Adsorption of Th(IV) on the modified multi-walled carbon nanotubes using central composite design

Adsorption of thorium onto nitric acid modified multi-walled carbon nanotubes was investigated by central composite design as a function of contact time, pH, initial thorium concentration and temperature. The results showed that optimum uptake capacity was 65.75$\pm$2.23 mg$\cdot$g${}^{\text{â}\text{ˆ}\text{’}1}$ with respect to pH=4, initial thorium concentration of 100 mg$\cdot$L${}^{\text{â}\text{ˆ}\text{’}1}$, 25 °C and 15 min contact time. Thermodynamic parameters [standard enthalpy ($\Delta$H${}^0$), entropy ($\Delta$S${}^0$), and free energy ($\Delta$G${}^0$)] were calculated, and the results indicated that adsorption was endothermic. Langmuir, Freundlich and Dubinin-Radushkevich isotherms have been investigated in order to characterize the adsorption process in the range of 25–100 mg$\cdot$L${}^{\text{â}\text{ˆ}\text{’}1}$ initial thorium concentration. The Freundlich isotherm is the best suited as a model because it has the highest correlation coefficient (R${}^2$=0.9485). The pseudo-second order kinetics well defined the adsorption process.