Study of Brønsted acid site in H-MCM-22 zeolite by temperature-programmed desorption of ammonia

Interaction of ammonia with H-MCM-22 zeolite (Si/Al = 24.5) was investigated by temperature-programmed desorption technique in order to obtain information on thermodynamics of the process. Average activation energy for desorption of ammonia from Brønsted acid sites of H-MCM-22 zeolite was estimated from the data obtained under conditions varying in heating rate and also flow rate of carrier gas. It resulted in value of E _d = 127 kJ mol^?1 for heat rate variation method, whereas flow rate variation led to E _d value of 111 kJ mol^?1. Obtained E _d values are compared with those reported in the literature for other zeolitic materials and discussed in the broader context of zeolite acidity. Comparison of E _d values estimated here for H-MCM-22 zeolite with corresponding data for other protonic zeolites shows that H-MCM-22 displays mediocre/lower activation energy for ammonia compared with other high-silica zeolites.     

Resorcinol - formaldehyde adsorption of cesium from Hanford waste solutions

Batch equilibrium measurements were conducted at 25±2 °C with a granular resorcinol-formaldehyde (RF) resin to determine the distribution coefficients (K _d ) for cesium. In the tests, Hanford Site actual waste sample containing radioactive cesium (¹³⁷Cs) and a pretreated waste sample that was spiked with non-radioactive ¹³³Cs were used. Initial concentrations of non-radioactive cesium in the waste sample were varied to generate an equilibrium isotherm for cesium. Majority of the tests were conducted at 25±2 °C using a liquid to solid phase ratio of 100 ml/g and at a contact time of 72 hours. Two additional tests were conducted at a liquid to solid phase ratio of 10 and at contact time of 120 hours. The measured distribution coefficient (K _d ) for radioactive cesium (¹³⁷Cs) was 948 ml/g, the K _d for non-radioactive cesium (¹³³Cs) was 1039 ml/g. The K _d for nonradioactive cesium decreased from 1039 to 691 ml/g with increased initial cesium concentration from 8 to 64 g/ml. Very little change of the K _d was observed at initial cesium concentration above 64 g/ml. The maximum sorption capacity for cesium on granular RF resin was 1.17 mmole/g dry resin. This value was calculated from the fit of the equilibrium isotherm data to the Dubinin-Radushkevich equation. Previously, a total capacity of 2.84 mmole/g was calculated by Bibler and Wallace for air-dried RF resin.This work was conducted at the Savannah River Technology Center in Aiken, SC, which is operated for the U. S. Department of Energy (DOE) by Westinghouse Savannah River Company under Contract DE-AC09-96SR18500. The Hanford River Protection Project-Waste Treatment Plant (RPP-WTP) funded this work. The authors are very grateful to Karen Palmer, Betty Mealer, and Yvonne Simpkins for their assistance in the experimental work.     

Resorcinol - formaldehyde adsorption of cesium from Hanford waste solutions

Batch equilibrium measurements were conducted at 25±2 °C with a granular resorcinol-formaldehyde (RF) resin to determine the distribution coefficients (K _d ) for cesium. In the tests, Hanford Site actual waste sample containing radioactive cesium (¹³⁷Cs) and a pretreated waste sample that was spiked with non-radioactive ¹³³Cs were used. Initial concentrations of non-radioactive cesium in the waste sample were varied to generate an equilibrium isotherm for cesium. Majority of the tests were conducted at 25±2 °C using a liquid to solid phase ratio of 100 ml/g and at a contact time of 72 hours. Two additional tests were conducted at a liquid to solid phase ratio of 10 and at contact time of 120 hours. The measured distribution coefficient (K _d ) for radioactive cesium (¹³⁷Cs) was 948 ml/g, the K _d for non-radioactive cesium (¹³³Cs) was 1039 ml/g. The K _d for nonradioactive cesium decreased from 1039 to 691 ml/g with increased initial cesium concentration from 8 to 64 g/ml. Very little change of the K _d was observed at initial cesium concentration above 64 g/ml. The maximum sorption capacity for cesium on granular RF resin was 1.17 mmole/g dry resin. This value was calculated from the fit of the equilibrium isotherm data to the Dubinin-Radushkevich equation. Previously, a total capacity of 2.84 mmole/g was calculated by Bibler and Wallace for air-dried RF resin.This work was conducted at the Savannah River Technology Center in Aiken, SC, which is operated for the U. S. Department of Energy (DOE) by Westinghouse Savannah River Company under Contract DE-AC09-96SR18500. The Hanford River Protection Project-Waste Treatment Plant (RPP-WTP) funded this work. The authors are very grateful to Karen Palmer, Betty Mealer, and Yvonne Simpkins for their assistance in the experimental work.     

