Electrostatic Adsorption Models

The adsorption-isotherm and ion-exchange models are of limited applicability when modeling complex and variable natural systems, particularly when the sorbates of interest are minor or trace ionic species ( 10 to 10 mol/kg), and the sorbents exhibit pH-dependent surface charge. For such conditions, the adsorption of trace ionic species often takes place against the net surface charge of the sorbent. This is the behavior of most toxic trace metal cations, including those of the heavy metals and radionuclides when adsorbed by positively charged metal oxyhydroxides, for example. 

A way to understand this behavior is to remember that a solid s net surface charge is the sum of negative and positive charge effects. Thus, below its ZPC, an oxide has a dominance of positive surface sites for anion adsorption. However, at this same pH the small number of coexisting negative sites can adsorb trace cations (see Fig. 10.4). 

Changes in aqueous speciation are not readily accommodated by isotherm and ion-exchange models. Multivalent species often occur in aqueous complexes adsorption depends on the sorptive behavior of the complexes. For example, cation-OH complexes are often strongly adsorbed by oxides and hydroxides of Fe(III), Mn(IV), Ti(IV), and Al(III). In contrast, fluoride, chloride, sulfate, and carbonate complexes of the same cations are usually weakly adsorbed, if at all, by the same solids. 

Complex adsorption models based on double-layer theory, which take a mechanistic and atomic-scale approach to adsorption, can be used to model and predict these observations. Such models have been called electrostatic adsorption models or surface complexation models. They can consider simultaneously such important system properties as changes in pH, aqueous complex formation and solution ionic strength (solution speciation), and the acid-base and complexing properties of one or more sites on several sorbing surfaces simultaneously. 

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Solving problems that involve the electrostatic adsorption models requires the solution of a number of simultaneous equations. These include intrinsic constant adsorption expressions (mass-action equations) such as just described, and mass- and material-balance and charge-balance equations that account for total surface sites and sorbate species associated with the surface and the solution (cf. Davis et al. 1978 Dzombak and Morel 1987 Allison et al. 1991). These equations are considered in more detail in discussions of the three models. 

A list of the input parameters used in the three electrostatic adsorption models and required by MINTEQA2 is given in Table 10.14. For measurements over a range of ionic strengths, the CC model requires one or more values for Ci (cf. Dzombak and Morel 1987). The DL model is both simpler... 

One can expect that A JJJ values in any of the electrostatic adsorption models would decrease, in the same order, for adsorption of these metal cations by other hydrous oxides, as well as by many other phases. 

Application of the Electrostatic Adsorption Models to Natural Systems... 

A variety of adsorption models, from A, to the electrostatic adsorption models in MINTEQA2, have been coupled with hydrologic transport models. Available coupled codes and their attributes have been described and compared, in some detail, by Mangold and Tsang (1991) (see also Lichtner et al. 1996) and will not be considered here. 

M1NTEQA2 includes three electrostatic adsorption models. What are they Compare their treatment of ... 

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