Mutual effect of U(VI) and Sr(II) with graphene oxides: Evidence from EXAFS and theoretical calculations
The competitive interaction of U(VI) and Sr(II) on graphene oxides (GOs) were studied by batch techniques, EXAFS analysis and DFT calculations. The batch results indicated that the decreased sorption of Sr(II) on GOs was observed at C[U(VI)] < 0.2 mmol/L and the enhanced sorption of Sr(II) was found at C[U(VI)] > 0.2 mmol/L, 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 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 U-C shell in the absence/presence of Sr(II) indicated that U(VI) tended to form inner-sphere surface complexes with GOs. For Sr(II) interaction, Sr-C shell was observed at low U(VI) concentration, but not formed at 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 energies of GO-U(VI) (e.g., -40.3 kcal/mol for inner-sphere coordination) was significantly lower than that of GO-Sr(II) (-16.4 kcal/mol), demonstrating that U(VI) was preferential towards GOs relative to Sr(II). These findings can provide a reliable prediction for the transport and fates of U(VI) and Sr(II) at water-GOs interface and open the doorways for the application of GOs.