Synthesis of NaX and NaY Zeolites from Tunisian Kaolinite as Base Catalysts: An Investigation of Knoevenagel Condensation

The synthesis of Faujasite‐type zeolites with high purity has been successfully performed from Tunisian kaolinite and the effects of different crystallization parameters on the final products were widely investigated. The alkaline fusion of kaolinite followed by hydrothermal treatment lead to zeolite NaX synthesis whereas the classic hydrothermal transformation of metakaolinite produces NaY zeolite. The results show that an increase in the synthesis temperature and time has improved the crystallization process of the zeolite NaX whereas the SiO₂/Al₂O₃ and the Na₂O/SiO₂ molar ratios were the key parameters to obtain a pure zeolite NaY. The highest specific surface areas obtained with the optimal crystallization conditions were 554 m2 g^?1 and 592 m2 g^?1 for respectively NaX and NaY zeolites. The basic properties of NaX and NaY zeolites were explored in the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate at 140 °C as a test reaction in the absence of solvent. The influence of ion exchange with cesium cation on the catalytic activity of prepared catalysts was also investigated. It was found that the NaX provided higher activity than that of NaY catalyst due to its lower Si/Al ratio whereas a cesium exchange conferred higher basicity to the prepared Na‐faujasite.     

Comparison of catalytic performance of synthesized EU-1 zeolite with dealuminated EU-1 zeolite for <Emphasis Type="Italic">m</Emphasis>-xylene isomerization reaction

EU-1 zeolite was synthesized with high purity by a hydrothermal method and under optimum conditions: synthesis time 72 h, temperature 200°C, and aging time 12 h. Then, the synthesized EU-1 zeolite was modified by dealumination with nitric acid and the changes of the properties such as surface area, pore volume, and Si/Al ratio were investigated. The catalytic performance of these two catalysts was studied and compared with a commercial mordenite catalyst for meta-xylene isomerization reaction in a fixed bed reactor. The results showed that the modification of catalyst with acid increases Si/Al ratio from 25 to 50 due to the removal of a number of aluminium atoms from the framework of zeolite. Also the catalyst surface area increased from 300.237 m² g^?1 for EU-1 to 333.639 m² g^?1 for modified EU-1. According to results, the modified Eu-1 had higher para/ortho ratio, meta-xylene conversion, and para-xylene yield than EU-1 and commercial mordenite in the meta-xylene isomerization reaction.     

SuperLig 644 equilibrium sorption data for cesium from Hanford tank waste supernates

SuperLig 644 ion exchange resin is currently being evaluated for cesium (¹³⁷Cs) removal from radioactive Hanford tank wastes. To assess the performace of the resin in column configuration, a multiple batch contact method was used to determine the equilibrium distribution coefficients (K _d) and percent removal for ¹³⁷Cs from highly alkaline waste solutions obtained from the Hanford Site. The equilibrium loading data were interpreted in terms of Freundlich and Dublin-Radushkevics (D-R) isotherms. The equations fit the experimental data remarkably well considering the complexity of the Hanford tank waste compositions. The mean energy of adsorption and total resin capacity were calculated. The mean free energy for adsorption of cesium from Hanford tanks was ~9 kJ/mol. The total exchange capacity of the SuperLig 644 resin ranged from 0.72 to 3.46 mmole/g resin, depending on the Hanford tank composition. The K _d results reveal that SuperLig 644 resin in highly selective for cesium in the presence of relatively high concentrations of sodium and potassium salts.     

Migration of 134Cs in unsaturated soils at a site in Egypt

To demonstrate the safety performance assessment for the disposal of ¹³⁴Cs radionuclide in a geological formation, several investigations were required to calculate the possible release of radionuclides into groundwater. This research examined the sorption behavior of radioactive cesium (¹³⁴Cs) in natural groundwater. Cesium chloride (10^-6 to 10^-2 mol^.l^-1) was used as a carrier, traced with ¹³⁴Cs radionuclide. Distribution coefficients of radiocesium for sorption and desorption were measured on natural soil samples of different grain size fractions (400 to 36 mm). Cesium sorption and desorption were found reversible at cesium chloride concentrations between 10^-6 mol^.l^-1 to less than 10^-3 mol^.l^-1. Sequential extraction procedures showed that the cesium sorption on soils were of various types: those easily desorbed, ion-exchanged, bound to carbonate, iron oxides, and organic matter. To demonstrate the safety performance assessment for the disposal of ¹³⁴Cs radionuclide in a geological formation, several investigations were required to calculate the possible release of radionuclides into groundwater. This research examined the sorption behavior of radioactive cesium (¹³⁴Cs) in natural groundwater. Cesium chloride (10^-6 to 10^-2 mol^.l^-1) was used as a carrier, traced with ¹³⁴Cs radionuclide. Distribution coefficients of radiocesium for sorption and desorption were measured on natural soil samples of different grain size fractions (400 to 36 mm). Cesium sorption and desorption were found reversible at cesium chloride concentrations between 10^-6 mol^.l^-1 to less than 10^-3 mol^.l^-1. Sequential extraction procedures showed that the cesium sorption on soils were of various types: those easily desorbed, ion-exchanged, bound to carbonate, iron oxides, and organic matter.     

