Experimental study of TiO2 nanoparticle adhesion to silica and Fe(III) oxide-coated silica surfaces

With the rapid expansion of industrial nanotechnology applications, engineered nanoparticles are being introduced into the environment before the controls on their fate and mobility are fully understood. In this study, we measured the adhesion of TiO2 nanoparticles onto silica and Fe(III) oxide-coated silica surfaces as a function of pH, nanoparticle concentration, and nanoparticle size. Batch TiO2 adhesion experiments were conducted at pH 3–8 and 0.01 M NaClO4 with TiO2 concentrations ranging from 10 to 200 mg/L. Three TiO2 size fractions, each containing a range of particle sizes, had initial average diameters of 16, 26, and 50 nm. Silica grains, both uncoated and coated with Fe(III) oxide, were used as the geosorbents. The extent of TiO2 nanoparticle adhesion increased with increasing nanoparticle concentration, and pH exerted a strong effect on the adhesion behavior of the nanoparticles onto the uncoated silica particles. At and below pH 5, TiO2 nanoparticle adhesion increased with increasing pH; at pH 6 and above, adhesion occurred independently of pH. In general, the differences in adhesion between the three nanoparticle sizes at a given pH were not large. Within a given size fraction, preferential adhesion of the larger TiO2 particles was suggested below pH 6, and preferential adhesion of the smaller TiO2 particles was suggested at and above pH 6. Experiments with the Fe-coated silica grains were conducted only with the 26 nm TiO2 nanoparticles, and, except at pH 6 where we observed significantly enhanced adhesion to the Fe-coated silica relative to the uncoated silica, the extents of nanoparticle adhesion onto the two geosorbents were the same within experimental uncertainty. The similarity in adhesion behaviors onto solids with such different surface chemistries suggests that the properties of the TiO2 nanoparticles, such as agglomeration, and not of the mineral surfaces, are primarily responsible for governing adhesion.