Removal of thorium(IV) from aqueous solutions by carboxyl-rich hydrothermal carbon spheres through low-temperature heat treatment in air

Abstract

The number of carboxyl groups on the surface of hydrothermal carbon spheres (HCSs) has been largely increased by simply heating at lower temperature in air. The textural properties of the carboxyl-rich hydrothermal carbon spheres were characterized using Boehm titrations, scanning electron microscopy, Fourier transform infrared spectrometer (FT-IR), and elemental analysis (EA). The result of Boehm titrations indicated that the content of carboxyl groups on HCSs increased significantly from 0.53 to 3.81 mmol/g after heat treatment at 300°C, which was also confirmed by FT-IR and EA qualitatively. The ability of heat-treated HCSs has been explored for the removal and recovery of thorium from aqueous solutions, and the influences of different experimental parameters, such as solution pH, heat treatment temperature, contact time, initial thorium concentration, temperature, and ionic strength on adsorption, were investigated. The Th(IV) sorption capacity of HCSs increased from 17.76 to 69.93 mg/g after heat treatment at 300°C for 5 h. Adsorption kinetics was better described by the pseudo-second-order model and the adsorption process was well defined by the Langmuir isotherm. Thermodynamic parameters indicated that the adsorption process was feasible, endothermic, and spontaneous in nature. Selective adsorption studies showed that the heat-treated HCSs could selectively remove Th(IV), and the selectivity coefficients were improved after heat treatment in the presence of coexisting ions, Na(I), Ni(II), Sr(II), Mn(II), Mg(II), Zn(II), and Hg(II). The adsorbed heat-treated HCSs could be effectively regenerated by 1.0 mol/L HCl solution for the removal and recovery of Th(IV). Complete removal (≈100%) of Th(IV) from 1.0 L artificial sea water containing 25.0 mg Th(IV) ions was possible with 1.5g heat-treated HCSs.