Abstract:

A thorough understanding of fine particles clogging mechanisms is one of the critical issues during artificial recharge. This study was conducted to examine the effects of solution ionic strength (IS) and flow velocity on colloid transport and deposition in saturated porous media. Column transport experiments were carried out with different flow rate at various solution IS. These experiments were designed to obtain the long-term breakthrough curves (BTCs) in order to determine parameters by Hydrus-1D modeling. The results indicated that the BTCs were rising with increasing flow rate at given solution IS during attachment stage and values of key parameters. The results showed that these parameter values were controlled by the effects of solution IS at given flow velocity conditions and the parameter value increases with the increase of ionic strength. The effect of flow velocity was more significant for the chemical conditions when IS equaled to 30 and 50 mmol·L^-1. An explanation for these observations was obtained from extended interaction energy calculations that considered chemical heterogeneity on the sand surface. Interaction energy calculation showed that the size of the energy barrier to attachment in the primary minimum (Φ_T) reduced with increasing IS. The enhanced residence time available at low flow velocity allowed the bacteria to gain enough thermal energy to overcome the strength by realizing a primary minimum attachment. In addition, the hydrodynamic torques were small at microscopic roughness locations and grain-grain contacts. Therefore, a subsequent increase in flow velocity followed by retention phase had a negligible effect on the particles deposited at these favorable attachment locations. Hence, this study provided a very systematic experimental and theoretical evidence that the colloidal retention depended on the chemical feature and the flow velocity. On the whole, the migration and retention behavior of colloidal particles during the recharge process was mainly controlled by the ionic strength. However, the hydrodynamic characteristics interrupted the control of ionic strength. During the artificial recharge engineering, the two factors should be considered together to against the clogging.

Key words: particle, saturated porous, flow velocity, transport, retention, clogging, suspensions, model

摘要：

人工回灌过程中的堵塞问题一直是影响其推广的瓶颈，目前回灌过程中大颗粒悬浮物导致的堵塞机理研究较多，对胶体类颗粒物的堵塞机理研究相对少。采用室内砂柱实验，研究不同离子强度和不同水流流速条件下胶体在饱和多孔介质中的迁移-滞留特征。选择大肠杆菌为实验胶体，设计在不同离子强度、不同水流条件下的砂柱回灌实验；运用Hydrus-1D软件模拟，拟合穿透曲线后得到表征胶体沉积的相关参数。实验结果表明，在相同的离子强度下，流速增大会促进胶体的迁移，穿透曲线峰值增高，胶体的吸附率减小。在中等离子强度条件下（IS=30、50 mmol·L^-1）流速对胶体的这种影响比在更低的离子强度（≤10 mmol·L^-1）或更高的离子强度（≥300 mmol·L^-1）条件下更为显著；相反地，同一流速条件下，离子强度从10 mmol·L^-1升高到300 mmol·L^-1时，胶体的吸附随着离子强度的增加而迅速增加。从胶体和介质相互作用势能来看，随着离子强度的增加，胶体和砂表面的相互作用增强，有利于胶体吸附在介质表面，增加介质堵塞的概率。但是，在一定的离子强度下，流速的增加产生的水动力剪切力有利于促进胶体的迁移，不利于胶体的吸附或阻塞，减少了微小颗粒堵塞的概率。模拟结果显示吸附速率系数k、最大固相沉积量S_max随着离子强度的增大而增大，随着流速的增大而减小。从整体上来看，回灌过程中胶体微粒的迁移滞留行为主要受控于离子强度，但水流因素会干扰离子强度的控制作用。在实际的人工回灌过程中，有效的预防堵塞需要将化学（降低离子强度）和水动力（增加回灌水流速）手段有效地结合起来。

关键词: 粒子, 饱和多孔介质, 水流速度, 迁移, 滞留, 堵塞, 悬浮系, 模型