Determination of chemical affinity of graphene oxide nanosheets with radionuclides investigated by macroscopic, spectroscopic and modeling techniques
The chemical affinity of graphene oxide (GO) nanosheets with radionuclides (Eu(III) and U(VI)) was determined by macroscopic, spectroscopic and modeling techniques. The macroscopic results showed that the adsorption of Eu(III) and U(VI) on GO nanosheets was independent of ionic strength, indicating that inner-sphere surface complexation predominated their adsorption. The maximum adsorption capacities calculated from a Langmuir model at pH 4.0 and T = 303 K were 208.33 mg U(VI) and 28.70 mg Eu(III) per gram of GO nanosheets, respectively. No hysteresis was observed for both Eu(III) and U(VI) on GO nanosheets when desorption was initiated by lowering solution pH. While desorption was induced by replacing the radionuclide supernatant liquid with radionuclide-free electrolyte solution, the adsorption–desorption hysteresis was observed for U(VI) but not for Eu(III), indicating that the chemical affinity of GO nanosheets with U(VI) was stronger than that of GO nanosheets with Eu(III). The adsorption behaviors of Eu(III) and U(VI) on GO nanosheets can be fitted by a double diffuse layer surface complexation model with the mononuclear monodentate >SOM(n−1)+ and >SOMOH(n−2)+ complexes, and larger log K values of U(VI) was observed as compared to those of Eu(III). According to the spectroscopic analysis, the irreversible adsorption of U(VI) on GO nanosheets at variable radionuclide concentrations was attributed to the oxygen-containing functional groups.