Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 222

confidence level). Based on these calculations, the SRS model was capable
of quantifying competitive adsorption for Ni and Cd. However, for both Ni
and Cd, the SRS model deviated considerably from experimental data for
high concentrations of the competing ions. This finding is consistent with
the application of SRS made earlier by Gutierrez and Fuentes (1993) and
illustrates the need for model improvement to better describe competitive
adsorption of heavy metals over the entire range of concentrations.
7.8.2 Zinc-Phosphate
Zinc availability and mobility in soils may be controlled by several interac-
tions with the soil-water environment. Primary sources of Zn contamination
include mining, smelting, and other industrial as well as anthropogenic fac-
tors (Adriano, 2001). A secondary source of Zn is through phosphate fertiliz-
ers, which often contain traces of heavy metals such as Cd, Cu, Mn, Ni, Pb, and
Zn. Zinc is also an essential micronutrient for plants and animals (Adriano,
2001). The understanding of the complex interactions of Zn in the environ-
ment is a prerequisite in the effort to predict their behavior in the vadose
zone. It is well accepted that several factors influence Zn adsorption, desorp-
tion, and equilibrium between the solid and solution phases. These factors
include soil pH, clay content, organic matter (OM), cation exchange capacity
(CEC), and Fe/Al oxides (Gaudalix and Pardo, 1995), among which, soil pH is
one of the most important factors (Barrow, 1987). Zn sorption increases and
Zn desorption reduces with increased pH (Rupa and Tomar, 1999; Tagwira,
Piha, and Mugwira, 1993). This may be because increasing pH increases the
negative charge of variable-charge soil for Zn adsorption (Saeed and Fox,
1979). On the other hand, pH affects Zn hydrolysis which would be preferen-
tially sorbed on soil surface (Bolland, Posner and Quirk, 1977).
Over the past two decades, phosphate has been observed to increase Zn
adsorption and decrease Zn desorption in soils (Agbenin, 1998; Rupa and
Tomar, 1999). Xie and Mackenzie (1989) reported that P sorption increased
soil CEC, resulting in increased Zn adsorption on three different soils. Saeed
and Fox (1979) reported that increased negative charge due to P sorption was
responsible for the observed increase in Zn sorption. Xie and Mackenzie (1989)
found phosphate sorption enhanced the correlation between Zn sorption and
soil OM and Fe content and postulated that enhanced Zn sorption may be a
consequence of either an increase in negative surface charge of soil particles,
creation of specific sites, or precipitation of hoepite. Ahumada, Bustamnte,
and Schalscha (1997), Pardo (1999), and Sarret et al. (2002) observed sig-
nificant increases in Zn fractions associated with organic matter and Fe/Al
oxides in the presence of P. Rupa and Tomar (1999) investigated Zn desorp-
tion kinetics as influenced by phosphate on alfisol, oxisol, and vertisol. They
tested several kinetic models and found that Zn release was best described
by the Elovich equation. A primary limitation of simple models such as