Environ. Sci. Technol. IQQO, 24, 647-654
(44) Snyder, L. R. In Separation and Purification; Perry, E.
S., Weissberger, A., Eds.; John Wiley and Sons: New York,
1978; pp 25-75. (45) Valvani, S. C.; Yalkowsky, S. H.; Amidon, G. L. J.Phys. Chem. 1976,80, 829-836.
Received for review March 21,1989. Revised manuscript received October 31, 1989. Accepted December 20,1989. This study was supported, in part, by funds from U.S. EPA Cooperative Agreement CR-81412-01-0. Approved for publication as Florida Agricultural Experiment Station Journal Series No.R-00246.
Cosolvency and Sorption of Hydrophobic Organic Chemicals P. Suresh C. Rao,” Linda S. Lee, and Rodolfo Pinal
Soil Science Department, University of Florida, Gainesville, Florida 326 11-015 1 Sorption of hydrophobic organic chemicals (HOCs) by two soils was measured from mixed solvents containing water plus completely miscible organic solvents (CMOSs) and partially miscible organic solvents (PMOSs). The utility of the log-linear cosolvency model for predicting HOC sorption from solvent mixtures was evaluated. Cosolvent effects of PMOSs on HOC solubility and sorption were compared. Nonpolar PMOSs (e.g., toluene, p-xylene, and TCE) have low aqueous solubilities and when present either as a cosolvent/cosolute in the aqueous phase or as a separate liquid phase do not significantly influence HOC sorption by soils. In contrast, polar PMOSs (e.g., nitrobenzene and o-cresol) have sufficiently high aqueous solubilities that significant decreases in HOC sorption can be measured. The presence of a CMOS increases the PMOS solubility which, in turn, is reflected in increased solubility and decreased sorption of HOCs. Results presented suggest that water-PMOS and sorbent-PMOS interactions need to be considered in predicting the cosolvency of a polar PMOS. Introduction Properties such as solubility, sorption, and transport of organic contaminants usually have been characterized in water and/or dilute electrolyte solutions. Furthermore, most of the available data are for systems involving a single organic contaminant. Such data have enabled the development of semiempirical, phenomenological approaches useful for predicting the environmental behavior of organic contaminants (1-3).At waste disposal/spill sites, where mixtures of multiple solvents and multiple contaminants may exist, such approaches need to be modified to account for the presence of cosolutes and cosolvents. The effect of organic cosolvents on solubility of nonpolar organic solutes was discussed in a previous paper (4).This paper deals with cosolvent effects on sorption of hydrophobic organic chemicals (HOCs) by soils. Mixed solvents are defined here as homogeneous solutions of various completely miscible (with water) organic solvents (CMOSs) and partially miscible organic solvents (PMOSs). Solvents with water plus one or more CMOSs and PMOS, and that form two distinct liquid phases, will be denoted here as biphasic solvents. The effects of CMOSs on solubility and sorption of HOCs, referred to as cosolvency, have received increasing attention in recent years. Yalkowsky and co-workers (5-10)investigated the theoretical and experimental aspects of cosolvency of CMOSs on HOC solubility. Rao and co-workers (11-16) and Fu and Luthy (17,18)extended this approach to study the influence of CMOSs as cosolvents on sorption and transport of HOCs. More recently, Walters et al. (19,20) examined the applicability of the cosolvency theory to explain experimental data for sorption of dioxins by soils from binary mixed solvents. 0013-936X/90/0924-0647$02.50/0
In contrast to work on mixed solvents involving CMOSs, few published data are available for HOC solubility, partitioning, and sorption from mixed solvents involving PMOSs, and from biphasic solvents. Pinal et al. (4)recently examined the cosolvency of PMOSs on HOC solubility in ternary mixed solvents (water-CMOS-PMOS). They concluded, on the basis of cosolvency, that PMOSs may be divided into two broad groups: (a) “conditional” PMOSs, which have low aqueous solubility and exhibit only minor effects on HOC solubility unless large concentrations of a CMOS are also present in the ternary mixture, and (b) “unconditional”PMOSs, which have large aqueous solubilities (in the order of lo4 pg/mL), and increase HOC solubility to a considerable extent. Nonpolar solvents such as benzene, toluene, and trichloroethylene were classified as conditional PMOSs, whereas more polar solvents, such as nitrobenzene and o-cresol, were considered to be unconditional PMOSs (4).In this paper, we investigated sorption of HOCs from binary and ternary mixed solvents with CMOSs as well as conditional and unconditional PMOSs. Sorption of HOCs from biphasic solvents (water-PMOS) was also investigated. Theory Cosolvcncy of CMOSs. For mixed solvents containing more than one CMOS, cosolvent effects on HOC solubility and sorption may be approximated by treating the mixed solvent as a linear combination of the component solvents (7,11,14,15). On the basis of data and theoretical approaches available to date, it can be stated that, with increasing volume fraction cf,) of a CMOS in a binary mixed solvent, HOC solubility increases and sorption decreases essentially in log-linear manner (8,12,14,17-20). Deviations from log-linear behavior are more apparent for some solvents than others, especially at f, > 0.5 (8-10,12,14). The log-linear model (5,ll) forms a basis for quantifying the cosolvency of CMOSs and PMOSs as described below. Decrease in HOC sorption with increasing f, in a binary solvent can be described by (11,18) log Ks,b = log Ks,w - u8,cfc
