Environ. sci. Technol. 1990, 2 4 , 1869-1875
Buttery, R. G.; Ling, L. C.; Guadagni, D. J. J . Agric. Food Chem. 1968,17, 385-389. Dong, S.; Dasgupta, P. K. Enuiron. Sci. Technol. 1986,20, 637-640. Betterton, E. A.; Hoffmann, M. R. Enuiron. Sci. Technol. 1988,22, 1415-1418. Zhou, X.; Mopper, K. Enuiron. Sci. Technol. 1990, 24, 1482-1485. Kieber, R. J.; Mopper, K. Enuiron. Sci. Technol. 1990,24, 1477-1481. Zhou, X.; Mopper, K. J . Geophys. Res., submitted. Walker, J. R. Formaldehyde, 3rd ed.; R. E. Kriger Publishing: Huntington, NY, 1975. Bell, R. P. Adu. Phys. Org. Chem. 1966, 4, 1-29. Mackay, D.; Shiu, W. Y.; Sutherland, P. R. Enuiron. Sci. Technol. 1979, 13, 333-337.
(23) Carey, F. A. Organic Chemistry; McGraw-Hik New York, 1987. (24) Friant, S. L.; Suffet, I. H. Anal. Chem. 1979,51,2167-2172. (25) Masterton, W. L. J . Solution Chem. 1975, 4, 523-534. (26) Lowe, D. C.; Schmidt, U. J . Geophys. Res. 1983, C88, 10844-10858. (27) Fushimi, K.; Miyake, Y. J. Geophys. Res. 1980, C85, 7533-7536. (28) Penkett, S. A. In Atmospheric Chemistry; Goldberg, E. D., Ed.; Springer-Verlag: New York; pp 329-355. Received for reuiew August 8,1989. Revised manuscript received March 6,1990. Accepted July 24,1990. This work was supported by grants from the Chemical Oceanography Program of the US. National Science Foundation (OCE 86-13940 and OCE 8917709).
Effect of pH and KCI Concentration on the Octanol-Water Distribution of Methylanilines C. Annette Johnson+ and John C. Westall" Department of Chemistry, Oregon State University, Corvallis, Oregon 9733 1-4003 The distributions of aniline, 4-methylaniline, 3,4-dimethylaniline, and 2,4,54rimethylaniline between octanol and water were determined as a function of pH and KC1 concentration in the aqueous phase. The data were interpreted in terms of a multicomponent equilibrium model with anilinium in the water-saturated octanol as free ions and ion pairs. The reactions are BH+ = B
+ H+
B=
K,
KO,
+ C1- = m++ a- K , BH+ + C1- = BHCl K2
BH+
where B represents the organic base, and the overbar denotes a species in the octanol phase. The implications of these results to the use of the octanol-water reference system for organic bases and to the sorbent-water distribution of organic bases in the environment is discussed.
Introduction Several hydrophobic ionizable organic compounds (HIOCs) are among compounds recognized recently as important contaminants of the environment ( I ) . These compounds include both organic acids and bases, which exist in both neutral and ionic forms in aqueous solutions, depending on solution pH: HA = H+ + A(1) BH+ = B
+ H+
(2) where HA represents an organic acid and B an organic base. Consideration of both the neutral species and the ionic species is necessary to understand the transport, fate, and effect of these compounds in the environment (2). Many environmental phenomena involving organic chemicals (e.g., sorption, biological availability, toxicity, etc.) have been related to the octanol-water partition constants of the chemicals. While these relations have been applied extensively for nonpolar compounds (e.g., refs Present address: EAWAG, CH-8600 Duebendorf, Switzerland. 0013-936X/90/0924-1869$02.50/0
3 and 4), relatively little is known about their applicability for ionic (amphiphilic) organic compounds. For the distribution of ionic compounds, the counterion must be taken into consideration. In this paper we address the octanol-water distribution of organic bases and organic cations. In related papers (5-8) we address the octanolwater distribution of inorganic ions and organic anions and illustrate the relation of the octanol-water distributions of organic anions to their sorbent-water distributions, for several environmental sorbents. Application in Environmental Chemistry. The sorption of organic bases to environmental sorbents can be interpreted through four types of mechanisms. (I) hydrophobic partitioning of the neutral organic base: B=B (3) where the overbar represents a sorbed species; (11) hydrophobic-like sorption of the organic base cation, with counter charge provided by a dominant inorganic counterion such as C1- (9): BH+ + C1- + P = YBH+Cl(4) where y represents a site or domain for hydrophobic-like sorption; (111)cation exchange of the organic base cation (BH+) with the dominant inorganic cation on the surface, such as Na+ (9-1 7): BH+ + XNa =
m+Na+
(5) where represents a cation-exchange site; or (IV) formation of a covalent bond between the organic base and the sorbent (18). Evidence for each of these mechanisms exists in the references cited. The results of this study are most directly related to mechanisms 1-111. The octanol-water reference system is of use in understanding the biological effects of organic bases and organic cations as well. Researchers in pharmaceutical and medicinal chemistry (19-21) have reported on the octanolwater distribution of organic bases a t different pH values and have discussed the application of the octanol-water reference system to physiological problems. The scope of this study is confined to the octanol-water distribution of a homologous series of organic bases. Application of these concepts to environmental sorbents and
0 1990 American Chemical Society
x
Environ. Sci. Technol., Vol. 24, No. 12, 1990 1869
Table I. Reactions Considered in the System Water, Octanol, Methylaniline, KCl, and HCl reaction
1. 2. 3. 4. 5. 6.
