Mutual promotion mechanism for adsorption of coexisting Cr(III) and Cr(VI) onto titanate nanotubes

Multilayered titanate nanotubes (TNTs) were shown to provide excellent adsorption of Cr(III), with a maximum adsorption capacity of 88.99 mg g(-1). Adsorption of Cr(III) and Cr(VI) was mutually promoted when the two ions coexisted at wider pH ranges of 3-6. Compared to single systems, the adsorption capacity of Cr(III) increased from 56.3 to 82.1 mg g(-1), whereas the Cr(VI) capacity increased from 0 to 9.8 mg g(-1) in the binary system at pH 5 with 0.6 g L-1 TNTs. Cr(III) adsorbed onto the TNTs mainly through ion-exchange with Na+ in the interlayers of TNTs, as indicated by the linear relationship between adsorption capacity of Cr(III) and the Na content of TNTs. However, double-layer adsorption occurred when Cr(III) and Cr(VI) coexisted. An ionic cluster of Cr(III) and Cr(VI) was formed initially and subsequently bridged with Cr(III) adsorbed on TNTs through Cr(111)-O-Cr(VI) linkages. X-ray photoelectron spectroscopy (XPS) and Raman analyses confirmed the ion-exchange mechanism and formation of Cr(III)-O-Cr(VI), and that the process did not change the basic [TiO6] structure of TNTs. Moreover, rearrangement of Cr(III) from the second layer to TNTs' surface was observed when adsorption of Cr(III) was unsaturated, resulting in damage to Cr(III)-O-Cr(VI) linkages and desorption of Cr(VI). (C) 2013 Elsevier B.V. All rights reserved.