Transport of surface-modified carbon nanotubes through a soil column

Nano-SiO2 transport and retention in saturated porous medium: Influence of pH, ionic strength, and natural organics

Nano silica (nSiO₂), induces potential harmful effects on the living environment and human health. It is well established that SiO₂ facilitates the co-transport of a variety of other contaminants, including heavy metals and pesticides. The current study focused on the systematic evaluation of the effects of multiple physicochemical parameters such as pH (5, 7, and 9), ionic strength (10, 50, and 100 mM), and humic acid (0.1, 1, and 10 mg/L) on the transport and retention of nSiO₂ in saturated porous medium. Additionally, the influent concentration of nSiO₂ (10, 50, and 100 mg/L) was also varied. Our experimental findings indicate that the size of nSiO₂ aggregates was directly related to the pH, ionic strength, HA, and particle concentration had a significant impact on the breakthrough curves (BTCs). The stability provided by the varying concentrations of pH and humic acid had a significant effect on the size of nSiO₂ aggregates and transport (C/C₀ > 0.7). The presence of a greater magnitude of negative charge on the surface of both nSiO₂ and quartz sand resulted in less aggregation and enhanced flow of nSiO₂ through the sand column. The Electrostatic and steric repulsion forces were the primary governing mechanisms in relation to the size of nSiO₂ aggregates, affecting the single-collector efficiency and attachment efficiency, which determined the maximal transport of nSiO₂. Conversely, a probable increase in Van der Waals force of attraction exacerbated the particle deposition and reduced their mobility for high ionic strength, and particle concentrations (C/C₀ < 0.1). The formation of large nSiO₂ aggregates, in particular, was principally responsible for the enhancement of nSiO₂ retention in sand columns over a broad range of IS and particle concentration. The interaction energy profiles based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were determined to understand the mechanism of nSiO₂ deposition. Aditionally, all the experimental BTCs were mathematically simulated and justified by the colloidal filtration theory (CFT). Considering the environmental ramifications, the transport behavior of nSiO₂ was further evaluated in various natural matrices such as river, lake, ground, and tap water. The nSiO₂ suspended in the river, lake, and tap water had significantly higher mobility (C/C₀ > 0.7), whereas groundwater indicated higher retention (C/C₀ < 0.3). The study advances our collective knowledge of physicochemical and environmental parameters that can affect particle mobility.