Sorption From Water Solution 731 General Equilibrium Characteristics

We begin by looking at the sorption data for relatively nonpolar organic compounds (solutes), and then look at the data for relatively polar organic compounds, because of some characteristic differences in their behaviors. Whereas the demarcation between polar and nonpolar compounds is not straightforward, polar compounds are considered to be those that possess significant polar groups in their molecular structures, and nonpolar compounds those that contain little or no polar groups. Some common strong polar groups are —OH, —NH2, —COOH, —CO—, and —NO2, as illustrated in Table 5.3. Polar groups enhance molecular interactions of the compounds by polar forces and H-bonding with each other and with other polar compounds. In general, the effect of a polar group in a molecule is more significant for small molecules than for large molecules. Polar organic solutes generally exhibit low partition from water to a water-immiscible (or partially miscible) organic phase relative to nonpolar solutes because the former have a high affinity for water. For instance, as shown in Table 5.3, the logKow (octanol-water) values of highly polar organic solutes are generally <2.

In water solution the sorption isotherms for relatively nonpolar organic compounds on soils or sediments are usually virtually linear, as has been demonstrated in a number of studies (see, e.g., Yaron and Saltzman, 1972; Chiou et al., 1979,1983; Karickhoff et al.,1979;Means et al.,1980;Schwarzenbach and Westall, 1981; Kile et al., 1995). Similar isotherm linearity has been reported for the soil uptake of volatile nonpolar pesticide vapors onto water-saturated soils (Leistra, 1970; Spencer and Cliath, 1970). In some studies where slight isotherm curvatures were shown (see, e.g., Mingelgrin and Gerstl, 1983), the extent of isotherm nonlinearity (either concave upward or downward) appears to be comparable with the normal range of data scatter and hence cannot be distinguished from a linear isotherm. This is especially true in sorption studies of low-organic-content soils when the solute uptake is computed by the difference in solute concentrations in water before and after equilibration; here the uncertainty in detecting small concentration changes is expected to be relatively large. Since the curvatures in allegedly nonlinear isotherms for some relatively nonpolar pesticides (e.g., ethylene dibromide and lindane) (Mingelgrin and Gerstl, 1983) are quite small and show no consistent shape, the results offer no clear evidence for strong solute adsorption over the concentration range studied.

0 400 800 1200 1600

0 400 800 1200 1600

Equilibrium Concentration, Ce (mg/L)

Figure 7.2 Sorption of benzene, 1,3-dichlorobenzene, and 1,2,4-trichlorobenzene from water on Woodburn soil (fom = 0.019) at 20°C. [Data from Chiou et al. (1983). Reproduced with permission.]

Equilibrium Concentration, Ce (mg/L)

Figure 7.2 Sorption of benzene, 1,3-dichlorobenzene, and 1,2,4-trichlorobenzene from water on Woodburn soil (fom = 0.019) at 20°C. [Data from Chiou et al. (1983). Reproduced with permission.]

While in previous studies the linear sorption isotherms were observed for solutes in the low concentration range, such linear isotherms also extend to high relative concentrations (CeISw) for sparingly water-soluble solutes, where Ce is the equilibrium solute concentration and Sw is the solute solubility in water. Figure 7.2 shows typical linear isotherms for the sorption of benzene, 1,3-dichlorobenzene, and 1,2,4-trichlorobenzene from water on a Woodburn soil which contains 1.9% SOM (fom = 0.019) (Chiou et al., 1983). The benzene isotherm is linear with Ce/Sw up to about 0.90. Similar linear isotherms for many halogenated organic liquids on a Willamette silt loam (fom = 0.016) (Chiou et al., 1979) are shown in Figure 7.3, where, for example, 1,2-dichlorobenzene exhibits linearity with CeISw up to 0.95. This wide isotherm linearity together with the dependence of soil sorption on fom is illustrative of solute partition into an organic phase (in this case, SOM) as the dominant sorption pathway. Here the low soil uptake of the low-polarity solutes results from both the low SOM content and the low partition efficiency of the solutes with relatively polar SOM; the isotherms are thus essentially linear rather than concave upward in shape (see Chapter 3, section 3.5). As noted, the sorption capacities (Q) of many of the solutes normalized to the SOM content are <10% of the SOM weight. The weak adsorption of nonpolar solutes on soil minerals may be attributed to the strong competitive adsorption of water for polar mineral surfaces. The normalized Kom values of these halogenated solutes (i.e., Kom = Kd/fom, where Kd is the soil-water distribution coefficient) and their water solubilities at 20°C are given in Table 7.1.

Figure 7.3 Sorption of halogenated organic liquids on Willamette silt loam (fom = 0.016) at 20°C. [Data from Chiou et al. (1979). Reproduced with permission.]
TABLE 7.1. Normalized Sorption Coefficients of Halogenated Organic Liquids from Water on Willamette Silt Loam (Kom) and Corresponding Liquid Solubilities in Water (SJ at 20°C

Compound

Sw (mg/L)

Kom

1,2-Dichloroethane

8450

19

1,2-Dichloropropane

3570

27

1,2-Dibromoethane

3520

36

1,1,2,2-Tetrachloroethane

3230

46

1,1,1-Trichloroethane

1360

104

1,2-Dibromo-3-chloropropane

1230

75

1,2-Dichlorobenzene

148

180

Tetrachloroethene

200

210

Source: Data from Chiou et al. (1979).

Source: Data from Chiou et al. (1979).