I. 8.
log K"
B=B BHt = B + Ht BHt + CI- = gt+ BHt + C1- = Ht + Cl- = Ht + Ht + Cl- = HCI KC + C1- = Kt + Kt + C1- = KCI
m
a-
at
KO, Ka K1 K2 KI(HC1) Kz(HC1) KI(KC1) Kz(KC1)
-6.15 -1.34 -7.78 -3.12
"log K from ref 8.
extension to other groups of compounds are discussed in the references cited above and in manuscripts in preparation. Octanol-Water Distribution of Organic Ions. Here we examine the octanol-water distribution of organic bases, specifically aniline and some of its derivatives. These compounds are important byproducts of coal gasification and shale oil extraction. They are important not only as environmental pollutants themselves, but also as representatives of a class of organic bases. The compounds studied were aniline (A), 4-methylaniline (4-MA), 3,4-dimethylaniline (3,4-DMA), and 2,4,5-trimethylaniline (2,4,5-TMA). The water phase contained either 0.05, 0.10, 0.15, or 0.20 M KC1, at pH values between 1 and 8, adjusted with HC1. Thus, the systems described in this paper contain water, octanol, a methylaniline, KC1, and HC1. The reactions that are used to describe the distribution of all of the species in the system are presented in Table I. The distribution of HCl and KCl in the octanol-water system has been discussed in a related paper (8). Partition of the neutral base between octanol and water can be expressed by the conventional octanol-water partition constant, KO,: KO, = [Bl/[Bl
(6)
However, to describe the complete distribution of the base, both the neutral and ionic species, as free ions and ion pairs, must be considered. Thus, we define the concentration distribution ratio, D , by
D = ([E]+
[mt]+ [ m - ] ) / ( [ B ] + [BHt])
(7)
The distribution ratio is approximately equal to the KO, when the ionic species are negligible in both phases.
Methods Materials. The octanol used in these experiments was Analyzed Reagent grade from Baker, used without purification. A comparison between this octanol used as received, this octanol purified according to the method of Karickhoff and Brown (22),and Aldrich Gold Label octanol revealed no detectable differences in the measured distributions. Deionized water from a Millipore Milli-Q system was used. The salts were from EM Science. The HC1 and NaOH Dilut-it standards were obtained from Fisher Scientific. Analytical grade aniline compounds and tetrabutylammonium hydroxide were obtained from Aldrich. Distributions. Distributions were determined in batch mode a t 25 "C in 25-mL Corex centrifuge tubes with PTFE-lined screw caps. All distribution tests were carried out in triplicate over a range of pH values (from 1.5 to 8) and KC1 concentrations (0.05, 0.10, 0.15, and 0.20). 1870
Environ. Sci. Technol., Vol. 24, No. 12, 1990
Tests were carried out initially to determine the necessity of presaturating the two solvents, octanol and water, with each other. It was found that the preequilibration of water with octanol wm not necessary but that significant errors could arise if octanol was not presaturated with water, because the octanol took up a significant volume of water and altered aqueous-phase concentrations. Dilute phosphate buffers, which were used to control the pH of the aqueous phase, were shown not to affect distributions. The procedure to determine the distribution of anilines consisted of the following steps: (i) octanol was saturated with water; (ii) solutions of water-saturated octanol with known concentrations of an aniline compound were prepared; the amount of aniline was set to ensure that both phases would contain at least 60 pM of aniline compound after equilibration; (iii) aqueous solutions with various concentrations of KC1 were prepared; values of pH were adjusted with 0.1 M HC1 below pH 4 and with 0.005 M K,HP04 and 0.1 M HC1 between pH 4 and 8; (iv) a quantity (10 mL) of each of the two solvents was added to a centrifuge tube, and the tube