Environ. Sci. Technol. 4994, 28, 1010-1014
Cosolvency Effects on Sorption of a Semipolar, Ionogenic Compound (Rhodamine WT) with Subsurface Materials Thomas S. Soerens and David A. Sabatini’ School of Civil Engineering and Environmental Science, The University of Oklahoma, Norman, Oklahoma 7301 9
The sorption of rhodamine WT (RWT), a semipolar ionizable fluorescent dye, from methanoljwater and acetonejwater mixtures was investigated using batch and column studies with two natural subsurface media. The utilities of cosolvency, solubility, and sorption theories were evaluated. The effect of cosolvents on sorption of RWT showed significant deviations from the log-linear cosolvency model. The observed cosolvency effects on sorption are qualitatively consistent with that expected for semipolar solutes or for organic acids. However, the quantitative application of either the semipolar cosolvency model or the cosolvency model with speciation could not adequately describe the observed behavior. The magnitude of the cosolvency effect differed for the two subsurface media. This difference can be attributed in part to pH differences but also suggests interactions of the ionic form of RWT with mineral surfaces. This study demonstrates the need for additional research to elucidate the important mechanisms involved in the sorption of semipolar and ionogenic compounds.
utility for all of these applications. Additionally, RWT is a semipolar ionogenic organic compound, and an evaluation of its sorption and cosolvency behavior will assist in understanding sorption processes for semipolar and ionogenic organic contaminants in general and will help develop an adequate conceptual model for the sorption of these compounds. The hypothesis evaluated in this research was that cosolvency effects on sorption of RWT would differ from the log-linear cosolvency model and that these effects could be explained by RWT’s semipolar and ionogenic character. The objectives of this research were to measure the batch sorption of RWT onto subsurface materials in water/ cosolvent mixtures, to evaluate the ability of batch data to predict the sorptive behavior in column studies, to evaluate the utility of the log-linear cosolvency model in describing cosolvency effects on sorption of RWT, and to evaluate the ability of solubility and cosolvency theory to predict cosolvency effects on sorption of RWT.
Theory Introduction The effects of cosolvents on sorption of nonionic hydrophobic organic compounds (HOCs) can generally be predicted by a log-linear cosolvency model proposed by Rao et al. (1) which is based on solubility cosolvency models. This model, however, may be inadequate for predicting sorption of ionic, ionogenic, or semipolar organic contaminants. The sorptive behavior of ionic, ionogenic (ionizable),and semipolar (Le., having a polarity between that of water and a cosolvent) organic contaminants has just begun to be studied. Sorption of ionic and ionogenic compounds is more complex than sorption of nonionic HOCs, and an adequate conceptual model for the sorption behavior of these compounds in the presence of cosolvents has yet to be developed. Conceptual models do not accurately predict the solubility of semipolar solutes in cosolvent mixtures, and the application of these models to sorption is untested. RWT is a semipolar, ionizable fluorescent dye that has seen extensive use as a water tracer in surface water, in karst terrain, and to a lesser extent in groundwater (2-7, 24,26), RWT exists as two isomers with different sorptive tendencies (9). Recently, fluorescent dyes (including RWT) have been suggested as adsorbing tracers for predicting the transport of sorbing contaminants in groundwater (8). Fluorescent dyes have been suggested for use in stored methanol fuel in order to trace subsurface leaks or spills. An understanding of the sorption and cosolvency behavior of RWT is important in assessing its
* Address correspondence to this author at the School of Civil Engineering and Environmental Science, 202 W. Boyd, Rm. 334, Norman, OK 73019. Telephone: (405)325-4273; Fax: (405)3254217; E-mail: sabatinia mailhost.ecn.ouknor.edu. 1010
Environ. Sci. Technol., Voi. 28, No. 6, 1994
