Competitive adsorption of U(VI) and Co(II) on montmorillonite: A batch and spectroscopic approach

The competitive adsorption of U(VI) and Co(II) on montmorillonite (Mt) was investigated by batch and spectroscopic techniques. The batch experiments indicated that competitive adsorption of U(VI) and Co(II) on Mt decreased with increasing ionic strength at low pH conditions, whereas no effect of ionic strength was observed at high pH, indicating that the outer-sphere and inner-sphere surface complexation dominated the competitive adsorption of U(VI) and Co(II) at low and high pH, respectively. In the binary-solute systems, Co(II) slightly inhibited U(VI) adsorption at low pH, whereas no effect of Co(II) on U(VI) adsorption was observed at high pH. The adsorption kinetics of U(VI) and Co(II) on Mt at single-solute systems were both well described by pseudo-second-order kinetic model due to the higher correlation coefficient (R² > 0.9999). The adsorption isotherms of U(VI) on Mt can be satisfactorily fitted by Freundlich model, while Langmuir model gave the better fits for Co(II) adsorption. The maximum adsorption capacities of U(VI) and Co(II) on Mt calculated from Langmuir model were 4.57 and 6.92 mg·g⁻¹ at pH 5.0 and 323 K, respectively. According to XPS analysis, hydroxyl groups of Mt were responsible for the competitive adsorption of U(VI) and Co(II). The thermodynamic parameters suggested that both the adsorption of U(VI) and Co(II) on Mt were spontaneous and endothermic processes. These observations are crucial for understanding the fate and transport of radionuclides on the clay mineral in the natural environment.