Long-term fate of ZnO-FexOy mix-nanoparticles through the saturated porous media under constant head condition

Abstract

The objective of this study is to evaluate the long-term fate of the nZnO-nFexOy mix nanoparticles through the natural sediment in the presence of humic acid (HA) under varying pH and natural groundwater conditions. To achieve the objectives, a series of experiments were carried out where 50 mg/l of nZnO-nFexOy mix suspensions were injected for 5 pore volumes (PVs) in that porous media in the presence of 10 mg/l HA under varying pH (6 and 8) followed by flushing with deionized and natural groundwater for another 95 PVs under constant head condition. The outcome of the study suggests that during the injection of the nZnO-nFexOy mix suspension, more nZnO particles retain when the suspension is prepared at pH 6 (>90%). With an increase in pH of nZnO-nFexOy mix suspension and background water, the long-term release of retained nZnO particles has increased significantly (from 29.97% at pH 6 to 95.89% at pH 8). The surface charge and the electrostatic repulsion are likely to govern the detachment and release of nZnO particles. Certain fraction (3.58–7.97%) of the Zn was also found to be dissolved and eluted at the outlet when the pH of background water is maintained at 6. In the case of nFexOy, extensive retention of particles is observed during injection at both pHs (6 and 8). The release of the retained particles is limited (6.34%) specifically at lower pH (pH 6). There is an increase in the release of nFexOy particles (24.76%) with an increase in the pH (pH 8) of both the suspension and background solution. When groundwater is used as the background water, a slight reduction in the release of Zn (22.04%) and Fe (2.06%) is observed at a pH of 6. However, at higher pH (pH 8), significantly less amount of retained particles (2.24% of Zn and 4.96% of Fe) are released. This is mainly due to the presence of co-ions in the groundwater which resulted in less negative charge of ENPs thus having less detachment and release of Zn and Fe particles. Overall, it could be concluded that there is a risk of release of Zn and Fe (especially at high pH) in the long run in the presence of organic matter when exposed in the porous media. The extent of release of Zn and Fe would be more at higher pH and might be less in the presence of other ions and under groundwater conditions.