Influences of Mineral Matter Organic Matter and Water

To better understand the sorption of an organic compound to a soil or a natural solid under a particular system condition, it is helpful to have a brief overview of important and unique sorption characteristics in relation to the soil or solid composition and the water content associated with it. The highly heterogeneous nature of soil samples from different geographic sources greatly complicates the resulting sorption of organic contaminants. This made it a formidable challenge for scientists to interpret the wide range of soil sorption data. In earlier studies of pesticide-soil interactions, the soil was generally assumed to be a single adsorbent, or at best a mixed adsorbent of some kind, analogous to other well-defined conventional adsorbents. Although this view reconciled to a large extent the sorptive behavior of organic pesticides on relatively dry soils and minerals, it ran into serious technical difficulties in explaining the sorption data with water-saturated soils. We shall see later that the different sorptive characteristics with relatively dry and water-saturated soils are directly responsible for the change in chemical activity, bioavailabil-ity, and toxicity of contaminants sorbed to the soil.

The adsorptive character of soils and minerals has been illustrated unequivocally in earlier studies on the vapor uptake of chloropicrin (Stark, 1948), ethylene dibromide (Hanson and Nex, 1953; Wade, 1954), and methyl bromide (Chisholm and Koblitsky, 1943) by water-unsaturated soils and minerals, in which the vapor uptake is suppressed by soil moisture. The uptake of parathion and lindane by soils from hexane solution exhibits a similar suppression by soil moisture (Yaron and Saltzman, 1972; Chiou et al., 1985). These observations indicate that organic compounds and water compete for adsorption on initially water-unsaturated soil minerals, which comprise most of the available surface area of the soil solid. Moreover, the isotherms measured for the uptake of pesticides from either the vapor phase or from a nonpolar solvent (e.g., hexane) on water-unsaturated soils and minerals are commonly nonlinear, characteristic of an adsorption process.

In keen contrast to the findings above, the uptake of the same nonionic compounds from water by soils, or from vapor phase by water-saturated soils, displays uniquely different features. Most notably, the extent of soil uptake for given organic compounds shows a strong dependence on the SOM content (Kenaga and Goring, 1980; Means et al., 1980; Kile et al., 1995). The uptake of organic vapors by wet soils displays a similar effect (Wade, 1954; Leistra, 1970). The predominant effect of SOM content in this case is demonstrated by the relative invariance of the sorption coefficients of given organic compounds among soils, or size fractions of soil, when the coefficients are normalized to the SOM content (Karickhoff et al., 1979; Kenaga and Eoring, 1980; Kile et al., 1995). The sorption isotherms of nonionic compounds on water-saturated soils are all relatively linear (Chiou et al., 1979; Karickhoff et al., 1979; Schwarzenbach and Westall, 1981;Sun and Boyd, 1991;Rutherford et al., 1992) and are not strongly temperature dependent, exhibiting only small exothermic heats of sorption (Mills and Biggar, 1969; Spencer and Cliath, 1970; Yaron and Saltzman, 1972; Pierce et al., 1974; Chiou et al., 1979). Moreover, the soil uptake of binary nonpolar solutes from water occurs without a significant competition between the solutes (Schwarzenbach and Westall, 1981; Chiou et al., 1983, 1985) in contrast to the strong competitive effects found in sorption by dry soil from the vapor phase and from nonpolar organic solvents (Chisholm and Koblitsky, 1943; Wade, 1954; Spencer et al., 1969; Mills and Biggar, 1969; Yaron and Saltzman, 1972; Chiou and Shoup, 1985; Chiou et al., 1985; Pennell et al., 1992; Thibaud et al., 1993).

Despite the fact that the relationship observed between soil uptake and SOM content had greatly simplified assessments on the uptake of nonionic organic compounds from water by soils, there was no single widely accepted view on the sorptive mechanism with SOM in the literature before 1979. Prior to that time, one popular view considered SOM as a high-surface-area adsorbent (Bower and Gschwend, 1952; Bailey and White, 1964) capable of adsorbing nonionic organic compounds by hydrophobic interactions (Weed and Weber, 1974; Browman and Chesters, 1977; Mingelgrin and Gerstl, 1983). Such a hydrophobic adsorption concept, however, is not supported by common adsorption criteria and in particular by the observed soil sorption data in aqueous systems. Moreover, the earlier accepted surface area for SOM (550 to 800m2/g), reported by Bower and Gschwend (1952) based on the ethylene glycol (EG) retention method, was later shown to be largely an artifact of the high solubility of polar EG in relatively polar SOM (Chiou et al., 1990,1993). Using some high-organic-content soils (peat and muck) as a model for SOM, the surface area of SOM as measured by the standard BET method (with N2 vapor as the adsorbate) is actually only about 1 m2/g (Chiou et al., 1990; Pennell et al., 1995), which is nearly three orders of magnitude lower than the value assumed earlier.

In attempts to reconcile the inconsistency in reported SOM surface areas, Pennell and Rao (1992) consider that the large difference between the values obtained by the polar-solvent retention method and by the standard BET-N2 method represents the internal surface of the SOM. Although the term internal surface has been used concurrently in surface science, it is well understood there that the internal surface, which extends inward the porous channels of a solid (e.g., activated carbon), is freely accessible to an inert gas such as N2, as noted in Chapter 6. Therefore, if an assumed internal surface is impervious to an inert gas, it is more a reflection of solvent penetration into the SOM solid matrix, as pointed out earlier by Brunauer (1945). As we will see later, the excess uptake of an organic vapor (especially, a polar vapor) over that of N2 gas by SOM is more properly interpreted in terms of the vapor partition.

