Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 227

properties sufficiently to affect Zn sorption. The latter was also proposed by
Tagwira, Piha, and Mugwira (1993). Thus, P application can affect Zn sorp-
tion resulting in increased specific sorption sites for Zn and subsequently
reducing Zn release or desorption. Furthermore, P-induced Zn retention
may be due in part to precipitation or co-precipitation involving the forma-
tion of solid-solution of ZnHPO 4 as an intermediate product (Agbenin, 1998).
Al-P precipitation products from reactions of P with Al-OM complexes may
form new sites to retain Zn on soil organic matter. Thus, P sorption on the
surface increased Zn sorption and restricted Zn desorption, depending on
soil pH, surface complexation, and soil precipitation.
Desorption or release following adsorption is presented as isotherms in
the traditional manner in Figures 7.23 and 7.24 for both soils. Distinct dis-
crepancies between adsorption and successive desorption isotherms are
clearly observed in the figures and indicate considerable hysteresis for Zn
release, the extent of which varied among the three soils. This observed
hysteresis was not surprising in view of the kinetic behavior of Zn and is
indicative of the nonequilibrium behavior of Zn retention mechanisms.
Significant irreversibility of Zn sorbed on mineral surfaces and soils has
been reported extensively in the literature, and this observed hysteresis may
be due in part to slow diffusion and kinetic ion exchange, as well as irrevers-
ible mechanisms.
Hysteresis results shown in Figures 7.23 and 7.24 illustrate the extent of
kinetics during release of Zn in the three soils. Webster exhibited limited
kinetics or very slow release where desorption isotherms exhibited little
release over time for all initial Zn concentrations considered. In contrast, for
the two acidic soils Zn release as a percentage of the total sorbed was 47% to
51% and 42% to 49% for Windsor and Olivier soil, respectively. For the neutral
Webster, only 9% to 11% of the sorbed Zn was released into the soil solution
after 28 d of successive desorptions. This is due to its much higher content
of clay with smectite as the predominating mineral, organic matter, Fe/Al
oxides, and CEC. Soils with high clay, OM, Fe/Al content, and CEC have
more available reactive sites and increased specific sorption and reduced Zn
desorption (Xie and MacKenzie, 1989). Furthermore, at low initial Zn concen-
trations, lower proportions of Zn were desorbed, indicating high affinity for
Zn in soils, whereas at high concentration, the percentage of Zn desorption
increased, indicating lower Zn affinity. Although higher Zn amounts were
sorbed as a result of the presence of P, similar release curves were obtained
for all P concentrations (Figures 7.23 and 7.24). Thus, the kinetics of Zn reten-
tion was not altered by the presence of increased levels of P. However, the
application of P resulted in reduced desorption of Zn for all three soils. This
is perhaps because the added P increased specific sorption sites on OM and
Fe/Al oxides surfaces where Zn was tightly sorbed. Moreover, the effect of
P on Zn sorption by Webster soil was more pronounced compared to the
acidic soils. This is likely due to the high clay, OM, Fe/Al content, and CEC
for Webster soil as discussed above.