Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 217

Metal ion competition is presented in the traditional manner as isotherms
and is given in Figures 7.16 and 7.17. These isotherms were described using
the Freundlich model in a similar manner to those for a single ion. The extent
of nonlinearity of Ni and Cd isotherms is depicted by the dimensionless
parameter n and was not influenced by input concentration of the competing
ion. Thus in a competitive system, the parameter n did not exhibit apprecia-
ble changes for both metal ions investigated. In contrast, K values exhibited a
decrease of sorption as the concentration of the competing ion increased, and
the extent of such a decrease was dissimilar for the three soils. For Windsor
and Olivier, Ni adsorption decreased significantly over the entire range of
concentrations of the competing ion (Cd). However, Cd adsorption was less
affected by the competing Ni ions for both soils. For the neutral Webster
soil, Ni was not appreciably affected by the presence of Cd, especially at low
Ni concentrations. This may be because, for a single-component system, Ni
adsorption was much stronger than Cd for Webster soil as discussed above.
Another explanation of the competitive Ni sorption behavior is perhaps
due to Ni-LDH precipitates, which may be considered irreversible on soils
and minerals (Voegelin and Kretzschmar, 2005). This process may lead to
significant long-term stabilization of the metal within the soil profile (Ford
et al., 1999). In acidic soils, Ni and Cd are both weakly bonded to soil par-
ticle surfaces and mainly forms outer spheres, which are available for cation
exchange. However, for the neutral Webster soil, Ni sorption may include a
fraction of inner-sphere complexation or Ni-LDH precipitates, both of which
are perhaps not available for competition via cation exchange.
Based on Sheindorf-Rebhun-Sheintuch (SRS) model predictions, the esti-
mated α Ni-Cd for Ni adsorption, in the presence of Cd, was larger than 1
for Windsor and Olivier soils, indicating noticeable decrease of Ni in the
presence of Cd. In contrast, α Ni-Cd for Ni adsorption on Webster soil was
less than 1, which is indicative of small influence of competing Cd ions.
These results are in agreement with the competitive sorption reported by
Antoniadis and Tsadilas (2007). Such small α Ni-Cd implies that Ni adsorption
in Webster soil was least affected in a competitive Ni-Cd system in compari-
son to the other two soils. Moreover, the estimated α Cd-Ni for Cd adsorption
was 0.61 for Windsor and 0.82 for Olivier, whereas the competitive coeffi-
cient of Cd/Ni was 4.00 for Webster soil. Although the SRS equation may be
regarded as a multicomponent model and does not imply certain reaction
mechanisms, differences of competitive sorption between the neutral and
the two acidic soils were illustrated based on the SRS models' competitive
selectivity parameters. In fact, Roy, Hassett, and Griffin (1986a) suggested
that the SRS parameters could be used to describe the degree of the compe-
tition under specific experimental conditions. Calculated results using the
estimated α Ni-Cd are given in Figures 7.18 and 7.19 and illustrate the capabil-
ity of the SRS model to describe experimental data for competitive adsorp-
tion of Ni and Cd. An F -test indicated that there was no statistical difference
between our experimental results and SRS model calculations (at the 95%