Removal of U(VI) by acid-oxidized biochar: batch experiments and spectroscopy study

With the rapid development of nuclear industry, radioactive wastes have been directly/indirectly released into the natural environment, threatening the health of human being and ecological balance. As one of the renewable materials, biochar has attracted increasing attention and been extensively applied to eliminate pollutants from aqueous solution, owing to its abundant pore structures and unique chemical properties. In this study, we used the carbonized coffee ground (CG) and acid-oxidized coffee ground (CG-AO) to remove U(VI) from solutions to evaluate their sorption performances. The sorption of U(VI) was investigated under various conditions (e.g., contact time, pH, initial U(VI) concentration and temperature). The results showed that the sorption process was strongly influenced by pH. The thermodynamic parameters derived from sorption isotherms revealed that the sorption of U(VI) on CG-AO was a spontaneous and endothermic process. The maximum sorption capacity of U(VI) on CG-AO was calculated to be 251.71 mg g−1 from Langmuir model at T = 298 K and pH 5.0, which was almost three times relative to the untreated CG (80.93 mg g−1). Combined with the X-ray photoelectron spectroscopy (XPS) analysis, the removal of U(VI) on CG-AO was mainly attributed to the formation of (O=C–O)2UO2, while the removal of U(VI) on CG was mainly via a combined effect of carboxyl and H2O molecules, resulting in the coordination of O=C–OUO2(OH).