Metal/metalloid adsorption, modeling

In the application of surface complexation models to clay minerals or to soils dominant in clays, the assumption is often made that metal ion adsorption occurs primarily on the aluminol and silanol groups of clay edges. The effect of the permanent charge sites on the adsorption process may not be considered. This simplification may be inappropriate, particularly for metal and metalloid anions, since repulsive electrostatic forces emanating from clay faces may spill over and affect the adsorption process on clay edges (Secor and Radke, 1985). 

Various empirical and chemical models of metal adsorption were presented and discussed. Empirical model parameters are only valid for the experimental conditions under which they were determined. Surface complexation models are chemical models that provide a molecular description of metal and metalloid adsorption reactions using an equilibrium approach. Four such models, the constant capacitance model, the diffuse layer model, the triple layer model, and the CD-MUSIC model, were described. Characteristics common to all the models are equilibrium constant expressions, mass and charge balances, and surface activity coefficient electrostatic potential terms. Various conventions for defining the standard state activity coefficients for the surface species have been... 

Adsorption to mineral surfaces such as Fe and Al (hydr)oxides has long been known to be an important process that limits the mobility of heavy metals and metalloid species in aqueous systems (e.g., Stumm 1992). The sorption of ionic species in MSWI bottom ash has been recently studied in detail by Meima Comans (1998, 1999). These authors used a sequence of selective chemical extractions to determine sorbent concentration, namely Fe and Al (hydr)oxides. Their model calculations suggested that Zn(II) and M0O4 sorbed to Fe (hydr)oxides, while Pb(II) and Cu(II) appeared to have a greater affinity for Fe (hydr)oxides. The sorption of Cd(Il) was found to be very weak. The interpretation of... 

Goldberg, S., and L. J. Criscenti. 2008. Modeling adsorption of metals and metalloids by soil components. In Biophysico-chemical processes of heavy metals and metalloids in soil environments. Ed. A. Violante, P. M. Huang, and G. M. Gadd. 215-264. Hoboken, NJ Wiley-Interscience. 

MODELING ADSORPTION OF METALS AND METALLOIDS BY SOIL COMPONENTS... 

The purpose of this chapter is to present several of the most common models used to describe metal and metalloid ion adsorption by soil components. Empirical models used in soil chemistry are described and their limitations discussed. Common chemical models used to describe metal adsorption on soil minerals are described and their advantages over empirical approaches discussed. Methods for obtaining model parameters are provided. Methods for establishing adsorption mechanisms and surface speciation are addressed. Limitations and approximations in the application of chemical models to natural systems are presented. 

VIBRATIONAL SPECTROSCOPY Infrared and Raman spectroscopies have proven to be useful techniques for studying the interactions of ions with surfaces. Direct evidence for inner-sphere surface complex formation of metal and metalloid anions has come from vibrational spectroscopic characterization. Both Raman and Fourier transform infrared (FTIR) spectroscopies are capable of examining ion adsorption in wet systems. Chromate (Hsia et al., 1993) and arsenate (Hsia et al., 1994) were found to adsorb specifically on hydrous iron oxide using FTIR spectroscopy. Raman and FTIR spectroscopic studies of arsenic adsorption indicated inner-sphere surface complexes for arsenate and arsenite on amorphous iron oxide, inner-sphere and outer-sphere surface complexes for arsenite on amorphous iron oxide, and outer-sphere surface complexes for arsenite on amorphous aluminum oxide (Goldberg and Johnston, 2001). These surface configurations were used to constrain the surface complexes in application of the constant capacitance and triple layer models (Goldberg and Johnston, 2001). 

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