Sorption of radioactive strontium on a silica-titania mixed hydrous oxide gel

A silica-titania (Si–Ti) mixed hydroxide gel with Ti to Si mole ratio of 11 was prepared. Studies on the sorption of radiostrontium from alkaline solutions having various concentrations of sodium were carried out with Si–Ti. The distribution coefficient (K _d) of strontium decreased with increasing concentrations of sodium from 0.1 to 4.0 mol·dm^–3. The plot of logK _d versus log [Na⁺] gave a slope of –0.55 as against the theoretical value of –2.0. This suggests that some other paths in addition to the simple stoichiometric ion exchange is also existing for sorption. From pH titrations the pH_pzc of Si–Ti was found to be 4.22 coinciding nearly with the pH sorption edge. An attempt was made to correlate the sorption results with the surface characteristics of the sorbent. It was found that sorption is favored when the surface of the sorbent transforms into an anionic form. Sharp increase in the sorption of strontium was observed when the equilibrium pH was between 7 and 10. This behavior could be attributed to the abrupt increase in net negative charge on the surface of the sorbent.     

Characterization of the pores in hydrous ferric oxide aggregates formed by freezing and thawing

Hydrous ferric oxides (HFO) are efficient sorbents for inorganic and organic pollutants and therefore have great potentials in environmental science and engineering applications. Freezing and thawing of HFO suspensions leads to the formation of dense HFO aggregates. It facilitates the handling and increases the drying rate of HFO. In this study, we used a combination of pycnometry, gas adsorption (N(2) gas, water vapor), and small-angle neutron scattering (SANS) to characterize the porosity and pore size distribution of dense HFO aggregates formed by freezing dialyzed HFO suspensions at -25 degrees C and thawing them at room temperature. The crystallinity of the HFO, which was a 2-line ferrihydrite, was not affected by this treatment. Wet sieving and laser diffraction analysis showed that the dense HFO aggregates had a unimodal size distribution with an average diameter of 235+/-35 microm. Increasing the freezing rate by cooling with liquid N(2) (-196 degrees C) resulted in much smaller aggregates with an average diameter of 20 microm. Adding NaNO(3) electrolyte to the HFO suspensions prior to freezing also resulted in the formation of smaller aggregates. The dense HFO aggregates formed at -25 degrees C had a porosity of 0.73+/-0.02 ll(-1). SANS revealed a unimodal size distribution of pores, with an average pore diameter of 2.0 nm. The diameter of the HFO crystallites was estimated by transmission electron microscopy to be 1.9+/-0.5 nm. Geometrical considerations taking into account the unit particle and average pore size suggest that the crystallites retain 1-2 layers of hydration water during the coagulation induced by freezing. Analysis by N(2) gas adsorption showed that drying the dense HFO aggregates induced a reduction in porosity by about 25% and shifted the pore size distribution to smaller diameters. Rewetting during water vapor adsorption did not induce significant changes of the aggregate structure. The specific surface area of the dry HFO aggregates was between 320 and 380 m(2)g(-1).     

Sorption studies of strontium on hydrous zirconium dioxide

Summary Hydrous zirconium dioxide was prepared to study the sorption behavior and the removal of strontium from radioactive waste solutions. The hydrous zirconium dioxide were identified and characterized. The parameters affecting the strontium adsorption were investigated. Sorption data have been interpreted in terms of Langmuir equations. Thermodynamic parameters for the sorption system have been determined at 303, 313, 323 and 333 K temperatures. The values ofDH°=102.30 kJ/mol andDG°=-26.03 kJ/mol at 313 K prove that the sorption of strontium on hydrous zirconium oxide is an endothermic and a spontaneous process.     

Sorption of cesium and strontium ions on hydrous titanium dioxide from chloride medium

Epiboron instrumental neutron activation analysis (EINAA) using flexible boron as thermal neutron filter, has been used to analyze several salt samples obtained from various markets in Ghana for iodine. The method involves the irradiation of samples in boron carbide-lined polyethylene vials at the outer irradiation site of the Ghana Research Reactor-1 (GHARR-1). The samples were then counted directly without any pre-treatment on a Canberra N-type HPGe detector. The qualitative and quantitative analyses were done using the 443 keV photopeak of ¹²⁸I. The precision and accuracy of the method have been evaluated and the detection limits of the various samples were calculated. The values of iodine determined in the iodized salt range between 10.0 and 210 ppm. For non-iodinated salts, iodine levels were below 500 ppb. The values obtained show great variations among the salt samples, sample collection time and from market to market. This results show that the method can be successfully applied in the determination of trace amount of iodine in salt samples without any chemical separation.     