A radiochemical study on the thermodynamics of cesium adsorption on potassium copper nickel hexacyanoferrate(II) from aqueous solutions

This paper reports a radiochemical study on the themodynamics of cesium adsorption on potassium copper nickel hexacyanoferrate(II) (KCNF) using¹³⁴Cs radionuclide as a radiotracer. The values of themodynamic parameters like H ^o , S ^o and G ^o have been calculated using the temperature dependence of cesium adsorption on KCNF. These values indicate the endothermic nature of the adsorption process. Freundlich and Langmuir izotems have shown their applicability for the adsorption of cesium on KCNF at different temperatures and their corresponding constants have been calculated and interpreted. The Dubinin-Radushkevich (D-R) equation has also been applied to the adsorption data and the values of mean free energies of the adsorption process at different temperatures have been evaluated. These values indicate that the adsorption of cesium on KCNF is expected to follow an ion exchange process.     

Diffusion Mechanism of Carbon Dioxide in Zeolite 4A and CaX Pellets

The adsorption kinetics and equilibria of CO₂ in commercial zeolite 4A and CaX pellets were theoretically and experimentally studied by a gravimetric method in the range of 273–313 K and 0.0–0.8 atm. The diffusion mechanism of an adsorbate into a pellet is composed of micropore and macropore diffusion due to the bidisperse structure of the pellet. When one diffusion mechanism played a more important role than the other in determining the overall diffusion rate, the diffusion rate was estimated by the nonisothermal monodisperse diffusion model (NMDM). However, when the combined effects of both mechanisms controlled the overall adsorption kinetics, the experimental uptake was analyzed by the nonisothermal bidisperse diffusion model (NBDM). The CO₂ diffusion in zeolite 4A pellets was controlled by micropore diffusion within the experimental pressure and temperature ranges. However, both macropore and micropore diffusion contributed to CO₂ diffusion in the zeolite CaX pellet. The overall CO₂ diffusion rate in zeolite CaX became faster as pressure increased mainly due to its highly favorable isotherm in the zeolite CaX. The micropore diffusion time constant of CO₂ in the zeolite CaX pellet was approximately one hundred times greater than that in the zeolite 4A pellet. In addition, the activation energy of micropore diffusion of CO₂ diffusion in the zeolite CaX pellet was smaller than that in the zeolite 4A pellet. In this study, the dimensionless parameter, , indicating the relative importance of macropore and micropore diffusion, was modified to consider non-zero coverage as an initial condition for each step in the gravimetric method. When is greater than 100, the overall adsorption rate is controlled by macropore diffusion. However, in cases where is less than 0.1, micropore diffusion is the dominant mechanism in the overall adsorption rate. In the case of a system with between these values, both macropore and micropore diffusion contributed to the overall diffusion rate.     

A cesium-selective electrode prepared from a crystalline synthetic zeolite of the mordenite type

Cesium-selective electrodes were prepared from a synthetic zeolite molecular sieve of the mordenite type. The membrane was made from zeolite crystals embedded in an epoxy resin. The response towards cesium ions -was linear from about 3 × 10^-5 to at least 0.1 mol l^-1, with almost Nernstian slope. The usable pH range was 3.5–9 for 0.01 mol l^-1 cesium solutions. The selectivity order was Cs > Ag, K > Na > Li for univalent ions. The response for divalent ions was not Nernstian, Cs > Ba > Ca > Cu. About two weeks after the hydration, there was a degradation of electrode performance indicated by increased detection limit and decreased slope.     

90Sr and 137Cs radioisotopes and heavy metal concentrations in pharmaceutical herbal plants from the Lublin (Poland) region

Radioactivity of ⁹⁰Sr and ¹³⁷Cs in several species of herbs collected at Fajsawice and Góra Puawska located in Lublin (Poland) region was examined. Depending on the type of herb the radioactivity of strontium ranged from 0.004 to 1.808 Bq/kg dry weight whereas that from cesium varied from 0.053 to 9.853 Bq/kg. Calculated transfer factors of the radioisotopes in the plants studied revealed much stronger accumulation of strontium than cesium. Concentration of calcium and potassium in these plants has no influence on the concentration of measured radioisotopes.     