Although the idea of solute partition to SOM was suggested earlier by Swoboda and Thomas (1968) as a possible mechanism for parathion uptake by soil from water, it did not gain widespread acceptance because of the lack of other supporting evidence. As a matter of fact, there had been serious misconception about the occurrence of linear sorption isotherms. As noted for sparingly water-soluble solutes and pesticides with soil, the isotherm linearity was thought by many to be a result of solutes' low concentrations in water that restricted the soil adsorption capacity in a low and linear range (Mingelgrin

Figure 7.4 Adsorption of selected halogenated organic liquids from water on Pittsburgh CAL (12 x 40) activated carbon at 20°C. [Data from Chiou (1981). Reproduced with permission.]

and Gerstl, 1983; Maclntyre and Smith, 1984). Although the adsorption of single solutes (and vapors) may be linear at very low relative concentrations (Ce/Sw) (i.e., in the Henry's law concentration region), the observed sorption linearity that extends over a wide range of Ce/Sw (as with the soil uptake from water) should not be confused with the linear range of an overall nonlinear adsorption isotherm. To make this point evident, one may, for example, compare the adsorption isotherms of 1,2-dichlorobenzene, 1,1,1-trichloroethane, 1,2-dichloropropane, and 1,2-dibromoethane on activated carbon (Figure 7.4) with their sorption isotherms on a Willamette silt loam (Figure 7.3). The isotherms of the compounds on activated carbon are linear only at very low equilibrium concentrations relative to their water solubilities, whereas the sorption isotherms on soils show no obvious indication of a curvature even at concentrations approaching saturation.

An important feature associated with the linear sorption of organic compounds to soil is that the molar heat of sorption of the compound is constant, independent of its loading on soil (Chiou et al., 1979). This effect may readily be understood in terms of the calculated (equilibrium) heat of sorption of a compound using its linear isotherms obtained at two temperatures. A schematic plot of the linear isotherms of a compound at temperature T1 and T2 (in Kelvin) is presented in Figure 7.5, with T2 > T1. Let Q be the mass of the compound sorbed by a unit mass of soil (or, more closely, by a unit mass of SOM) and Ce be the concentration in water of the compound in equilibrium with a given Q on soil. Each linear isotherm is assumed to cover a wide range

Water Sorption Isotherm

Ce(A,T1) Ce(A,T2) Ce(B,T1) Equilibrium Concentration, Ce

Figure 7.5 Schematic plot of the linear solute sorption from water by soil (Q) versus the equilibrium solute concentration (Ce) at temperatures T1 and T2, with T2 > T1.

Ce(A,T1) Ce(A,T2) Ce(B,T1) Equilibrium Concentration, Ce

Figure 7.5 Schematic plot of the linear solute sorption from water by soil (Q) versus the equilibrium solute concentration (Ce) at temperatures T1 and T2, with T2 > T1.

of Ce relative to the compound solubility in water at the system temperature. The isotherms are drawn such that the soil uptake at T1 is greater than that at T2, as is usually observed for most organic solutes; however, a reverse temperature dependence may take place if the compound shows abnormal (i.e., exothermic) heat of solution in water over a temperature range, as noted with 1,1,1-trichloroethane (Chiou et al., 1979).

The molar isosteric heat of solute sorption at a given uptake capacity can be obtained by use of the Clausius-Clapeyron equation (4.16). At the capacity QA, for example, the equation gives

DH(Qa ) = -R 'n'C- ^^ T')] (7.1)

where R is the gas constant (8.31J/mol-K) and Ce(A,T2) and Ce(A,T1) are the equilibrium concentrations corresponding to QA at temperature T2 and T1, respectively. The molar heat of sorption at capacity QB [i.e., DH(QB)] can be obtained similarly by substituting Ce(B,T2) for Ce(A,T2) and Ce(B,T1) for Ce(A,T1) in Eq. (7.1). Because the isotherms are linear, one finds that Ce(A,T2)/Ce(A,T0 = Ce(B,T2)/Ce(B,T1) and hence that DH (Qa) = DH (Qb). By repeating the same calculations at other loadings, one thus concludes that the molar heat of sorption is constant and independent of the loading capacity. Because of the linearity of the isotherms, the concentration ratio in Eq. (7.1) is equal to the ratio of the sorption coefficient (the slope of the isotherm) at T1 to that at T2.

The calculated molar heats of sorption for most organic compounds and pesticides on soil in water are generally less exothermic than -DHw and are largely independent of sorption capacities (Mills and Biggar, 1969; Yaron and Saltzman, 1972; Pierce et al., 1974; Chiou et al., 1979,1985). The same is true of the sorption of organic compounds from the vapor phase by water-saturated soils, such as found for ethylene dibromide (Wade, 1954) and lindane (Spencer and Cliath, 1970), in which the heats of sorption are less exothermic than the heats of vapor condensation (-DHv). These results are inherently consistent with the conceived partition uptake of nonionic organic compounds by the soil organic matter of water-saturated soils.

The relation between DH and DHw for soil sorption in aqueous systems can be explained readily by the temperature dependence of the normalized isotherms, as shown in Figure 7.6, using the relative solute concentration (CJSw) as the abscissa. By such a normalized plot, one usually finds a reverse temperature dependence for the sorption of organic solutes to soil (Yaron and Saltzman, 1972; Mills and Biggar, 1969); that is, at a given solute loading on soil,

Relative Concentration, CJSw

Figure 7.6 Schematic plot of the linear solute sorption from water by soil (Q) versus the relative solute concentration (CJSw) at temperatures T1 and T2, with T2 > T1.

Relative Concentration, CJSw

Figure 7.6 Schematic plot of the linear solute sorption from water by soil (Q) versus the relative solute concentration (CJSw) at temperatures T1 and T2, with T2 > T1.

which can be expressed as d ln Sw d ln C, dT > dT

Since one finds that d ln Sw DHw

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