For weak organic acids, Lee et al. (10) have shown that because the fraction of neutral organic present, &, for a monoprotic acid can be defined in terms of pH and pK,:
4, = [HAIj([HA] + [A-I) = (1+ lQPH-PKa)-i (I) the overall sorption of a weak organic acid can be described by
K,
zz
Kn4n+ Ki(1- 4J
(2)
where K, and Ki are the sorption coefficients for the neutral and ionic species of the organic acid, respectively. The following log-linear model has been used to describe cosolvency effects on sorption of HOCs (I): log K , = log K, - aufc
(3)
where K (L/g) is the linear sorption partition coefficient, a, is defined as the cosolvency power, f c is the fraction cosolvent, a is a constant (unity if Raoult’s law is obeyed), and the subscripts m, w, and c denote mixture, water, and cosolvent, respectively. Applying the solubility theory to sorption of semipolar, ionic, or ionogenic solutes may not be as accurate as for strictly nonpolar HOCs due to the possible impacts of solvent-sorbent interactions and nonhydrophobic sorption mechanisms (27, 28). The sorption of ionogenic compounds in cosolvent mixtures is affected by speciation changes which occur as pKa rises with increasing cosolvent fraction. The overall sorption from cosolventiwater mixtures can be expressed by incorporating changes in ionic speciation into the log-linear cosolvency model (11):
K , = Kw,,4,Pn + K,,i(l - 4,)Pi 0013-936X/94/0928-1010$04.50/0
(4)
0 1994 American Chemical Society
3
,
5
1
-
-
-
-
Table 1. Sorbent Properties parameter
.4 t,=0.71
organic contenta % TOC by mas8 cation-exchange capacity (Mequiv/100 g of dried soil) median grain size diameter d~ (mm) uniformity coefficient dm/dlo
7-
5 t,=040 -0 5-1 0
0'1
1 04
0'2 0 3 0.5 06 0 7 Acetone Volume Fraction
08
09
where
p, = 10-*"c,jo
0.11 2.1
0.18 2.3
Analysis by ManTech Environmental Technology, Ada, OK. Table 2. Properties of Rhodamine WT molecular formulaa molecular weight0 log Kowb solubility in HzOC( % ) excitlemit wavelengtha (nm) pKad chemical structurea
CaHaOaN2Na 509 -1.33 18-22 558/583 5.1
A-
(5)
The log-linear model has proven accurate in most investigations involving the sorption or solubility of nonpolar HOCs, although deviations from log-linear cosolvency have been noted in some studies (12,13). The observation that semipolar solutes usually show a parabolic shape in the log of solubility versus cosolvent fraction curves indicates that the log-linear equation is not applicable to these systems (14). While deviations from log-linearity can be found in regular solubility theory, Yalkowsky and Roseman (14)point out that the magnitude of the solubility increases are not predicted well in water/ cosolvent systems even though the general shape of the solubility curves can be predicted in a qualitative way by regular solution theory. In both the dielectric constant and the extended Hildebrand solubility approaches (16), the mixed solvent is treated as a linear combination of water and cosolvent
where f c is the fraction cosolvent and P is a measure of polarity (e.g., log KO,). The maximum in the solubility curve is attained when the polarity of the solute (P,) is equal to the polarity of the mixed solvent (14). Rearranging eq 6 with P, = P,, the fraction cosolvent a t which solubility is a maximum, f 0 , can be expressed as
f,, = (P, - P,)/(Pc- P,)
0.070 f 0.008 0.012 f 0.001 2.5 1.9
1
Figure 1. Predicted cosolvency plot for solutes of various polarities in acetonelwater mixtures (fco = [log KOw,*- log Ko,,w]/[log K O , , log K o w w I h
p, = 10-%,Jc
GWS
CRA
(7)
For solutes with a polarity between that of a cosolvent and that of water, fco falls between 0 and 1. Figure 1shows expected solubility curves for solutes of various polarities in an acetone/water mixture. Notice that for solutes of log KO, € -0.24, a maximum is observed in solubility. Assuming that sorption is inversely proportional to solubility, sorption of a semipolar compound would be expected to be a t a minimum a t a cosolvent volume fraction of fco. Materials and Methods
Materials. Two sorbents were used in this study-an alluvial sand sample taken adjacent to the Canadian River (south of Norman, OK) (CRA)and a sample from an outcrop of the Garber-Wellington sandstone aquifer
k H 5 h
(CZH5)ZG a
From ref 6. From ref 24. From ref 25. From ref 9.
