Effects of particle size and crystalline phase on lead adsorption to titanium dioxide nanoparticles

The adsorption of lead to TiO2 materials with different particle sizes and compositions was investigated to examine the factors controlling metal adsorption to nanoparticles. Experiments were conducted with four different TiO2 materials. Three of the materials were nanoparticles, with sizes ranging from 20 to 33 nm, and the fourth was a bulk material with a 520-nm particle size. Batch adsorption experiments were conducted over a broad range of pH (2-8) and total lead concentration (10(-8)-10(-4) M) to provide a comprehensive dataset for modeling equilibrium adsorption. Modeling was conducted using both the Langmuir isotherm model and a surface complexation model. Adsorption capacity was most effectively evaluated using the Langmuir model, and adsorption affinity was assessed best through surface complexation modeling. On a mass basis, the adsorption capacity to the nanoparticles was always higher than to the bulk material. However, when adsorption capacity was normalized to surface area, the bulk material had a higher capacity. Among the nanoparticles, the capacity was higher for a material that was pure anatase than for the two materials that were predominantly anatase with additional amounts of rutile. Surface complexation modeling determined a similar trend for adsorption affinity. Adsorption was strongest to the bulk material, followed by the pure anatase nanoparticles, and then the mixed anatase-rutile nanoparticles. For each of the materials, a surface complexation model with only one adjustable parameter provided good fits to the experimental data.