was capped and mixed for 90 min at 25 "C with a Model G24 New Brunswick environmental incubator shaker; the two phases were separated by centrifugation for 5 min at 7500g a t 25 "C (IEC B-20A centrifuge); (v) an aliquot of octanol was removed for back-extraction of chloride into deionized water; (vi) the pH of the aqueous phase was measured; (vii) concentrations of the neutral aniline and the total aniline in both water and octanol were determined. Analysis for Anilines. Aniline concentrations were determined spectrophotometrically with a Hewlett-Packard 8451A diode array spectrophotometer. Absorption maxima of neutral aniline in water were observed at 206, 232, and 282 nm; no maxima above 200 nm were observed for the anilinium ion. Thus, total concentrations of aniline in water were determined by absorbance measurements at 282 nm after addition of 100 pL of 1.0 M NaOH to the aqueous solutions (2.55 mL). Base was added to all standards and blanks as well. In octanol, absorption maxima of neutral aniline were observed at 218,238, and 292 nm; maxima of anilinium were observed at 224 and 272 nm. Neutral species concentrations were determined directly a t 292 nm, and total aniline concentrations were determined after addition of 100 pL of octanol saturated with tetrabutylammonium hydroxide. An estimate of the concentration of anilinium ions was obtained by subtraction of the concentration of neutral species from the total concentration. Analysis for Chloride in Octanol. Chloride concentrations in octanol were determined by back-extraction into water and analysis by ion chromatography. Aliquots of octanol (5 mL) were removed from the equilibrated and separated octanol-water mixtures and placed in clean centrifuge tubes containing 5 mL of deionized water. Equilibration and separation were carried out as previously described, although centrifugation was found not to be essential. The chloride concentration of the aqueous phase in the back-extraction step was determined by ion chromatography with a mobile phase of 8 mM phthalic acid, a Wescan anion column (269.001) and a Wescan Model 213A detector. Quantitation was by peak height. Phosphate concentration was determined simultaneously as a check that the phosphate buffers were not affecting the distribution of the anilines. Determination of pH. The pH values of solutions were determined with a Ross combination electrode (Orion Model 8102) and an Orion Model 701A mV meter. The electrodes were calibrated by incremental addition of 0.01
M HCl to each (0.05, 0.10, 0.15, or 0.20 M) of the KCl solutions. The pH values for calibration were in the range pH 4.5-3.5. Standard potentials and slopes were determined by nonlinear regression from the calibration data, and the log [H+]values of aqueous solutions were determined from the calibration constants. The calibration in this manner results in direct determination of hydrogen ion Concentration, rather than hydrogen ion uctiuity. Activity Coefficients. For the aqueous phase, the activities of H+, K+, C1-, and BH+ were calculated from the Davies equation (8). For the nonaqueous phase, all activity coefficients were set equal to 1. This approximation for the octanol phase is generally good since the ionic strength in that phase never exceeded 0.0006 M. However, the range of applicability of this approximation is still limited, since the dielectric constant of the medium is so low, 8.1 at 25 "C. For example, the logarithm of the activity coefficient calculated from the Debye-Huckel limiting law for these conditions ( I = 0.0001 M, 6 = 8.1, T = 298 K) is log f = -0.15, for a monovalent ion (8). The methods for activity correction used in this study were oriented toward ease of use and an adequate description of the experimental data. A more thorough approach could have been used, but it would have been more cumbersome for general use.