Table 3. Solvent Properties
Rohrschneider polarity indexa dielectric constantb solubility parameterb log KWb
water
methanol
acetone
9.0 81.0 23.4 -4.00
6.6 32.6 14.7 -0.77
5.4 20.7 10.0 -0.24
From ref 12. From ref 14.
(GWS) east of Norman, OK. Table 1 lists pertinent parameters of the sample sorbents. The chemical structure of RWT and some of its properties are given in Table 2. RWT used in this study was obtained as a 20% solution (Pylam Products, Inc.; Garden City,NJ). Allsolutions in this study were prepared with a background electrolyte concentration of 0.01 N CaC12. Dyes were analyzed using a Turner Model 10 portable fluorometer according to USGS procedures (17). The emission filters used were 23A and 3-66with excitation filter 5-46, and reference filter 16, and a clear quartz lamp was used. The cosolvency studies used deionized water (with 0.01 N CaC12 added) with various fractions of reagent-grade methanol or acetone. These solvents represent two extreme classes of polarity; methanol is a proton donor, while acetone is a proton acceptor (12).Some properties of the solvents are listed in Table 3. B a t c h Methods. Batch studies were conducted by placing a constant mass of soil (15 g) and a constant volume of RWT solution (30 mL) a t varying concentrations with various fractions of cosolvent in 45-mL reactors with screw caps and shaking overnight (19-22 h) a t room temperature (23 "C). Preliminary studies indicated equilibrium conditions had been obtained. The initial and equilibrium concentrations for each reactor were determined, and the mass of dye adsorbed was determined by mass balance. Previous studies have shown that, with appropriate Environ. Scl. Technol., Vol. 28, No. 6, 1994
1011
controls, all losses could be attributed to sorption (8).RWT concentrations (4-5) for each soil and cosolvent fraction combination were used to calculate avalue of K,,the linear partition coefficient. Duplicates were evaluated for each set of conditions, and chemical and soil blanks were measured for each isotherm study. Solid-liquid separation was conducted by centrifuging for 40 min. The final pH was measured with an Orion 91-06 pH probe. In preliminary studies, nearly complete sorption of RWT onto the two sorbents was observed for methanol fractions of 0.8 and 0.99. This is consistent with the observations of Lee et al. (11). A different soilisolution ratio would be needed to quantify the sorption at these cosolvent fractions (18). Because RWT exists in two isomers that have different sorptive tendencies, changing the soil/solution ratio changes the relative contribution of each isomer to the overall sorption, and a direct comparison of the Kp values at different soil/solution ratios is not valid without separate K, values for the individual isomers. The study was therefore restricted to a range of f c = 0-0.5, which is sufficient to evaluate the hypotheses and achieve the objectives in this study. Column Methods. The laboratory column studies were conducted using CRA material in a Kontes Chromaflex glass column 2.5 cm in diameter and 15 cm in length. RWT solutions of 100 pg/L with acetone fractions of 0,0.3, and 0.5 were used. The column breakthrough and elution were run under saturated conditions with a flow rate of 2.0 mL/min (pore water velocity = 24 cm/h). Chemfluor FEP flex tubing (0.125-in. X 0.063-in.) was used with a Masterflex peristaltic pump. A time-controlled fraction collector was used to obtain discrete samples for fluorescent dye analysis and pH measurement. Before the RWT runs, a conservative tracer (chloride), monitored by an in-line chloride electrode, was run through the column to evaluate the hydraulics of the column. For the column runs with fractions of acetone, water/acetone mixtures at that fraction were pumped through the column for several days, thus preequilibrating the column