Ramazan Donat

Thermal and kinetic analysis of uranium salts

Thermal decomposition of U(C2O4)2·6H2O was studied using TG method in nitrogen, air, and oxygen atmospheres. The decomposition proceeded in five stages. The first three stages were dehydration reactions and corresponded to removal of four, one, and one mole water, respectively. Anhydrous salt decomposed to oxide products in two stages. The decomposition products in nitrogen atmosphere were different from those in air and oxygen atmospheres. In nitrogen atmosphere UO1.5(CO3)0.5 was the first product and U2O5 was the second product, while these in air and oxygen atmospheres were UO(CO3) and UO3, respectively. The second decomposition products were not stable and converted to stable oxides (nitrogen: UO2, air–oxygen: U3O8). The kinetics of each reaction was investigated with using Kissinger–Akahira–Sunose and Flynn–Wall–Ozawa methods. These methods were combined with modeling equations for thermodynamic functions, the effective models were investigated and thermodynamic values were calculated.

Thermal and kinetic analysis of uranium salts: Part III. Uranium(IV) oxalate hydrates

The thermal decomposition kinetics of UO2C2O4·3H2O were studied by TG method in a flowing nitrogen, air, and oxygen atmospheres. It is found that UO2C2O4·3H2O decomposes to uranium oxides in four stages in all atmosphere. The first two stages are the same in the whole atmosphere that correspond to dehydration reactions. The last two stages correspond to decomposition reactions. Final decomposition products are determined with X-Ray powder diffraction method. Decomposition mechanisms are different in nitrogen atmosphere from air and oxygen atmosphere. The activation energies of all reactions were calculated by model-free (KAS and FWO) methods. For investigation of reaction models, 13 kinetic model equations were tested and correct models, giving the highest linear regression, lowest standard deviation, and agreement of activation energy value to those obtained from KAS and FWO equations were found. The optimized value of activation energy and Arrhenius factor were calculated with the best model equation. Using these values, thermodynamic functions (ΔH*, ΔS*, and ΔG*) were calculated.

Removal of thorium(IV) from aqueous solutions by natural sepiolite

Abstract Natural sepiolite has been tested as a potential sorbent for the removal of Th(IV) from aqueous solutions by batch technique. Effects of various parameters on the adsorption process have been investigated. Under optimum conditions, Th(IV) was adsorbed with high adsorption efficiency. The results indicated that sorption of Th(IV) on sepiolite was strongly affected by pH values and temperature. The adsorption patterns of thorium on the sepiolite adsorbent followed the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherms. The thermodynamic data (ΔH°, ΔS°, ΔG°) are calculated from the temperature-dependent sorption isotherms. The results suggest that sorption process of Th(IV) on sepiolite is spontaneous and endothermic.

Adsorption of U(VI) ions from aqueous solutions by activated carbon prepared from Antep pistachio (Pistacia vera L.) shells

Abstract Antep pistachio (Pistacia vera L.) shells an abundant and low cost natural resource in Turkey was used to prepare activated carbon by physiochemical activation and carbon dioxide (CO2) atmosphere as the activating agents at 700°C for 2 h. The adsorption equilibrium of U(VI) from aqueous solutions on such carbon has been studied using a batch system. The parameters that affect the U(VI) adsorption, such as particle size of adsorbent, contact time, of pH of the solution, and temperature, have been investigated and conditions have also been optimized. The equilibrium data for U(VI) ions’ adsorption onto activated carbon well fitted to the Langmuir equation, with a maximum monolayer adsorption capacity of 8.68 mg/g, The Freundlich and Dubinin–Radushkevich (D–R) isotherms have been applied and the data correlated well with Freundlich model and that the adsorption is physical in nature (Ea=15.46 kJ/mol). Thermodynamic parameters [ΔHs=11.33 kJ/mol, ΔS=0.084 kJ/molK, ΔG (293.15 K)=−13.29 kJ/mol] showed the endothermic heat of adsorption and the feasibility of the process.

Simultaneous extraction and stripping of uranium ions via multi-dropped liquid membrane system

An investigation on the separation of U(VI) from aqueous solutions via a multi dropped liquid membrane (MDLM) and its model is presented. A new method that is more advantageous than other liquid membrane techniques is applied. Di(2-ethylhexyl) phosphoric acid was used as a highly selective carrier for the transport of U(VI) ions through the MDLM. Maximum percentages of U(VI) extraction and stripping achieved were 98.9 % using 1.459 × 10–3 M D2EHPA as the extractant, 1 M NH4HCO3 as the stripping solution. The influences of pH and temperature on donor and acceptor phases, the influence of carrier phase concentrations and membrane’s flow velocity were investigated. For the description of relationships of U(VI) concentration in particular phases with time, a model based on the assumption of consecutive first-order reactions was proposed. The kinetic parameters (k1, k2, Rmmax, tmax, Jdmax, Jamax) were calculated for the interface reactions assuming two consecutive, irreversible first-order reactions. The activation energy value was calculated as 10.2 kcal mol-1 for extraction. The value of calculated activation energy indicates that the process is chemically controlled by U(VI) ions. The experiments have demonstrated that D2EHPA derivative is a good carrier for U(VI) transport through MDLM in the study.