[Ammonium Adsorption Characteristics in Aqueous Solution by Titanate Nanotubes]

Titanate nanotubes (TNTs) were synthesized via a hydrothermal method using P25 and NaOH as the raw materials. The composition and morphology of the nanotubes were characterized by X-ray diffraction and transmission electron microscopy. The adsorption characteristics and the rules of ammonium in aqueous solutions were tested in the static system. The results showed that when the alkali concentration was 10 mol·L^-1, titanate nanotubes with a length of approximately 120 nm and a diameter of approximately 8 nm were obtained. The equilibrium adsorption capacity of ammonium was 10.67 mg·g^-1. When the pH ranged between 3 and 8, TNTs effectively adsorbed ammonium. The equilibrium adsorption time was 1 h, and this followed the pseudo second-order model. The results from the intra-particle model also showed that the adsorption process of ammonium by TNTs was controlled by surface adsorption and inter-particle diffusion. The Temkin model gave the best fit for the adsorption of ammonium onto TNTs. The thermodynamic experiments showed that the adsorption of titanate nanotubes on ammonium was a spontaneous endothermic process. Coexisting anions and cations had an inhibitory effect on the adsorption of ammonium. The order of influence was SO₄^2- > Cl^- > H₂PO₄^- and K⁺ > Na⁺ > Ca²⁺, respectively. The adsorption effect of ammonium by regenerated TNTs remained more than 88.64% after five repeat usages. The results of Fourier transform infrared spectroscopy showed that the ammonium adsorption mechanism of titanate nanotubes was ion-exchange between NH₄⁺ and Na⁺ in the TNTs. Titanate nanotubes can effectively remove ammonium from water because of their good recycling capacity and large adsorption capacity.