Molazadeh Marziye, Calabro Guilherme, Liu Fan, Tassin Bruno, Rovelli Lorenzo, Lorke Andreas, Dris Rachid, Vollertsen Jes
Aalborg University, Section of Civil and Environmental Engineering, Department of the Built Environment, Thomas Manns Vej 23, 9220 Aalborg Øst, Denmark.
LEESU, Ecole des Ponts, Univ Paris Est Creteil, Marne-la-Vallée, France.
Sci Total Environ. 2024 Feb 10;911:168540. doi: 10.1016/j.scitotenv.2023.168540. Epub 2023 Nov 19.
Intrinsically floating microplastics (MP) such as polyethene (PE) or polypropylene (PP) are among the most common MPs found in aquatic sediments. There must hence be mechanisms that cause lighter-than-water MPs to deposit despite them being buoyant. How these MPs end up in the sediment bed is only partly understood. This study explores how turbulence in the water can affect the vertical movement of buoyant MP and bring them in contact with the bed. The deposition of PE (995 kg m) in slow-flowing water (average flow velocities of 1.85 and 4.17 cm s) was measured by tracking them and analyzing their motion in an open, rectangular, glass-sided flume. Flow characteristics in terms of turbulent kinetic energy and shear velocity were measured by particle image velocimetry. Experiments were conducted at a water depth of 27 cm and at various hydraulic conditions created by adjusting inflow speeds and using different bed materials: medium gravel, fine gravel, medium sand, cohesive sediment, and glass. The results showed that the vertical velocity of the MPs in the turbulent flow regimes varied over 4 orders of magnitude from their predicted rising velocity in quiescent water (laminar flow). Turbulence mixing resulted in distribution throughout the water column with a substantial quantity consistently subject to downward vertical transport, which in turn increased the chance of the PE particles encountering the bed and potentially getting immobilized. This work provides a plausible explanation and further experimental validation for the concept of mixing induced transfer of MPs from the water surface to the sediments of shallow waters.
诸如聚乙烯（PE）或聚丙烯（PP）等本质上漂浮的微塑料（MP）是在水生沉积物中发现的最常见的微塑料之一。因此，必然存在一些机制，使得比水轻的微塑料尽管具有浮力却仍能沉积下来。这些微塑料最终如何进入沉积床目前仅得到部分理解。本研究探讨了水中的湍流如何影响漂浮微塑料的垂直运动并使其与床体接触。通过在一个开放的、矩形的、玻璃壁面的水槽中追踪并分析聚乙烯（密度为995 kg/m³）在缓慢流动的水中（平均流速为1.85和4.17 cm/s）的运动来测量其沉积情况。通过粒子图像测速法测量了湍动能和剪切速度方面的流动特性。实验在水深27 cm以及通过调整流入速度和使用不同床体材料（中砾石、细砾石、中砂、粘性沉积物和玻璃）所产生的各种水力条件下进行。结果表明，在湍流状态下微塑料的垂直速度与其在静止水中（层流）预测的上升速度相比变化超过4个数量级。湍流混合导致微塑料在整个水柱中分布，大量微塑料持续受到向下的垂直输运，这反过来增加了聚乙烯颗粒与床体接触并可能被固定的机会。这项工作为微塑料从水面通过混合作用转移到浅水沉积物这一概念提供了合理的解释和进一步的实验验证。
