Investigation of thorium adsorption with multi-walled carbon nanotubes

In this day and time, nuclear power is becoming increasingly important with the growing energy crisis. Therefore, uranium and thorium draw attention as nuclear fuel. Reactor types that based on the thorium fuel cycle are under development due to the presence of thorium in the earth's crust more than uranium. High combustion rate, radiation and chemical stability, higher thermal conductivity than uranium, less production of minor actinides and plutonium and low fabrication costs properties of thorium makes up it as nuclear fuel in an advantageous situation. In addition, a significant proportion of world reserves of thorium (380000 tonnes ThO2) is located in Eskisehir in Turkey. Hence, researches done with thorium gain importance. Thorium removal from radioactive wastes, that occur when thorium fuel cycle is used, is important because of toxicity and long half-life of it. Therefore, various methods are applied for removal of thorium from liquid wastes, such as liquid-liquid extraction, ion exchange, precipitation and adsorption. Among of these, the importance of adsorption is the separation of the elements in trace amounts from a large volume of solution. Many types of adsorbents (biosorbents, synthetic sorbents, activated carbon, composite materials, fiber types, clay, zeolite etc.) are used in adsorption applications. Carbon nanotubes have found many usage areas after the discovery in 1991 by Iijima. Carbon nanotubes have great potential for the adsorption of metal ions from aqueous solutions because of their porous structure, large surface area, low density, high mechanical, thermal and chemical stability. For these reasons, researches done related to the usage of carbon nanotubes as an adsorbent material become important. The scope of this study is to examine the utility of multi-walled carbon nanotubes as an effective adsorbent in nuclear waste management and to determine optimum conditions by examining the adsorption of multi-walled carbon nanotubes to recover from thorium waste solutions. In this study, carbon nanotubes used as adsorbent, which modified with HNO3 , and parameters affecting the adsorption of thorium (pH, initial thorium concentration, temperature and contact time) were examined. Furthermore, instead of individually observing each variable, "experimental design" technique was used and optimum conditions for thorium adsorption were determined as pH 4, initial thorium concentration 100 mg.L-1, temperature 25oC, shaking time 15 minute. Optimum uptake capacity was calculated to be 65.75 mg.g-1 in 10 different tests performed under these conditions. Adsorption isotherms (Langmuir, Freundlich, Dubinin-Radushkevich) were investigated and the highest R2 value (R2 = 0.95) was obtained at the Freundlich isotherm. When the thermodynamic properties (standard enthalpy change, standard entropy change, Gibbs standard free energy change) were examined, it was found that the process was endothermic and voluntary. It has been determined that the pseudo second-order velocity equation is suitable for adsorption of thionium on multi-walled carbon nanotubes. Keywords: Adsorption; Thorium; Carbon Nanotubes; Response Surface Methadology