Facilitated transport of microplastics and nonylphenol in porous media with variations in physicochemical heterogeneity

Microplastics transport in soils: A critical review

“…In general, MPs migration is proportional to MP concentration. Xu et al (2022aXu et al ( , 2022b showed that higher concentration of MPs (for two sizes, 0.1 μm and 2 μm) resulted in higher mass recovery rate, suggesting that this is due to adsorption of some MPs on soil pores which then depleted the amount of further available sites for adsorption, allowing the remaining MPs to traverse the soil. Contradictory findings for very small MPs (≤0.1 μm) are probably due to increasing dominance of mechanisms co-interacting with MP concentration for smaller MPs, that inhibit MPs transport, including (i) aggregation and deposition (Y. ; (ii) adsorption on soil particles due to the increase in MP specific surface area (Shen et al, 2019); as well as clogging of pores by small MPs.…”

“…Soil organic matter enhances MP transport (Ivanic et al, 2023;Gao et al, 2021;Dong et al, 2021;Hou et al, 2020;Xu et al, 2022a;.…”

“…However, Hou et al (2020) demonstrated that MP mass recovery rate does not always increase with FA concentration, because for high values of FA (>5 mg/L) other factors such as hydrophobicity and buoyancy can dominate MP transport over electrostatic forces (Hou et al, 2020). Soil colloids were shown to promote MP mass recovery rate by filling the concave area of the soil's pores (in the contact between the grains), thereby reducing the soil's surface roughness and increasing the repulsive force between MPs and pore walls (Xu et al, 2022a;.…”

“…Wang et al, 2022c). Unlike biochar and nitrogen fertilizers, nonylphenol increases MP mass recovery rate, as it increases the MPs Zeta potential, and also possibly because of the hydrogen bonds forming between MPs and nonylphenol (Xu et al, 2022a). Perfluorooctanoic Acid (PFOA; a surfactant, considered an emerging pollutant) decreased the breakthrough concentration of negatively-charged MPs decreasing their Zeta potential hence promoting their adsorption in the soil; an opposite effect-increasing breakthrough concentration-was observed for positively-charged MPs, due to the reversal of their surface charge, limiting their adsorption in the soil (Rong et al, 2023).…”

“…These metrics could be further categorized into (a) flux-and (b) depth-based, where in both one could use either mass or number of particles. Flux-based metrics include: (i) "transport rate" (Gui et al, 2022), also termed "mass recovery rate" (Xu et al, 2022a) or "mass balance" (Rong et al, 2022): the percentage of MPs traversing the sample, by weight; (ii) "loss ratio" (Rehm et al, 2021): same as (i), using number of MPs. A variant of (ii) is "MP concentration" , which is the number of MPs, however per mL of water measured at the outlet; (iii) "total mass recovery" (Y. : the ratio of MP concentration to that of a tracer (measured from the two breakthrough curves, BTC); (iv) "breakthrough concentration" (Dong et al, 2021): MP concentration relative to that in a injected suspension via which MPs were introduced (measured from the BTC).…”