Kinetics of metal ion sorption on clinoptilolite

From an investigation into the sorption kinetics of Na⁺, K⁺, and Sr²⁺ ions from standard solutions on finely dispersed clinoptilolite incorporated in a highly permeable inert polyacrylamide gel we have determined the characteristic kinetic size of clinoptilolite and the diffusion coefficients for Na⁺, K⁺, and Sr²⁺ in the clinoptilolite microcrystallites and transport pores. Diagrams have been constructed to enable a valid selection of kinetic models for sorption of the ions from any solutions on clinoptilolite.Institute of Chemistry, Far Eastern Branch of the Russian Academy of Sciences, 690022 Vladivostok. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 117975 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 273–277, February, 1992.     

Cation exchange and sorption properties of crystalline alpha-titanium(IV) phosphate

The ion-exchange and sorption properties of alpha-titanium bis(monohydrogen orthophosphate) monohydrate of composition Ti(HPO(4))(2).H(2)O was studied in aqueous electrolyte solutions of KCl over the temperature range of 300-320 K, varying the pH and metal ion concentration in the solution. The data were explained on the basis of the law of chemical equilibrium and the metal ion sorption data were fitted to Langmuir parameters. Further, the extent of sorption was found to increase with increasing temperature and metal ion concentration in the selectivity order Fe(3+)>Cu(2+)>Co(2+)>Mn(2+)>Cr(3+)>Ni(2+). The values of Langmuir constants were used to calculate the various thermodynamic parameters, such as DeltaG(0), DeltaH(0), and DeltaS(0), during the sorption process.     

Kinetics of water vapor adsorption by ion exchange polyacrylonitrile fibres

Kinetics of water vapor adsorption by ion exchange PAN fibres has been studied. Adsorption can be described by an exponential kinetic equation. Experimental results permit the assumption that the surface of the materials studied behaves as uniformly nonhomogeneous with more strongly expressed entropy nonhomogeneity, connected with cross-linking of the modified fibres. The dependence between the kinetic characteristics and the solubility of the materials in DMF shows that the main factor affecting unfavorably the adsorption kinetics is cross-linking. Increase of N-containing anion exchange groups reduces the effect of adsorption delay due to cross-linking.

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Some ion exchange properties of amorphous titanium (IV) phosphate

An amorphous titanium (IV) phosphate exchanger has been synthesised and characterized through elemental analysis, infrared, and thermogravimetric techniques. Divalent cations Mn, Co, Ni, Cu, Zn, Cd, Hg and Pb in bidistilled water have been exchanged with the proton of the exchanger matrix using a batch method. From these results the thermodynamic selectivity constants and distribution coefficients were calculated, whose values indicate a high selectivity for Pb and Hg. Both cations were successfully separated from a mixture containing all cations studied through column separation experiments.     

Kinetics of hydrazine exchange in the pentacyanohydrazineferrate(II) ion

Summary Rate constants and activation parameters for the release of hydrazine (both neutral and protonated) coordinated to the Fe(CN) ₅ ^3– moiety are reported, together with the corresponding stability constants. The rates of formation of the complexes have been evaluated indirectly. The results confirm the general dissociative mechanism for pentacyano(ligand) ferrate(II) ions but some significant differences are found on comparing the data with those from several coordinated diamines. The acidity of complexed N₂H ₅ ⁺ is enhanced byca. two orders of magnitude with respect to free N₂H ₅ ⁺ , its rate of release being greater than that of N₂H₄ by a factor of 31. A solvation controlled model for the dissociation of [Fe(CN)₅L]^n– is shown to be operative in its general features, although specific charge effects are shown to influence the rate of both the dissociation and the formation reactions.     

