Comment on “Thermodynamics of Organic Chemical Partition in Soils” SIR In recent papers in Environ. Sci. Technol. ( I , 21, Spurlock and Biggar presented a “general partition model” in order to accommodate both linear and nonlinear partition of nonionic solutes in water to soil organic matter (SOM). Sorption data of several substituted ureas on soils were presented. The relatively high water solubilities of some of these solutes enabled the authors to investigate solute sorption at much lower relative concentrations (ratios of equilibrium solute concentrations to solute water solubilities) than experimentally feasible with less soluble solutes. They showed that when solute relative concentrations were lowered by about 3 orders of magnitude to levels of the sorption coefficients increased by about a factor of 3.5. The enhanced sorption coefficients at low concentrations are attributed to specific interactions of polar substituted ureas with polar humic groups, which are assumed to be most powerful at low concentrations owing to “site”availability. The wide isotherm linearity as commonly found for relatively nonpolar solutes is ascribed to the absence of such specific interactions. In their general partition model, Spurlockand Biggar (1, 2) proposed a Freundlich-type equation to relate the sorbed concentration to solute equilibrium concentration in water at a given temperature as
+
In C$l = [ l / ( m 111 InXy,
+ In Kf
(1)
where d1is the solute volume fraction in the sorbed (humic) phase, Xis the solute mole fraction in water, yw is the solute activity coefficient in water (Raoult’s convention), Kf is a Freundlich coefficient, and l / ( m 1) corresponds to the Freundlich exponent. Setting In 41y1 = In X ~ Wwhere , y1 is the solute activity coefficient in the sorbed phase at 41 (Raoult’s convention), results in
+
In Kf = In
= -In y l *
(2)
where is the solute volume fraction solubility as a subcooled liquid in the sorbed phase and y1* is the corresponding solute activity coefficient. By these relations, the variation of solute activity coefficient y l in relation to c$~ is given as
In y1 = m In #1
+ ( m + 1) In y l *
(3)
and 8 In y l / 8 In C$l = m
(4)
One basic problem with the nonlinear partition model as expressed by eqs 3 and 4 with m > 0 is that it leads to unrealistic consequences. Here, the isotherms are concave downward (on a linear scale) and do not attain linearity in the limit of zero 41;i.e., y 1 will not be constant as the solute approaches infinite dilution. This conclusion is in conflict
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with dilute solution theorythat the solute activitycoefficient must be constant at infinite dilution. Moreover, while In y l * values are positive at the point of saturation for all substituted ureas (suggesting endothermic heats of solution), eq 3 would give negative In y1 at very small 41 (suggestingexothermic heats of solution), This transition, again, cannot be reconciled with ordinary solution theory nor with Flory-Huggins theory (in terms of the Flory parameter x). These unusual characteristics appear to be artifacts of the assumed mathematical model. Although one may find nonlinear solute partition isotherms in certain systems, isotherms in this case are usually concave upward, such as found for solutes exhibiting a large solubilityin solvents or amorphous polymers ( 3 , 4 ) . Under this condition, the compatibility of the solute with solvent or polymer increases with the amount of solute partitioned in the medium. Thus, while fenuron gives a constant octanol-water partition coefficient (Kow)over 3 orders of magnitude in concentration (2),one may expect to find an increase in KOWnear saturation for solutes that are relatively soluble in octanol. The nonlinear isotherms as observed for substituted ureas at low relative concentrations with soils are not in conformity with this type of isotherm nonlinearity; they appear to result from mechanisms other than specific solute-humic interactions. A suggested alternative hypothesis is that there are two simultaneous processes for the sorption of nonionic organic solutes by soil: a linear partition in SOM and a nonlinear (concave-downward) adsorption on soil minerals. In the original Chiou et al. model (5,s), the soil is regarded as a dual sorbent in which the organic matter functions as a partition medium and the mineral matter functions as an adsorbent. Linear isotherms (and other characteristics) as observed for the sorption of relatively nonpolar solutes from water are taken as a basis for the dominance of solute partition in SOM because of the strong suppression by water of mineral adsorption. Previously, this model has not explicitly considered the situation in the limit of extremely low relative concentrations. If one considers the possibility of a small amount of residual mineral adsorption in water solution (particularlyby polar solutes),then one can account both for the predominance of partition in SOM at moderate to high relative concentrations and for the possible predominance of residual adsorption (and hence a small nonlinear sorption) at low relative concentrations. Consider now the effect of solute polarity and relative concentration on the sorption behavior of polar and nonpolar solutes. Mineral adsorption is expected to be more effectively suppressed by water for nonpolar solutes because of their unfavorable polarities for competitive adsorption on polar mineral surfaces; partition would predominate for soils with a significant SOM content and at moderate to high relative concentrations. Note that the relative contribution by linear partition increases rapidly with increasing concentration. On the other hand, the residual mineral adsorption should be relatively more pronounced for polar solutes than for nonpolar solutes.