Results and Discussion Dependence of log D o n pH. The effect of pH on the distribution of methylanilines between octanol and water is shown in Figure 1. The logarithm of the concentration distribution ratio is plotted against the logarithm of the hydrogen ion activity for aniline, 4-methylaniline, 3,4-dimethylaniline, and 2,4,5-trimethylaniline in KC1 solutions of four different concentrations: 0.05, 0.1, 0.15, and 0.20 M. A regular dependence of log D on log uH+ is seen, and three domains can be recognized. At high pH, the neutral species, B, is the predominant form of the methylanilines in both the aqueous phase and the octanol. The distribution is independent of pH and aqueous-phase ionic strength, and the value of D is simply the conventional octanol-water partition constant, KO,. At intermediate pH, the protonated species, BH+ is the predominant form in the aqueous phase, while the neutral species is the predominant form in the octanol phase. The value of log D decreases linearly with log uH+ in this domain. At the knee in the curve between the high-pH domain and the intermediate-pH domain, the value of pH is equal to the value of pK, for the reaction BH+ = B + H+ (Table I, reaction 2). At low pH, the protonated form of the methylanilines is dominant in both phases. The value of log D is independent of pH, but does increase with the concentration of KCl. The salt concentration dependence is due to the transfer of the anilinium ion from water to the -octanol phase with the counterion C1-. Both free ions (BH+, C1-) and ion pairs (BHCl) contribute to this effect. Interestingly, the unsubstituted anilinium ion does not appear to enter the organic phase under the conditions of the study, above pH 2 a t an ionic strength of 0.1 M. Chemical Equilibrium Model. The distribution of the methylanilines as a function of pH and KCl concentration, which was described qualitatively above, can be described quantitatively with a two-phase chemical equilibrium model. The model is defined completely by reactions 1-8 in Table I and the material balance equations for all of the components. The constants K,, KO,,K1,and K2 were determined by a weighted nonlinear least-squares optimization procedure,
21-
log
aHt
2r
lI*-"-"h\
1:
-8
-+
-k -; -'4
-3 -2 log aH+
I
I
I
I
I
-7
-6
-5
-4
-3
-I
I
-2
-1
1
log aH+
-'t
-21 -0
I
-7
I
-6
1
1
-5
-4
I
-3 log
I
-2
-1
aHt
Figure 1. Distribution ratios of aniline and methylaniline compounds between octanol and water as a function of pH at I = 0.05 (0), 0.10 (O), 0.15 (X), and 0.20 (+) M KCI. The symbols repesent individually measured data points and the continuous lines represent calculations from the model.
FITEQL 2.0 (23). The data available from these experiments were the activities of H+, K+, and C1- in the water phase, the total concentrations aniline in the water phase, and the total concentrations of aniline and C1- in the octanol phase. The errors in these concentration data were estimated to be 2% for H+ and 1%for K+ and C1-. For the aniline data, error was estimated from the calibration data. The object function was the weighted sum of squares of residuals (i.e., calculated - experimental) of the total aniline in the water phase, total aniline in the octanol phase, and total chloride in the octanol phase and was subject to the constraints of chemical equilibrium (reactions in Table I) and electroneutrality in both phases. Thus, all of the experimental data described above were incorporated into the chemical equilibrium model or the object function. The values of KO,, K,, K1, and K , determined by this method are presented in Table 11;the constants for reactions 5-8 in Table I were determined in a related study (8). The solid lines in Figure 1 were computed from this model. The excellent agreement between the lines and the experimental data support the validity of our interpretation. Environ. Sci. Technol., Vol. 24, No. 12, 1990
1871
Table 11. Equilibrium Constants Determined from Experimental Data in Figure 1" log K , log KO, log K1 1% Kz -5.17 (0.01) 1.40 (0.01) -4.84 (0.03) -0.22 (0.01) 4-MA 3,4-DMA -5.28 (0.01) 1.84 (0.01) -4.15 (0.02) 0.23 (0.01) 2,4,5-TMA -5.09 (0.01) 2.27 (0.01) -3.88 (0.03) 0.69 (0.01) Standard deviation ( n = 25) in parentheses.