Kinetics of metal ion exchange between citric acid and serum transferrin

The exchange of Fe(3+), Tb(3+), In(3+), Ga(3+), and Al(3+) between the C-terminal metal-binding site of the serum iron transport protein transferrin and the low-molecular-mass serum chelating agent citrate has been studied at pH 7.4 and 25 degrees C. The removal of Ga(3+), In(3+), and Al(3+) follows simple saturation kinetics with respect to the citrate concentration. In contrast, removal of both Fe(3+) and Tb(3+) shows a combination of saturation and first-order kinetic behavior with respect to the citrate concentration. The saturation component is consistent with a mechanism for metal release in which access to the bound metal is controlled by a rate-limiting conformational change in the protein. The first-order kinetic pathway is very rapid for Tb(3+), and this is attributed to a direct attack of the citrate on the Tb(3+) ion within the closed protein conformation. It is suggested that this pathway is more readily available for Tb(3+) because of the larger coordination number for this cation and the presence of an aquated coordination site in the Tb(3+)-CO(3)-Tf ternary complex. There is relatively little variation in the k(max) values for the saturation pathway for Tb(3+), Ga(3+), Al(3+), and In(3+), but the k(max) value for Fe(3+) is significantly smaller. It is suggested that protein interactions across the interdomain cleft of transferrin largely control the release of the first group of metal ions, while the breaking of stronger metal-protein bonds slows the rate of iron release. The rates of metal binding to apotransferrin are clearly controlled in large part by the hydrolytic tendencies of the free metal ions. For the more amphoteric metal ions Al(3+) and Ga(3+), there is rapid protein binding, and the addition of citrate actually retards this reaction. In contrast, the nonamphoteric In(3+) ion binds very slowly in the absence of citrate, presumably due to the rapid formation of polymeric In-hydroxo complexes upon addition of the unchelated metal ion to the pH 7.4 protein solution. The addition of citrate to the reaction accelerates the binding of In(3+) to apoTf, presumably by forming soluble, mononuclear In-citrate complexes.     

Preparation of ammonium intercalated vanadyl phosphate by redox intercalation and ion exchange

Redox intercalation of NH₄⁺ into vanadyl phosphate dihydrate (VOPO₄·2H₂O) leads to a two-phase (NH₄)_xVOPO₄·H₂O (x=0.2−0.9) compound with interlayer distances of 6.7 and 6.4 Å. Ammonium ions can be incorporated into the interlayer space of VOPO₄ also by an ion exchange, starting from alkali-metal redox-intercalated vanadyl phosphate Me_xVOPO₄·yH₂O (Me=Li, Na, K, Rb). Several phases are formed during the ion exchange, one of them with the interlayer distance corresponding to the original Me_xVOPO₄·yH₂O phase, the second one corresponding to formed (NH₄)_xVOPO₄·H₂O. In addition, a third phase is formed by the ion exchange when Li_0.98VOPO₄·1.98H₂O or Rb_0.60VOPO₄·1.3H₂O are used as starting compounds. An opposite ion exchange of NH₄⁺ for Me⁺ starting from (NH₄)_xVOPO₄·H₂O does not proceed.     

Radioanalytical separation and size-dependent ion exchange property of micelle-directed titanium phosphate nanocomposites

Nanocomposite titanium-phosphate (TiP) of different sizes was synthesized using Triton X-100 (polyethylene glycol-p-isooctylphenyl ether) surfactant. The materials were characterized by FTIR and powdered X-ray diffraction (XRD). The structural and morphological details of the material were obtained by scanning electron microscopy (SEM) and transmission electron microscopy. The SEM study was followed by energy dispersive spectroscopic analysis for elemental analysis of the sample. The important peaks of the XRD spectra were analyzed to determine the probable composition of the material. The average size distribution of the particles was determined by dynamic light scattering method. Ion exchange capacity was measured for different metal ions with sizes of the TiP nanocomposite and size-dependent ion exchange property of the material was investigated thoroughly. The nanomaterial of the smallest size of around 43 nm was employed to separate carrier-free ^137mBa from ¹³⁷Cs in column chromatographic technique using 1.0 M HNO₃ as eluting agent at pH 5.     

Study of lithium ion exchange by two synthetic zeolites: Kinetics and equilibrium

We examined the exchange of univalent cations (Na+ and H+) retained on two commercially available synthetic zeolites with Li+ ions present in aqueous solutions in contact with the solids with a view to preparing effective controlled-release pharmaceutical forms. The studied zeolites were manufactured by Merck and featured channel diameters of 0.5 (Zeolite 5A, Ref. 1.05705.250, designated Z-05 in this work) and 1.0 nm (Zeolite 13X, Ref. 1.05703.250, designated Z-10 here). The XRD technique revealed that Z-05 possesses an LTA structure derived from that of sodalite and Z-10 a faujasite-type structure. Their exchange capacities were found to be 2.72 and 3.54 meq/g. The Z-Na + Li(+) / Z-Li + Na(+) and Z-H + Li(+) / Z-Li + H(+) ion-exchange processes were found to be reversible and their kinetic laws to obey the equation (-dC/dt) = k(a) x C(n) x (1-theta) - (k(d) x theta), with n = 1 for Z-10 and n = 2 for Z-05. Based on the equilibrium results, the overall processes involve one (with Z-05) or two single ion-exchange processes (with Z-10). In both cases, the equations that govern equilibrium are direct results of the kinetic laws. Thus, the first process-the one with only Z-05-involves the retention of Li+ cations at anionic sites on the outer surface of the solid and their access to the larger pores; the second process-which occurs with Z-10 only-involves the retention of lithium(I) cations within the zeolite channels. In both systems, the exchange with Li+ (from the aqueous solution) is easier than that with H+; this is consistent with our kinetic, equilibrium, and thermodynamic results.     

