Influence of pH, ionic strength and humic acid on competitive adsorption of Pb(II), Cd(II) and Cr(III) onto titanate nanotubes

Titanate nanotubes prepared by alkaline hydrothermal method were characterized by N-2 adsorption/desorption, X-ray diffraction, high resolution transmission electron microscopy and Fourier transform infrared spectroscopy. Competitive absorption of Pb(II), Cd(II) and Cr(III) onto titanate nanotubes were systematically investigated, and interactions between metal ions and TNTs were studied using X-ray photoelectron spectroscopy (XPS). Special attention was paid to the influence of pH, ionic strength and humic acid concentration on competitive adsorption. As results, pH was found a key factor influencing the adsorption process due to its effect on metal ions speciation and surface charge of TNTs. Furthermore, the ionic strength enhanced selectivity of TNTs to Pb2+, which could be ascribed to combined action of steric hindrance of Na+, changes of zeta potential of TNTs and the radius of hydrated ion of metal ions. Significant influence of humic acid (HA) was also revealed and attributed to the HA-metal molecular (HA-M) formed through complexation between metal ions and HA. The HA-M, with both positive and negative charges, would alter the adsorption capacity of TNTs, in which HA-M's stability was of primary importance. Nevertheless, the existence of HA didn't mean the significant change of surface properties of TNTs, which could be proved by neglected change of zeta potentials of TNTs, similar FTIR spectra of the absorbed TNTs with HA and the TNTs-com but quite different HA spectra. Overall, the adsorption mechanism was confirmed to be dominated by ionic exchange with Ne/H. in the interlayer of TNTs but modified by complexation with existence of HA. (C) 2012 Elsevier B.V. All rights reserved.