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.