Environ. Sci. Technol, 1984, 18, 295-297
CORRESPONDENCE Comment on “Partition Equilibria of Nonionic Organic Compounds between Soil Organic Matter and Water” SIR: The article of Chiou et al. ( I ) stated that sorption of nonionic organic compounds on soils from water solution occuxs by equilibrium partitioning into a soil organic phase and that adsorption by minerals is relatively unimportant in wet soils. Evidence presented by Chiou does not prove a partitioning model of sorption to the exclusion of a physical adsorption model. The partitioning model apparently was chosen as a convenient explanation of the effect of soil organic matter on sorption of nonionic compounds and its relation to the sorbate octanol-water partition coefficient. Chiou et al. (1) have assumed without proof that a particular relation between organic content and sorption is valid for all soils. Although there is a correlation between organic content and sorption, considerable variation exists between soils (2, 3). Hamaker et al. ( 4 ) observed that KO, values for soils with high organic content are usually lower than KO, values for “normal” soils and that soils with very low organic content give high KO, values. Shin et al. (5) found that adsorption of DDT on soil extracted with ether or alcohol was considerably enhanced over that for the unextracted soil. Solvent extraction removed selected organic species with little change in total organic content. This change in soil sorption with change in soil organic matter composition is a strong argument against a partitioning model. Adsorption of nonionic organic compounds by soil minerals is not insignificant. MacIntyre et al. (3) and Pierce et al. (6) observed significant adsorption by natural and synthetic clay minerals. Natural sediments whose organic matter has been removed by H202digestion show adsorption ranging from 25% to 150% of that for nondigested sediments (3,5-7). Anomalously high KO, values calculated for soils with very low organic content may be explained by adsorption by mineral phases. Application of the Flory-Huggins theory, or of a partitioning model, to soil organic matter requires several unproven assumptions and is not needed to explain the variation of the measured distribution coefficient with sorbate solubility. Chiou’s relationship of KO,with sorbate solubility provides no information about KO,because only one soil of fixed organic content and composition was studied. The distribution constant-sorbate solubility relation is reflective of the degree of incompatibility of nonionic solutes with water. Compounds of low solubility are excluded from the water structure and associate with surfaces, producing higher distribution coefficients. Chiou et al. (I) use the observed linearity of adsorption isotherms and absence of competitive effects as evidence for a partitioning mechanism. Such behavior is also consistent with a physical adsorption model in which each of the soil minerals and organic phases is considered as a separate adsorbent. Isotherms based on physical adsorption models can have a wide linear range. Isotherms are linear when there is considerable free substrate available to adsorb more sorbate molecules. The sparing water solubility of nonionic organics may limit maximum concentrations to values still within the linear portion of the adsorption isotherm. Noncompetitive sorption of 1,3-di-
chlorobenzene and 1,2,4-trichlorobenzenecan be predicted by a physical adsorption model. MacIntyre et al. (3) observed similar noncompetitive sorption for components of hydrocarbon mixtures on clays and sediments. The clays contained little organic matter so partitioning into an organic phase was unlikely. The affinity for adsorption sites of sorbate molecules of such size and molecular structure must be similar since van der Waals forces predominate. Failure to observe competitive adsorption is therefore not surprising.
Literature Cited Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Enuiron. Sci. Technol. 1983, 17, 227-231. Means, J. C.; Hassett, J. J.; Wood, S. G.; Banwart, W. L. In “Carcinogenis, Polynuclear Aromatic Hydrocarbons”; Jones, P. W.; Leber, P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1979; Vol. 1, pp 327-340. MacIntyre, W. G.; Smith, C. L.; deFur, P. 0.;Su, C. W. Air Force Eng. Seru. Cent., Eng. Seru. Lab., [Tech. Rep.] ESL-TR (U.S.) 1982, ESL-TR-82-06, 1-53. Hamaker, J. W.; Thompson, J. M. In “Organic Chemicals in the Soil Environment”; Marcel Dekker: New York, 1972; Vol. 1, Chapter 2. Shin, Y. 0.; Chodan, J. J.; Wolcott, A. R. J. Agric. Food Chem. 1970,18, 1129. Pierce, R. H., Jr.; Olney, C. E.; Felbeck, G. T., Jr. Geochim. Cosmochim. Acta 1974, 38, 1061-1073. Meyers, P. A.; Quinn, J. G. Nature (London)1973, 244, 23-24.
Wllilam G. MacIntyre,” Craig L. Smith
College of William and Mary Virginia Institute of Marine Science School of Marine Science Gloucester Point, Virginia 23062
SIR: In their argument against the partitioning as the major process of soil sorption from water, MacIntyre and Smith (1)cited (i) the variation of KO, values among soils and (ii) the enhancement of soil sorption with (partial) removal of soil organic content. They thereby suggested that (iii) the adsorptive contribution by soil minerals in aqueous systems is important. They also questioned (iv) the significance of the log K,,-log S , (water solubility) relationship derived by application of the Flory-Huggins theory for solutes in soil humic phase and (v) the criteria for partition equilibria based on isotherm linearity (or, more strongly, failure to observe nonlinearity) and absence of solute competition even at high relative concentrations in soil sorption. In a recent article, Mingelgrin and Gerstl (2) have made similar inquiries to the validity of the partition concept. We have stated quite clearly in our paper (3) that the KO,values of a compound in different soils can vary to some extent since the composition of the humic components is not constant. Another important factor not illustrated before that affects the KO, value is the effect on the water solubility of relatively insoluble solutes by humic components that get dispersed from soil into water (either in dissolved state or in association with colloidal soil particulates) ( 4 , 5 ) ,and which may not be effectively spun
Not subject to U.S. Copyright. Published 1984 by the American Chemical Society
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down even at high gravity. The resulting effect on the apparent KO, can be rationalized as follows. The concentration of the solute in humic-containing water (Ca) differs from that in pure water (C,) by the relation C: = C, + xKC, (1) where x is the mass of the dispersed humic components per unit volume of water and K is the concentration ratio of the solute between the dispersed humic materials and water. The magnitude of x depends obviously on factors such as the organic content and composition of the soil and the water-to-soil ratio used in sorption experiments. K appears to have the same magnitude as Komon the basis of the data of Carter and Suffet (6) and Wershaw et al. (7) for DDT in humic-containing water in comparison with the KO, value of DDT (8). In practically all sorption studies, no distinction has been made between Ca and C,, and Ca has been used as the abscissa scale in the isotherm plot of soil uptake vs. equilibrium concentration in water. The amount sorbed is normally calculated by difference based on Ck. When x is moderately low (say,
