Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 138

the hydroxyl reaction rate constant, p K . Amphoteric hydroxyl sites strongly attract water, creating a
layer of tightly bound water at the surface (vicinal water). Organic molecules interact i rst with the
charged surface of the vicinal water layer; direct adsorption of the compound onto the mineral
surface must be preceded by displacement of the water (Bennett, 2004b).
3.3.2.2 Molecular Properties Affecting Adsorption
The size of a molecule inl uences its potential to be adsorbed to solid surfaces. Large molecules
have more sites where van der Waal's forces * contribute to attraction of the molecule to charged soil
particles, due to temporary dipoles resulting from time-varying electron distribution or polarity
induced by charges on soil particles. The positive regions in the temporary dipole attract electrons
in the adjacent soil surface, resulting in a net attraction of the compound to the soil surface (Carey,
1987; Dragun, 1988). For organic chemicals with molecular weights greater than 400-500, van der
Waal's forces become the dominant adsorption mechanism in nonsandy soils (Dragun, 1988).
Solubility in water, exemplii ed by hydrophilic compounds, also controls the degree to which
organic compounds are adsorbed to soil organic matter. Molecular fragments including carbon,
hydrogen, and the halogens chlorine, bromine, and iodine are hydrophobic, whereas fragments
including nitrogen, sulfur, oxygen, and phosphorus are generally hydrophilic. The solubility of com-
pounds with both hydrophilic and hydrophobic groups depends on which fragments are dominant
and the geometric arrangement of the molecule (Dragun, 1988). In general, highly soluble and mis-
cible compounds are less prone to adsorption.
The molecular charge of an organic chemical can also play an important role in adsorption onto
soil surfaces if the molecule is polar. Many of the stabilizer compounds do not carry a sufi cient
charge to be affected by electrostatic sorption. Some stabilizer compounds including amines and
quaternary ammonium compounds may react in acidic waters to form functional groups that carry
a positive charge to make them prone to adsorption. Compounds with hydroxyl or carboxyl groups
may acquire a negative charge in alkaline waters and become adsorbed by oxide surfaces on
alumina-silicate clay minerals (Dragun, 1988).
Hydrogen bonding can also affect organic chemical adsorption onto soil surfaces. Hydrogen
bonding occurs where a hydrogen atom serves as a bridge between two electronegative atoms, with
a covalent bond on one end and an electrostatic bond on the other. For example, hydrogen bonding
may join a polar organic molecule to an adsorbed cation through a water molecule in the cation's
primary hydration shell. This type of hydrogen bonding occurs with pyridine as well as with ketones
and amides in soils containing smectite clays (Dragun, 1988).
3.3.2.3 Distribution Coeffi cients and Sorption Isotherms
Distribution coefi cients provide a quantitative description of partitioning between chemicals in
solution and soils, bottom sediments, suspended sediments, and other solid media. The distribution
coefi cient, K d , describes a linear relationship between the concentration of a chemical in solution
and its concentration adsorbed to soil surfaces. At low concentrations, adsorption is a linear process,
where the amount adsorbed is directly proportional to the amount in solution (Bennett, 2004b).
Sorption potential increases with increasing values of K d . Sorption isotherms become nonlinear and
sorption decreases as the concentration increases, particularly for polar and ionizable compounds
and for soils low in organic carbon or high in clay (Suthersan, 2002). The value of K d is restricted to
the system for which that value was obtained; it will vary in different soils.
The relationship between the quantity of a chemical absorbed at constant temperature per quan-
tity of adsorbent, q e , and the equilibrium concentration of adsorbate in solution, C e , is called the
adsorption isotherm (Snoeyink, 1999). The distribution coefi cient K d is the slope of the sorption
* Van der Waal's forces, also called London dispersion forces, are the weak electrostatic charges within an otherwise
neutral molecule resulting from movement of electrons forming temporary dipoles or charge concentrations.