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Although this unsuppressed residual adsorption may be insigmficant compared with partition to SOM at appreciable relative concentrations, it would predominate over partition at very low relative concentrations to give a nonlinear isotherm (because the linear partition diminishes more rapidly at high dilution than the residual adsorption of a concave-downward shape). This nonlinear sorption may be expected to follow the Freundlich equation. By this hypothesis, one may expect to find similar isotherm nonlinearity at high dilution for other watersoluble polar solutes on soils. For example, the isotherms of 2-chlorophenol and 2,4-dichlorophenol on soils (7) show enhanced sorption coefficients at low relative concentrations but exhibit essentially constant coefficients at moderate to high relative concentrations. Similarly, the relatively high sorption coefficients of aniline and phenol as measured at low relative concentrations (8)may quite possibly include contributions from unsuppressed mineral adsorption, although polar solutes are expected to exhibit greater partition than nonpolar solutes in relatively polar SOM (9). By contrast, the isotherm of benzene on soil is linear over about 2 orders of magnitude below its saturation (5). Thus, the cited increased sorption coefficients of substituted ureas at low relative concentrations result possibly from the predominance of residual mineral adsorption over partition at high dilution. By the same argument, if soils contain small amounts of high-surfacearea carbonaceous materials (e.g.,charcoal) relative to their SOM and mineral contents, nonlinear adsorption of solutes from water on these materials could also predominate over partition in SOM at high dilution because such adsorption would be only weakly suppressed by water. Lastly, a small amount of solute adsorption on glass walls, if not properly corrected, will also contribute to the isotherm nonlinearity at low relative concentrations, while it would have a relatively insignificant effect at high concentrations. In their sorption experiments with substituted ureas, Spurlock and Biggar (2) stated that sorbed phase concentrations were calculated by difference, with
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total extractions performed on selected samples to verify mass balance. Although they indicated that independent experiments showed no measurable sorption to vial walls or septa for fenuron and neburon, it is not clear whether these blanks were carried out in the low concentration region where they would be most critical. At this point, the relative merit of all the alternative mechanisms (including that from Spurlock and Biggar) remains to be settled by further experiments.
Acknowledgments I thank Dr. Frank C. Spurlock for sending me the accepted manuscripts prior to their publication in Environmental Science and Technology. I also thank Dr. Milton Manes, Professor Emeritus, Kent State University, for stimulating discussion of the subject matter.
Literature Cited (1) Spurlock, F. C.; Biggar, J. W. Enuiron. Sci. Technol. 1994, 28, 989. (2) Spurlock, F. C.; Biggar, J. W. Enuiron. Sci. Technol. 1994, 28, 996. (3) Eichinger, B. E.; Flory, P. J. Trans. Faraday SOC. 1968,64,2035. (4) Chiou, C. T.; Lee, J.-F.;Boyd, S. A. Enuiron. Sci. Technol. 1992, 26, 406. (5) Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Enuiron. Sci. Technol. 1983, 17, 227. (6) Chiou, C. T.; Shoup, T. D. Enuiron. Sci. Technol. 1985,19, 1196. (7) Boyd, S A . ; Mikesell,M. D.; Lee,J.-F. In ReuctionsandMouement
of Organic Chemicals in Soil; Sawhney, B. L., Brown, K., Eds.; Special Publication 22; Soil ScienceSocietyof America;Madison, Wi, 1989; Chapter 8, pp 209-228. (8) Briggs, G. G. 1.Agric. Food Chem. 1981, 29, 1050. (9) Chiou, C. T.; Kile, D. E. Enuiron. Sci. Technol. 1994, 28, 1139.
Cary T. Chiou U.S. Geological Survey Denver Federal Center, MS 408 Denver, Colorado 80225 ES940452+
