Transport and aggregation of Al2O3 nanoparticles through saturated limestone under high ionic strength conditions: measurements and mechanisms

Aluminum oxide (Al2O3) nanoparticles (NPs) are being utilized in a broad range of applications; thus, noticeable quantities of these particles are being released into the environment. Issues of how and where these particles distribute into the subsurface remain major challenges. In this study, mechanisms governing the transport and aggregation of Al2O3-NPs (Alpha-40 nm) through saturated limestone porous media under different ionic strength conditions were evaluated. For this aim, 50 mg of Al2O3-NPs was dispersed in 1 L of different electrolyte solutions including NaCl and CaCl2. Ionic strength range was selected from deionized water up to 500 mM. Breakthrough curves in the column effluent were measured by UV-VIS spectrometry. It was found that the presence of NaCl and CaCl2 in the suspensions led to formation of ion bridges among NPs. Thus, the stability of Al2O3-NPs significantly declined and NPs started to flocculate and form bigger clusters. Furthermore, ionic strength caused considerable delay in NPs breakthrough in the effluents and reduction of NPs recovery. CaCl2 compared to NaCl was found more effective in instability and deposition of Al2O3-NPs. In addition, the obtained results from transport experiments were checked against classical filtration and Derjaguin-Landau-Verwey-Overbeek (DLVO) theories. The results were found to be in agreement with named theories.