(1)
where K is the equilibrium sorption coefficient; us,, represents the cosolvency power of the CMOS; the subscripts w, c, and b stand for water, cosolvent (CMOS), and binary solvent, respectively; and s stands for HOC as the solute. For a mixture of water and multiple CMOSs, eq 1 may be extended as follows (11,14,15): log K8,m = log Ks,w - Cus,ifi
(2)
where the subscript m denotes mixed solvent and i the ith cosolvent, and where the other terms are as defined earlier. Rubino and Yalkowsky (8) and Pinal et al. (4) have shown that us,,values can be viewed as the hypothetical partition coefficients for the HOC between a CMOS and
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 5, 1990 847
water. The logarithm of the ratio of HOC solubilities in pure CMOS (SS,,) and in pure water (SS,,) is then equal to us,,: Qs,c
= 1%
(SS,C/SS,W)
(3)
Morris et al. (10) have shown that u’s can be correlated to HOC octanol-water partition coefficient (Pow) values as follows: u = a log Pow +b (4) where a and b are empirical constants unique for a given CMOS. Other properties of CMOSs and HOCs may also be used to estimate Q values (8-10). Cosolvency of PMOSs. The aqueous-phase concentration of a PMOS in a biphasic solvent is at its solubility value (SP,,).The log-linear increase in PMOS concentration of the aqueous phase with the addition of a CMOS can be predicted by ( 4 ) log Sp,b = log sp,w -k
up,fc
(5)
where S is PMOS solubility; the subscript p stands for PMOS, and the other terms have been already defined. Note that the notation used in eq 5 implies that the PMOS is treated as a liquid solute in a binary solvent (CMOSwater). Throughout this paper, whenever a HOC is present, the PMOS will be treated not as a cosolute but as a cosolvent. The cosolvency power of a CMOS for a PMOS is represented by Q ~ , which ~ , can be estimated as (4) Qp,c
= log
(Sp,cdSp,w)
(6)
where the PMOS solubilities in 50:50 (v/v) CMOS-water and Sp,w, respecand pure water are represented by SP,,( tively. Most PMOSs are totally miscible in many CMOSs. Thus, instead of neat solvent solubilities, we use the initial slope, which is based on data in the practical range of 0 < f , 5 0.5, and not the terminal slope (i.e., from neat solvent solubility data) as was the case for eq 3. This choice is based on the recommendation of earlier workers (4-6). Assuming that the cosolvent effects of a PMOS are log-linear, and that the cosolvencies of a CMOS and a PMOS are additive, HOC sorption from a ternary solvent mixture (water-CMOS-PMOS) can be predicted by
1%
Ks,t
= 1% K , , -
(QsJc
+ %,pfp)
(7)
where f p is the volume fraction of PMOS, Q ~ is, the ~ cosolvency power of PMOS for HOC, and the other terms are as defined earlier. Note from eq 5 that the maximum value that fp can reach, denoted as f,,,,, is dependent on f c as follows: fp,max
tSp,w/lOOOP)lO‘pJc
(8)
where p is the PMOS liquid density. For conditional PMOS, fp ;= fp,,,, but this may not be the case for unconditional PMOS. Note that, iff is not negligibly small, f, in eq 7 should be replaced by [(l - f p ) f c ] . value needed in eq 7 can Similar to eqs 3 and 6, the be estimated as Qs,p = 1% ( S s , p / S s , w ) (9) where S , , and S , , are the HOC solubilities in pure PMOS and in pure water, respectively. Impacts of Solvent-Solvent and Solvent-Sorbent Interactions. In interpreting cosolvent effects on solubility and sorption of HOCs, the foregoing discussion focused on solvent-solvent (miscibility), solvent-solute (solubility), and solute-sorbent (sorption) interactions. 648
Environ. Sci. Technol., Vol. 24, No. 5, 1990
Table I. Properties of Soils Used in Sorption Studies
soil Webster Eustis
particle size distrib, % silt clay sand
CEC, cmol/g
OC,
pH
21.2 3.2
6.0 4.7
37.2 3.4
2.23 0.39
40.8 1.3
38.0 95.5
%
