Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 212

Moreover, for the case where one assumes negligible competition among
the various ions, Equation 7.26, omitting the subscript i , is reduced to the
second-order formulation discussed earlier.
7.8 Experimental Evidence
7.8.1 Nickel and Cadmium
Several studies on Ni and Cd indicate that they exhibit somewhat similar
sorption behavior on minerals and soils. These two cations have lower affini-
ties for soil colloids and are generally considered as weakly bonded metals
(Atanassova, 1999). A consequence of weakly bonded heavy metals ions such
as Cd and Ni is that ion competition may result in their enhanced mobility
in the soil environment. Moreover, a number of studies reported varying Cd
and Ni affinities in soils and minerals. Several studies indicated that for some
soils Cd is of higher affinity than Ni (Gomes et al., 2001; Echeverría et al., 1998;
Papini, Saurini, and Bianchi, 2004). Moreover, cation exchange was consid-
ered as the major sorption mechanism for both ions. Echeverría et al. (1998)
and Antoniadis and Tsadilas (2007) reported that Ni adsorption was stron-
ger than Cd and was related to hydrolysis of divalent ions capable of forming
inner-sphere complexes with clay lattice edges. Other studies with minerals,
such as kaonilite, montmorillonite, and goethite, indicated stronger affinity
for Cd than Ni (Barrow, Gerth, and Brummer, 1989; Puls and Bohn, 1988).
For hematite, kinetic sorption results indicated that Ni is of stronger affinity
than Cd (Jeon, Dempsey, and Burgos, 2003). Schulthess and Huang (1990)
showed that Ni adsorption by clays is strongly influenced by pH as well as
silicon and aluminum oxide surface ratios. Moreover, in recent studies using
X-ray absorption fine structure (XAFS) and high-resolution transmission
electron microscopy (HRTEM) techniques, Ni-Al layered double hydroxide
(LDH) was considered responsible for the sorption behavior for pH above
6.5 on pyrophyllite and kaolinite surfaces (Scheidegger, Lamble, and Sparks,
1996; Eick, Naprstek, and Brady, 2001). They suggested that Al dissolved at
high pH values could be responsible for Ni precipitate on clay surfaces. The
formation of surface-induced precipitates may play an important role in the
immobilization of Ni in nonacidic soils. However, surface-induced precipi-
tates were not found for Cd in nonacid soils. This suggests that competitive
behavior of Cd and Ni in neutral and alkaline soils may have different char-
acteristics from their behavior in acidic soils.
Modeling competitive adsorption between Ni and Cd in soils has been at
best sparse. Nevertheless a few scientists have utilized variable charge sur-
face models and surface complexation models. Barrow, Gerth, and Brummer
(1989) successfully utilized a variable-charge surface model in an effort to
describe Ni, Zn, and Cd adsorption in a goethite-silicate system. A modified