Mechanism comparisons of transport-deposition-reentrainment between microplastics and natural mineral particles in porous media: A theoretical and experimental study

The migration and distribution of microplastic particles (MPs) in the natural environment has attracted global attention in recent years. However, little is known about the transport-deposition-reentrainment differences between MPs and natural mineral particles in porous media. In this study, polystyrene (PS) and silica (SiO 2 ) particles, representing model MPs and natural mineral particles, respectively, were selected to study the responses of different particle types to changes in specific particle size and flow velocity. Three typical particle sizes and various flow velocities were chosen to compare and delineate the transport-deposition-reentrainment characteristics of PS and SiO 2 in a packed-bed laboratory column. Collector efficiency was calculated using Tufenkji and Elimelech (TE) equation. The particle fractions released from the collector surfaces were predicted using DLVO theory and force analysis. Two types of particles were attached in the secondary minimum, which were either retained on the collector surface or reentrained to the fluid. The staged elution experiment wherein the flow velocity was increased experienced a period of flow shock, thus breaking the force balance of the particle. An increase in the flow velocity resulted in various degrees of particle elution. The breakthrough experiment at a specific flow velocity showed that the corresponding velocity alteration in staged elution experiment contributed to reentrainment to varying extents. When the effect of gravity on particle deposition was negligible, the particle size was larger, and the lower the velocity for releasing the particles. However, the opposite tendency was observed when considering the effect of gravity on particle deposition. Moreover, the deposition, mainly due to gravity, easily causes particle reentrainment as the flow velocity increases. This study further predicts and reveals the nature of transport and deposition differences between MPs and natural mineral particles, which helps to further assess the risk and potential of groundwater contamination with MPs of different sizes. • The velocity alteration caused differences in elution of PS and SiO 2 . • Gravity significantly affects deposition for large-size or high-density particles. • Gravity-dominated deposition easily leads to the reentrainment of particles.

近年来，微塑料颗粒（MPs）在自然环境中的迁移和分布引起了全球关注。然而，对于MPs和天然矿物颗粒在多孔介质中的迁移 - 沉积 - 再悬浮差异知之甚少。在本研究中，分别选取聚苯乙烯（PS）和二氧化硅（SiO₂）颗粒作为典型的MPs和天然矿物颗粒，以研究不同颗粒类型对特定粒径和流速变化的响应。选择了三种典型粒径和多种流速，以比较和描述PS和SiO₂在填充床实验室柱中的迁移 - 沉积 - 再悬浮特性。使用Tufenkji和Elimelech（TE）方程计算收集器效率。利用DLVO理论和力分析预测从收集器表面释放的颗粒比例。两种类型的颗粒附着在次最小能量处，它们要么保留在收集器表面，要么再悬浮到流体中。在流速增加的分级洗脱实验中经历了一个流动冲击阶段，从而打破了颗粒的力平衡。流速的增加导致了不同程度的颗粒洗脱。在特定流速下的穿透实验表明，分级洗脱实验中相应的流速变化在不同程度上促进了再悬浮。当重力对颗粒沉积的影响可忽略不计时，粒径越大，释放颗粒所需的流速越低。然而，当考虑重力对颗粒沉积的影响时，观察到相反的趋势。此外，主要由重力引起的沉积随着流速的增加容易导致颗粒再悬浮。本研究进一步预测并揭示了MPs和天然矿物颗粒之间迁移和沉积差异的本质，这有助于进一步评估不同尺寸的MPs对地下水污染的风险和可能性。 • 流速变化导致PS和SiO₂洗脱的差异。 • 重力对大尺寸或高密度颗粒的沉积有显著影响。 • 以重力为主的沉积容易导致颗粒再悬浮。