However, Figure 1 is not a complete test of the correspondence between the model and the data. The object function of the optimization procedure is based on total aniline in the water phase, and total aniline and total chloride in the octanol phase. Thus, a measure of the goodness of fit may be obtained by the comparison of experimental and calculated values, as illustrated in Figure 2a and b, for total chloride and total aniline in the octanol phase. Calculated total concentrations are within 10% of the experimental values for all experiments in the study. A further independent test of the agreement between the model and the data is the total anilinium concentration in octanol, shown in Figure 2c. These values were determined from independent experimental data that were not used in the parameter optimization procedure. Total anilinium was calculated as the difference between total aniline (determined by UV after addition of base) and neutral aniline (determined by UV before addition of base). A t low concentrations the subtraction of two relatively large numbers causes increased scatter in the data, but the overall agreement is very good. Covariance. The constants given in Table I1 show the standard deviation estimated for the constants. This estimate is obtained from the diagonal elements of the inverse of the normal matrix (24) and shows that the model fits the data well. However, the covariance between two constants, which is given by the off-diagonal elements of the inverse of the normal matrix, is not shown. Whereas the covariance of K , with other parameters is relatively small, the covariance between other constants, particularly between K , and K 2 ,is significant and indicates that other combinations of values can yield an acceptable representation of the data. Therefore, it is important that the constants be used together as a complete set. The covariance is an unavoidable consequence of the range of data that is available for determination of the constants. If the domains of predominance of two species overlap extensively, covariability in the constants results. In this study, the free ions and ion pairs overlap. To decrease the overlap, experiments could be conducted at lower and higher total concentrations, where the free ions and ion pairs, respectively, would be dominant. However, at low concentrations analytical difficulties arise, and a t high concentrations uncertainties about activity coefficients arise. Thus, we accept some covariability in the constants. Speciation in the Water-Saturated Octanol. The multicomponent equilibrium model can be used to illustrate the effect of pH and ionic strength on the speciation of the methylanilines in the octanol phase, as shown in Figures 3 and 4. For the calculations for these figures, the total initial concentration of the 2,4,5-trimethylaniline was set to 1000 pM in the aqueous phase, and equal volumes of the aqueous and nonaqueous phases were assumed; the reactions and constants in Tables I and I1 were used. The calculated fractional distribution of 2,4,5-TMA in water-saturated octanol is shown in Figure 3 as a function of pH and KC1 concentration of the aqueous phase. The 1872
Environ. Sci. Technol., Vol. 24, No. 12, 1990
0
1000
h
I z
Y
500 X
Om
0
-%
C
200
- 0 4
lPm O =
0
It 200
400
Figure 2. Comparison of calculated and experimentally observed concentrations of (a) total CI- in the octanol phase, (b) total methylaniline in the octanol phase, and (c) ionic methyhnilinium in the octanol phase. The model was defined by free H', K', and CI- concentrations in the water phase, the total methylaniline in both phases, and the reactions and constants in Tables I and 11.
figure shows that anilinium predominates over neutral aniline at pH values below 2.5-2, and that the free anilinium ion and the anilinium ion pair are of approximately equal significance. The calculated speciation of 2,4,5-TMA and inorganic ions in water-saturated octanol as a function of pH is illustrated in Figure 4. The water-phase KCl concentration was set to 0.1 M. This figure clearly shows the predominance of ionic methylanilinium species at low pH values. Values of K , and K,. The values of log KO,in Table I1 agree well with established linear free energy relations (25),with A log K = 0.43 per CH, group. The values of log K , determined from the octanol-water distribution (Table 11) are quite similar to those determined in this laboratory by direct potentiometry: -5.04 (4-MA), -5.14
I
'
I
I
I
Table 111. Equilibrium Constants for Reactions 9 and 10 in Water-Saturated OctanoP
4-MA 3,4-DMA 2,4,5-TMA
log ii:,
log E,
4.62 4.38 3.57
-5.08 -5.44 -5.09
Constants calculated from values in Tables I and 11.
0.4
1 BH+ -1
+ CI2
1
log
laq aHt Flgure 3. Calculated distribution of 2,4,5-TMA species in the octanol phase as a function of pH and KCI concentration in the water phase. The species shown are the neutral compound (-), the free ion (- * -), and the ion pair (---). The total initial concentration of 2,4,5-TMA in the water phase was 1000 pM, and equal volumes of octanol and water phases were imposed. The reactions and constants in Tables I and I1 were used.
-
= BHCl
KO,
B BH+
+ CI-
=
Ei+ + Ei-
0 '
-5
1
2
3
log KO, Figure 5. Partition constants of the methylanllinium Ions (CMA, 3,C DMA, 2,4,5-TMA) with CT as a function of compound K,. Data from Table 11. (A) Partition constants for the ion pairs, reactlon 4 In Table I . (B) Partition constants for the free ions, reaction 3 in Table I.
log aHt Flgure 4. Calculated distribution of inorganic and 2,4,5-TMA species in octanol as a function of aqueous-phase pH. The total initial concentration of 2,4,5-TMA in the water phase was set to 1000 MM, and equal volumes of octanol and water phases were assumed. The reactions and constants in Tables I and I1 were used. The waterphase KCI concentration was set to 0.10 M.
