Transport and Retention of Metal Oxide Nanoparticles in Saturated Porous Media
We investigate the behavior of four types of untreated metal oxide nanoparticles in saturated porous media. The transport and retention of Fe3O4, TiO2, CuO, and ZnO were measured in a series of column experiments. Vertical columns, 20 cm in height, were packed with uniform, spherical glass beads. Initial experiments demonstrated that when nanoparticles were introduced to the column as a dry powder, placed on the inlet surface with an hydraulic head being built up above them, the nanoparticles remained virtually immobile, with complete retention at the top 5 mm near the column inlet. All subsequent experiments were carried out with an inlet flow condition that introduced nanoparticles as a pulse suspended in aqueous solutions. Breakthrough curves of nanoparticles were measured using UV-vis spectrometry; the experiments proved to be highly reproducible in repeated tests. Following completion of some experiments, the mass of nanoparticles retained in each column was measured to ensure consistency. Different factors affecting the mobility of the nanoparticles such as ionic strength, addition of organic matter (humic acid), flow rate and pH were investigated. The experiments showed that mobility varies strongly among the nanoparticles, with TiO2 demonstrating the highest mobility. For example, at solution concentrations of 0.01 M NaCl, TiO2 had the highest mobility, with 62% of the nanoparticles exiting the column; 52%, 16% and only 1.4% of the CuO, Fe3O4, and ZnO nanoparticles reached the column outlet. But nanoparticle mobility is also strongly affected by the experimental conditions. Increasing the ionic strength to 0.1 M NaCl, only 13%, 8.3%, 6.2% and 1.2% of the TiO2, CuO, Fe3O4 and ZnO nanoparticles, respectively, emerged from the columns. This behavior can be attributed to the suppression of the electrical double layer by the added ions. Under conditions of higher ionic strength, attractive van der Waals forces are dominant over repulsive electrostatic interactions, leading to enhanced aggregation - and thus reduced mobility - of the nanoparticles. On the other hand, addition of humic acid increases the nanoparticle mobility significantly: aqueous solutions with 0.01 M NaCl and 60 mg/L humic allowed 98%, 98%, 74% and 62% of the TiO2, CuO, ZnO and Fe3O4 nanoparticles, respectively, to exit the columns. Lower flow rates again led to reduced mobility, while changes in pH had little effect. Overall, in natural systems, it is expected that the presence of humic acid in soil and aquifer mateirals, and the ionic strength of the resident water, will be key factors determining nanoparticle mobility.
