Page 227 - Partition & Adsorption of Organic Contaminants in Environmental Systems

218 CONTAMINANT UPTAKE BY PLANTS FROM SOIL AND WATER

(8.3)
C s = K d C w
where K d is the soil-water distribution coefficient specific to a contaminant on
a given soil. K d is largely independent of C w when the ratio of C w to S w (water
solubility) is moderately large, but may be a function of C w for certain
contaminant–soil systems at very small C w /S w (Chiou and Kile, 1998 and
references therein). On the other hand, as rationalized below, reasonable and
convenient estimates of C w can be achieved for relatively nonpolar contami-
nants on most soils from the observed effect of soil organic matter (SOM) on
contaminant sorption to water-saturated soils.
As discussed in Chapter 7, an ordinary soil is a dual sorbent for nonionic
organic compounds of limited water solubilities, in which the mineral matter
acts as an adsorbent and the SOM as a partition medium. As described earlier,
the soil uptake of relatively nonpolar contaminants from water occurs mainly
by partition into SOM because of the suppression by water of their adsorp-
tion on mineral matter. Thus, the effective concentration of a contaminant in
a hydrated soil is the concentration that is normalized to the SOM content of
the soil, that is,
C som = C s f som (8.4)

where C som is the SOM-normalized contaminant concentration in soil and f som
is the weight fraction of the SOM in soil. Assuming local equilibrium for con-
taminants between soil particles and interstitial water, the contaminant C w in
soil interstitial water, which will be designated as the driving force for con-
taminant transport, is related to C som as follows:

C w = C som K som (8.5)

where K som is the contaminant partition coefficient between SOM and water
(here the subscript som is used to replace the subscript om in earlier K om in
order to contrast K som from K pom ); K som is practically concentration independ-
ent, with the possible exception at very low C w /S w for some special soils, such
as those containing a significant amount of high-surface-area carbonaceous
material (HSACM), as elucidated in Chapter 7 (see section 7.3.7). The mag-
nitude of K som is determined by the contaminant and SOM properties. We have
seen that for contaminants of limited water solubilities, the K som values are
usually much greater than 1. Since the SOM properties for soils from widely
dispersed geographic sources have been found to be relatively comparable, as
illustrated in Chapter 7 (see section 7.3.2), Eq. (8.5) should allow for a fairly
general assessment of the influence of soil sorption on the contaminant uptake
by plants. Thus, while the direct determination of C w, or the evaluation of C w
from the established relation with C s, helps to capture the possible nonlinear-
sorption effect, sufficiently accurate estimates of C w can readily be obtained
for low-polarity contaminants from C som and K som for soils sufficiently high in
SOM and low in HSACM if C w and C som are close to being at equilibrium. The