(3,4-DMA), -5.02 (2,4,5-TMA). Equilibrium Constants for Reactions i n WaterSaturated Octanol. The formation of ion pairs in octanol can be written according to the reaction
for which the constants are given in Table 111. These values are reasonably similar to those reported for ion-pair formation for alkali-metal chlorides (8)and alkali metals and organic acids (7). The acid dissociation reaction of the organic bases in water-saturated octanol can be calculated from the constants in Tables I and 11:
m+= + H+ ra= Kl(HC1)K,Ko,/Kl
(9)
The values determined for water-saturated octanol (Table 111) are quite close to those determined for the aqueous phase (Table I). This result is perhaps not too surprising
in view of the charge symmetry in reaction 6. Values of K 2a n d K1. One of the key questions with respect to the values of K1 and K2 is the degree of correlation with Kow The values of K2 in Table I1 for the three methylanilines are shown to be directly proportional to K, in Figure 5a: log K2 = log KO, - 1.60. This relationship indicates that, among the methylanilines, the differences in K2 simply reflect the differences in the hydrophobicities of the compounds; there appear to be no differences in the ion pairs over and above those attributable to hydrophobicity. In contrast, the values of K1 in Table I1 for the three compounds appear not to be directly proportional to KO,, as shown in Figure 5b. However, the differences in the correlations in Figure 5a and b may not be significant due to the strong covariability between K1 and K2, as discussed next. Alternative Approaches to Models. In the model discussed above (Tables I and 11), the parameters K,, Kow, K1, and K2were determined directly from the data for each compound, without any constraints on the way the constants should vary among compounds. An alternative approach is to impose linear free energy relationships on KO,, and even K1 and K2, and to determine constants consistent with these constraints. Such constraints reduce the covariability in the parameters. The near-linearity of the plots in Figure 5 and the high degree of covariability among the constants support the use of this alternative approach. Indeed this approach does yield constants that vary slightly from those reported and that do adhere strictly to linear free energy relationships. However, these relationships do not appear to be generally valid for octanol-water partition of ionic species (6, 7); they happen Environ. Sci. Technol., Vol. 24, No. 12, 1990
1873
to apply to those methylaniline compounds that are chemically very similar to each other. Thus, we note that the relationships are applicable for compounds that are quite similar to each other, but do not recommend them for general use. Applications. This study was conducted to characterize the octanol-water distribution of organic bases as a function of pH and ionic composition of the water phase. The results of the study are applicable in many areas of environmental chemistry, as discussed below. In dealing with octanol-water distributions, it is important to recognize that the ionic species of organic acids and bases do enter the octanol phase. This study has shown that, in the octanol-water distribution of methylanilines, the concentrations of methylanilinium ions in octanol are significant compared to those of the neutral methylanilines a t pH values approximately 2 pH units below the pK,, a t KC1 concentrations ranging from 0.05 to 0.2 M. If this result is applicable to other organic bases, as we might expect, and we consider a typical environmental pH range of 5-9, we see that the ionic species could be the dominant species in the octanol phase for a large variety of amines with high pK, values (e.g., pK, 2 7). This result has two important impacts on the use of the octanol-water reference system: (i) if one sets out to determine the octanol-water partition constant for the neutral species of a base, one should be aware that there are conditions under which the concentration of the ionic species in the octanol phase is significant; (ii) if one sets out to determine the octanol-water partition constant of an organic ion, one must consider the nature and concentration of the counterion. Furthermore, this study illustrates how octanolwater distribution can be used to determine the pK, of sparingly soluble compounds. These points are obvious in view of this study, but they have been overlooked in earlier studies. In the introduction, we listed four mechanisms by which organic bases might become associated with particle surfaces: (I) hydrophobic sorption of the neutral molecule, (11) hydrophobic-like sorption of the organic base cation with a dominant anion for counter charge, (111) ion exchange of the organic base cation with a dominant cation on particle surface, and (IV) formation of a covalent bond. The dominant mechanism among these four depends on many factors, particularly the pK, of the base, hydrophobicity of the base, organic carbon content and cationexchange capacity of the sorbent, and the reactivity of the base. A survey of representative literature on sorption of organic bases and organic cations on environmental materials (9-1 7) reveals no universally dominant mechanism-a variety of