Page 119 - Partition & Adsorption of Organic Contaminants in Environmental Systems

110 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

co-workers (1979, 1981) that the SOM behaves primarily as a partition
medium, rather than a conventional solid adsorbent. In addition to the rec-
ognized 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 sorp-
tion coefficients (K om) or soil-organic-carbon-normalized sorption coefficient
(K oc) exhibit generally lower limiting sorption capacities on SOM. By appli-
cation of the Flory–Huggins model to account for solute solubility in (amor-
phous) soil organic phase, Chiou et al. (1983) developed a partition equation
to account for the magnitudes of the observed sorption coefficients. This analy-
sis led to the recognition that the primary factor affecting the sorption coef-
ficients 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
(K om or K oc) and octanol–water partition coefficient (K ow) of the solutes was
recognized to be the consequence that the solute solubility in water is the
major determinant of both K om and K ow 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
2
area (about 1m /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 simultane-
ous 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 adsorp-
tion on minerals (for dry soil) or the partition into SOM (for water-saturated
soil). For dry soil, as shown in Figure 7.1a, the much greater nonlinear adsorp-
tion with mineral matter than the linear partition with SOM gives rise to a