Microplastics and nanoplastics barely enhance contaminant mobility in agricultural soils

Globally, a massive amount of plastic waste is projected by 2050, with a significant portion not being recycled or incinerated. This leads to the ubiquitous accumulation of plastics in various environmental systems, including farmland soils. Agricultural practices contribute substantially to MNP pollution in farmland through sources such as biosolids, compost application, and plastic mulching films. MNP can contain various intentionally added substances and can absorb organic contaminants, raising concerns about their role in facilitating the transport of these contaminants to deeper soil layers and groundwater. While some studies express concern about MNP acting as vectors for contaminant relocation, a comprehensive examination of this issue is lacking. For MNP-facilitated transport to be relevant, several conditions must be met: sufficiently high particle concentrations, contaminants of concern, greater particle mobility than the contaminant, and slow contaminant desorption during MNP travel time. This study aims to assess the conditions under which MNP might affect organic contaminant relocation in farmland soils, particularly focusing on scenarios where their influence would be negligible.

Existing research on nanoparticle and colloid mobility in soil provides a framework for understanding MNP mobility. Particle mobility is influenced by flow conditions, solution chemistry, and soil and particle properties. Maximum transport is expected with zero attachment efficiency to the soil matrix and negligible physical removal. Particle size significantly affects mobility; particles around 1 µm are most mobile, while larger particles are restricted to larger pores or preferential flow paths. Smaller nanoplastics' mobility might be limited by diffusion. The contaminant desorption rate is crucial; fugacity differences between the plastic and surrounding soil phases drive desorption. Intra-particle diffusion (IPD) and diffusion through the aqueous boundary layer (ABLD) affect desorption rates. The Damköhler number (Da), the ratio between transport and desorption timescales, is used to assess the relevance of particle-facilitated transport. Da < 0.01 indicates decoupled transport (MNP facilitating transport), while Da > 100 indicates equilibrium (MNP not facilitating transport).

This study calculated Damköhler numbers (Da) using two diffusion models (IPD and ABLD) for MNP ranging from 100 nm to 1 mm. Two flow velocity scenarios were considered: slow (1 m a⁻¹) representing average soil water flow and fast (1 m h⁻¹) representing preferential flow paths. The analysis considered the influence of particle size and the apparent diffusion coefficient (Dapp) for IPD or the partitioning coefficient (Kpw) for ABLD. The calculated Da values were compared to literature values of Dapp and Kpw for various polymer-contaminant combinations. To estimate the ABL thickness (δ), the Reynolds number (Re) was calculated for both flow conditions considering variations in grain diameter (dg) and kinematic viscosity of water (νaq). Laminar flow was assumed for both scenarios. A range of Dapp and Kpw values from the literature were used, and a constant D value was adopted for calculations involving ABLD. The study examined a range of particle sizes from 100 nm to 1 mm to encompass both nanoplastics and microplastics. Two flow regimes were simulated to capture a wide range of soil water flow conditions: a slow regime representing average soil flow (1 m/year) and a fast regime representing preferential flow paths (1 m/hour). The models considered various factors, including IPD-limited and ABLD-limited mass transfer processes. The Da values were then compared to thresholds to determine whether the transport was predominantly decoupled, in equilibrium, or in a kinetic region.

Under slow flow conditions, MNP did not enhance contaminant mobility. For IPD-limited desorption, only larger microplastics (100 µm–1 mm) showed potential for contaminant relocation in some specific scenarios (e.g., polyethylene and polystyrene additives, and PCB in polyvinylchloride). For ABLD-limited mass transfer, microplastics <100 µm did not facilitate transport for most contaminants (logKpw < 5.5). Only highly hydrophobic PCB with larger polyethylene microplastics (>1 mm) showed decoupled transport. Under fast flow conditions, MNP might contribute to the mobility of highly hydrophobic contaminants (logKow >5). Nanoplastics <1 µm did not enhance mobility for most contaminant-polymer combinations. Microplastics <100 µm did not facilitate transport for contaminants with logDapp > −12; decoupled transport was observed only for larger microplastics (>100 µm) of specific polymer-contaminant combinations (polyethylene, polystyrene, and polyvinylchloride with PAH, PCB, and additives). For ABLD-limited mass transfer, nanoplastics <1 µm only facilitated transport for contaminants with logKpw >7 (mostly PCB). Microplastics >100 µm could facilitate transport of contaminants with logKpw > 4 (hydrophobic PAH, PCB, and some pesticides). The study identified that even under fast flow conditions, nanoplastics generally do not enhance contaminant transport. For microplastics, the possibility of enhanced transport exists under very specific conditions: fast flow (preferential flow paths), glass-like polymers, and very hydrophobic, slowly desorbing contaminants. ABLD-limited desorption is the rate-limiting factor for transport of highly hydrophobic contaminants (logKow > 5).

The findings suggest that MNP do not significantly enhance the vertical mobility of most organic contaminants in farmland soils to a degree that endangers groundwater. The models used have limitations, neglecting particle-soil interactions and simplifying polymer morphotypes (assuming spherical particles). Soil water flow velocities vary greatly, and preferential flow paths offer faster transport regimes. However, even in preferential flow paths, nanoplastics largely reach equilibrium before reaching deeper soil layers. Microplastics show potential for enhanced transport under specific conditions, but their mobility is restricted by natural filtration mechanisms. The irregular shapes of real-world MNP further limit their mobility. Factors like biofilm formation and UV-induced aging can alter desorption rates, but do not fundamentally change the overall conclusion. While MNP may carry contaminants into the soil, the overall abundance of plastics is relatively low compared to other soil phases, limiting their role in contaminant uptake. The study highlights the need for further research on the risks associated with MNP-derived contaminants in the topsoil, plant uptake, and effects on the soil microbiome.

This study demonstrates that, in most cases, micro- and nanoplastics do not significantly enhance contaminant mobility in farmland soils, challenging previous concerns about their role in groundwater contamination. The limited conditions under which MNP-facilitated transport is relevant involve fast preferential flow paths and very hydrophobic contaminants. Future research should focus on the risks posed by MNP-associated contaminants within the topsoil, including plant uptake and impacts on soil microorganisms.

The models used in this study simplify the complex soil environment by neglecting several factors, including particle-soil interactions, the irregular shapes of most MNP, and the influence of biological processes. The assumed spherical particle shape could overestimate transport for some particle types (e.g., fibers). The model also assumes homogeneous soil and does not fully capture the heterogeneity of real-world soil environments. The limited availability of comprehensive experimental data for all polymer-contaminant combinations also poses a limitation.