Removal of Cd²⁺ from water containing Ca²⁺ and Mg²⁺ using titanate nanotubes modified by carbon

Calcium and magnesium ions usually exist in natural water. When Cd²⁺ is removed from water by adsorption, it will be inhibited by these two ions. Titanate nanotubes (TNTs) have an effective adsorption capacity for Cd²⁺ due to extraordinary ion exchange property. However, TNTs also adsorb Ca²⁺ and Mg²⁺ in water. In this study, carbon-modified TNT (TNT/C) and TNT/C further treated with acid (TNT/HC) were synthesized by hydrothermal synthesis. The transmission electron microscope (TEM) images show that TNT/C or TNT/HC still keep nanotube morphology. The experimental results show the order of adsorption amount to Cd²⁺ is TNT (171.56 mg/g) > TNT/C (166 mg/g) > TNT/HC (159.88 mg/g) when there is no Ca²⁺ or Mg²⁺. But when there is 0.1 M Ca²⁺ or Mg²⁺ in the water, the order of Cd²⁺ adsorption capacity becomes TNT/HC (44.28, 49.04 mg/g) > TNT/C (58.84, 69.32 mg/g) > TNT (65.52, 70.6 mg/g). It indicates that the surface carbon modification can alleviate the hindrance of Ca²⁺ or Mg²⁺ to Cd²⁺ removal. This is because the carbon on the surface of TNT captured part of Ca²⁺ or Mg²⁺; it made more Cd²⁺ be successfully absorbed by TNT through ion exchange. This mechanism was confirmed by the X-ray photoelectron spectroscopy (XPS) spectra analysis. The results of this paper can provide ideas for the adsorption and removal of Cd²⁺ in water in the presence of Ca²⁺ or Mg²⁺.