Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 233

in Figure 7.28. Formation of surface complexation between Fe/Al (hydro)
oxides and As(V)/P restricted the accessibility of those surface sites for fur-
ther adsorption (Hingston, Posner, and Quirk, 1971; Smith, Naidu, and Alston,
2002). Because adsorption of both anions takes place preferentially on high
affinity sites at low surface coverage, the competition is expected to be great-
est at low As(V) and P concentrations, which is consistent with observations.
The SRS equation (7.15) was employed to quantify competitive adsorption
of As(V) and P. Specifically, the Freundlich K F and N were taken from the
single-anion adsorption data and utilized to obtain the competitive coeffi-
cients α ij by fitting the competitive adsorption data to Equation 7.15 using
nonlinear least square optimization. We should emphasize that the SRS
equation should only be regarded as an empirical model and the confor-
mity of this equation does not imply certain reaction mechanisms. In their
original paper, Sheindorf, Rebhun, and Sheintuch (1981) defined α ij as sym-
metrical values, that is, a ij = 1/ a ji . However, Roy, Hassett, and Griffin (1986a,
1986b) suggested that the coefficients should be regarded as empirical val-
ues describing the degree of competition under specific experimental condi-
tions. Furthermore, Barrow, Cartes, and Mora (2005) used nonlinear curve
fitting to determine the competitive coefficients between As(V) and P and
they found that the coefficients were not symmetrical.
Results from the kinetic batch experiments are presented in Figures 7.29
and 7.30 in order to illustrate the competitive sorption kinetics between
As(V) and P by the soils. The extent of As(V) sorbed by all soils were
significantly reduced as concentrations of P in the applied solution
increased. Moreover, both As(V) and P exhibited strongly time-depen-
dent adsorption behavior, which is depicted by the continued decrease
of concentration with reaction time. Observed retention kinetics of As(V)
and P in Figures 7.29 and 7.30 is likely due to the heterogeneity of the
soil surface where multiple chemical and physical processes take place.
Chemical reaction rates of surface complexation between anions and
metal oxides are considered rapid. Using a pressure jump relaxation tech-
nique, Grossl et al. (1997) calculated a kinetic rate constant of 10 6.3 s -1 for
the formation of a monodentate inner-sphere surface complex on goethite
surface. In addition, a forward rate constant of 15 s -1 was associated with
succeeding reaction for the formation of a bidentate mononuclear surface
complex. Because of their rapid reaction rates, surface complexation is not
a rate-limiting step of As(V) and P adsorption in soils. However, different
types of surface complexes (e.g., monodentate, bidentate, mononuclear,
binuclear) can be formed on oxide surfaces at high or low surface cov-
erage. This heterogeneity of sorption sites may contribute to adsorption
kinetics observed in most experiments, that is, where sorption takes place
preferentially on high affinity sites and followed by slow sorption to sites
of low sorption affinity.
Recent adsorption studies suggested that surface precipitation, that is,
three-dimensional growth of a particular surface phase, may occur for both