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.     

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 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.     

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.     

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.     

Sorption properties of oxides IX: Effect of anions on the sorption of uranium (VI) on hydrous oxides

Effect of anions such as nitrate, chloride, sulphate and carbonate on the sorption of U(VI), from aqueous solutions on hydrous oxides of Ti(IV), Ce(IV) Zr(IV) and Th(IV) has been studied. The sorption of U(VI) is markedly reduced in the presence of anions, like carbonate, whichform strong complexes with UO ₂ ^{2 +} in solution. The results are explained in terms of a competition for free UO ₂ ^{2 +} between surface hydroxyl groups and ligands (anions) present in solution. The sorption of U(VI) on these hydrous oxides was also studied from a bicarbonatecarbonate mixture. Sorption was less under conditions when tricarbonate complex of U(VI) was formed, but increased at higher pH values (>9), presumably due to the formation and sorption of hydroxo complexes of U(VI).     

Anion exchange in hydrous oxides

Commercially available synthetic hydrous oxides (Ferrox, Zirox, Alumina, Oxti, and Oxtain) have been used to study the sorption of some anionic species from simulated radioactive waste. Results of the analysis showed varying degrees of uptake for the anions in various media solutions. Sorption was most effective in the solution pH 2–7 range following the amphoteric nature of these materials. The presence of competing anions had a marked effect on the sorption properties of the material.     

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.     

Organophosphonate-functionalized nanosized magnetic iron oxides as sorbents for heavy metal cations

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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.     

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.     

Phase composition analysis of hydrous aluminium oxides by thermal analysis and infrared spectrometry

A general method for determination of the phase composition of hydrous aluminium oxides by thermal analysis and infrared spectrometry, and determination of the transformation temperature of mixtures of Al(OH)₃ and AlOOH into -Al₂O₃ are described.     

Inorganic particulates in removal of heavy metal toxic ions--part X: rapid and efficient removal of Hg(II) ions from aqueous solutions by hydrous ferric and hydrous tungsten oxides

A radiotracer technique has been used to study the removal of Hg(II) ions from aqueous solutions by synthesized and well-characterized hydrous ferric oxide (HFO) and hydrous tungsten oxide (HTO). Adsorptive concentration (10(-4)-10(-8) mol dm(-3)), pH (ca 4.0-10.0) and temperatures (303-333 K) were examined for assessing optimal conditions for removal of these ions. The uptake of Hg(II) ions, fitted well with the Freundlich isotherm, increased with increasing temperature. No significant desorption took place in the present temperature range. The presence of other anions and cations affected the uptake of Hg(II) markedly. Irradiation of hydrous ferric oxide and tungsten oxide by using a 11.1 x 10(9)Bq (Ra-Be) neutron source having a neutron flux of 3.9 x 10(6)cm(-2)s(-1) with the associated gamma-dose rate of 1.72 Gy h(-1) did not influence the extent of adsorption of Hg(II) significantly.     

Thermal and sorption behavior of polyacrylonitrile supported hydrous titanium dioxide

Summary Modification of the physico-chemical properties of hydrous titanium dioxide (TiO₂) was conducted by using binding polyacrylonitrile (PAN) for the preparation of larger size particles having higher granular strength. The thermal behavior of the obtained composite has been studied by thermogravimetric and differential thermal analysis (TG/DTA). Sorption behavior of the TiO₂-PAN composite for removal of some hazardous radionuclides has been studied at different conditions such as, pH, contact time, ion concentrations and reaction temperature as well as the drying temperature. The effects of interfering ions as well as some complexing agents on the distribution ratio of the sorption process have been determined. As a result of the obtained data the optimum conditions for the removal of the studied radionuclides were recommended.