Page 187 - Partition & Adsorption of Organic Contaminants in Environmental Systems

178 CONTAMINANT SORPTION TO SOILS AND NATURAL SOLIDS

systems, both dissolved and suspended organic matters would influence the
behavior of sparingly soluble solutes to varying degrees, depending on their
sources, concentrations, and compositions.
The enhanced solute solubility by DOM offers a logical explanation to the
K* dependence on the soil/sediment–water ratio of solutes observed in
d
soil–water mixtures. Alternatively, Di Toro (1985) hypothesized that collisions
between solute-loaded suspended soil/sediment particles induce desorption of
the solute from the particles, the extent being related to the suspended parti-
cle concentration, particle organic carbon fraction (f oc) and solute K oc.
However, the idea that the solute sorption coefficient is affected by particle
collision is unprecedented, as it violates the principle of equilibrium that the
net amount of solute sorbed ought to be independent of particle collisions.
From kinetic theory, the root-mean-square velocity of a substance (suspended
particle or solute) is inversely related to the square root of its molecular
weight; therefore, the important collision would be between suspended parti-
cles and solute molecules rather than between suspended particles themselves.
Solute–particle collision affects solute concentrations (in both solution and
particle phases) only before the system reaches equilibrium; it poses no net
effect on concentration once equilibrium is reached. Thus, although the parti-
cle collision model gives a mathematical fit to Eq. (7.25), it lacks a physical
basis to explain the influence of dissolved and suspended organic matter on
solute solubility and the subsequent effect on solute sorption coefficient.

7.3.9 Influence of Surfactants and Microemulsions
The discharge of household and industrial organic wastes into the environ-
ment may produce complex mixtures of dissolved and suspended organic
matter in natural water. A class of human-made dissolved and/or colloidal
organic matter of special interest is that of surface-active agents (surfactants)
due to their huge discharge quantities. The unique molecular structures of sur-
factants enable them to form stable aggregates (called micelles) above certain
concentrations in water. Thus, whereas some surfactants or their degradation
products could become potential pollutants in surface water or groundwater,
their presence may also affect the solubility and partition behavior of other
pollutants. The ability of a surfactant to form micelles is characterized by its
critical micelle concentration (CMC). At concentrations below CMC, the dis-
solved surfactant is all in monomeric form; above the CMC, the amount of
surfactant in excess of the CMC forms micelles. Unlike truly dissolved
ordinary chemicals, which exist in monomeric form, the surfactant micelle
offers a relatively large microscopic nonpolar environment to allow for solute
partition (i.e., solubilization) (Rosen, 1978). This effect could greatly promote
the (apparent) water solubility of otherwise relatively insoluble solutes, as
compared to the effect of a dissolved natural organic matter on solute water
solubility. As such, surfactants also provide a potential means to remove con-
taminants from contaminated soils (Sabatini et al., 1995), provided that the