Adsorption mechanisms of thallium(I) and thallium(III) by titanate nanotubes: ion-exchange and co-precipitation.

Hydrothermally-synthesized titanate nanotubes (TNTs) are found to be excellent at adsorption of highly toxic thallium ions. Uptake of both thallium ions is very fast in the first 10 min. The adsorption isotherm of Tl(I) follows the Langmuir model with maximum adsorption capacity of 709.2 mg g(-1). Ion-exchange between Tl(+) and Na(+) in the interlayers of TNTs is the primary mechanism for Tl(I) adsorption. Excess Tl(+) undergoes further exchange with H(+). The adsorption mechanism is different for Tl(III), and involves either ion-exchange with Na(+) at low Tl(III) concentration or co-precipitation in the form of Tl(OH)3 with TNTs at high Tl(III) concentration. XPS analysis indicates that the ion-exchange process does not change the basic skeleton [TiO6] of TNTs, whereas Tl(OH)3 precipitation increases the percentage composition of O within the surface hydroxyl groups. XRD analysis also confirms the formation of Tl(OH)3 on TNTs at high initial concentration of Tl(III). Coexisting Na(+) and Ca(2+) hardly inhibit adsorption, indicating good selectivity for thallium by TNTs. Furthermore, TNTs can be reused efficiently after HNO3 desorption and NaOH regeneration, making TNTs a promising material to remove thallium from wastewaters. This study also confirms that co-precipitation is another important adsorption mechanism for easily hydrolytic metals by TNTs.