Highly selective ferric ion sorption and exchange by crystalline metal phosphonates constructed from tetraphosphonic acids

With the motivation of searching for highly selective ferric ion sorbents, two open-framework and microporous materials, {[Pb7(HEDTP)2(H2O)] x 7H2O}n (1) and {[Zn2(H4EDTP)] x 2H2O}n (2) [H8EDTP = N,N,N',N'-ethylenediaminetetrakis(methylenephosphonic acid)], have been synthesized and structurally characterized. The structure of compound 1 results from the seven crystallographically different lead atoms that are bridged by two HEDTP(7-) ligands to yield a three-dimensional microporous framework with tunnels along the a and b axes. Compound 2 features a layer architecture built of square waves along the a axis. The layers are connected by hydrogen bonds between uncoordinated phosphonate oxygen atoms to form a three-dimensional supramolecular network, with one-dimensional tunnels along the a axis. Both compounds 1 and 2 exhibited high ion sorption and exchange capacities for millimolar concentrations of Fe(III). Specifically, when 0.01 g of 1 (or 2) was added to 5 mL of a 1 mM metallic chloride aqueous solution and the mixture was allowed to stand for 2 days at room temperature, compound 1 adsorbed nearly 100% of Fe(III) and compound 2 adsorbed 96.8% of Fe(III). They were also found to adsorb ferric ions selectively over other metal ions, such as Ca(II), Cr(II), Mn(II), Cu(II), Zn(II), Cd(II), etc. Their special ferric ion uptake capacities may be attributed to the cation exchange, coordination bonding, and electrostatic attraction between ferric ions and metal phosphonates.     

Inorganic ion exchange membranes consisting of microcrystals of zirconium phosphate supported by Kynar®

Membranes consisting of insoluble acid salts of tetravalent metals are attractive for their potential use in some technological processes where organic ion exchange membranes cannot be employed because of their degradability under drastic conditions (e.g. high temperature, high doses of ionizing radiation, high concentration of oxidizing agents, etc.). p]A brief survey of crystalline insoluble acid salts of tetravalent metals and membranes consisting of these compounds is presented, and Zr(HPO₄)₂ · H₂O— Kynar® membranes are examined in detail. Although the ratio of surface protons to internal protons is small (～10^?3), the mobility of the protons in the surface of microcrystals is much higher than that of internal protons (≥ 10⁴ times), so that most of the electric transport occurs at the surface. Thus, these heterogeneous membranes behave as homogeneous porous membranes, the pores being the free spaces between adjacent flat microcrystals of Zr(HPO₄)₂ · H₂O. For this reason the concentration potential depends essentially on the nature of the counterions present in the surface. This was confirmed by preparing membranes with microcrystals of Zr(HPO₄)₂ · H₂O whose surface protons were partially or totally replaced by Cs⁺ions. An equation has been derived which fits the experimental data and permits a calculation of the counterion composition at the surface of the microcrystals from measurements of membrane potential. Some possibilities for enhancing the internal transport are discussed.     

Kinetics of solid-phase H<Superscript>+</Superscript>/M<Superscript>+</Superscript> ion exchange on hafnium hydrogen phosphate

The kinetics of solid-phase reactions between hafnium hydrogen phosphate and alkali metal chlorides were studied by thermogravimetry with subsequent analysis of leaving gases. For sodium and potassium chlorides, the reaction occurs in two stages; the first stages produces MHHf(PO₄)₂, and the second yields the MHf₂(PO₄)₃ NASICON phosphate. For rubidium chloride, the reaction is one stage and produces Rb₂Hf(PO₄)₂. For cesium chloride, the reaction is an analogue of the reaction for rubidium chloride, but has two stages. Kinetic data were used to determine interdiffusion coefficients for hydrogen and alkali-metal ions at various temperatures and the activation energies of interdiffusion.     

Kinetics of the anion exchange involving ethylmercury(II) hydroxide and bromide ion

The temperature-jump method has been used to study the kinetics of the exchange reaction EtHgOH + Br⁻⇌EtHgBr + OH⁻ in aqueous alkaline solution at 25°C and ionic strength 0.10M. The rate constants for the forward and the reverse reactions are smaller than those obtained for the corresponding reactions involving methylmercuric hydroxide, providing further evidence for the occurrence of the associative mechanism previously suggested. The rate constants for the reaction EtHgOH⇌EtHg_aq⁺ + OH⁻ have been estimated and an approximate value for the rate constant of the reaction between EtHg_aq⁺ and bromide ion is also reported.