Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 223

Elovich is that different rates of reaction are needed to describe Zn adsorp-
tion from Zn desorption or release. Multireaction kinetic models are capable,
given only one set of reactions rate parameters, of describing adsorption and
desorption or release and have been successfully used to describe kinetic
behavior for several heavy metals and contaminants (Amacher, Selim, and
Iskandar, 1988; Darland and Inskeep, 1997; Barnett et al., 2000). None of the
multireaction-type models have been tested for their capability to describe
Zn reactivity in the complex soil environment in the presence of P.
Similar to the previous case study for Ni-Cd, three surface soils with con-
trasting properties were chosen for this study: a Webster Loam, Windsor
sand, and Olivier loam. Adsorption of Zn was studied using the batch
method as described by Selim and Amacher (1997). The duration of adsorp-
tion was 1 day. Release or desorption commenced following the 1 day
adsorption step using sequential or successive dilutions. To study the influ-
ence of P on Zn adsorption as well as release, the Zn batch experiments
were also carried out where different levels of P concentrations were added
in the solutions.
A family of Zn isotherms is represented in Figure 7.20 for each soil, where
different initial P concentrations were added (from 0 to 100 mg L -1 ) (see Zhao
and Selim, 2010). Distinct differences in the amount of Zn sorbed among the
different soils were observed. Highest sorption was observed for the neutral
Webster soil. Strong retention of Zn is observed as the soil pH increased
(Harter and Naidu, 2001: Barrow, 1987). Moreover, Webster soil is a fine
loamy Haplaquoll with 3.7% calcium carbonates and the presence of carbon-
ates enhances Zn sorption in soils (see Mesquita and Vieira e Silva, 1996). The
Olivier and Windsor soils exhibited lower sorption capacities where Windsor
soil showed least sorption. Hinz and Selim (1994) reported stronger retention
for Zn by Olivier compared to Windsor which may be due to higher CEC
caused predominately by smectitic clays. In contrast, Windsor is an Entisol
and contains parent material that has not been completely weathered to sec-
ondary minerals and hence lower sorption capacity for Zn. Increased CEC
results in more negatively charged sites to adsorb Zn (Kurdi and Doner, 1983;
Cavallaro and McBride, 1984).
The influence of P on increased Zn sorption was clearly manifested in
the isotherms shown in Figure 7.20 where similar trends were observed
for all soils. Saeed and Fox (1979) showed that P fertilization increased Zn
adsorption by soils from Hawaii that contained colloids predominantly of
the variable-charge type. Their results support the hypothesis that phos-
phate additions to soils increased Zinc adsorption by increasing the negative
charges on iron and aluminum oxides. Wang and Harrell (2005) reported that
zinc sorption was enhanced by H 2 PO 4 - as opposed to Cl - or NO 3 - in acid soils.
The overwhelming increase of total Zn sorption with P addition was consis-
tent with other studies (Agbenin 1998; Pardo, 1999; Rupa and Tomar, 1999; Xie
and Mackenzie, 1989). Moreover, the influence of P on increased Zn sorption
for Windsor and Webster soils was much greater than for Olivier soil.