Adsorption from electrolyte solutions Surface complexation models

Criscenti and Sverjensky (1999, 2002) continued to build the internally consistent set of triple layer model equilibrium constants developed by Sverjensky and Saliai (1996) and Sahai and Sverjensky (1997a,b) by reexamining sets of adsorption edge and isotherm data for divalent metal cation adsorption onto oxide surfaces. In contrast to previous investigations, they found tliat the adsorption of transition and heavy metals on solids such as goethite, y-ALOs, corundum, and anatase, which have dielectric constants between 10 and 22, was best described by surface complexes of the metal with the electrolyte anion. Metal (M +j adsorption from NaNOs solutions is described by... 

In the models discussed in the previous sections the metal electrode has a constant contribution to the equilibrium constant(s) of the adsorption process(es). However, the role of the metal is more complex, especially in the case of solid electrodes. Up to now two effects have been analyzed in some detail electron spillover from the metal into the electrolyte solution, and the heterogeneity of the electrode surface. 

The above-cited example on Cd/hematite indicates that some groups perform titrations in the presence of solutes different from innocent electrolytes. Such titrations may yield important macroscopic information on the proton balance of the suspension in the presence of such a solute (Table 2). However, the exact proton stoichiometry of some surface complex can rarely be inferred, because this would require that only one complex exists and that the protonation states of the surface groups, which are not contributing to that particular surface complex, are not affected by the adsorption process. This can, at best, be assumed in a quaUtative interpretation but can be quantitatively handled with the mean field approximation and the corresponding assumptions inherent to the respective computer programs. In fitting some models to adsorption data, proton data will constitute an independent and very valuable dataset representative of the system however, they may be restricted to sufficiently high solute to sorbent ratios. 

Since it is difficult to assess the actual surface potential at mineral surfaces in complex mixtures of phases, it will usually not be possible to determine the appropriate description of the electrical double layer (EDL) that is required for more complex SCM, such as the DDL or the triple layer model (TLM) (Davis et al., 1998 Bolt van Riemsdijk, 1987). It is known that the surface charge of mineral phases in natural waters is very different from that observed in simple electrolyte solutions. For example, the adsorption of major ions in natural waters (e.g., Mg ", Ca ", SO and silicate) and the formation of organic coatings are known to cause large changes in the point-of-zero-charge (pHp2c) and isoelectric point (pHjEp) of mineral phases (Davis Kent, 1990). 

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