mechanisms has been invoked to explain results for a variety of materials. In some cases, potentially important mechanisms appear to have been overlooked. An understanding of the mechanism is particularly important for interpolating or extrapolating results to other compounds or other conditions. In the attempt to understand the adsorption mechanism, the energy of adsorption is often broken down into hydrophobic and electrostatic components (2). The hydrophobic component has been characterized for nonionic compounds through complementary octanol-water and sorbent-water partitioning studies. However, similar complementary studies for the hydrophobic component for amphiphilic ions are still rare (7), despite the recent interest in the environmental behavior of amphiphilic ions such as surfactants, dyes, and organic acid and base ions. The results of this octanol-water study are a first step in this direction; with the results of parallel studies with 1874
Environ. Sci. Technol., Vol. 24, No. 12, 1990
environmental sorbents, we will be able to assess the significance of the different adsorption mechanisms (particularly mechanisms I-111), as illustrated below. For example, if mechanism I1 is dominant, can the distribution of the organic ion (and counterion) between the sorbent and water be correlated with the distribution ratio between octanol and water? If mechanism I11 is dominant, to what extent can the sorbent-water distribution of a homologous series of amphiphilic cations (surfactants) be predicted from an electrostatic term, which is constant for the series, and a hydrophobic term, which varies with homologue? Then how does one estimate the magnitude of the hydrophobic term? The study of HIOCs provides an ideal opportunity to estimate the effect of charge, since the neutral and cationic forms are structurally very similar, differing only in the presence of H+. These questions will be addressed in manuscripts in preparation. Brownawell et al. (9) have studied the adsorption of permanently charged organic cations to soils and subsurface materials and found evidence for sorption of the organic cation with an inorganic anion (mechanism 11),as well as sorption by cation exchange (mechanism 111). The extent of mechanism I1 compared to that of mechanism I11 was greater for sorbents with a higher organic carbon contents, as might be expected. Almost 30% of the sorbed amphiphilic cation appeared to be sorbed through mechanism 11, at concentrations far below the CEC, for the sorbent with 2.3% organic carbon. Perhaps ultimately octanol-water partitioning of the organic cations can lead directly to prediction of K's for mechanism 11. As more studies are carried out on sorption and biological effects of amphiphilic ionic and ionogenic compounds (e.g., ionic surfactants and organic acids and bases), researchers will need reference systems, just as they have for nonpolar compounds-this study provides the foundation for the reference system. One final application of this study is in the reactivity of organic compounds. The correlation of partition constants for neutral species and the corresponding ionic species may offer information on charge delocalization in organic ions and solvation of the organic ion in the water-saturated octanol. For example, Schwarzenbach et al. (6) have found that the ratio of the octanol-water distribution ratios of the neutral and ionic forms [log (DHA/DA-)] of a series of nitrophenols correlates well with the Hammett u constant. This sort of information may help to clarify differences in ion-exchange properties of organic cations and differences in reactivity. As far as the environmental chemistry of the anilines themselves, recent work has shown that mechanism IV, formation of covalent bonds, is very important for association of anilines with many sorbents. Thus, while this study is important in the elucidation of partition mechanisms through the use of methylanilines as representative organic bases, the environmental fate of methylanilines per se may depend on other factors as well.
Conclusions This study has shown that, in the octanol-water distribution of methylanilines, methylanilinium as free ions and ion pairs is present in the octanol phase in significant quantities at pH values approximately 2 units below the pK, at KCI concentrations ranging from 0.05 to 0.2 M. For the methylaniline compounds that were studied, the partition constants of the free ions and ion pairs correlated well with the partition constants of the neutral species; however, this relationship is not expected to be seen in
Environ. Sci. Technol. 1990, 24, 1875-1877
general for dissimilar organic bases. Registry NO. 4-MA, 106-49-0; 2,4,5-TMA, 137-17-7;A, 62-53-3; 3,4-DMA, 95-64-7; KC1, 7447-40-7; water, 7732-18-5; octanol, 111-87-5.
Literature Cited (1) Zachara, J. M. Selection of Organic Chemicals for Subsurface Transport; DOE/ER-0206; U.S. Department of Energy, Office of Health and Environmental Research, Ecological Research Division; Washington, DC, 1984. (2) Westall, J. C. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley: New York, 1987; pp 3-32. (3) Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241-248. (4) Schwarzenbach,R. P.; Westall, J. C. Enuiron. Sci. Technol.
