Adsorption of Cu(II) and Cd(II) on titanate nanomaterials synthesized via hydrothermal method under different NaOH concentrations: Role of sodium content

A series of titanate nanomaterials (TNMs) were synthesized via hydrothermal reaction under different NaOH concentrations (4–15 mol/L), and then used to remove Cu(II) and Cd(II) from aqueous solution. Transmission electron microscope (TEM) revealed that the morphology of the TNMs successively presented as nano-granules, nanoplates, nanotubes, nanosheets and nano-blocks with increasing NaOH concentration. X-ray diffraction (XRD) analysis indicated that the crystal phase of the TNMs changed as mixture of TiO2 and crystalline titanate, pure titanate and amorphous phase with increasing NaOH concentration. The TNMs which possessed titanate phase presented excellent adsorption performance, with maximum adsorption capacity of ca. 120 mg/g and 210 mg/g for Cu(II) and Cd(II) respectively. Moreover, the adsorption capacity showed a good correlation with the sodium content of TNMs, with R2 of 0.994 and 0.986 for Cu(II) and Cd(II) respectively. Therefore, the primary adsorption mechanism was ion-exchange between Na+ and metal ions, thus ONa groups were the main adsorption sites. Other mechanisms like ion-exchange with H+ and complexation by OH also could not be ignored. Furthermore, specific surface area and average pore diameter was found to have no definite relationship with adsorption capacity for the TNMs with titanate phase.