Outlines of the ion exchange characteristics of hydrous oxides

This work gives a critical review of the behaviour of hydrous oxides as ion exchangers. The various physicochemical properties of this important class of adsorbents and the effect of preparation procedures are summarized. Data on the ion exchange kinetics and thermodynamics are presented and discussed. Selectivity is also discussed in terms of the different parameters. Both aqueous and mixed media are considered. In addition, the different sorption mechanisms, some relevant sorption models, and potential applications in chemical separations are given. Moreover, this review attempts to point to the experimental and theoretical confusions now alarmingly met in the literature.     

Anion coordination and cation packing in oxides

It is shown by considering a few examples that oxide structures are usefully described in terms of their cation packings and the coordination of the anions by these cations. This, together with a consideration of nonbonded repulsions between the atoms, leads simply to a rationalization of some crystal structures and coordination numbers, and to an understanding of the volume changes in certain phase transitions.     

Sorption properties of oxides VIII: sorption of uranium on hydrous oxides

Sorption of unranium from nitrate solution on four hydrous oxides, namely, hydrous titanium, zirconium, cerium and thorium oxides (TiO₂, ZrO₂, CeO₂, ThO₂) has been studied at pH 3.5 as a function of uranyl concentration and temperature. The sorbed species was uranyl ion in the case of hydrous TiO₂ and CeO₂, while in the case of hydrous ZrO₂ and ThO₂, electrolyte sorption involving uranyl nitrate predominated. Sorption site densities calculated from saturation capacities, evaluated from Langmuir adsorption isotherm and surface areas, bear a direct relationship with the heat of sorption values. Results indicate that, in terms of sorption site density, hydrous CeO₂ seems to be a better sorbent than hydrous TiO₂ for the sorption of uranium.     

Adsorption characteristics of metal-EDTA complexes onto hydrous oxides

The adsorption characteristics of a variety of metal-EDTA complexes onto hydrous oxides, principally aluminum oxide (γ-Al₂O₃), were examined in aqueous solution. Adsorption of these complexes increased with increasing proton concentration due to the formation of surface complexes between EDTA and the surface hydroxo groups, specifically the AlOH₂⁺ surface groups. The pH-dependent adsorptive behavior and the magnitude of adsorption of the “free” EDTA species were similar to those of the metal complexes. The results also showed that the adsorption of “free” EDTA was exothermic, while the adsorption of Ni(II)-EDTA complexes was endothermic in the lower pH region (3.5) and exothermic at higher pH values (6.0). This implied that the surface preferred the NiHEDTA⁻¹ species rather than the NiEDTA⁻² species. Specific adsorption of the metal complexes was evidenced by the charge reversal exhibited by the γ-Al₂O₃ particles at the highest surface loadings. A quantitative model was formulated based on the pH-dependent speciation of the oxide surface, speciation of the metal complexes in solution, and ζ potential measurements. This model proved valid over a wide range of pH (3–10) and for both high (>50% coverage) and low (<10% coverage) surface loadings.     

Mechanism of metal-ion sorption on hydrous metal oxides

A model for treating the sorption of metal ions on hydrous metal oxides was established based on the assumptions that these materials are weakly acidic cation exchangers and have a discrete exchanger phase. The experimental results of the sorption of metal ions on the hydrous niobium(V) and tin(IV) oxides are found to be consistent with the formulas derived from the model by considering that the charge balance and the mass action law hold in the exchanger phase and cations are sorbed by the distribution between this phase and the bulk aqueous phase.     

Anion exchange in mixed solvent systems

The diffusion of chlorocomplexes of some corrosion and fission products in anion exchange beads has been studied in mixed solvent media. The effects of variables on the kinetics of the exchange process by the batch and flow technique were examined. The strongly basic anion exchanger Dowex 2X8 in its Cl⁻ form was used in organic solvent—water—hydrochloric acid solutions. The dependence of the exchange rate on temperature, the viscosity of the solution, the mean resin particle diameter and the composition of the solution was studied. Film and particle diffusion coefficients were calculated from the experimental data. The results provide valuable data for the design of separation procedures.     

Adsorption of microamounts of ruthenium on hydrous iron oxides

The adsorption of Ru on amorphous Fe(OH)₃, -Fe₂O₃ and Fe₃O₄ have been measured as a function of the pH and the time of aging. The adsoprtion of Ru increases markedly in the 3–5.5 pH range. At higher pH values, -Fe₂O₃ shows different behaviour with respect to Ru adsorption. The influence of EDTA, citrate and oxalate on the adsorption of Ru on Fe₃O₄ has also been investigated. Possible mechanisms of the adsorption of Ru on hydrous iron oxides are discussed in the light of the results obtained in the course of this study and of those of other researchers.     

Electrical conductivity and chemical reactivity of hydrous ferric oxides

Hydrous ferric oxide was synthesised under alkaline conditions, aged in distilled water and calcined. Characterisation was attempted through x-ray, differential thermal analysis, thermogravimetric and infrared analyses. Electrical conductivity was measured and chemical reactivity towards hydrazine investigated. It was found that ageing as well as calcination facilitated crystallisation toa-Fe₂O₃ and thereby decrease the reactivity, which in turn appears to be strongly influenced by the three types of water,viz. absorbed, bound and crystallisation.     