Specific water-cosolvent and sorbent-cosolvent interactions were not addressed. Note that as,, and us values in eqs 3 and 9 are estimated from the terminal s g p e of the corresponding log-linear solubilization curves, whereas cP,, values are estimated as the initial slope. As noted earlier, this was done to avoid the problems associated with complete miscibility for most CMOS-PMOS mixtures. For solubilization of HOCs in water-CMOS mixtures, measured data closely follow “ideal” log-linear behavior. Departures from log-linearity can be related, at least in a semiquantitative manner, to bulk solvent properties that reflect changes in the cohesiveness of the solvent mixture (8-10). For mixed solvents containing highly soluble PMOSs as the cosolvents, deviations of measured data from predictions based on the log-linear model are more evident, presumably because of water-PMOS interactions ( 4 ) . The solvophobic theory of sorption is based on the premise that the thermodynamic activity of HOCs in solution is the primary factor controlling their sorption (1, 2, 11) and that sorbent-CMOS interactions are not significant. Adoption of this view would imply that the magnitude of cosolvent effects on HOC solubility and sorption are equal. Literature data (12,15,18,19) indicate that, although this is not quite the case, sorbent-CMOS interactions are apparently of minor consequence. Data will be presented here to assess the impacts of PMOSwater and PMOS-sorbent interactions on HOC sorption. Dissolution of sorbent organic matter fractions by the cosolvent may alter the hydrophobic nature of the sorbent as well as increase the dissolved organic carbon content in the solution phase. However, such effects appear to alter HOC sorption to a small extent in comparison with the effects on HOC activity in the solution phase. Thus, we conclude that CMOS-sorbent interactions are of minor consequence in determining HOC sorption from CMOSwater mixtures. Materials and Methods
Solutes, Solvents, and Sorbents. Nonpolar, polycyclic aromatic hydrocarbons used as sorbates were as follows: biphenyl, naphthalene, anthracene, fluoranthene, and pyrene. In addition, the substituted urea herbicide diuron was also used. Completely miscible organic solvents (CMOSs) used were as follows: methanol and dimethyl sulfoxide (DMSO). The partially miscible organic solvents (PMOSs) employed were as follows: trichloroethylene (TCE), toluene, p-xylene, 1-octanol, chlorobenzene, nitrobenzene, and o-cresol. Properties of these solutes and solvents have been given by Pinal et al. ( 4 ) . Two surface soils, Eustis fine sand from Florida and Webster silty loam from Iowa, were used as the sorbents in this study. Both of these soils have been used in our previous studies of HOC sorption from mixed solvents (12, 13, 25). Selected properties of these soils are listed in Table I. The nonlabeled crystalline compounds used for the sorption studies were all of >99% purity, except for naphthalene and diuron, which were >98% pure. The CMOSs used were all high-purity, HPLC-grade solvents purchased from either Aldrich or Fisher Scientific. Nitrobenzene (certified grade) and o-cresol (purified grade)
were used as received from Fisher Scientific; all other PMOSs were double-distilled, All CMOS-water solutions were prepared on a volume basis with graduated burets. The [14C]anthracene and [14C]diuron used for the sorption studies were uniformly ring-labeled (>98% purity) and were purchased from Sigma (Pathfinders). The specific activities for anthracene and diuron were 13.1 and 12.1 mCi/mM, respectively. The liquid scintillation cocktail used in the radioassays was Scinti-Verse 11, obtained from Fisher Scientific. Sorption from Mixed Solvents. All sorption studies were performed in triplicate and carried out in 1-dr (5-mL) glass vials with Teflon-lined caps. Samples were equilibrated by tumbling on a rotary shaker for at least 24 h at room temperature (23 f 1 "C). Prior to analysis, samples were centrifuged a t a minimum of 300 RCF for 25 min. Batch-equilibration techniques were used to measure sorption of [14C]anthraceneand [14C]diuronby Eustis and Webster soils from 2575 and 50:50 methanol-water mixtures with and without TCE. Sorption of [14C]anthracene and [14C]diuron by Webster soil from 5050 methanolwater mixtures with and without nitrobenzene and chlorobenzene was also measured. In addition, sorption of various nonlabeled (I2C) solute mixtures of diuron, anthracene, naphthalene, biphenyl, fluoranthene, and/or pyrene by Eustis and/or Webster soils was measured from several CMOS/water solutions with and without a dissolved PMOS. The soil to solution ratio was optimized for each soil-solute-solvent combination, to achieve 30-70% reduction in initial concentrations. The headspace was minimized in all batch systems to reduce volatile losses. Sorption from Biphasic Solvents. Sorption studies with biphasic solvents were conducted with [ 14C]diuron on Eustis and Webster soils in aqueous solutions with the following PMOSs: 1-octanol, toluene, p-xylene, and TCE. Companion studies were conducted to measure [14C]diuron sorption from aqueous solutions saturated with a PMOS (but with no excess phase present). Liquid-liquid partitioning of [14C]diuronwas also measured in PMOS-water biphasic solvents. In experiments involving sorption from biphasic solvents, 2 g of Webster soil or 3 g of Eustis soil were equilibrated with 4 mL of 0.01 N CaC12solution and 0.5-1 mL of PMOS. Experiments with similar soil/solution ratios, but with no PMOS phase, were also conducted. Soil samples were equilibrated in 1-dr (5-mL) glass vials equipped with Teflon-lined screw