Mechanism of perfluorooctanoic acid on microplastic transport in Northeast farmland soils: Based on irrigation mode

To investigate the mechanisms driving the migration and diffusion of microplastics (MPs) within agricultural soils subjected to perfluorooctanoic acid (PFOA) contaminated irrigation, this study, albic soil, luvisol, and chernozem were selected as porous media, with four treatment conditions established: a control group, two different PFOA concentration levels, and varying PFOA concentrations coexisting with biochar (denoted as: L0, L10, L10+BC, L20, L20+BC). Simulations mimicking real agricultural irrigation were conducted using outdoor soil column experiments to investigate MPs migration flux under varied scenarios, subsequently constructing a multifactorial soil MPs migration response function to elucidate PFOA's inducement mechanisms on MPs migration. The results indicate that the addition of biochar and high concentrations of PFOA can inhibit the migration of MPs to deeper soil layers. The average MPs concentrations in the L0 and L10 treatments were 497.56 mg/kg and 406.84 mg/kg, respectively. Compared to the L10 treatment, the average MPs concentrations in the luvisol increased by 14.37 % and 4.09 % in the L10+BC and L20 treatments, respectively. Additionally, the average MPs concentration in chernozem under the L10 treatment was 437.28 mg/kg, while the MPs concentrations in albic soils and luvisol were 5.57 % and 6.96 % lower than in chernozem, respectively. This further confirms that the migration ability of MPs in chernozem is the weakest under wastewater irrigation conditions. Furthermore, soil water flux, PFOA concentration, and MPs migration flux corresponded to a two-dimensional Gaussian function. As soil water flux increases and PFOA concentration decreases, there is a positive driving effect on the increase in MPs migration flux. Ultimately, an enhanced model for delineating MPs transport in multi-component porous media was formulated, underscoring advection's predominance over molecular diffusion and mechanical dispersion in influencing MPs transport. The study indicates that adding biochar and controlling PFOA concentrations can reduce MPs migration, protect soil and the environment, and provide scientific basis for agricultural pollution control.