Effects of particle size and surface chemistry on plastic nanoparticle transport in saturated natural porous media

The Mobility of Plastic Nanoparticles in Aqueous and Soil Environments: A Critical Review

“…For the simplest systems, this loss of PNP stability can be attributed to the electrostatic charge screening effects of the cations. In the typical suspension containing predominantly negatively charged PNPs, increasing the concentration of metal cations in solution will limit the Debye radius of each particle’s net electrostatic effect. ,, This process minimizes the energy barrier that prevents the particles from approaching each other to the point at which van der Waals attraction will cause them to aggregate (Figure b). ,,, Therefore, PNPs are generally less stable in the water column as the ionic strength increases. ,,,− Multivalent cations (e.g., Ca, Mg, and Fe) will have a stronger charge screening effect at a given mass concentration than monovalent cations (e.g., Na). , In natural solutions, the high ionic strength of seawater, for example, causes a more dramatic increase in PNP aggregation compared to river water and groundwater. ,, In one study, the measured hydrodynamic diameter of unmodified polystyrene PNPs (net negative surface charge) in seawater was nearly doubled compared to that of river water and groundwater due to aggregation . Increasing the ionic strength will, in most cases, limit the fluid mobility of PNPs by decreasing the electrostatic repulsion between particles, causing increased aggregation.…”

“…Dissolved organic matter (DOM), including organic acids, proteins, and a variety of other materials, exhibits a highly variable impact on PNP mobility depending on the PNP and DOM characteristics and the solution chemistry. In the simplest cases with DOM and PNPs with opposite net surface charges, increasing concentrations of DOM (e.g., humic acid, alginate, or albumin) may decrease the absolute ζ potential of PNPs as the DOM becomes attached to the particles. ,,,, The resulting decrease in electrostatic repulsion between particles will increase particle aggregation, and under certain conditions, the average particle hydrodynamic diameter may increase as much as 2 orders of magnitude (Figures b and c). ,− However, when DOM increases to excess, the organic material may begin to completely coat the PNPs. , This coating can push the net surface charge passed the isoelectric point and impart a non-zero absolute ζ potential onto the particles, causing them to once again electrostatically repel each other, ultimately reducing aggregation. ,, In addition to the electrostatic effects, hydrophobic interactions between attached DOM may cause PNP aggregation, while in other cases, steric repulsion between these attached organics may reduce aggregation . For example, bovine serum albumin was found to limit the aggregation of unmodified polystyrene PNPs due to steric hindrance, while positively charged lysozyme increased aggregation due to adsorption and bridging effects .…”

“…These attachment mechanisms between PNPs and the substrate are often similar to the interactions between PNPs and colloidal or aqueous components. In a simple system consisting of only pure water and clean silica sand, positively charged PNPs will be attracted to and retained by the negatively charged sand without any additional intervening processes. , Conversely, the electrostatic repulsion between negatively charged PNPs and the sand will stabilize the PNPs in solution and may improve their mobility. ,,,, Unmodified polystyrene (100 nm) nanoparticles, for example, were found to exhibit behavior similar to a conservative tracer during transport through a natural desert soil (with low OM and clay content) at a low ionic strength and a high pH, which is attributed to the negative ζ potential of both the soil and PNPs . When the fluid phase has a more complex composition, the presence of metal cations can limit the electrostatic repulsion between the particles through charge screening, increasing the relative effects of van der Waals interactions between PNPs and the substrate. ,,, Cations, like Ca 2+ , and dissolved organic matter may also be adsorbed by the PNPs and/or the substrate, mitigating the repulsion between them and facilitating sorption, or directly causing adsorption via cation or polymer bridging. ,,,, While DOM typically limits PNP mobility in soils, particulate organic matter, and in some cases even DOM, can actually increase PNP mobility by becoming attached to the PNP and/or substrate surfaces, resulting in steric or electrostatic repulsion between the PNPs and the substrate. ,,− While clean sand can provide initial insights into the mechanisms affecting PNP mobility in soil, natural systems are much more complex.…”

“…One such behavior, known as blocking, produces trends in which PNP mobility continues to gradually increase following the initial transport of the particles through the porous medium. ,,, A soil can have only a finite amount of surface sites available for PNP and PNP heteroaggregate attachment. As these sites become occupied during prolonged exposure to PNPs, the soil will begin to approach its retention capacity and further PNP attachment will decrease, with a corresponding increase in PNP transport (Figures e and a). ,,,, Larger attached PNPs may even block adjacent, available sites via steric repulsion, compounding this effect . This mechanism is especially relevant in systems in which PNP–PNP interactions are limited by the aqueous conditions (i.e., low ionic strength and DOM content), preventing the attached PNPs from interacting with the suspended PNPs. ,,,, In one example, 200 nm carboxylated polystyrene PNPs exhibited blocking behavior during transport through seawater-saturated sand because the repulsion between PNPs was stronger than the repulsion between PNPs and the sand substrate .…”

Enhanced oil recovery by nanoparticles flooding: From numerical modeling improvement to machine learning prediction

“…Moreover, they found out that the flow velocity affects the density such that the flow velocity declines as the number of aggregates enlarge. Shaniv et al (2021) examined the polystyrene nanoparticles transport in fully water-saturated soil. They highlighted that particle size and surface texture can affect polystyrene mobility in the soil.…”