scientific. The 1-octanol used in the octanol-water experiments was ACS grade, also purchased from Fisher Scientific, and distilled twice with a glass distillation apparatus prior to use. Scinti-Verse I1 scintillation cocktail obtained from 656
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Fisher Scientific was used in the liquid scintillation counting (LSC) assay. Equilibrium Sorption Isotherms. Equilibrium sorption isotherms were measured by the batch-equilibration method (19). All sorption isotherms were measured at room temperature (7' = 22-25 "C). Preliminary time studies with Eustis soil indicated that a 4-h shaking time on a platform shaker at low speed was sufficient to attain equilibrium; however, samples were shaken overnight (16-24 h). A LFER relationship for estimating sorption rate constants as a function of equilibrium sorption constants was presented by Brusseau and Rao (36). Based on their results, the contact time in most of our batch studies was judged to be more than sufficient to attain sorption equilibrium. Exceptions to this might have been the batch studies with Webster soil and Pahokee muck in aqueous solutions a t pH C 3, where the largest sorption coefficients were measured. A longer contact time may have been required to achieve true equilibrium; however, based on the empirical relationship given (36), approximately 75-95% equilibration was achieved. Batch studies at an ionic strength ( p ) of 0.015 were conducted by equilibrating the sorbents with ['*C]- and [12C]PCP-supplemented 0.01 N CaC1, solutions. Glass vials were used for the soils and glass centrifuge tubes were used for the SAz-1 clay samples. All vials and tubes were fitted with Teflon-lined septa inserts in screw-top caps. Amber vials were used when possible to avoid losses via photolysis of PCP; otherwise, samples were routinely kept from light during equilibration. The stock 0.01 N CaCl, solutions were adjusted to various pH values with concentrated HC1 or 1M NaOH in very small amounts, such that changes in ionic strength (p)were negligible. For soils where more than one ionic strength was used (Eustis, Webster, Lincoln, and Kidman), p was varied by changing the CaCl, concentrations. Batch isotherms in mixed solvents were measured for Webster soil at pH < 3 with 30,50, and 70% methanol by volume GL = 0.015), and at pH > 9 with 10,20,30, and 40% methanol by volume (p = 0.015). Note that the observed pK, values increase with methanol content (37). Dey at al. (38)observed a positive shift of 2 units for the pK, of p-nitrobenzoic acid going from 0 to 70% methanol content. Such pK, shifts for pH regions of 9 presented here would not significantly effect the percent ionized species. The ratio of soil mass to solution volume was varied depending on the magnitude of sorption expected to obtain at least a 30% decrease in PCP concentration in the solution phase. Following equilibration, the solution and solid phases were separated by centrifuging the soil samples at 250 RCF in glass vials in a Sorvall RT6000 centrifuge and the SAz-1 clay samples at 2800 RCF in glass centrifuge tubes in a Sorvall5CRC centrifuge. Aliquots (0.5-1 mL) of the supernatant were taken from each sample to which 20 mL of Scinti-Verse I1 was added followed by mixing. The concentration of PCP was then assayed by LSC methods employing a Searle Delta 300 liquid scintillation counter. Soil samples were resuspended and pH was measured with a Corning Model 130 pH meter and a Fisher Scientific or Orion combination microelectrode. The sorption coefficients, K (mL/g), were estimated by fitting the sorption data to a linear isotherm: S, = KC,, where S , and C, are sorbed (pg/g) and solution (pg/mL) concentrations, respectively, at equilibrium. The solution concentrations were directly determined, whereas S, values were determined by difference: S, = (Ci - C,)( VIM), where Ci and C, are the initial and equilibrium concentrations
Table 11. Sorbents, Organic Carbon Contents (OC), and References for the Compiled Literature Data sorbent
Predicted (Eq. 8) Predleted (Eq. 7) I
0
1
2
3
4
5
6
7
8
9
1 0 1 1 1 2 1 3 1 4
PH
Flgurr 1. Sorption data for PCP obtained over a broad pH range for a number of clays and soils and compiled from the literature.
(pg/mL) of PCP in solution, respectively; Vis the solution volume (mL); and A4 is the soil mass (g). The K values determined in binary mixed solvents will be referred to as K,,, The coefficients of determination (r2) values were greater than 0.8 for all sorbents, with most values exceeding 0.98. Octanol-Water Partitioning. Octanol-water partition coefficients (K,) were determined for [14C]PCPin various electrolyte solutions at pH > 10, where PCP is completely ionized. The pH was adjusted with either NaOH, Ca(OH),, or KOH. Ionic strengths (calculated) were then achieved by adding either NaC1, CaC12, Ca(C104),, or KC1. All octanol-water studies were conducted in 25-mL glass Kimax centrifuge tubes with screw tops fitted with Teflon-lined septa. An octanol to aqueous solution volume ratio of 1 : l O was used in all cases. Samples were equilibrated on a rotary shaker for 4 h followed by centrifugation for 25 min a t 250 RCF. Aliquots of 100 pL and 1 mL of the octanol and aqueous phases, respectively, were analyzed for 14C activity as stated previously. The octanol phase was sampled by using a Drummond micropipet with a glass tip. The aqueous phase was sampled after all the octanol was removed by using an Oxford 1-mL pipet with a plastic tip. The pH of each bulk electrolyte solution was measured prior to equilibration and found to have pH values of 110. The initial concentration of PCP in the aqueous phase was 4.43 pg/mL (16.6 pM). There were no measurable differences for quenching and counting efficiencies between the octanol and aqueous phases; therefore, partition coefficients were estimated by calculating the ratio of the 14Cactivity measured (in cpm/mL) in the octanol phase and in the aqueous phase.
Results and Discussion Sorption from Aqueous Solutions. Effects of pH. All of the PCP sorption data collected in this study, combined with data from the literature (34,39-43) are shown in Figure 1as a plot of K , versus pH. It is important to note that much of the literature data that were reviewed are not included here because critical auxiliary information, such as pH and organic carbon content values, was not reported along with the PCP sorption data. References for the literature data shown in Figure 1are given in Table 11. The curves depicting the decrease in K,,p as estimated by eqs 7 and 8 are also shown in Figure 1. The log Koc,n value for the neutral species, needed in both eq 7 and eq 8, was estimated by averaging all of the values available for sorption a t pH
