Contaminants and Land Environment Sustainability Indicators - Sustainable Practices in Geoenvironmental Engineering - page 327

species, and the subscripts ox and red refer to the oxidized and reduced i th species. At a
temperature of 25°C, the relationship between Eh and pE will be obtained as Eh = 0.0591 pE.
Figure 9.12 (from Yong, 2001) shows the pE -pH diagram for Fe and water with a maximum
soluble Fe concentration of 10 −5 M. As can be seen, the valence state of the reactants is a
function of the pH- pE status.
Speciation and complexations are central to the processes that control the fate of heavy
metals in soils. Speciation refers to the formation of complexes between heavy metals
and ligands in the aqueous phase (porewater). In a soil-water system, speciation provides
competition between the ligands and the reactive soil solids for sorption of heavy metals.
Various dissolved solutes in the porewater participate in the aqueous and surface complex-
ation that are characteristic of the interactions between the solutes and the reactive soil
particles. These interactions impact directly on the predictions of contaminant transport,
and especially modeling procedures that rely on the use of simple partition coeficients.
For example, Cl − ions, sulfates, and organics can form complexes with heavy metals. The
result of this is seen as a lesser amount sorbed onto the soil particles—i.e., a lower adsorp-
tion isotherm performance—and a larger amount of the target heavy metal transported in
the porewater. In other words, the environmental mobility of heavy metals is enhanced
with speciation and complexation. Studies on Cd adsorption by kaolinite soil particles
indicate that the Cd that were not adsorbed by the soil were in the form of CdCl ,CdCl
2

−
and CdCl 2− (Yong and Sheremata, 1991). The amount of Cd not adsorbed by the kaolinite
soil particles in the presence of Cl − was due to (a) a decrease in activity due to the presence
of NaCl, (b) competition from Na + for adsorption sites, and (c) complexation of Cd 2+ as
negative and neutral chloride complexes. With increasing pH values, there is a tendency
for Cd to be removed from the porewater as hydroxides, and if the pH is above the precipi-
tation pH of Cd, precipitation of the Cd onto clay particle surfaces is likely. At pH values
higher than the isoelectric point (iep), Cd is more likely to remain in solution in the presence

0

3

25

Water oxidized

20

15

Fe 3+

Aerobic zone

10

Fe 2+

5

Fe(OH) 3

0

-5

Fe(OH) 2

Anaerobic
zone

-10

Water reduced

-15

0 2 4 6 8 10 12 14

pH

FIGURE 9.12
pE -pH chart for Fe and water with maximum soluble Fe concentrations of 10 −5 M. The zone sandwiched between
the aerobic and the anaerobic zones is the transition zone. (From Yong, R.N., Geoenvironmental Engineering:
Contaminated Soils, Pollutant Fate and Mitigation , CRC Press, Boca Raton, 307 pp., 2001.)