Department of Civil and Environmental Engineering, University of California, 1 Shields Avenue, #2001, Davis, California 95616, United States.
Department of Chemical Engineering, University of Chemistry and Technology, Technicka 5, Prague 166 28, Czech Republic.
Environ Sci Technol. 2020 May 5;54(9):5813-5821. doi: 10.1021/acs.est.9b05744. Epub 2020 Apr 1.
Colloid deposition in granular media is relevant to numerous environmental problems. Classic filtration models assume a homogeneous pore space and largely ignore colloid aggregation. However, substantial evidence exists on the ubiquity of aggregation within porous media, suggesting that deposition is enhanced by it. This work studies the deposition process in relation to aggregate size and structure. We demonstrate that aggregation is induced at typical groundwater velocities by comparing the repulsive DLVO force between particle pairs to the hydrodynamic shear force opposing it. Column experiments imaged with high-resolution X-ray computed tomography are used to measure aggregate structure and describe their morphology probability distribution and spatial distribution. Aggregate volume and surface area are found to be power-law distributed, while Feret diameter is exponentially distributed with some flow rate dependencies caused by erosion and restructuring by the fluid shear. Furthermore, size and shape of aggregates are heterogeneous in depth, where a small number of large aggregates control the concentration versus depth profile shape. The range of aggregate fractal dimensions found (2.22-2.42) implies a high potential for restructuring or breaking during transport. Shear-induced aggregation is not currently considered in macroscopic models for particle filtration, yet is critical to consider in the processes that control deposition.
胶体在颗粒介质中的沉积与许多环境问题有关。经典的过滤模型假设孔隙空间是均匀的，在很大程度上忽略了胶体聚集。然而，大量证据表明聚集在多孔介质中普遍存在，这表明沉积过程会受到聚集的促进。本工作研究了与聚集大小和结构有关的沉积过程。我们通过比较颗粒对之间的排斥 DLVO 力与抵抗它的流体剪切力，证明了在典型的地下水速度下会诱导聚集。使用高分辨率 X 射线计算机断层扫描进行的柱实验用于测量聚集结构，并描述其形态概率分布和空间分布。聚集物的体积和表面积呈幂律分布，而费雷特直径呈指数分布，其中一些流速依赖性是由侵蚀和流体剪切的重构引起的。此外，聚集物的大小和形状在深度上是不均匀的，少数大聚集物控制着浓度随深度的分布形状。发现的聚集分形维数范围（2.22-2.42）意味着在运输过程中具有很高的重构或断裂的潜力。剪切诱导聚集目前尚未在颗粒过滤的宏观模型中考虑，但在控制沉积的过程中是必须要考虑的。
