Absorption of Cr(VI) onto amino-modified titanate nanotubes using 2-Bromoethylamine hydrobromide through SN2 reaction

Abstract

Unlike the complex reaction of grafting amino groups using harmful organic solvents, we proposed an environmental friendly method for effective amino grafting on titanate nanotubes (TNTs) with 2-Bromoethylamine hydrobromide (2-Bh) through a two-step SN2 reaction in pure water solution. The amino-modified titanate nanotubes (TNTs-RNH2) were characterized by Transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Under optimal conditions, the molar ratio of NaOH/2-Bh was 1.2 for step 1 and 1.3 for step 2 with corresponding reaction time of 12h and 24h, respectively. The adsorption isotherm of Cr(VI) onto TNTs-RNH2 well fits Langmuir model with maximal adsorption capacity of 69.1mg/g, which is almost five times larger than that of fresh TNTs. The slight decrease in adsorption capacity with NO3- concentration was attributed to competition from ions, suppression of electric double layer and changes of Cr(VI) speciation. The wide adsorption pH range was due to much larger point of zero charge (8.2) and electrostatic attraction between positively charged TNTs-RNH2 and Cr(VI) with different speciations. The products could be well reused due to simple desorption at pH 10 with about 20% loss of the adsorption capacity after three recycles without any regeneration treatment.