Adsorption multivalent ions

The adsorption of ions with a strong nonculombic interaction, such as chemical interaction (multivalent metal ions on oxides), leads to so-called specific adsorption. Multivalent ions may adsorb on the one or a few hydroxyl groups, but the number of the occupied adsorption places (groups) does not exceed the valency of the ion, and usually, because of the steric reason is not bigger... 

Adsorption Isotherms of Surfactants in Solutions of Multivalent Ions. 139... 

The theory is extended to multivalent ions, in which case an inversion of the surface charge can occur with increasing electrolyte concentration. Depending on the sign of the surface charge, the adsorption of dipolar molecules on the surface can increase or decrease the repulsion [7.6]. When association equilibria are taken into account... 

The adsorption of multivalent ions is often a very complicated process [19]. In the liquid phase, these ions may exist in the several forms, depending on the presence and a concentrations of other ions. For example with multivalent anions the changes of pH may produce a change in the number of H+ ions connected with them, whereas hydrolyzable cations will change the number of coordinated hydroxyl groups. The presence of other ions may radically change the ionic composition of the solution and thus, also the adsorption properties of the ion. 

Adsorbing on the surface of the metal oxide multivalent ions, may occupy one or more adsorption sites. Usually the number of places occupied by the single ion is not bigger than two [67]. The process of the ion adsorption may go through the reaction with surface groups, which can also react with H+ ions, whereas adsorbing anions will liberate OH- ions. 

The process of the specific adsorption of the multivalent ions is described by the SCM model, however, because of different adsorption behavior of the ions, the model presents difficulties. It may happen, that the ion adsorbing on the one crystalline face will adsorb as inner-sphere complex and on the other as a outer-sphere complex. For example Pb2+ on the face (1102) a—AI2O3 form the inner-sphere complex and on the face (0001) the outer-sphere complex [148]. 

Figure 9.10. Schematic of zeta potential reversal curve A is a normal curve with curve B is reversed owing to an inner-sphere adsorption (adsorption within the slipping plane) of multivalent ions producing 2 (adapted from Taylor and Ashroft, 1972).

Just like metal adsorption onto activated carbons and biomaterials, different ions are absorbed on ion-exchange sites with different affinities. In other words, ion exchange resins are selective for ions. Multivalent ions normally are more strongly absorbed from dilute solutions than ions of lower valence (e.g., Ca " vs Na+). 

With modelling attempts of specific adsorption it is important to distinguish between s.a. ions that adsorb at the same adsorption sites as the protons, and s.a. ions that adsorb on independent sites. The first type of specific adsorption has been assumed in sections 4.5. to 4.7. Spectroscopic studies on metal ion adsorption [83-85] support this view. The description presented in sections 4.5. to 4.7. is adequate for monovalent ions. However, modelling of complexation of multivalent ions with surface groups is faced with several complications. 

Metal hydroxides in general are anion-selective in acid solution and turn to be cation-selective beyond a certain pH, called the point of the iso-selectivity, pHpjS it is pHpjS = 10.3 for ferric oxide and pHpis = 5.8 for ferric-ferrous oxide [72]. Adsorption of multivalent ions may also control the ion selectivity of hydrous metal oxides because of its effect on the fixed charge in the oxides. For instance, hydrous ferric oxide, which is anion-selective in neutral sodium chloride solution, turns to be cation-selective by the adsorption of such ions as divalent sulfate ions, divalent molybdate ions, and trivalent phosphate ions [70,73]. It is worth emphasizing that such an ion-selectivity change due to the adsorption of multivalent ions frequently plays a decisive role in the corrosion of metals. 

Thus, the mechanism of the supporting influence of cationic surfactants on microflotation and flotation can be different. In microflotation the electrostatic barrier can be decreased, in flotation the contact angle can be increased. Naturally, both effects manifest themselves simultaneously. Li Somasundaran (1990, 1992) observed a bubble recharge due to adsorption of multivalent inorganic cations. Thus, their application is recommended in order to increase the contact angle and to stabilise bubble-particle aggregates. Naturally, selective adsorption of multivalent ions at the water-air interface is important. But even in the absence of adsorption selectivity under equilibrium conditions a deviation from equilibrium can happen due to the increase of adsorption within the r.s.c. This is important for the precalculation of increase of the contact angle caused by cation adsorption. 

Appendix 7A The Approximate Integration of the Differential Equation of Adsorption of Multivalent Ions... 

Stern layer adsorption with multivalent ions... 

Multivalent ions are believed to enhance natural organics adsorption. The effect depends on the organic tj pe. Clark and Jucker (1993) determined that the effect of calcium on FA is lower than with HA. Binovi (1983) found that the gel layer formed on RO membranes was composed primarily of organics and iron. Nystrom efal (1994) filtered HA with 3 mgL iron and while a gel layer was formed, the flux decline was low. 

Recently it has been also shown that the surface tension of micellar solutions above the CMC can respond to the transition between spherical and rodlike micelles taking place in micellar solutions of certain surfactants in the presence of multivalent ions, such as Al [65], The qualitative explanation of this phenomenon is connected with the ability of ion to bind three surfactant headgroups, and, consequently, to lower the area per headgroup. According to Israelachvili et al., [11] this can induce a transition from spherical to rodlike micelles. On the other hand, the new micelles adsorb additional Al ions from the bulk. This leads also to a lower adsorption of Al at the solution-gas interface because the competitive adsorption of the counterions follows the same tendency as their bulk concentration. The desorption of Ap causes sharp increase of the surface tension with increasing molar ratio. 

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