Adsorption from electrolyte solutions oxide surfaces

As I see, an essential problem in this field is that the complex aspect of adsorption from electrolyte solutions shown in Fig. 1 has not been widely accepted. I have hardly found a systematic analysis of all probable simultaneous equilibria in a given adsorption system in the relevant literature. In most cases the solution condition (e.g., pH)-dependent dissolution of the solid phase, the surface precipitation, and the speciation in the aqueous phase are omitted in the evaluations either without mentioning them or with reference to some reasoning. To demonsteate some outcomes, it is worth inspecting a simple case of the surface-charge titration of a common aluminum oxide in detail. 

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... 

The nature of the problem in establishing a mechanistic model of the oxide-electrolyte interface, in which chemical and electrostatic energies are described explicitly, can be appreciated by consideration of the adsorption reaction depicted in Figure 2. The adsorption of a hydrogen ion from the bulk of a monovalent electrolyte is considered. The oxide-solution interface is divided conceptually into four regions the bulk oxide (not shown in the figure), the oxide surface at which the adsorption reaction takes place, the solution part of the double layer containing the counterions, and the bulk of solution. 

The pH value at which the oxide surface carries no fixed charge, i.e. Oj = 0, is defined as the point of zero charge (PZC) . A closely related parameter, the isoelectric point (lEP), obtained from electrophoretic mobility and streaming potential data, refers to the pH value at which the electrokinetic potential equals to zero The PZC and lEP should coincide when there is no specific adsorption in the iimer region of the electric double layer at the oxide-solution interface. In the presence of the specific adsorption, the PZC and lEP values move in opposite directions as the concentration of supporting electrolyte is increased. ... 

In general, two important types of processes occur at the electrode surface in contact with electrolyte solution containing electroactive substances when an appropriate potential is applied a charge (electron) transfer process that causes oxidation or reduction of the substances and an adsorption-desorption process in which adsorbable species from the solution phase are attached to the electrode surface through replacement of preadsorbed species such as solvent molecules. Electrochemical adsorption is characterized by competitive processes depending on the electrode potential. Furthermore the adsorbed state of a species, particularly its orientation to the electrode surface, affects redox reactivity. In situ studies on the adsorption of bioactive substances on an electrode surface are thus of great interest from a bioelectroanalytical standpoint. 

Adsorption of Pb(II) on the oxide surfaces (CsiOj = 0.2 wt.%) was performed from the aqueous solution of Pb(C104)2 at the initial Pb(II) concentration of 10 , 10 or 10 M (concentration of radioactive species °Pb(lI) was 10 M) with addition of a neutral electrolyte (10 MNaC104) using a Teflon cell (50 cm ) temperature-controlled at T = 25 + 0.2°C. The pH values were adjusted by addition of 0.1 M HCl or NaOH solutions. Determination of gamma radioactivity of the solution containing... 

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