(15) Karickhoff, S. W.; Brown, D. S. J. Enuiron. Qual. 1978, 7, 246-252. (16) Brown, D. S.;Combs, G. J.Enuiron. Qual.1985,14,195-199. (17) Zierath, D.; Hassett, J. J.; Banwart, W. L.; Wood, S. G.; Means, J. C. Soil Sci. 1980, 129, 277-281. (18) Simmons, K. E.; Bollag, J. M. Enuiron. Sci. Technol. 1989, 23, 115-121. (19) Ezumi, K.; Kubota, T. Chem. Pharm. Bull. 1980,28,85-91. (20) Sherrer, R. A. In Pesticide Synthesis through Rational
Approaches;Magee, P., Kohn, G. K., Menn, J. J., Eds.; ACS
Symposium Series 255; American Chemical Society: Washington, DC, 1984; Chapter 14. (21) Mayer, J. M.; Testa, B.; van dewaterbeemd, H.; Bornand-Crausaz, A. Eur. J . Med. Chem. 1982,17,461-466. (22) Karickhoff, S. W.; Brown, D. S. Determination of Octano11 Water Distribution Coefficients, Water Solubilities, and Sediment/ Water Partition Coefficients for Hydrophobic Organic Pollutants; EPA-600/4-79/032; U.S. En-
1981, 15, 1360-1367. (5) Westall, J. C.; Leuenberger, C.; Schwarzenbach,R. P. Environ. Sci. Technol. 1985, 19, 193-198. (6) Schwarzenbach,R. P.; Stierli, R.; Folsom, B. R.; Zeyer, J. Environ. Sci. Technol. 1988, 22, 83-92. (7) Jafvert, C. T.; Westall, J. C.; Grieder, E.; Schwarzenbach,
R. P. Environ. Sci. Technol., this issue. (8) Westall, J. C.; Johnson, C. A.; Zhang, W. Enuiron. Sci. Technol., this issue. (9) Brownawell, B. J.; Chen, H.; Collier, J. M.; Westall, J. C. Environ. Sci. Technol. 1990, 24, 1234-1241. (10) Zachara, J. M.; Ainsworth, C. C.; Felice, L. J.; Resch, C. T. Enuiron. Sci. Technol. 1986, 20, 620-627. (11) Zachara, J. M.; Felice, L. J.; Sauer, J. K. Soil Sci. 1984,138, 209-219. (12) Moreale, A.; Van Bladel, R. Soil Sci. 1979, 127, 1-9. (13) Cloos, P.; Moreale, A.; Broers, C.; Badot, C. Clay Miner. 1979, 14, 307. (14) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Wiley: New York, 1974; pp 136-238.
vironmental Protection Agency. Environmental Research Laboratory: Athens, GA, 1979. (23) Westall, J. FITEQL-A Computer Program for Determination of Chemical Equilibrium constants from Experimental Data. Version 2.0. Report 82-02; Department of Chemistry, Oregon State University; Corvallis, OR, 1982. (24) Bevington, P. R. Data Reduction and Analysis f o r the Physical Sciences; McGraw-Hill: New York, 1969. (25) Handbook of Chemical Property Estimation Methods; Lyman, W. J., Reehl, W. F., Rosenblatt, D. H., Eds.; McGraw-Hill: New York, 1982. Received for review April 19,1990. Revised manuscript received July 11, 1990. Accepted July 17, 1990. This research was supported by the Ecological Research Division, Office of Health and Environmental Research (OHER), US.Department of Energy (DOE) under Contract DE-ACOG-76RLO 1830 as part of OHER’s Subsurface Science Program.
COMMUNICATIONS A Model of Humin James A. Rice’ and Patrick MacCarthy Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401
Introduction
Humin is defined as the fraction of humic materials that is insoluble in an aqueous solution at any pH value (1,2). The nature of humin has remained something of an enigma. Despite the fact that it typically comprises 50% or more of the organic carbon in soil (3), sediment ( 4 ) ,and peat (5), it has been the subject of comparatively little research interest. For example, only 409 (4%)of the 10315 citations under the keywords humic acid, fulvic acid, and humin in all volumes of Chemical Abstracts through Vol. 1 0 3 (1985) pertain to humin (6). Furthermore, a substantial portion of these 409 citations do not refer to humin as it has been traditionally defined, and in many cases the term, as used, does not even refer to an extract of a soil, sediment, or similar substrate. The comparative lack of *Present address: South Dakota State University, Department of Chemistry, Brookings, SD 57007-0896. 0013-936X/90/0924-1875$02.50/0
interest in the study of humin may be the result of its insolubility and consequent difficulty of its separation from nonhumic materials. However, in recent years there has been a growing interest in the nature of humin (2,4, 7,8). By definition, humin is obtained as the solid residue that remains after centrifugation of the alkali extract of a humus sample (e.g., ref 1). T o separate the organic components of humin from the inorganic components, the humin is generally subjected to extensive digestion with a mixture of concentrated hydrofluoric and hydrochloric acids (I). As a result of this treatment the inorganic material is decomposed, but the organic constituents are also likely to undergo significant changes. Despite the previously limited interest in the study of humin, several models have been proposed to describe its nature. It has been described as humic and/or fulvic acid complexed to inorganic colloids or clay minerals (9-13), a “high molecular weight Polymer” (I), ya 1ignoProtein” (14), “a melanin” (8,15), or “plant and fungal residues in
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