Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 369

electroneutrality and is stoichiometric. Calculations or determinations of the proportion
of each type of exchangeable cation to the total cation exchange capacity of the soil can
be made using exchange equilibrium equations such as the Gapon relationship shown in
Equation 9.1.
From the electrostatic point of view, physical adsorption (or sorption) of contaminants in
the porewater (or from incoming leachate) by soil fractions is due to the attraction of posi-
tively charged contaminants such as the heavy metals to the negatively charged surfaces
of the soil fractions. This type of adsorption is called nonspeciic adsorption. By deinition,
we can refer to nonspeciic adsorption when ions are held by the soil particles primarily by
electrostatic forces. This distinguishes it from speciic adsorption , which is another way of
identifying chemisorption , a process that involves covalent bonding between the contami-
nant and the soil particle (generally mineral) surface. Examples of nonspeciic adsorp-
tion are the adsorption of alkali and alkaline earth cations by the clay minerals. By and
large, cations with smaller hydrated size or large crystalline size would be preferentially
adsorbed.
10.5.2.3 Solubility and Precipitation
The contaminants affected by solubility and precipitation processes are mostly heavy met-
als. The pH of the soil-water system plays a signiicant role in the fate of heavy metal con-
taminants because of the inluence of pH on the solubility of the heavy metal complexes.
According to Nyffeler et al. (1984), the pH at which maximum adsorption of metals occurs
varies according to the irst hydrolysis constant of the metal (cationic) ions. When the ionic
activities of heavy metal solutes in the porewater of a soil exceed their respective solubility
products, precipitation of heavy metals as hydroxides and carbonates can occur. The two
stages in precipitation are nucleation and particle growth. This will generally be under
slightly alkaline conditions. The precipitate will either form a new separate substances in
the porewater or will be attached to the soil solids. Gibbs phase rule restricts the number
of solid phases that can be formed.
Factors involved in formation of precipitates include soil-water system pH, type and
concentration of heavy metals, presence of inorganic and organic ligands, and the individ-
ual precipitation pH of heavy metal contaminants. In the solubility-precipitation diagram
shown in Figure 10.8 for a metal hydroxide complex, the left-shaded area marked as soluble
identiies the zone where the metals are in soluble form with positively charged complexes
formed with inorganic ligands. The right-shaded soluble area contains the metals in solu-
ble form with negatively charged compounds. The precipitation region in-between the two
shaded areas contains various metal hydroxide species.
Figure 10.9 shows heavy metal precipitation information using data reported by
MacDonald (1994). Transition from soluble forms to precipitate forms occurs over a range
of pH values for the three heavy metals. The onset of precipitation can be as early as a pH
of about 3.2 in the case of the single heavy metal species (Pb). The process of precipitation
is a continuous process that begins with onset at some early pH and inally concludes at
somewhat higher pH value, generally around pH 7 for most metals. The inluence of other
metal species in the precipitation process is felt not only in terms of when onset pH occurs,
but also in the rate of precipitation in relation to pH change. Figure 10.9 shows that the
onset of precipitation of Zn as a single species is about pH 6.4 and that reduces to about
pH 4.4 when other metals are present. Given that the experiments were conducted with
equal amounts of each of the three heavy metals, it is expected that the concentrations of
the other metals would also have an effect on modiication of the onset pH. Precipitation