Contaminants and Land Environment Sustainability Indicators - Sustainable Practices in Geoenvironmental Engineering - page 322

9.4.2 Basic Elements of Interactions between Dissolved Solutes and Soil Fractions
Interactions occurring between contaminants in the porewater, which are essentially dis-
solved solutes and reactive soil particle surfaces, are responsible for the transfer of these
solutes from the porewater to the soil particle surfaces. These interactions or processes are
called partitioning . Molecular interactions governing sorption of contaminants are essen-
tially electrostatic in nature. They are coulombic interactions between nuclei and electrons.
Of particular importance are the interatomic bonds such as the ionic, covalent, hydrogen,
and van der Waals. Ionic forces are coulombic forces. These are forces between positively
and negatively charged atoms and the bonds formed are called ionic or electrovalent
bonds. The simplest example of ionic bonding is between a sodium atom and a chlorine
atom, resulting in the formation of NaCl. The strength of the attractive forces, and hence
the strength of the ionic bonds decrease as the square of the distance separating the atoms.
Another example of ionic bonding is the bond established between the oxygen from a
water molecule to the oxygen on a clay particle's surface. This is due to the hydrogen atom,
which can attract two electronegative atoms, and the ionic bond formed is called the hydro-
gen bond . In comparison to other bonds between neutral molecules, the hydrogen bond is a
strong bond. Hydrogen bonding between two oxygen atoms, which are electronegative, is
important in bonding layers of clay minerals together, in holding water at the clay surface,
and in bonding organic molecules to clay particle surfaces.
Van der Waals forces of attraction can be categorized into three components: (1) Keesom
forces developed as a result of dipole orientation, (2) Debye forces developed due to induc-
tion, and (3) London dispersion forces. Adsorption of organic anions onto clay particle
surfaces can be in the form of (a) anion associated directly with cation or (b) anion asso-
ciated with cation via a water bridge, referred to as a cation bridge . The process consists
of replacement of a water molecule from the hydration shell of the exchangeable cation
by an oxygen or an anionic group, e.g., carboxylate or phenate of the organic polymer.
Hydrogen bonding to the oxygens of siloxane (mica-type) surfaces of clay particles are
generally weak bonds. Adsorption of the organic anion is readily reversible by exchange
with chloride or nitrate ions.
Cation exchange in soils refers to the exchange of positively charged ions associated with
clay particle surfaces. The process is stoichiometric and electroneutrality at the clay par-
ticle surfaces must be satisied. Cations will be attracted to the reactive soil particle sur-
faces in accordance with the relationship, M s / N s = M o / N o = 1, where M and N represent
the cation species and the subscripts s and o represent the surface and the bulk solution,
respectively. Exchangeable cations are cations that can be readily replaced by other cations
of equal valence or by two of one half the valence of the original one. Thus, for example, if
a clay containing sodium as an exchangeable cation is washed with a solution of calcium
chloride, each calcium ion will replace two sodium ions, and the sodium can be washed
out in the solution.
The quantity of exchangeable cations held by the soil is called the cation exchange capac-
ity (CEC) of the soil and is expressed as milliequivalents per 100 g of soil (meq/100 g soil).
One milliequivalent is equal to 6.023 × 10 20 cation exchange sites in the soil. The CEC is
a measure of the amount of amount of negatives sites associated with the soil fractions.
The predominant exchangeable cations in soils are calcium and magnesium, with smaller
amounts of potassium and sodium. The valence of cations plays a signiicant role in the
exchange process. Higher valence cations will show greater replacing power. The higher
the charge, the higher is its attraction to exchange sites. The converse also holds true, i.e.,
higher valence cations at the surfaces of clay particles will be more dificult to replace. The