caps. The headspace in these vials was small. The vials were tumbled endover-end on a rotary shaker for -20 h and centrifuged at 1200 RCF for -25 min. Aliquots of the aqueous and PMOS phases were assayed for 14Cactivity as described in the next section. In all of the above experiments, PMOS concentrations in the aqueous phase were not measured, but instead were assumed to be a t the solubility limit. Analytical Methods. Concentrations of the 14C-labeled compounds in aqueous solutions and in the PMOS phase were measured by liquid scintillation radioassay methods as described in an earlier paper (12). A Searle Delta 300 liquid scintillation counter was used to measure 14C activity. Reversed-phase liquid chromatography (RPLC) techniques employed to analyze solution-phase concentrations of the nonlabeled HOCs were identical with those described previously ( 4 ) . Data Analysis. The amount of an HOC sorbed by soil, S , was estimated from the difference in initial and final concentrations in the solution phase. Losses via volatiiization or degradation, as well as HOC sorption on the
container, were found to be negligible, as determined from mass-balance determinations in preliminary studies. The HOC sorption data conformed to the linear isotherm model:
S = K,,,C,
(10)
where S is the equilibrium sorbed-phase concentration, C, is the equilibrium solution-phase concentration, and Ks,m is the sorption coefficient. Least-squares techniques were used to estimate K,, values, and in all cases, the coefficient of determination (9) values were greater than 0.9 and the 95% confidence intervals were generally within 5%. In the analysis of the data for HOC sorption from biphasic solvents, linear partitioning between water and the PMOS phases was assumed; this liquid-liquid partition coefficient, P , was experimentally measured. The amount of HdC sorbed (S)was calculated by using the following mass-balance equations: T = (VpCp+ V,C, + mS) (11)
c, = p p w c w S = ( l / m ) [ T - (V,Cp + V,C,)]
(12) (134
where V, and V , are the volumes of PMOS and water, respectively; C, and C, are HOC concentrgtions in PMOS and water, respectively; m is mass of soil; S is sorbed HOC concentration; and T i s total mass of HOC added. Note that HOC concentrations in both the aqueous and the PMOS phases were measured, to check on the validity of using eq 13a or eq 13b.
Results and Discussion HOC Sorption from Mixed Solvents. In our earlier papers (12-15), the log-linear decrease in sorption coefficient (K,,.,,J with increasing f , in binary, ternary, and quinary mixed solvents consisting of water and several CMOSs was reported. Here, we examine the impact of a solubilized PMOS on HOC sorption from ternary mixed solvents (i.e., water, CMOS, and PMOS). Data for [14C]diuronand [14C]anthracene sorption by Webster and Eustis soils from 2575 methanol-water solutions are shown in Figure l. Isotherms for [14C]anthracene sorption by Eustis and Webster soils from 5050 methanol-water are shown in Figure 2. The presence of 2200 pg/mL TCE in the 257.5 mixture did not appear to have a measurable effect on diuron or anthracene sorption by either Eustis soil or Webster soil (Figure 1). However, with -7300 pg/mL TCE present in the 5050 methanolwater mixture (Figure 2), sorption of anthracene was suppressed; the magnitude of this decreased sorption appeared to be greater for the Webster soil than for the Eustis soil. Sorption isotherms measured for [12C]-and [14C]anthracenewere essentially identical (Figure 2). Data for sorption of [I2C]naphthaleneand [12C]biphenyl,shown in Figure 3, confirm the trends noted for anthracene and diuron. The cosolvent effects of two other PMOSs, chlorobenzene and nitrobenzene, on sorption of [14C]diuronand [ 14C]anthracene by Webster soil from 5050 methanolwater are shown in Figure 4. The PMOS concentrations in the ternary solvents were -5000 pg/mL for chlorobenzene and 10 000 pg/mL for nitrobenzene; these PMOS concentrations are close to the solubility limits in 5050 methanol-water. Sorption of both diruon and anthracene was decreased by -30-40% in the presence of nitrobenzene and chlorobenzene. Isotherms for anthracene N
Environ. Sci. Technol., Vol. 24, No. 5, 1990
649
25
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Oluron Sorptlon 25:75 MothrnokWator
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,,I’
0.05
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”
n
0
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0.01 0.015 Solution Conc. (ug/mL)
0.02
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Figure 1. Equilibrium isotherms for sorption of (A) [ 14C]diuronand (e) [ ‘‘Clanthracene by Eustis and Webster soils from 2 5 7 5 methanolwater mixtures with and without TCE. 0.18
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Solutlon Conc. (ug/mL)
Flguro 2. Equilibrium isotherms for sorption of (A) [“C]- and [’%Ianthracene by Eustis soil and (B) [I4C]- and [‘%]anthracene by Webster soil from 5050 methanol-water mixtures with and without TCE.
