Page 193 - Partition & Adsorption of Organic Contaminants in Environmental Systems

184 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

removal may be described by Eqs. (7.27) and (7.28), provided that the sur-
factant solution readily reaches the surfaces of soil particles and penetrates
into their void spaces. For the NAPL-saturated soil, the sorption of surfactants
to the soil should be small compared to the amount of NAPL retained by soil
and therefore should not significantly alter the overall NAPL sorption by the
soil. On the other hand, if the contaminants exist at subsaturated levels in the
soil or a natural solid (i.e., no NAPL or excess contaminant phase is formed),
the applied surfactant solution may produce disparate results, depending on
the system, as described below.
Let us consider the situation for a contaminant at subsaturated level in a
soil–water system to which a surfactant is applied. To begin with, one must
recognize that there are fundamental differences in molecular properties
between surfactants and common organic compounds that affect their inter-
actions with soils or other solids. Most commercial and certain natural sur-
factants contain powerful ionic or polar head groups (as needed to keep them
stabilized in water) that are attached to relatively long nonpolar tails. The high
polarity and molecular weight of many surfactants (e.g., nonionic and cationic
surfactants) enable them to compete more efficiently against water for adsorp-
tion onto certain soil minerals,besides their partition into SOM. In the absence
of a separate contaminant phase, adsorption of the surfactant on minerals may
induce the uptake of other contaminants onto or into the mineral-adsorbed
surfactant phase and consequently affect the distribution coefficient of a con-
taminant. The simultaneous effects of soil-sorbed surfactants on contaminant
sorption and unsorbed aqueous surfactants on contaminant dissolution have
been addressed by Sun and Boyd (1993), Chiou (1998), and Lee et al. (2000)
to delineate the consequent change in contaminant sorption coefficient. By
this account, the apparent sorption coefficient (K*) of a subsaturated con-
d
taminant in a soil–water mixture with a surfactant is given as

K d + f K sf
sf
*
K d = (7.29a)
1 + X mn K mn + X K mc
mc
or, in a more useful alternative form, as

* 1 +
sf
K d fK sf K d
= (7.29b)
mc
K d 1 + X mn K mn + X K mc
where K d is as defined before; f sf the mass fraction of the sorbed surfactant in
soil; and K sf is the solute (contaminant) sorption coefficient between sorbed
surfactant and water. The term f sf is for the sum of adsorbed and partitioned
surfactant. Here K sf is not a direct function of f sf , but rather, a function of the
aggregation state of the sorbed surfactant molecules. In principle, only the
adsorbed surfactant can form a molecular aggregation, the extent being