This investigation explores the vertical transport demeanor of polypropylene (PP) microplastics via fluvial sand soil columns under drip irrigation and common farming amendment conditions. Fluorescently labeled PP particles (35–40 µm) were implanted in the upper 2–5 cm of four 31.5-cm-deep soil columns amended with 5% by weight of nitrogen–phosphorus–potassium (NPK) fertilizer, compost, humic acid, and a control (unamended fluvial sand). Breakthrough curves (BTCs) constructed from fluorescent microscopy-based quantification (λ = 529 nm) revealed amendment-dependent transport dynamics. Gaussian fits to raw BTCs yielded high coefficients of determination (R2 = 0.991 for NPK, 0.987 for compost, 0.989 for control, and 0.985 for humic acid), confirming accurate temporal modeling. The NPK-treated column exhibited early, sharp peaks, implying strong advective transport and a faster flow. In contrast, humic acid significantly retarded particle mobility by enhancing electrostatic and hydrophobic interactions, resulting in delayed and flattened BTCs. Heatmap analysis of vertical retention further confirmed higher particle accumulation near the surface in humic-amended columns. Chemical integrity of PP particles pre- and posttransport was verified using attenuated total reflectance–Fourier transform infrared and micro-Raman spectroscopy, with characteristic peaks observed at −2,950, 1,450, and 1,050 cm−1. Field-emission scanning electron microscopy revealed an irregular surface and spherical particle morphology conducive to advective flow and potential clogging. These results indicate that soil amendments significantly influence microplastic mobility, with conceivable implications for leaching risk to groundwater. The study integrates imaging, spectroscopy, and column-scale modeling to provide mechanistic insights into microplastic fate in amended agricultural soils and highlights modification selection as a key factor in mitigating subsurface plastic defilement.
Khan et al. (Fri,) studied this question.