Mutual effect of U(VI) and Sr(II) on graphene oxides: evidence from EXAFS and theoretical calculations
The competitive interaction of U(VI) and Sr(II) on graphene oxides (GOs) was studied by batch techniques, EXAFS analysis and DFT calculations. The batch results indicated that decreased sorption of Sr(II) on GOs was observed at C[U(VI)] < 0.2 mmol L−1 and enhanced sorption of Sr(II) was found at C[U(VI)] > 0.2 mmol L−1, whereas the presence of Sr(II) did not affect U(VI) sorption on GOs. The increased sorption of Sr(II) at C[U(VI)] > 0.2 mmol L−1 resulted from the new available sites provided by the precipitated U(VI) or adsorbed hydrolyzed U(VI) species according to EXAFS analysis. The occurrence of a U–C shell in the absence/presence of Sr(II) indicated that U(VI) tended to form inner-sphere surface complexes with GOs. For the Sr(II) interaction, a Sr–C shell was observed at a low U(VI) concentration, but not formed at a high U(VI) concentration, indicating the shift of inner-sphere to outer-sphere surface complexes with increasing U(VI) concentration. According to DFT calculation, the binding energy of GO–U(VI) (e.g., −40.3 kcal mol−1 for inner-sphere coordination) was significantly lower than that of GO–Sr(II) (−16.4 kcal mol−1), demonstrating that U(VI) was preferentially bound to GOs relative to Sr(II). These findings can provide a reliable prediction of the transport and fates of U(VI) and Sr(II) at the water–GO interface and open doorways for the application of GOs.