Dispersion and transport of microplastics in three water-saturated coastal soils

Microplastics transport in soils: A critical review

“…Overall, coarser texture promotes MP transport; this was found in terms of mass recovery rate (Dong et al, 2022;Wu et al, 2020), transport rate (Gui et al, 2022), number of MPs traversing the sample (Xing et al, 2021), and loss ratio (Rehm et al, 2021). For a given MP size, larger soil particles, which in turn imply larger pore sizes, enhance the MPs mobility, in terms of mass recovery rate (Dong et al, 2022;Hou et al, 2020), maximum depth (Gao et al, 2021;Ranjan et al, 2023), and MP outlet concentration .…”

“…This is rather intuitive: for a given size of soil pores, smaller MPs can traverse further (Gao et al, 2021;Ranjan et al, 2023;O'Connor et al, 2019). In contrast, for MPs ≲1μm, increasing MP size increases MP mobility (transport rate in Gui et al, 2022; mass recovery rate in Rong et al, 2022). The reason that the trend in MPs size vs. mobility reverses for very small MPs is linked to the fact that for very small particles, other forces and mechanisms start to become dominant.…”

“…The reason that the trend in MPs size vs. mobility reverses for very small MPs is linked to the fact that for very small particles, other forces and mechanisms start to become dominant. In particular, further decreasing MPs size below ~1μm decreases their Zeta potential and hence the repulsive energy barrier between MPs and soil particles (Gui et al, 2022), enhancing their adsorption to the soil particles. Due to the complexity of these microscopic mechanisms, the critical size for which the mobility trend reverses can depend on other factors and hence is not univocal (e.g., 1 μm in Rong et al (2022) vs. 2 μm in vs. 10 μm in ).…”

“…Soil texture also affects soil Zeta potential. In most cases, both soil and MPs surfaces are negatively charged; in such cases, coarser soils have higher Zeta potential (Dong et al, 2022;Rong et al, 2022;Gui et al, 2022;Li et al, 2023), and thus stronger repulsion which retains fewer MPs, promoting their mobility (in terms of mass recovery rate in Wu et al, 2020). MP surface roughness was found to reduce MP penetration depth, due to increased friction with the soil pores (Ranjan et al, 2023), however this effect is subject to interference with other mechanisms and properties, including water flow rate and MP concentration (Hou et al, 2020).…”

“…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).…”