Sorption of hafnium on hydrous titanium oxide using radiotracer technique

The sorption of hafnium on hydrous titanium oxide (TiO₂·1.94 H₂O) has been studied in detail. Maximum sorption of hafnium can be achieved from a pH 7 buffer solution containing boric acid and sodium hydroxide using 50 mg of the oxide after 30 minutes shaking. The value ofk _d, the rate constant of intraparticle transport for hafnium sorption, from 0.01M hydrochloric and perchloric acid and pH 7 buffer solutions has been found to be 17 mmole·g^–1·min^–2. The kinetics of hafnium sorption follows Lagergren equation in 0.01M HCl solution only. The values of the overall rate constantK=6.33·10^–2 min^–1 and of the rate constant for sorptionk ₁=6.32·10^–2 min^–1 and desorptionk ₂=2.28·10^–5 min^–1 have been evaluated using linear regression analysis. The value of correlation factor() is 0.9824. The influence of hafnium concentration on its sorption has been examined from 4.55·10^–5 to 9.01·10^–4 M from pH 7 buffer solution. The sorption data followed only the Langmuir sorption isotherm. The saturation capacity of 9.52 mmole·g^–1 and of a constant related to sorption energy have been estimated to be 2917 dm³·mole^–1. Among all the additional anions and cations tested only citrate ions reduce the sorption significantly. Under optimal experimental conditions selected for hafnium sorption, As(III), Sn(V), Co(II), Se(IV) and Eu(III) have shown higher sorption whereas Mn(II), Ag(I) and Sc(III) are sorbed to a lesser extent. It can be concluded that a titanium oxide bed can be used for the preconcentration and removal of hafnium and other metal ions showing higher sorption from their very dilute solutions. The oxide can also be employed for the decontamination of radioactive liquid waste and for pollution abatement studies.     

Effect of gamma-ray irradiation on the sorption property of hydrous ferric oxide and ferric phosphate

The equilibirum sorption capacity of hydrous ferric oxide and ferric phosphate has been observed to increase on irradiation with γ-rays by 5.2 and 6.6%, respectively. The rate of sorption increases differently in the irradiated exchanger materials depending upon their characteristics nature; e.g., when the concentration of the Zn(ammine) solution is 0.10M, increase in the F-values, caused by irradiation is significantly more striking in the case of hydrous ferric oxide than in ferric phosphate. On the other hand, at lower external concentration of the Zn(ammine) ion (0.01M), the increase in F-values is more significant in the case of ferric phosphate.     

Sorption kinetics and diffusion of cadmium in calcium hydroxyapatites

The kinetics of Cd Sorption from aqueous solution on a synthetic calcium hydroxyapatite was investigated at 18, 28 and 75 °C in batch experiments, using also X-ray diffraction, electron microscopy and nuclear microprobe analysis. Cadmium is incorporated into the hydroxyapatite structure via diffusion and substitution for calcium ions, but exchange equilibrium is not achieved even after 12 days of contact at 75 °C and sorption is only partly reversible. Values of diffusion coefficients were estimated from X-ray line broadening and sorption kinetics. The fact that Cd is incorporated into the bulk of the apatite with partial reversibility is important in the context of the safe storage of used sorbent material after decontamination of water polluted by cadmium.     

Sorption and diffusion in heteropolyoxometalates : I. Aromatic hydrocarbons

The sorption capacities and diffusivities of benzene, toluene, xylene, mesitylene, and m-diethylbenzene have been measured at 293, 308, and 323 K on three solid heteropolyoxometalates, 12-tungstophosphoric, 12-molybdophosphoric, and 12-tungstosilicic acids, and their ammonium salts. Heats of adsorption were calculated from the former data. The microporous salts show considerably larger sorption capacities than the nonporous solid acids, not surprisingly, and the former show sorption properties which are evidently related to both the micropore size distributions and the nature of the sorbate molecules. Diffusivities of aromatic hydrocarbons were found to decrease with the increase in molecular weight and kinetic diameter, and were also dependent on the particular heteropolyoxometalate being examined. Heats of adsorption, in general, show no evidence of chemisorption effects. However, the sorption of mesitylene and of m-diethylbenzene on ammonium 12-tungstosilicate was found to be irreversible. Little or no evidence was found for the penetration of aromatic hydrocarbons into the bulk structure of the solid heteropolyacids.     

EXAFS study of Zn sorption mechanisms on hydrous ferric oxide over extended reaction time

The sorption species and coordination environment of zinc sorbed on to hydrous ferric oxide (HFO) did not change for aging times up to six months. At an initial concentration of 10(4-) M, Zn formed innersphere surface complexes on the surface of HFO. Zn was tetrahedrally coordinated with oxygen atoms at ZnO bond distance of approximately 1.94-1.97 A with coordination number of approximately 3.8-4.7. In the second shell Zn appeared to be coordinated with Fe with a bond distance of approximately 3.42-3.49 A. At an initial concentration of 10(3-) M, both innersphere and polynuclear complexes were feasible sorption products. The first shell was tetrahedrally coordinated with about four oxygen atoms at a bond distance of 1.96 A. The second shell could be attributed to either ZnFe or ZnZn correlations with almost the same bond distance of 3.42-3.44 A.     