at the given fraction cosolvent before introducing the RWT. The presence of cosolvent is not expected to significantly change the hydrodynamics of the column (19). Reproducibility of column results for sorption of RWT has been demonstrated by Shiau et al. (9). pKL Measurement. The conditional ionization constants (pK,') for RWT in methanol/water and acetone/ water mixtures were measured at cosolvent fractions of 0, 0.3, and 0.5 by a titrametric method similar to that described by Shiau et al. (9). The method of Rubino and Berryhill (20) was used to calculate adjustments for the effect of the cosolvents on pH; however, the effect was negligible in the range studied ( f c up to 0.5). Results and Discussion
Batch data for each set of conditions were fit to a linear isotherm (q = K,C,;where q is the mass of dye adsorbed per unit mass of sorbent, and C, is the equilibrium concentration of the dye) by least-squares regression. The linear assumption adequately described the data, and there was no observable correlation between nonlinearity and media type or cosolvent. The correlation coefficient (r2) values for the linear isotherms were greater than 0.94 in all cases, with most greater than 0.99. Typical isotherms 1012
Envlron. Sci. Technol., Vol. 28, No. 6, 1994
=~ 0 4-
0
100
300
260
500
400
600
700
Ceq (wil)
Figure 2. RWT batch isotherms (CRA media, methanol) for various cosolvent volume fractions (fc).
GWS-MeOH
-2 6
*
-2 8 I
g 5-
CRA-MeOH
-3
E
-Is)
L
GWS-Ace! x
__0
005
01
015
02
I
CRA-Acet 025
03
035
04
045
05
Cosolvent Volume Fraction
Figure 3. RWT batch sorption cosolvency plot for two cosolvents-acetone (Acet) and methanol (Me0H)-and two mediaGWS and CRA. (Symbols represent data, and lines are model predictions).
are shown in Figure 2. The fitted K , values ranged from 10-3.6 f 10-5.1 L/g to 10-2.5 f 104.2 L/g. As seen in Figure 3, a plot of log K , versus f c is nonlinear with the sorption reaching a minimum at a cosolvent fraction between 0.25 and 0.5. Acetone caused a more significant decrease in sorption than did methanol, and the CRA media was subject to a more significant cosolvency effect than was the GWS media. The difference between acetone and methanol cosolvency effects is consistent with the results of other investigators (13)and is expected based on the different properties of acetone and methanol. The curved shapes of the log K , vs f c plots (Figure 3) are qualitatively consistent with the cosolvency behavior expected for semipolar solutes (14) and for organic acids (11).
Using the reported value of log KO,= -1.33 as a linear measure of polarity for RWT, the values of f c o (fc corresponding to maximum solubility) according to eq 7 would be 0.71 for acetone and 0.83 for methanol. Under simple solvophobic assumptions, ,the sorption of RWT should reach a minimum at these values. The sorption of RWT was observed to reach a minimum at lower values of f c (the observed fco values for sorption would indicate log KO,= -2.8 for RWT according to eq 6), suggesting that there is some other factor increasing sorption with increasing fc. The effect of changes in ionic species due to the rise in pK, appears to be a plausible explanation
Table 4. pH and pK.' Results
PH
@Il
pKL
CRA
GWS
CRA
GWS
0.3 0.5 facet= 0.3 facet= 0.5
4.89 5.11 5.51 5.26 5.94
7.25 7.20 7.16 7.06 7.04
6.29 6.17 6.06 5.91 5.92
0.00435 0.00806 0.0219 0.0156 0.0736
0.0383 0.0801 0.220 0.183 0.512
0 fmeth=
fmeth =
j
0.8
fc
0.7
+ -
I&:+
li
om
0.2-
Table 5. Fitted Parameters from Batch Data
CRA-MeOH CRA-acet GWS-MeOH GWS-acet
K"
Ki
0.49 f 0.15 0.57 f 0.14 0.059 f 0.009 0.069 f 0.009
0.00091 f 0.00037 0.00017 f 0.00015 0.00070 f 0.00028 0.00009 f 0.00003
ffgn
0.1-
an1
3.7 f 2.0 0 9.3 f 5.0 -0.9 f 0.8 1.8 f 0.8 0 6.4 f 1.3 -3.0 f 0.3
