Agriculture Reference
In-Depth Information
the mathematical form and the number of parameters are chosen to be as
simple as possible (Bradl, 2004).
Surface-complexation models (SCMs) are chemical models based on a
molecular description of the electric double layer using equilibrium-derived
adsorption data. To apply SCMs, it is required to provide the amount of reac-
tive surface of each type of sorbent as well as the density of the reaction sites.
In reality, experimental characterization of soil minerals is time consuming
and data is rarely available. In addition, the application of SCMs requires the
estimation of an extensive list of specific surface species and their thermo-
dynamic reaction constants. Moreover, basic to SCMs such as MINTEQA2
and PHREEQC is that equilibrium conditions are assumed dominant, that is,
LEA (local equilibrium assumption) is valid. The drawback of SCM models
is that they are limited to describing equilibrium-type reactions and do not
account for kinetic sorption-desorption processes in soils. In heterogeneous
soils with a variety of sorbents having different reactivities, time-dependent
sorption is often observed. Kinetics or nonequilibrium sorption may arise
due to the heterogeneity of sorption sites on matrix surfaces. In fact, various
types of surface complexes (e.g., inner sphere, outer sphere, monodentate,
bidentate, mononuclear, or binuclear) with contrasting sorption affinities can
be formed on mineral surfaces with metals and metalloids. This heteroge-
neity of sorption sites may contribute to the observed adsorption kinetics
where sorption takes place preferentially on high-affinity sites, followed sub-
sequently by slow sorption on sites with low sorption affinity. Furthermore,
the diffusion of ions to reaction sites within the soil matrix has been proposed
as an explanation for time-dependent adsorption by many researchers (e.g.,
Fuller, Davis, and Waychunas, 1993; Raven, Jain, and Loeppert, 1998). The
PHREEQC model simulates sorption kinetics with first-order kinetic equa-
tions. PHREEQC was not developed to simulate nonlinear adsorption kinet-
ics. Recent work of Zhang (2013) represents efforts to account for kinetics by
relaxing the equilibrium assumption in SCMs.
Surface-complexation models have been used to describe an array of
equilibrium-type chemical reactions, including proton dissociation, metal
cation and anion adsorption reactions on oxides and clays, organic ligands
adsorption, and competitive adsorption reactions on oxide and oxide-like sur-
faces. Application and theoretical aspects of SCMs are extensively reviewed
by Goldberg (1992) and Sparks (2003). SCMs are chemical models based on
molecular description of the electric double layer using equilibrium-derived
adsorption data. They include the constant capacitance model, triple-layer
model, Stern variable surface charge models, among others. SCMs have been
incorporated into various chemical speciation models. The model MINEQL
was perhaps the first where the chemical speciation was added to the triple-
layer SCM. Others include MINTEQ, SOILCHEM, HYDRAQL, MICROQL,
and FITEQL (see Goldberg, 1992).
All these chemical equilibrium models require knowledge of the reactions
involved and associated thermodynamic equilibrium constants. Due to the
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