Studies on the separation of cesium-137 from the acidic fission product waste solutions on a new complex inorganic exchanger (Zr−P-APW)

A new inorganic exchanger zirconiumphosphate-ammonium phosphotungstate (Zr–P-APW) has been synthesized in granular form suitable for column work. TheK _d values for different metal ions were determined and the affinity order was found to be CsRb>Zr>Ce> rare earths. Sodium exchange capacity, pH-titration curve, breakthrough capacities for cesium (both in pure HNO₃ and in different types of simulated nuclear wastes) and elution of cesium from the Zr–P-APW column have been studied. A selective method for the removal of cesium from other radioactive fission products has been developed. The exchanger was found to be stable to a -radiation dose of 10⁸ rads in presence and absence of 2M nitric acid.     

Facile synthesis of 5A zeolite from attapulgite clay for adsorption of <Emphasis Type="Italic">n</Emphasis>-paraffins

5A zeolites were facilely synthesized from attapulgite clay and sodium aluminate precursors. The optimum synthesis condition for 4A zeolite (Na-form) were H₂O/attapulgite ratio of 40:1 volume/mass, NaOH/attapulgite mass ratio of 2.35:1, the crystallization time was 4 h at 80–85 °C. The 4A zeolite was converted to related 5A zeolite (Ca-form) through ionic exchanges using calcium chloride solution with the Si/Al mole ratio of 1.3. SEM images demonstrated that as-synthesized 5A zeolites are ordered cubic crystals, average crystals length dimension is 1–2 μm. And the zeolites product had a specific surface area of 482 m² g^?1 and total pore volume of 0.274 cm³ g^?1. The static adsorption experiments showed that the equilibrium adsorption capacities of n-decane and n-pentadecane on produced 5A zeolite were 0.253 and 0.510 g g^?1, respectively. And the adsorption equilibrium time of n-decane and n-pentadecane on 5A zeolite were 45 and 60 min, respectively. The experimental adsorption data of n-decane and n-pentadecane on three zeolites could be properly fitted by the Langmuir–Freundlich isotherm model.     

Sorption-desorption of radiocesium on various sorbents in presence of humic acid

In general, the amount of radiocesium sorbed by the five sorbents with 0.01 mol·dm^–3 NaCl was in order zeolite > NiFeCN–SiO₂ > montmorillonite > aerogel > silica gel. Addition of humic acid solution to the sorbents depressed the sorption of cesium by all sorbents, except for NiFeCN–SiO₂ was not seen, with the greatest effect showing to the aerogel. The presence of humic acid resulted in an enhanced desorption of cesium from zeolite, NiFeCN–SiO₂ and to a lesser extent from montmorillonite and silica gel. The order of cesium retention following desorption for both sorbent and sorbent/humic-acid mixtures was zeolit > NiFeCN–SiO₂ > montmorillonite > silica gel. The presence of humic acid resulted in decreasing of distribution coefficient values for both sorption and desorption processes.     

Derivative Enthalpies of Adsorption of p-Xylene and m-xylene onto NaY and BaY Zeolites at 150°C: Contribution to the prediction of adsorption selectivity

The derivative enthalpies of adsorption of m-xylene and p-xylene onto the NaY and BaY zeolites were measured at 150°C, then compared with those obtained at 25°C, and finally used to predict the selectivity of adsorption of xylene mixtures. Significant differences were observed as the temperature was elevated: for the NaY zeolite, the adsorbate-adsorbate interactions became prevalent, in contrast with the BaY zeolite, between zeolite and derivative interactions were stronger. The difference between the adsorption derivative enthalpies of the two xylenes displayed an abrupt variation from 2 molec. ^–1 for both zeolites, the filling from which selectivity towards m-xylene for the NaY zeolite and towards p-xylene for the BaY zeolite appeared. The preferentially adsorbed xylene was closely connected with the sense of this difference, which changed with the zeolite.This revised version was published online in November 2005 with corrections to the Cover Date.     