and fluoranthene sorption by Webster soil in 5050 DMSO-water with and without o-cresol are shown in Figure 5. The addition of 45 O00 Kg/mL o-cresol resulted in a 50% decrease in sorption for anthracene and fluoranthene. Note that o-cresol concentration was about half 650
Environ. Sci. Technol., Voi. 24, No. 5, 1990
I
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1 1.5 2 Solutlon Conc. (ug/mL) e 4. Equilibrium isotherms for sorption of (A) [12C]diuronand (B) anthracene by Webster soil from 5050 methanol-water mixtures with and without chlorobenzene and nitrobenzene. 0.5
its solubility limit of 96 OOO pg/mL in this solvent mixture. Cosolvent effects of PMOSs on HOC sorption from ternary mixtures are summarized in Table 11. In all cases, the presence of a PMOS resulted in a decrease in HOC sorption. Such decreases ranged from -1% (for anthracene in 5050 methanol-water with TCE) to more than 100% (fluroanthene and anthracene in 5050 DMSO-water
0
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1~
\
Table 11. Cosolvent Effects of CMOSs a n d PMOSs on Sorption
- Anthracane Sorptlon - Web8terSo11 - 50:5DDMSO/watar
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with o-cresol). The magnitude of the PMOS cosolvency on HOC solubility follows the order of increasing solubility or polarity of the PMOSs (4). Results presented in Table I1 suggest that this trend is also true for PMOS effects on HOC sorption by soils. The largest cosolvent effect of a conditional PMOS was -55% decrease in sorption of diuron by Webster soil from 50:50 methanol-water with TCE. Note that fairly high CMOS concentrations are needed to achieve this result. Thus, we may conclude that conditional PMOS are likely to have negligible effects on HOC sorption from predominantly aqueous solutions (f, < 0.3). However, cosolvent effects of TCE on diuron sorption appear to be consistently greater than those on HOCs (anthracene and fluoranthene). This could be an indication of specific solutesolvent interactions that favor diuron solvation by all three solvents (water, CMOS, and PMOS). Additional data are needed to confirm this speculation. Comparison of Cosolvent Effects on Solubility and Sorption. The assumption that the log-linear model is applicable over the entire range of cosolvent contents implies that solvent-solvent interactions are unimportant. The actual effect, or magnitude of cosolvency, of a PMOS can be defined as on solubility (Esolub) Esolub = log (ss,t/ss,b) = fl‘s,pfp (14) where and Ss,brepresent HOC solubilities in ternary and binary solvent mixtures, respectively. The term 0in eq 14 accounts for the deviations arising from any PMOS-water interactions. Note that @ 1indicates that such interactions are unimportant because the us,pvalue, estimated as the terminal slope of a solubilization profile, provides adequate predictions of PMOS cosolvency. An expression analogous to eq 14 can be used to define the effects of a PMOS on HOC sorption (Esorp): Esorp = log (Ks,b/Ks,t) = abus,pfp (15) where Ks,band Ks,t are the sorption coefficients for the
ANT ANT ANT ANT NAP NAP BIP BIP BIP BIP DIU DIU DIU DIU ANT ANT ANT ANT DIU DIU ANT ANT ANT DIU DIU DIU ANT ANT FLU FLU
MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH DMSO DMSO DMSO DMSO
f, 0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50 0.50 0.50 0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50
PMOS
CPMOS,
K,,,,
Pg/mL
mL/g
soil E,
68.3 65.7 7.38 6.32 0.77 0.57 1.00 0.77 0.17 0.15 0.93 0.65 0.39 0.32 11.53 8.10 2.98 2.93 0.97 0.46 7.67 4.90 3.75 0.60 0.42 0.32 6.75 2.93 6.75 2.92
W W E E W W W W E E W W E E W W E E W W W W W W W W W W W W
TCE
2200
TCE TCE TCE TCE TCE TCE TCE
2200
TCE
2200
TCE
2200
TCE
7300
TCE
7300
0.50 0.50
TCE
7300
0.50
ClBz NIT
4800 10400
ClBz NIT
4800 8600
CRE
48000
CRE
48000
3000 3000 3700
0.50
0.50 0.50 0.50 0.50
0.50 0.50 0.50 0.50
0.016 0.067 0.130 0.113 0.054 0.155 0.085 0.153 0.007 0.324 0.194 0.310 0.154 0.273 0.362 0.363
a Abbreviations: ANT, anthracene; MeOH, methanol; W, Wester soil; E, Eustis soil; TCE, trichloroethylene; NAP, naphthalene; BIP, biphenyl; DIU, diuron; ClBz, chlorobenzene; NIT, nitrobenzene; DMSO, dimethyl sulfoxide; CRE, o-cresol; FLU, fluoranthene.
