Hydroxyl ion adsorption

With regard to the anodic dissolution under film-free conditions in which the metal does not exhibit passivity, and neglecting the accompanying cathodic process, it is now generally accepted that the mechanism of active dissolution for many metals results from hydroxyl ion adsorption " , and the sequence of steps for iron are as follows ... 

All ions in the system, whether specifically adsorbed or not, must be expected to saturate partially the ion exchange capacity arising from these relatively pH-independent sources. For this reason an isoelectric point defined in terms of hydrogen and hydroxyl ion adsorption is hypothetical, and in any real system, the pH at which zero surface charge is observed will depend upon the system composition. 

For most oxides there is a pH at which the proton and hydroxyl ion adsorption just compensate each other, this pH is called the point of zero charge or pzc [1]. In general... 

From an electrostatic point of view, the specific adsorption of cations is explained as an inhibition of the proton adsorption and an induced hydroxyl ion adsorption. For the original proton... 

P. C. Yates, Thermodynamic Stabilization of Highly Dispersed Systems Prevention of Particle Growth of Colloidal Amorphous Silica by Hydroxyl Ion Adsorption, presented at the Divisional Colloid Chemistry Symposium on Colloidal Silical and Silicates. 137th Meeting of the American Chemical Society, Cleveland, Ohio, April 13, I960. 

Activators enhance the adsorption of collectors, eg, Ca " in the fatty acid flotation of siUcates at high pH or Cu " in the flotation of sphalerite, ZnS, by sulfohydryl collectors. Depressants, on the other hand, have the opposite effect they hinder the flotation of certain minerals, thus improving selectivity. For example, high pH as well as high sulfide ion concentrations can hinder the flotation of sulfide minerals such as galena (PbS) in the presence of xanthates (ROCSS ). Hence, for a given fixed collector concentration there is a fixed critical pH that defines the transition between flotation and no flotation. This is the basis of the Barsky relationship which can be expressed as [X ]j[OH ] = constant, where [A ] is the xanthate ion concentration in the pulp and [Oi/ ] is the hydroxyl ion concentration indicated by the pH. Similar relationships can be written for sulfide ion, cyanide, or thiocyanate, which act as typical depressants in sulfide flotation systems. 

The surface of activated alumina is a complex mixture of aluminum, oxygen, and hydroxyl ions which combine in specific ways to produce both acid and base sites. These sites are the cause of surface activity and so are important in adsorption, chromatographic, and catalytic appHcations. Models have been developed to help explain the evolution of these sites on activation (19). Other ions present on the surface can alter the surface chemistry and this approach is commonly used to manipulate properties for various appHcations. 

Since carbon is a supplier of electrons, it acquires a positive charge in the process. In order to maintain electrical neutrality, carbon attracts the hydroxyl (OH-) ions. When the aqueous solution contains metal anions that have a greater affinity towards carbon, the hydroxyl ions are exchanged. In support of this theory, mention may be made of the observation that no adsorption takes place in the absence of oxygen and that hydrogen peroxide is liberated when oxygen is bubbled through an aqueous slurry of charcoal. 

Titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy are two techniques which have seldom been applied to study reactions at the solid-liquid interface. In this paper, we describe these two techniques and their application to the investigation of salicylate ion adsorption in aqueous goethite (a-FeOOH) suspensions from pH 4 to 7. Evidence suggests that salicylate adsorbs on goethite by forming a chelate structure in which each salicylate ion replaces two hydroxyls attached to a single iron atom at the surface. 

Abstract In this chapter, the depression mechanism of five kinds of depressants is introduced respectively. The principle of depression by hydroxyl ion and hydrosulphide is explained which regulates the pH to make the given mineral float or not. And so the critical pH for certain minerals is determined. Thereafter, the depression by cyanide and hydrogen peroxide is narrated respectively which are that for cyanide the formation of metal cyanide complex results in depression of minerals while for hydrogen peroxide the decomposition of xanthate salts gives rise to the inhibitation of flotation. Lastly, the depression by the thio-organic such as polyhydroxyl and poly carboxylic xanthate is accounted for in detail including die flotation behavior, effect of pulp potential, adsorption mechanism and structure-property relation. 

Similar types of electric double layer may also be formed at the phase boundary between a solid electrolyte and an aqueous electrolyte solution [7]. They are formed because one electrically-charged component of the solid electrolyte is more readily dissolved, for example the fluoride ion in solid LaFs, leading to excess charge in the solid phase, which, as a result of movement of the holes formed, diffuses into the soUd electrolyte. Another possible way a double layer may be formed is by adsorption of electrically-charged components from solution on the phase boundary, or by reactions of such components with some component of the solid electrolyte. For LaFa this could be the reaction of hydroxyl ions with the trivalent lanthanum ion. Characteristically, for the phase boundary between two immiscible electrolyte solutions, where neither solution contains an amphiphilic ion, the electric double layer consists of two diffuse electric double layers, with no compact double layer at the actual phase boundary, in contrast to the metal electrode/ electrolyte solution boundary [4,8, 35] (see fig. 2.1). Then, for the potential... 

Acid/hase potentiometry enables the surface charge density to be measured. This involves comparison of the titration curves obtained for the suspension of oxide at several different ionic strengths (10 10" M) with that of the electrolyte alone, followed by calculation of the net consumption of protons or hydroxyl ions (mol g ) at each pH. The data is presented as a plot of excess of acid or base (Fh - Toh ) mol g or mol m ) vs pH (adsorption isotherm) or as a plot of surface charge, cr, (coulombs m ) vs pH (charging curve) (Figure 10.5). 

Adsorption of anions on oxides is usually accompanied by the uptake of protons (or the release of hydroxyl ions). The ratio between the number of protons that are coadsorbed and the level of anion adsorption is not usually stoichiometric. Studies of adsorption of oxyanions on goethite as a function of pH appear to indicate that, provided only one adsorbed species is present, the proton anion ratio is related to the mode of adsorption (Rietra et al., 1999). It follows that oxyanions which adsorb with a similar proton/anion stoichiometry, have a similar adsorption mechanism. 

Identical maximal values for the adsorption by blood charcoal were found for the acids HCl, HNO3, H2SO4 in the presence of their neutral salts, viz. a 45 % adsorption in a O Ol molar solution. Of interest is the fact that similar results were obtained for the adsorption of hydroxyl ions, e.g. caustic potash and soda, in the presence of excess potassium and sodium chlorides, both hydrogen and hydroxyl ions exhibiting equal adsorption. 

If the solid diaphragm material adsorbs both hydrogen and hydroxyl ions it is evident that electric endosmose will cease when equal ionic adsorption has taken place, the double layer potential or electrokinetic potential being at this point zero and the diaphragm is at the isoelectric point. 

On the assumption that at a glass-electrolyte interface hydroxyl ions are adsorbed, the fraction of the original charge neutralised by adsorption can be calculated from the above equations as follows. From (a) and (b)... 

If Se032" is the species determining adsorption and the reaction mechanism involves exchange with hydroxyl ions it can be shown that... 

The IEP(s) characteristic of the hydroxylated surface is the pH at which the hydrogen ion adsorption density on the hydroxylated surface sites alone is zero. To achieve zero total charge when 07 5 0, however, Yh must be increased by adjusting the pH toward the acid side of the IEP(s) until eVH = —07 hence the ZPC is smaller than the IEP(s). 

In developing Equations 13 and 18 it was assumed that a hydrogen ion adsorption model would adequately describe charge development by either mechanism—i.e., amphoteric dissociation of hydroxylated surfaces or adsorption of hydroxo complexes from solution. This assumption was... 

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