Cesium ion-selective electrodes based on 1,3-alternate thiacalix[4]biscrown-6,6

Four thiacalix[4]biscrown ethers with 1,3-alternate conformation were examined for the potentiometric responses in poly(vinyl chloride) membrane electrodes. Their potentiometric selectivities toward potassium and cesium ions over other alkali, alkaline earth, and transition metal ions were measured by the fixed interference method (FIM). Among the ionophores, 1,3-alternate thiacalix[4]biscrown-6,6 showed a high selectivity for cesium over potassium ion and so was optimized as a Cs⁺-selective electrode. The electrode exhibited a linear response with a near Nernstian slope of 57.6 mV per decade in the concentration range of 1.0×10⁻⁶ to 3.2×10⁻² M. It was suitable for use in aqueous solution in a wide range of pH 2.5-12.5 and had a fast response time of ca. 5 s. On the basis of 1,3-alternate thiacalix[4]biscrown-6,6, the electrode has a wide linear range and selectivity for cesium ion over potassium ion better than those previously reported with other ionophores.     

"Decoking" of a "coked" zeolite catalyst in a glow discharge

'Decoking' of a 'coked' zeolite catalyst in a glow discharge in oxygen is investigated. The 'decoking' process involves reactions of atomic oxygen (O atoms) with 'coke' and yields gases such as CO, CO₂ as well as other gaseous products that could be easily pumped out.Three different modes of discharge were investigated including a static mode, a flowing-gas mode, and a periodic-purge mode where the oxygen and other gaseous products of the discharge were replaced by fresh O₂ gas after short but regular intervals of time. In some cases, additional heating was also used to provide base temperatures of the order of 100 °C to facilitate penetration of oxygen atoms into the inner layers and cages of the zeolite catalyst.This paper presents some results of spectroscopic analytical techniques used to monitor the atomization of oxygen, oxidation of 'coke', and to confirm the process of 'decoking'. More specifically, radiation emission on the 3 s ⁵S– 3p ⁵P transitions of O around 777.2–777.5 nm were selected for monitoring the atomization of O₂. On the other hand, X-ray photo-electron spectroscopy (XPS) was used to determine the amount of residual carbon and extent of 'decoking'. Furthermore, evolution of CO and CO₂ gases as a function of time was systematically monitored in real time. For CO, the 451.1 nm band head belonging to the B¹ - A¹ bands of the Angstrom system of the CO spectrum was used, while for CO₂, the band head at 353.4 nm belonging to the CO₂⁺ spectrum was used. The rates of evolution of CO and CO₂ were related to the rate of 'decoking' of the catalyst. It is noted that in the periodic-purge mode, about 63% of the total yield of CO from a given sample of the catalyst appears in the first 3-min exposure to discharge whereas it takes up to 15 min to remove nearly 94% of the removable carbon under our experimental conditions.     

The biological half-life of137Cs in snails

The biological half-life of¹³⁷Cs in snails Helix pomatia after a single administration of contaminated diet has been investigated. The calculation was based on the retention of¹³⁷Cs in snails in vivo. It was found that loss of cesium from snails can be explained by a two-exponential retention curve leading to biological loss constants _{B 1}=0.27 d^–1 and _{B 2}=0.024 d^–1, which correspond to biological half-lives of T_{B 1}=2.5 d and T_{B 2}=28.5 d respectively. The equation describing the retention of¹³⁷Cs in snails is also presented.     

Evaluation of cesium sorption on natural mordenite

The kinetic and equilibrium sorption behaviors of Cs⁺ on mordenite, a zeolite which can sorb cesium well, were investigated by using the batch method. Cesium-137 and the stable CsNO₃ were used as tracer and carrier to study the influences on Cs⁺ sorption behaviors by changing Cs⁺ initial concentration, pH value, particle size of mordenite and experimental temperature. The equilibrium was reached in 3 days and the saturated amount of cesium sorbed is about 0.19 kg Cs/kg NM. The sorption data at 25°C and 90°C were fitted to Freundlich sorption model and nonlinear isotherms were found. However, linear isotherm was applicable with a Cs⁺ initial concentration less than 10^–3M. The decrease of Cs⁺ sorption at elevated temperature suggested the sorption reaction was exothermic. The use of centrifugation to separate the liquid from solid phases in traditional batch techniques was not suitable to the kinetic experiment of Cs sorbed by mordenite for lower concentrations.     

Determination of cesium, manganese and terbium by intracavity laser spectroscopy

The statistical characteristics of the intracavity laser spectrometer are investigated by analysis of cesium in aqueous solutions. The error distribution of measurements is normal. The relative standard deviation of the analysis in the middle of the dynamic range is equal to 9.7%. The intracavity spectral analysis was carried out at low contents of cesium, manganese and terbium in aqueous solutions. The limits of detection are: 5 gl^-1 (Cs), 20 gl^-1 (Mn) and 1000 gl^-1 (Tb). Owing to the high spectral resolution of the spectrometer used the hyperfine structure of the thallium absorption line (535.05 nm) was recorded.