binary and ternary solvent mixtures, respectively, with a being an empirical constant. Note that, in eqs 14 and 15, the term E is a measure of cosolvency of a PMOS in a ternary mixture. It is defined such that E 1 0. This was done to facilitate a direct comparison between cosolvent effects on solubility and on sorption. However, it should be realized that cosolvency usually implies an increase in solubility, and a decrease in the sorption of HOCs. The term a in eq 15 is an empirical parameter that accounts for the interactions between the PMOS and the sorbent, whereas 0corrects for PMOS-water interactions. There are, therefore, two necessary conditions for using the terminal slope ( c T ~ , to ~ ) predict cosolvent effects of a PMOS on HOC sorption: (a) negligible PMOS-water interactions (0= l),and (b) negligible sorbent-PMOS interactions (a = 1). A plot of measured values for E (eqs 14 and 15) versus fp, along with a calculated “reference” line (with a = 0 = 1)can be used to distinguish between the cosolvent effects of a PMOS on HOC solubility and sorption. As can be seen in Figure 6, for anthracene solubility and sorption in DMSO-water mixtures, E values increase linearly with increasing volume fraction of o-cresol over the range 0 5 fp 5 0.19. The particular solvent mixtures in this case are 50.50 DMSO-water for Webster soil and 2575 DMSO-water for Eustis soil. The slopes of the two lines for anthracene solubility and sorption, however, differ considerably from each other and from the “reference” line, indicating that a > 1 and @ > 1. By use of a value of 4.87 for us,p(based on the terminal slope of the solubilization profile), the calculated values of @ and a are 1.64 and 1.5, respectively. Therefore, it may be concluded that both Environ. Sci. Technol., Vol. 24, No. 5, 1990 651
1.6
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Anthracene Solublllty and Sorptlon
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7 Toluene
0 w/phase
-
A
p-Xylene
1 TCE
'
whhare w/phase
i
~~
WEBSTER SOIL
f
-4
EUSTIS SOIL
____ I
0 0
0.05
I
1
01
0 15
0 0.2
0
2
4
PMOS Volume Fraction (fp )
Figure 6. Cosolvency of 0-cresoi on the solubility and sorption of [ '*C]anthracene in dimethyl sulfoxide-water mixtures.
water-PMOS and soil-PMOS interactions are important in determining the cosolvency of o-cresol. Preferential solvation of unconditional PMOSs by water has been suggested ( 4 ) as a possible explanation for the fact that cosolvency of unconditional PMOS is greater than expected (i.e., p > 1)from the log-linear model. Recall that CMOS-sorbent interactions play only a minor role in influencing HOC sorption and that, for several CMOSs, a values less than unity have been reported (12, 18, 19). With respect to the effect of a PMOS on the sorbent, it is possible for the PMOS to become incorporated into the soil in sufficiently high amounts to actually modify the polarity of the soil organic matter. Such an effect can be expected from a PMOS but not from a CMOS. If we consider log Pow of the cosolvent as an index of its polarity, most CMOSs have a log Pow on the order of -0.5, whereas several unconditional PMOSs have log Powvalues ranging between 1.5 and 2 (e.g., 1.92 for o-cresol). Therefore, the tendency for an unconditional PMOS to be sorbed by soil organic matter will be from 100 to 300 times greater than for a CMOS. Furthermore, if we consider that the organic matter content of the two soils used ranges from 0.39% (Eustis) to 2.23% (Webster), it is quite possible that the sorbed PMOS can act as a "cosolvent" in the organic phase. Since octanol (log Pow= 3.15) mimics organic matter quite well, we can expect that a sorbed unconditional PMOS with log Pow< 3.15 will effectively increase the sorbentphase polarity and, thus, will likely further suppress HOC sorption. Most of the foregoing discussion, and that in our earlier papers (12-15), suggest that cosolvency of CMOSs and PMOSs leads to increased solubility and decreased sorption of HOCs. This is indeed true for nonpolar organic solutes in mixtures of polar solvents, particularly water and CMOSs. Cosolvent effects on ionizable organic solutes can also be described by the log-linear model (18, 21, 22). Recent studies have shown that with increasing methanol content decreases in sorption of weakly acidic organic solutes, such as a-naphthol (18)and pentachlorophenol (21), are also log-linear. Decreases in the sorption of organic bases, such as quinoline, from mixed solvents can also be described by the log-linear model, as shown recently by Zachara et al. (22). HOC Sorption from Biphasic Solvents. Rao and Lee (14)measured the sorption of two herbicides, terbacil and atrazine, on Webster soil from biphasic solvents (water plus n-pentane or toluene as the PMOS). They reported that identical sorption isotherms were obtained with and without a PMOS present as a separate liquid phase. Aqueous solubilities of n-pentane and toluene are 800 and 505 Kg/mL, respectively, (24). Therefore, for both of these 652
Environ. Sci. Technol., Vol. 24, No. 5, 1990
6
8
10
Solutlon Conc. (us/mL)
Figure 7. Equilibrium isotherms for sorption [ "C]diuron by Webster soil from aqueous solutions saturated with several PMOSs, and from biphasic solvents.
