Highly efficient adsorption of Cr(VI) from aqueous solutions by amino-functionalized titanate nanotubes

• A new adsorbent for Cr(VI) with titanate nanotube structure was prepared.
• Cr(VI) adsorption capacity on NH2-TNTs was much larger than that on HTNTs.
• Ion-exchange with NO3- was responsible for efficient adsorption of Cr(VI).
• The reduced Cr(III) was all bonded with amino groups.

For highly efficient removal of Cr(VI) from aqueous solutions, amino-functionalized titanate nanotubes (NH2-TNTs) with excellent adsorption performance have been synthesized by covalently grafting [1-(2-amino-ethyl)-3-aminopropyl]trimethoxysilane (AAPTS) onto protonated titanate nanotubes (HTNTs) with great amounts of surface hydroxyl groups. TEM and XRD results confirmed that the nanotubular morphology and crystal structure of HTNTs and NH2-TNTs were preserved. FTIR spectra demonstrated that AAPTS was covalently bonded on the surface of HTNTs. Batch adsorption experiments showed that pseudo-second-order kinetics model and Langmuir isotherm model fitted the adsorption data very well for both materials, and the Cr(VI) adsorption capacity on NH2-TNTs calculated by Langmuir model was up to 153.85 mg g−1 at initial pH 5.4 and 30 °C, much larger than that on HTNTs (26.60 mg g−1). Moreover, uptake of Cr(VI) ions onto NH2-TNTs could be completed within only 5 min for 95% adsorption of the maximum. Influence of different species of Cr(VI) under varying pH was also considered. FTIR and XPS analysis indicated that Cr(VI) ions were first exchanged with NO3- linked on the positively charged amino groups and then partially reduced to Cr(III). Afterwards, Cr(III) were then totally chelated with amino groups and no Cr(III) was detected in the solution after Cr(VI) adsorption at pH range of 1–12.