Along with these unique features for soil uptake from water, we also recall that water vapor exhibits a generally much greater adsorption than an organic (benzene) vapor on various dry minerals, as elucidated in Chapter 6. Based on this disparity, one would then expect water, as a solvent, to strongly suppress the adsorption of an organic solute onto a soil mineral, because the adsorption process is competitive. Thus, the many outstanding features in the sorption of organic solutes from water solution, such as the virtual isotherm linearity and the dependence of the uptake on soil organic content, can only be reconciled readily and logically with the partition-dominated solute uptake by SOM.

7.2.2 Soils as a Dual Sorbent for Organic Compounds

A major advance in the description of the sorption process of organic compounds with soil (or sediment) started with the proposition by Chiou and co-workers (1979, 1981) that the SOM behaves primarily as a partition medium, rather than a conventional solid adsorbent. In addition to the recognized dependence of soil sorption on SOM content, they showed that the sorption of relatively nonpolar solutes from water is essentially linear from low to high relative concentrations (ratios of equilibrium concentrations to solute solubilities) and that the equilibrium heats of sorption for the solutes are less exothermic than their heats of condensation from water. In related studies (Chiou et al., 1983,1985), they also showed that the soil sorption of binary solutes from water exhibits no significant solute competition. The inability of the soil mineral fraction to adsorb nonionic organic compounds from water significantly is attributed to strong dipole interactions of water with minerals, which suppress the adsorption of these compounds on minerals. In keeping with the idea of solute partitioning into SOM, it was observed that solutes (as liquids or supercooled liquids) with higher SOM-normalized sorption coefficients (Kom) or soil-organic-carbon-normalized sorption coefficient (Koc) exhibit generally lower limiting sorption capacities on SOM. By application of the Flory-Huggins model to account for solute solubility in (amorphous) soil organic phase, Chiou et al. (1983) developed a partition equation to account for the magnitudes of the observed sorption coefficients. This analysis led to the recognition that the primary factor affecting the sorption coefficients of slightly water-soluble organic compounds is the solubility of the compounds (as liquids or supercooled liquids) in water. The frequently observed empirical correlation between the normalized sorption coefficient (Kom or Koc) and octanol-water partition coefficient (Kow) of the solutes was recognized to be the consequence that the solute solubility in water is the major determinant of both Kom and Kow values (Chiou et al., 1982b, 1983). The notion that the SOM acts essentially as a partition medium for the organic solute uptake is reinforced by the later finding that the SOM has a low surface area (about 1m2/g) (Chiou et al., 1990), which is far too small to account for solute uptake by SOM by adsorption.

The different characteristics in the sorption of nonionic organic compounds from aqueous and nonaqueous systems are reconciled with the postulate that the soil (or sediment) behaves as a dual sorbent: The mineral matter functions as a conventional adsorbent and the SOM as a partition medium (Chiou et al., 1979,1981,1983,1985; Chiou and Shoup, 1985). The linear isotherms and other characteristics in aqueous systems are attributed to the solute partition into SOM and a concomitant suppression of adsorption on mineral matter by water. The nonlinear isotherms and higher sorption capacities on dry soils are ascribed to adsorption on minerals, which predominates over the simultaneous partition in SOM. The schematic plots in Figure 7.1 depict the relative sorptive effects of mineral matter and SOM of a mineral soil that contains a moderate amount of SOM (say, 1 to 2%). The scales for these two effects are not exact but are drawn to emphasize the dominant role of either the adsorption on minerals (for dry soil) or the partition into SOM (for water-saturated soil). For dry soil, as shown in Figure 7.1«, the much greater nonlinear adsorption with mineral matter than the linear partition with SOM gives rise to a

Richard Rush Rod Nail

Relative Aqueous Concentration, Ce/Sw

Figure 7.1 Schematic plots of contributions by adsorption on soil mineral matter and by partition to soil organic matter to the uptake of an organic compound by a mineral soil. (a) Uptake from vapor phase on dry soil; (b) Uptake from water solution on wet soil.

Relative Aqueous Concentration, Ce/Sw

Figure 7.1 Schematic plots of contributions by adsorption on soil mineral matter and by partition to soil organic matter to the uptake of an organic compound by a mineral soil. (a) Uptake from vapor phase on dry soil; (b) Uptake from water solution on wet soil.

high and nonlinear overall soil sorption (the sum of the two contributions). For water-saturated soil, as shown in Figure 7.1b, the mineral adsorption is sharply suppressed by water and the partition in SOM predominates to produce a relatively low and linear total sorption.

The specific roles of SOM and mineral matter provide the point of departure for understanding the diverse and often seemingly contradictory sorption behaviors of organic contaminants with soils from water solution, from organic solvent solution, and from the vapor phase. These topics and related systems are treated in some detail later. Since there is a continuum of the organic matter content in soil, the analysis and discussion of the sorption data in aqueous systems is restricted to soils (or sediments) having more than 0.1 to 0.2% organic content so that the effect of SOM on contaminant uptake is significant enough to be reliably quantified.

Continue reading here: Sorption From Water Solution 731 General Equilibrium Characteristics

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