Dluron Sorptlon/Euatlr SOH
' J
0 Blphaslc (TCE)
10
onv 2
4
0
8
,
10
0.2 '
'
12
0 .
14
Solutlon Conc. (ug/mL)
Figure 8. Equilibrium isotherms for sorption of ["C]diuron by Webster soil from biphasic (wetting phase indicated in parentheses) and monophasic solvents. The inset shows data points in the low concentration range (C, < 0.4 kg/mL).
PMOSs, the aqueous-phase concentrations are mole fraction. This is not sufficiently large to have a measurable impact on HOC sorption, as suggested by earlier workers (2, 4, 26). Sorption of [14C]diuronby Webster and Eustis soils was measured from dilute aqueous electrolyte solutions (0.01 N CaC1,) saturated with a PMOS, and from several biphasic solvents. The PMOSs used were 1-octanol, toluene, p-xylene, and TCE, all classified as conditional PMOS ( 4 ) . The sorption data are shown in Figure 7. Sorption of [14C]diuronby Eustis soil was measured from 0.01 N CaC12 saturated with TCE, and also from a biphasic solvent (0.01 N CaCl,-TCE). These experiments were repeated with [14C]diuron being added to air-dry soil either in aqueous solution or in TCE. The measured sorption isotherms, shown in Figure 8, suggest that [14C]diuronsorption from aqueous and biphasic solvents was similar. Also, changing the initial wetting solvent from 0.01 N CaClz to TCE had no influence on the measured equilibrium sorption isotherms. However, preliminary studies did suggest that the time required to attain sorption equilibrium was longer for the biphasic solvents compared to the aqueous solutions. The reasons for this effect are unknown, and further investigations are currently underway. Results in Figures 7 and 8 confirm our previous findings (14) that the presence of a conditional PMOS, either dissolved in the aqueous phase or present as a separate liquid phase, did not influence HOC sorption by Eustis and Webster soils. This is not likely to be the case for unconditional PMOS. When an excess of an unconditional PMOS is present, its aqueous-phase concentration will be
at the solubility limit. Results presented in earlier sections suggest that such PMOS concentrations are large enough to have a cosolvent effect on HOC sorption. Evaluation of Other Effects of PMOSs. Rao and Lee (14) suggested that some PMOSs may also coat the sorbent and provide an additional sink (or sorptive volume) for HOCs; thus, the presence of a PMOS may result in an apparent increase in HOC sorption by a soil. Assuming that the PMOS coatings behave similarly to the bulk PMOS, no apparent effect would be observed if the sorbed concentrations were calculated on the basis of the solution concentration and the initial mass of solute. However, an apparent increase would be observed if the amount of solute sorbed were directly measured by extracting the sorbed HOC from soil. In order to qualitatively assess the likelihood of PMOS coating the soil, a simple experiment was conducted with two dyes: sudan red (a strongly hydrophobic dye) and methylene blue (a cationic dye). The PMOSs used in this study were TCE, 1-octanol, and o-cresol. Sudan red was dissolved in a PMOS and methylene blue was dissolved in an aqueous solution. The aqueous solubility of sudan red and the solubility of methylene blue in the PMOSs were small. The PMOSs with the red dye were added to air-dry Eustis soil, followed by the addition of aqueous solution with the blue dye. If the soil samples are colored dark red, PMOS coatings on the soil surfaces could be surmised. However, in all cases, the soil was noticed to acquire a dark blue color within a few minutes, similar to the behavior observed when aqueous solutions with the blue dye were first added to the air-dry soil. These results suggest that water effectively displaces the PMOS from the sorbent surfaces, and that HOC sorption by the soil is apparently from a water-soil interface. Further experiments are needed to confirm these observations and to preclude the possibility that small patches of PMOS coatings remained on the soil. Rao and Lee (14) also discussed the effects of microemulsified PMOS, in which case the observed aqueousphase HOC concentration would be overestimated due to the HOC associated with the microemulsified PMOS in the aqueous phase. This results in an apparent decrease in sorption in the presence of PMOS. These