Macroscopic and Microscopic Investigation of U(VI) and Eu(III) Adsorption on Carbonaceous Nanofibers

The adsorption mechanism of U­(VI) and Eu­(III) on carbonaceous nanofibers (CNFs) was investigated using batch, IR, XPS, XANES, and EXAFS techniques. The pH-dependent adsorption indicated that the adsorption of U­(VI) on the CNFs was significantly higher than the adsorption of Eu­(III) at pH < 7.0. The maximum adsorption capacity of the CNFs calculated from the Langmuir model at pH 4.5 and 298 K for U­(VI) and Eu­(III) were 125 and 91 mg/g, respectively. The CNFs displayed good recyclability and recoverability by regeneration experiments. Based on XPS and XANES analyses, the enrichment of U­(VI) and Eu­(III) was attributed to the abundant adsorption sites (e.g., −OH and −COOH groups) of the CNFs. IR analysis further demonstrated that −COOH groups were more responsible for U­(VI) adsorption. In addition, the remarkable reducing agents of the R-CH₂OH groups were responsible for the highly efficient adsorption of U­(VI) on the CNFs. The adsorption mechanism of U­(VI) on the CNFs at pH 4.5 was shifted from inner- to outer-sphere surface complexation with increasing initial concentration, whereas the surface (co)­precipitate (i.e., schoepite) was observed at pH 7.0 by EXAFS spectra. The findings presented herein play an important role in the removal of radionuclides on inexpensive and available carbon-based nanoparticles in environmental cleanup applications.