for this effect. This effect would cause the overall sorption to exhibit a curved shape in the plot of log K, versus f c . The average pKL and batch pH values and the resulting C$n values at cosolvent fractions of 0,0.3, and 0.5 are shown in Table 4. In water alone, the pH values for both media were well above the pKL, causing RWT to be almost entirely in the ionized form. The GWS media had a pH approximately 1pH unit lower than the CRA resulting in &, values for GWS about 1 order of magnitude higher than for CRA for all cosolvent fractions. At facet = 0.5, pH for GWS is nearly equal to pKL, and thus the fractions of RWT in the neutral and in the ionized forms are nearly the same (& i= 0.5). In the CRA solution at facet = 0.5, more than 90% of the RWT is in the ionized form. Values for the model parameters (abn,aci, Kn, Ki) were fitted for each cosolvent/soil combination, for each cosolvent with the two soils, and for each soil with the two cosolvents using the Levenberg-Marquardt nonlinear regression procedure (21) with various combinations of constraints. The best fit models are shown as lines in Figure 3, and the fitted parameters are given in Table 5 along with their approximate 95 % confidence intervals. Although in this case no quantitative conclusions should be drawn from this procedure due to the small number of degrees of freedom in fitting several parameters with a small number of data points (notice the lack of precision in some of the parameters), it does provide useful information on the general applicability of the model. For methanol, the most significant regression occurred with aui constrained to be equal to zero. For acetone, the model fitting failed to account for all of the increase in sorption with increasing acetone fraction unless a negative value of acri was used with a positive value of au,. This would seem to violate the assumptions of the model. RWT is significantly different from the compounds for which the model was developed in that it is relatively polar and hydrophilic (log KO, = -1.33, solubility > 20%)and that the solubility plot for the ionic and neutral forms are likely to be nonlinear. Thus it is out of the range of compounds for which the models and estimation techniques were developed (the correlations of Morris et al. (22) would give dMeOH = 0.17 and Oacet = -0.84). Note however that for carboxylic acids, relatively hydrophobic compounds, Lee et al. (11) measured the model parameters directly but found that the cosolvency-speciation model did not adequately predict the observed rise in sorption with increasing methanol fraction. It seems apparent that there
In-
[3
m
I
OW
)O
10
Figure 4. RWT column breakthroughcurves for three acetone volume fractions (f,) with CRA media.
Table 6. K,, Values from Column and Batch Tests (CRA Media) KP
KP
fraction acetone
column
batch
(L/g)
(L/g)
column KIK,
batch KIK,
columnibatch
0 0.3 0.5
0.00529 0.00049 0.00110
0.00266 0.00022 0.00053
0.092 0.207
0.083 0.200
1.99 2.21 2.07
are other mechanisms a t work in the sorption of these compounds. In this study, significant differences in the magnitude of cosolvency effects between the two sorbents were observed. The fact that these differences could not be explained completely by pH differences suggests that sorbent-specific sorption mechanisms are active in the sorption of RWT. Sabatini and Austin (8)observed that the sorption of RWT was much higher than predicted based on correlations with KO, and OC, that sorption of RWT was affected by ionic strength and valence of ionic species in solution, and also that sorption still occurred after oxidation of the soil organic fraction. It appears that the mineral surfaces are involved in adsorption of RWT and that the cosolvents affect the mineral surfaces or RWT in some way so as to cause more sorption (or a lesser decrease in sorption) onto GWS material than onto CRA material. The breakthrough curves for the RWT column runs are shown on a semi-log