Evaluation of desorption of Pb (II), Cu (II) and Zn (II) from titanium dioxide nanoparticles

The adsorption-desorption of toxic compounds onto engineered nanoparticles is an important process that governs their potential as sorbents for treatment applications, their toxicity and their environmental risks. This study was aimed to investigate the desorption of Pb (II), Cu (II) and Zn (II) from commercially prepared nano-TiO2 (anatase) using batch techniques, with the evaluation of isothermal, kinetic and thermodynamic properties. Results showed that desorption was pH dependent and that more than 98% of all metals desorbed at pH 2. Short term kinetic studies were fit with a pseudo second order model and showed that a significant amount of desorption occurred in the first fifteen minutes. Surface complexation modeling determined a trend of adsorption affinity to be Pb > Zn > Cu and with adjustable surface complexation constant (K-int) provided good fit to the experimental data. The thermodynamic studies found that desorption was exothermic and non-spontaneous in most cases. The XPS study showed that no change in oxidation state occurred due to desorption and suggested that Pb desorption was due to inner-sphere surface complexation. The results suggest three important points that will improve the capabilities of researchers to understand Pb (II), Cu (II) and Zn (II) adsorption-desorption to nano-TiO2: (1) the desorption of metals was enhanced at lower pH values suggesting its potential to be regenerated for treatment applications; (2) the possible mechanism for adsorption-desorption varies for different metals; and (3) nano-TiO2 could interact with metals in the environment if released due to their high sorption capacity and reversible adsorption at lower pH values which could affect the fate and behavior of metals in the environment and enhance nanoparticle toxicity. (C) 2012 Elsevier B.V. All rights reserved.