Transport mechanisms of contaminants released from fine sediment in rivers

• Published in: Acta mechanica Sinica Vol. 31; no. 6; pp. 791 - 798
• Main Authors: Cheng, Pengda, Zhu, Hongwei, Zhong, Baochang, Wang, Daozeng
• Format: Journal Article
• Language: English
• Published: Beijing The Chinese Society of Theoretical and Applied Mechanics; Institute of Mechanics, Chinese Academy of Sciences 01-12-2015
• Subjects: Adsorption; Classical and Continuum Physics; Computational Intelligence; Contaminants; Desorption; Diffusion; Engineering; Engineering Fluid Dynamics; Fluid flow; Mathematical models; Molecular diffusion; N-S方程; Research Paper; Sediments; Theoretical and Applied Mechanics; 对流扩散; 水动力条件; 污染物释放; 沉积物-水界面; 河流底泥; 细颗粒泥沙; 迁移机制; Cohesive; Coupling; Diffusion; Adsorption/desorption; Seepage boundary layer
• ISSN: 0567-7718; 1614-3116
• DOI: 10.1007/s10409-015-0520-8

Contaminants released from sediment into rivers are one of the main problems to study in environmental hydrodynamics. For contaminants released into the overlying water under different hydrodynamic conditions, the mechanical mechanisms involved can be roughly divided into convective diffusion, molecular diffusion, and adsorption/desorption. Because of the obvious environmental influence of fine sediment （D90 = 0.06 mm）, non-cohesive fine sediment, and cohesive fine sediment are researched in this paper, and phosphorus is chosen for a typical adsorption of a contaminant. Through theoretical analysis of the contaminant release process, according to different hydraulic conditions, the contaminant release coupling mathematical model can be established by the N-S equation, the Darcy equation, the solute transport equation, and the adsorption/desorption equation. Then, the experiments are completed in an open water flume. The simulation results and experimental results show that convective diffusion dominates the contaminant release both in non-cohesive and cohesive fine sediment after their suspension, and that they contribute more than 90 % of the total release. Molecular diffusion and desorption have more of a contribution for contaminant release from unsuspended sediment. In unsuspension sediment, convective diffusion is about 10-50 times larger than molecular diffusion during the initial stages under high velocity; it is close to molecular diffusion in the later stages. Convective diffusion is about 6 times larger than molecular diffusion during the initial stages under low velocity, it is about a quarter of mole- cular diffusion in later stages, and has a similar level with desorption/adsorption. In unsuspended sediment, a seepage boundary layer exists below the water-sediment interface, and various release mechanisms in that layer mostly dominate the contaminant release process. In non-cohesive fine sediment, the depth of that layer increases linearly with shear stress. In cohesive fine sediment, the range seepage boundary is different from that in non-cohesive sediment, and that phenomenon is more obvious under a lower shear stress.