Agriculture Reference
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competitive surface complexation model developed by Papini, Saurini, and
Bianchi (2004) was adopted to describe competitive adsorption of Pb, Cu,
Cd, and Ni by an Italian red soil. Equilibrium and kinetic ion exchange type
models were employed by several investigators to describe sorption of heavy
metals in soils (Abd-Elfattah and Wada, 1981; Hinz and Selim, 1994). Here the
affinity of heavy metals increases with decreasing heavy metal fraction on
exchanger surfaces. Using an empirical selectivity coefficient it was shown
that Zn affinity increased up two orders of magnitude for low Zn surface
coverage in a Ca-background solution (Abd-Elfattah and Wada, 1981). The
Rothmund-Kornefeld approach incorporates variable selectivity based on the
amount of metal sorbed. Based on the Rothmund-Kornefeld approach, Hinz
and Selim (1994) showed strong Zn and Cd affinities at low concentrations.
Another type of competitive adsorption modeling is that based on the
Freundlich approach. The Shenindrof-Rebhun-Sheituch (SRS) equation was
developed to describe competitive or multicomponent sorption where it was
assumed that the single-component sorption follows the Freundlich equation
(Sheindorf, Rebhun, and Sheituch, 1981). The derivation of the SRS equation
was based on the assumption of an exponential distribution of adsorption
energies for each component. Gutierrez and Fuentes (1993) concluded that
the SRS approach was suitable in representing competitive adsorption of
Sr, Cs, and Co in a system comprising Ca-montmorillonite suspensions.
Recently, Antoniadis and Tsadilas (2007) used the SRS successfully to pre-
dict competitive sorption of Cd, Ni, and Zn in a Greek vertic xerochrept soil.
They found Zn was strongly retained and competition suppressed the sorp-
tion of the three metals.
In the example we present here, three surface soils with contrasting prop-
erties were chosen for study: a Webster Loam, Windsor sand, and Olivier
loam. Webster soil has a pH of 6.92 and cation exchange capacity (CEC) of 27.0
cmol kg -1 . For Windsor sand, the pH is 6.11 and the CEC is 2.0 cmol kg -1 , and
for Olivier loam they are pH of 5.8 and CEC of 8.6 cmol kg -1 . Batch adsorption
of Ni and Cd in single and binary Ni-Cd systems was carried out in the tra-
ditional methods (Selim and Amacher, 1997). Different molar ratios of Ni/Cd
for a wide concentration range were applied to investigate competitive Cd
and Ni in all soils. Sorption isotherms for single ions as well as binary sys-
tems were modeled using the Freundlich and competitive approaches. A
wide range of concentrations of the competing ions is necessary to delineate
the adsorption characteristics for different heavy metals as well for model-
ing multicomponent competitive systems.
Sorption isotherms Ni and Cd are shown in FigureĀ 7.13 for Olivier, Windsor,
and Webster soils. These isotherms exhibit highly nonlinear behavior indic-
ative of strong affinities at low heavy metal concentrations. For all three
soils, the overall shape of the isotherms suggests some similarities in sorp-
tion mechanisms of the two cations. The Freundlich approach was used to
describe both Ni and Cd isotherms. For both Ni and Cd the Freundlich well
described the isotherms with coefficients of correlation ( R 2 ) ranging from
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