Metal ions association with hydrous oxide surfaces

Metal oxides selectively adsorb divalent cations even at solution pH values lower than the PZC of metal oxides. The mechanism of metal ion association with hydrous-oxide surfaces involves an ion-exchange process in which the adsorbed cations replace bound protons. Specifically, adsorbed cations raise the value of PZC of oxides. pH affects adsorption of metal cations, either by changing the number of sites available for adsorption or by changing the concentration of the cation species (Me +, MeOH+, Me(OH)2) that are preferentially adsorbed (Jackson, 1998). 

There is increasing evidence that several oxides, especially electroch-emically formed hydrous oxides, exhibit potential-pH behavior, as derived from cyclic voltammetry data, close to 1.5(2.303R71/F) rather than the more commonly accepted 2.303RTIF. Such behavior has been associated, for example, with increasing acidity of the oxy species at the oxide surface accompanying the increase in degree of oxidation of the central metal ion [12,142],... 

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