Page 159 - Partition & Adsorption of Organic Contaminants in Environmental Systems

150 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

the linear partition to SOM (Chiou, 1995; Gustafsson et al., 1997; Chiou et al.,
1998); (3) the availability of compound-specific “internal holes” (or internal
pores) in SOM for adsorption of specific solutes in addition to solute partition
into the water-saturated SOM (Pignatello and Xing, 1996; Xing et al., 1996;
Xing and Pignatello, 1997); and (4) the occurrence of specific interactions for
polar solutes with limited active sites in SOM in addition to solute partition
to SOM, the former effect approaching saturation at lower solute concentra-
tions (Spurlock and Biggars, 1994). Some pertinent experimental data are pre-
sented below to give readers a brief overview of the problem involved.
The sorption data on some reference samples (a soil, peat, and soil humic
acid) from Xing et al. (1996) indicate that the nonlinear sorption tends to be
more pronounced for polar solutes (e.g., atrazine and prometon) than for low-
polarity solutes (e.g., TCE), the data being based on their Freundlich (or log-
log) plots. On the Cheshire fine sandy loam used by Xing et al. (1996), TCE
exhibits essentially no nonlinearity, with concentrations ranging from
-4
-1
<0.1mg/L (<10 in C e/S w) to >100mg/L (>10 in C e/S w). By comparison, the
sorption of phenanthrene on some soils and shales reported by Young and
Weber (1995) exhibits significant nonlinearity at low concentrations. It thus
appears that the extent of nonlinear sorption for nonpolar solutes tends to
depend on the soil source and to be smaller in magnitude relative to that for
polar solutes. In addition, Xing et al. (1996) noted that in the binary-solute
systems a coexisting polar solute (prometon) strongly suppresses the non-
linear sorption of the nominal polar solute (atrazine), whereas a coexisting
nonpolar solute (TCE) exerts only a small suppressing effect. To account for
these observations, Xing et al. (1996) proposed that different sets of
compound-specific internal holes exist in SOM for adsorption of different
compounds in addition to their conventional partition into bulk SOM.
To contemplate on the sources of sorption nonlinearity, Chiou and Kile
(1998) presented extensive sorption data utilizing several polar and nonpolar
compounds on a peat (organic) soil and a mineral soil. The compounds studied
and their physicochemical properties are listed in Table 7.10.To minimize com-
plications from the interactions of polar solutes with minerals, most of the

TABLE 7.10. Physicochemical Properties of Selected Organic Compounds Used for
Detection of Nonlinear Sorption to Florida Peat and Woodburn Soil
Compound Abbreviation S w (mg/L) logK ow pK a
Phenol PHL 87,000 1.46 9.89
3,5-Dichlorophenol DCP 8,050 3.23 7.85
Monuron MON 275 1.98 <-1
Diuron DUN 38 2.68 <-1
Ethylene dibromide EDB 3,520 1.99 —
Trichloroethylene TCE 1,100 2.53 —
Lindane LND 7.8 3.75 —
Source: Data from Chiou et al. (1998) and references therein.