Preparation of polymer-supported hydrated ferric oxide based on Donnan membrane effect and its application for arsenic removal

In the present study a novel technique was proposed to prepare a polymer-supported hydrated ferric oxide (D201-HFO) based on Donnan membrane effect by using a strongly basic anion exchanger D201 as the host material and FeCl3-HCl-NaCl solution as the reaction environment. D201-HFO was found to exhibit higher capacity for arsenic removal than a commercial sorbent Purolite ArsenX. Furthermore, it presents favorable adsorption selectivity for arsenic removal from aqueous solution, as well as satis- factory kinetics. Fixed-bed column experiments showed that arsenic sorption on D201-HFO could re- sult in concentration of this toxic metalloid element below 10 μg/L, which was the new maximum con- centration limit set recently by the European Commission and imposed by the US EPA and China. Also, the spent D201-HFO is amenable to efficient regeneration by NaOH-NaCl solution.     

Autoxidation of poly-4-methylpentene-1. I. The role of diffusion in autoxidation kinetics

An apparatus is described for the measurement of oxygen uptake into a polymer sample at constant oxygen pressures in the range 20–1000 mm Hg. Measurements of the rate of oxygen uptake into poly-4-methylpentene-1 show that the rate is accurately first-order in oxygen pressure over the range 50–800 mm pressure for temperatures ranging from 122 to 154°C and film thickness in the range 0.001–0.025 cm. A theoretical treatment of the kinetics of a reaction in which oxygen diffuses into both faces of a thin film, in which it is consumed by a first-order reaction shows that the oxidation rate ρ per unit area of film surface is given by ρ = ρ_∞ tanh ßL/2 where ρ_∞ is the limiting oxidation rate for a thick film, L is the film thickness, and ß = (k/D)^1/2, k being the oxidation rate constant and D the diffusion constant. Values of D and the activation energy for diffusion calculated from autoxidation data are in good agreement with values determined directly.     

Ferric hydrous oxide sols I. Monodispersed basic iron(III) sulfate particles

The methods of preparation of basic ferric sulfate sols consisting of particles uniform in shape of extremely narrow size distribution are described in detail. To produce such sols, acidic solutions containing ferric ions and sulfate ions were aged at elevated temperatures for a few hours. Solids formed from solutions containing a mixture of a ferric salt with a metal sulfate consisted of Fe₃(SO₄)₂(OH)₅· 2H₂O, which is the basic formulation for the alunite mineral group, whereas particles formed from ferric sulfate solutions also included Fe₄(SO₄)-(OH)₁₀ in varying proportions. The morphology of the particles was strongly dependent on the [Fe³⁺]: [SO₄²⁻] ratio in solution. Changes in the cation (K⁺, NH₄⁺, Na⁺) of the sulfate salt used in the mixture with ferric nitrate solutions greatly affected the particle size and also exhibited some effect on the lattice parameters. Certain cations (Mg²⁺, Ni²⁺, Cu²⁺) completely inhibited particle formation. During the first few hours of growth of the Fe₃(SO₄)₂-(OH)₅· 2H₂0 particles their diameters increased essentially linearly with time, indicating that the rate determining step was the surface reaction. The relevance of these systems to the study of corrosion of iron and steel is discussed.     

Sorption of uranium on freshly prepared hydrous titanium oxide and its utilization in determination of 236U using accelerator mass spectrometry

Journal of Radioanalytical and Nuclear Chemistry - The determination of 236U with accelerator mass spectrometry (AMS) requires efficient separation methods with high concentration factors. The...     

Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. I. Surface limited desorption kinetics in amorphous solid water

The adsorption and desorption kinetics of N2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programed desorption (TPD), and reflection-absorption infrared spectroscopy. The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness indicating that N2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage-dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, approximately 0.5 microm). The success of the rescaling model indicates that N2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption.     

Identification of unknown diffusion coefficient in pure diffusive linear model of chronoamperometry. I. The theory

Coefficient identification problem for diffusion equation u _t (x, t) = (D(x)u _x (x, t)) _x arising in chronoamperometry is studied. The adjoint problem approach is developed for the case when the output measured data is given in the form of left/right flux. Analytical formulas for determination of the values D(0), D(L) at the endpoints x = 0; L, of the unknown coefficient D(x), via the solution v(x, t) of the constant coefficient equation v _t (x, t) = D v _xx (x, t) is obtained. The integral identity relating solutions of the forward and corresponding adjoint problems is derived. This integral identity permits one to prove the monotonicity and invertibility of input-output map, as well as formulate the gradient of the cost functional via the solutions of the direct and adjoint problems.