effects will become increasingly more important at higher microemulsion concentrations and/or large solvent-solvent partition coefficients. Note that the addition of a CMOS will reduce the surface tension of the aqueous phase (81, thus increasing the possibility of microemulsion formation. The presence of PMOS emulsions in the CMOS-water mixture could lead to erroneous evaluation of the cosolvency of the PMOS. Our data suggest that this is unlikely, at least for the CMOSs and PMOSs we examined. Most of the PMOSs used in this study absorb in the UV spectrum. The presence of even small amounts of a microemulsified PMOS in the analyte solution from sorption experiments would dramatically alter the chromatograms obtained; the PMOS peak should appear much larger than that corresponding to the saturation concentration. Such effects were not observed in the present study, however, suggesting that the formation of PMOS microemulsions was relatively unimportant. Summary
Cosolvency of several PMOSs on sorption of HOCs from ternary solvents (water-CMOS-PMOS) were examined in a series of batch experiments. Conditional PMOSs had a minor influence on HOC sorption, unless large PMOS concentrations were maintained by increasing the CMOS content. The exception to this was diuron sorption; spe-
cific solute-solvent interactions (which increase solubility) were proposed as likely reason for this result. Cosolvency of the unconditional PMOSs on HOC sorption was greater than for conditional PMOSs; such effects were even greater than cosolvent effects observed on HOC solubility, suggesting that PMOS-orbent interactions play a significant role. HOC sorption from aqueous solutions saturated with a conditional PMOS and from biphasic solvents (waterPMOS) were similar. This suggests that the presence of the conditional PMOS as a separate liquid phase had minimal impacts on HOC sorption. Increase in HOC solubility in the presence of cosolvents is reflected by decreased sorption by soils and increased mobility of HOCs. The cosolvency of PMOSs on the fate of hydrophobic contaminants is expected to be most pronounced in the “near-field” region of waste disposal sites, where a variety of cosolvents are present in high concentrations. The same properties that cause unconditional (polar) PMOSs to have greater cosolvency than conditional (nonpolar) PMOSs also lead to their lower sorption and greater mobility in soils. Significant concentrations of unconditional PMOS may therefore be found a t considerable distances from disposal sites; hence, the effects of PMOSs on solubility and sorption of HOCs may need to be considered in “far-field” regions as well. Glossary
empirical constants equilibrium solution concentration (pg/mL) completely miscible organic solvent HOC concentration in PMOS (pg/mL) HOC concentration in aqueous phase (pg/mL) expected magnitude of cosolvency observed magnitude of cosolvency on solubility observed magnitude of cosolvency on sorption volume fraction of CMOS volume fraction of PMOS hydrophobic organic chemical sorption coefficient in binary solvent mixture bL/d sorption coefficient in mixed solvent (mL/g) sorption coefficient in ternary solvent mixture (mL/g) mass of sorbent (8) partially miscible organic solvent octanol-water partition coefficient PMOS-water partition coefficient HOC sorbed concentrations (pg/g) PMOS solubility in a binary solvent mixture bg/mL) PMOS solubility in 50:50 water-CMOS mixture (pg/mL) PMOS solubility in water (pg/mL) HOC solubility in neat CMOS (pg/mL) HOC solubility in neat PMOS (pg/mL) HOC solubility in ternary solvent mixture (pg/mL) HOC solubility in water (pg/mL) total HOC mass (bel volume of PMOS ( m ~ ) volume of water (mL) emipirical constants PMOS liquid density (g/mL) cosolvency power of CMOS for PMOS cosolvency power of CMOS for HOC cosolvency power of PMOS for HOC Registry NO.BiP, 92-52-4;NAP, 91-20-3;ANT, 120-12-7; FLU, 206-44-0; Diu, 330-54-1; MeOH, 67-56-1; DMSO, 67-68-5; TCE, 79-01-6; ClBz, 108-90-7; NIT, 98-95-3; CRE, 95-48-7; 1-octanol, 111-87-5; toluene, 108-88-3;p-xylene, 106-42-3. Literature Cited (1) Karickhoff, S. W. Chemosphere 1981, 10, 833-846. Environ. Sci. Technol., Vol. 24, No. 5, 1990
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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