plot in Figure 4. The chloride breakthrough curve for CRA, also shown in Figure 4, is observed to be sigmoidal and complete by 2 pore vols. Breakthrough curves for RWT show a plateau in the curve at CIC, equal to about 0.43-0.48. This plateau is consistent with that observed in other RWT studies (8,9) and shows that the ratio of the two isomers of RWT is not significantly different in acetonelwater mixtures than in water alone. In the breakthrough curves for RWT in 0.3 and0.5 fraction acetone mixtures, the plateau is seen, but it is not as lenghty as in the f c = 0 run. As predicted by the batch experiment results, less sorption is seen for 0.3 and 0.5 fraction acetone mixtures with the f c = 0.3 showing less sorption than f c = 0.5. By integrating the breakthrough curves, Kp values were computed from mass balance. The column Kpvalues and the corresponding batch test values are shown in Table 6. ColumnK, values were about twice the batch K, values. Although typically batch values of Kp are greater than column values (23),Sabatini and Austin (8)observed this same phenomenon for RWT with other media. This Environ. Sci. Technol., Vol. 28, No. 8 , 1994
1013
Table 7. Comparison of Several RWT Studies investigator
K , (mL/g)
K , (mL/g)
Trudgill ( 3 ) 54 1000-1600 Sabatini and Austin (8) 2.7-9.7 1400-3700 present study, batch 2.7 (CRA), 2.8 (GWS) 3924, 23140 present study, column 5.3 7560 present study,K, 494 (CRA), 59 (GWS) 7.56 X lo6, 5.33 X 106
difference can be explained by the fact that RWT exists in two isomers which have different sorptive tendencies but are indistinguishable fluorometrically (9). Table 7 compares the K, and KO,values of this study to those of other studies. KO,values range from 1000 for a soil with high organic content ( 3 )to 23140 in this study for a soil with low organic content (GWS). This large variation suggests that normalizing the sorption coefficient to organic content is not valid in the case of RWT. However, normalizing the K, values for the two media used in the present study gives K,,,, values that are relatively close to one another as seen in Table 7. The extremely high value for K,,,, shows that the neutral form of RWT is very highly sorbed as expected for large neutral organic molecules. Preliminary data suggested that prolonged exposure to acetone changes the sorbent in some way as to greatly increase RWT sorption. This change could be due to speciation changes at the mineralsurface or to the swelling of clays or organic matter. It is interesting that, although the presence of cosolvents is generally expected to decrease sorption, the cosolvent can change the sorbent in a way which increases sorption. Some hypotheses to explain this are mentioned by Lee et al. (111, although they have not as yet been thoroughly investigated and were not investigated as part of this study. Implications of This Research The nearly complete sorption of RWT from solutions with very high fractions of methanol suggests that RWT would not be very useful as a subsurface methanol tracer. RWT undergoes acid dissociation, which is affected by pH and pK, and apparently adsorbs to mineral surfaces. Thus, a simple organic carbon referenced sorption coefficient cannot be utilized. The complexity of predicting RWT sorption makes RWT less promising as an adsorbing groundwater tracer. The sorption of RWT is complex and involves sorbate-solvent, sorbate-sorbent, and solvent-sorbent interactions. The results of this research suggest that care must be taken when making predictions or assumptions about the sorption behavior of RWT or other hydrophobic ionogenic organic compounds in the presence of cosolvents. Additional research needs to be done to elucidate the important mechanisms involved in the sorption of semipolar and ionogenic compounds, and an improved and more detailed model must be developed.