Anion exchange behavior of nobelium

The anion exchange behavior of ²⁵⁵No produced by the ²⁴⁸Cm(¹²C,5n) reaction has been investigated in 11.5M HCl, 4M HCl and 8M HNO₃ to deduce the behavior of 'primordial ²⁵⁷No' produced via ²⁴⁸Cm(¹⁸O,5n)²⁶¹Rf ²⁵⁷No before anion exchange separation in the ²⁶¹Rf experiment. It was found that ²⁵⁵No is mostly eluted from the anion exchange column with these solutions. The contribution of 'primordial ²⁵⁷No' to the -decay events of ²⁶¹Rf in the ²⁶¹Rf experiment was evaluated based on the present results.     

Adsorption of trace elements or radionuclides on hydrous iron oxides

Factors that influence the adsorption of trace elements or radionuclides on hydrous iron oxides were investigated. The adsorption of monovalent cations (Cs⁺, Rb⁺) on hydrous iron oxides is not strongly pH-dependent and it can be regarded as nonspecific. On the other hand, the adsorption of Ag⁺, divalent cations (Zn²⁺, Cd²⁺, Mn²⁺, Sr²⁺) or trivalent cations (Cr³⁺, La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, Er³⁺, Yb³⁺) is strongly pH-dependent. The regularities of the adsorption of these cations on hydrous iron oxides are discussed. Also, the differences in the adsorption behaviour of some divalent and trivalent cations are explained. Freshly precipitated iron(III) hydroxide can be used for the decontamination of radionuclides from low-level waste solutions. However, the efficacy of decontamination depends on the oxidation state and the chemical properties of radionuclides.     

Adsorption of zinc(II) on hydrous iron oxides

The adsorption of Zn²⁺ ions on amorphous Fe(OH)₃ and -Fe₂O₃, as a function of pH, has been investigated. In the pH region corresponding to the formation of positively charged Zn-hydroxy complexes, an abrupt increase in adsorption was observed. The influence of EDTA and glycine on the adsorption of Zn²⁺ by -Fe₂O₃ has also been investigated. Strong suppression of the adsorption of Zn²⁺ was observed for high [EDTA or Gly]/[Zn²⁺] concentration ratios. The results of the adsorption of Zn²⁺ in the presence of an organic ligand were explained by the formation of Zn-EDTA or Zn-glycine complexes and also by the occupation of adsorption sites by the free organic ligand.     

Ion exchange behaviour of hydrous tin oxide: Kinetics of anion exchange

The kinetic behaviour of Cl(-), Br(-) and SCN(-) exchanges on hydrous tin oxide have been investigated under conditions of particle diffusion and the limited bath technique. Values for the diffusion coefficients, energy of activation and entropy of activation have been calculated. The data obtained have been compared with those reported for other organic and inorganic exchangers.     

Thermodynamics of anion exchange on hydrous zirconia: IV. Calorimetric determination of the heat of anion exchange on hydrous zirconia

Ion-exchange processes Cl^?/NO^?₃ and SO^2?₄/NO^?₃ were studied by calorimetric measurements. Experiments were so performed to obtain stoichiometric equilibrium concentration in order to calculate equilibrium constants, and the heat studied ion-exchange reaction. As a result ?11.03 and ?21.70 kJ mol^?1 values were obtained for standard enthalpy changes of Cl^?/NO^?₃ and SO^2?₄/NO^?₃ ion exchange processes, respectively.These values, together with these obtained from distribution measurements were used for calculation of free energy and standard entropy changes of investigated processes.     

Anion exchange kinetics of uranium in sulphate media

Experiments have shown that the sorption of uranium from acidic sulphate solutions onto strong base anion exchange resins is particle diffusion controlled in the uranium concentration range 0.0001–0.004M. A simplified diffusion model, based on Fick's Law, fits the kinetic data at each concentration. The rate of sorption falls significantly at lowered solution concentration. This corresponds with a lowered equilibrium loading of uranium and can be predicted with the Nernst-Planck equations using the measured self diffusion coefficient of uranium (1.65 × 10⁻⁸ cm²/s) and sulphate ions. The importance of this lowering of uranium sorption on the design of ion exchange equipment is stressed.     

Anion exchange separation of bismuth and lead

Zusammenfassung Wismut und Blei können mit Hilfe des Anionenaustauschers Dowex 1 von mehreren anderen Elementen abgetrennt werden. Zur Elution der Elemente nacheinander werden methanolische oder wäßrige Lösungen von Salpeter- und Salzsäure verwendet. Das beschriebene Verfahren wurde zur Blei- und Wismutbestimmung in Stahl und Meeressedimenten benutzt.     

Effect of in-situ γ-radiation on the sorption properties of hydrous oxides

Sorption of Li⁺, Cs⁺ and Sr²⁺ on hydrous titanium oxide and magnetite as a function of pH in the presence and absence of -rays has been studied. pH-titrations of the irradiated hydrous oxides have also been carried out. It has been shown that the sorption characteristics of these oxides are not altered by -irradiation and the uptake of metal ions is governed by the equilibrium pH of the solution. Certain physical changes observed on irradiating of the hydrous oxide samples in water are discussed.