Adsorption of U(VI) by multilayer titanate nanotubes: Effects of inorganic cations, carbonate and natural organic matter

Abstract

Titanate nanotubes (TNTs) were synthesized through a hydrothermal method and tested to remove U(VI) from water. TEM characterizations indicate that the multilayer nanotubes have an inner and outer diameter of 4.5 and 9nm, respectively. The material exhibited excellent adsorption capacity (Langmuir Qmax=333mgg−1) for U(VI) and rapid adsorption kinetics. XRD, XPS and Raman analyses of TNTs before and after U(VI) uptake revealed that the high capacity is attributed to the abundant –ONa functional groups located in the interlayers of TNTs and ion exchange between Na+ and cationic uranyl species is the key mechanism. The chemical formula of TNTs was determined to be Na0.92H1.08Ti3O7·1.18H2O, which turns to (UO2)0.58(OH)0.70Na0.16H1.38Ti3O7·1.18H2O upon U(VI) adsorption. The optimal pH for U(VI) ranges from 4 to 6. The presence of Ca2+ and CO32− at elevated concentrations may inhibit U(VI) due to competitive adsorption and formation of anionic and electro-neutral complexes. However, humic acid (HA) promoted U(VI) adsorption because adsorbed HA facilitated binding with U(VI). Moreover, HA can greatly alleviate the competitive effects of Ca2+ and carbonate on adsorption. Spent TNTs can be efficiently regenerated through a sequential acid–base treatment process using dilute HNO3 and NaOH solution. TNTs appear promising for removal and recovery of U(VI).