Literature Cited (1) Rao, P. S. C.; Hornsby, A. G.; Kilcrease, D. P.; Nkedi-Kizza, P. J. Environ. Qual. 1985, 14, 376-383. (2) Bencala, K. E.; Rathbun, R. E.; Jackman, A. P.; Kennedy, V. C.; Zellweger, G. W.; Avanzino, R. J. Water Resour. Bull. 1983, 19, 943-950. (3) Trudgill, S. T. Hydrol. Processes 1987, 1, 149-170. (4) Hofstraat, J. W.; Steendiik, M.: Vriezekolk. G.: Schreurs. W.; Broer, G. J. A. A.; Wijnstok, N. Water'Res. 1991, 25; 883-890. Garklavs, G.; Toler, L. G. Open-File Rep.-U.S. Geol. Suru. 1985, NO. 84-856. Behrens, H. in Proceedings of the 5th International Symposium On Underground Water Tracing; Institute of Geology and Mineral Exploration: Athens, Greece, 1986; pp 121-133. Drexhage, K. H. In Dye Lasers, 3rd ed.; Schafer, F. P., Ed.; Springer-Verlag: Berlin, 1990; pp 155-200. Sabatini, D. A.; Austin, T. A. Ground Water 1991,29,341349. Shiau, B. J.; Sabatini, D. A,; Harwell, J. H. Ground Water 1993, 31, 913-920. Lee, L. S.; Rao, P. S. C.; Nkedi-Kizza, P.; Delfino, J. J. Enuiron. Sei. Technol. 1990, 24, 654-661. Lee, L. S.;Bellin, C. A.; Pinal, R.; Rao, P. S. C. Enuiron. Sci. Technol. 1993,27, 165-171. Nkedi-Kizza, P.; Rao, P. S. C.; Hornsby, A. G. Environ. Sei. Technol. 1985,19, 975-979. Zachara, J. M.; Ainsworth, C. C.; Schmidt, R. L.; Resch, C. T. J. Contam. Hydrol., 1988,2, 343-364. Yalkowsky, S. H.; Roseman, T. J. In Techniques of Solubilization of Drugs; Yalkowsky, S . H., Ed.; Marcel Dekker, Inc.: New York, 1981; pp 91-134. Rubino, J. T.; Yalkowsky, S. H. Pharm. Res. 1987,4, 231236. Martin, A.; Wu, P. L.; Adjei, A.; Lindstrom, R. E.; Elworthy, P. H. J. Pharm. Sei. 1982, 71, 849-856. Wilson, J. F.; Cobb, E. D.; Kilpatrick, F. A. Fluorometric Procedures f o r Dye Tracing; United States Geological Survey, Techniques of Water-Resources Investigations, Book 3 (A12); U.S. Geological Survey: Denver, CO, 1986. Roy, W. R.; Krapac, I. G.; Chou, S. F. J.; Griffin, R. A.
Batch-Type Procedures for Estimating Soil Adsorption of Chemicals;EPA/530-SW-87-006-F;U.S. EPA: Washington, DC, 1987. Wood, A. L.; Bouchard, D. C.; Brusseau, M. L.; Rao, P. S. C. Chemosphere 1990,21, 575-587. Rubino, J. T.; Berryhill, W. S. J.Pharm. Sei. 1986,75,182186. Draper, N. R.; Smith, H. Applied Regression Analysis, 2nd ed.; Wiley: New York, 1981. Morris, K. R.; Abramowitz, R.; Pinal, R.; Davis, P.; Yalkowsky, S. H. Chemosphere 1988,17, 285-298. Bouchard, D. L.; Wood, A. L.; Campbell, M. L.; NkediKizza, P.;Rao, P. S. C. J.Contam. Hydrol. 1988,2,209-223. Smart, P. L. NSS Bull. 1984, 46, 21-33. McVoy, C. W. M.S. Thesis, University of Florida, Gainesville, 1985. Smart, P. L.; Laidlaw, M. S. Water Resour. Res. 1977,13, 15-33. Jafvert, C. T.; Westall, J. C.; Grieder, E.; Schwarzenbach, R. P. Environ. Sci. Technol. 1990,24, 1795-1803. Westall, J. C.; Leuenberger, C.; Schwarzenbach, R. P. Environ. Sei. Technol. 1985, 19, 193-198.
Acknowledgments
The authors thank Linda Lee for helpful suggestions and discussions. Kevin Williams is thanked for helping perform the pK,' titrations.
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Environ. Sci. Technol., Vol. 28, No. 6, 1994
Received f o r review April 19, 1993. Revised manuscript received December 14, 1993. Accepted February 25, 1994.' Abstract published in Advance ACS Abstracts, April 1, 1994.
