Zevi Yuniati, Dathe Annette, McCarthy John F, Richards Brian K, Steenhuis Tammo S
Department of Biological and Environmental Engineering, Cornell University, Ithaca, New York 14853, USA.
Environ Sci Technol. 2005 Sep 15;39(18):7055-64. doi: 10.1021/es048595b.
Colloids have long been known to facilitate the transport of contaminants in soils, but few direct observations have been made of transport and retention in unsaturated porous media. Studies have typically been limited to evaluation of column breakthrough curves, resulting in differing and sometimes conflicting proposed retention mechanisms. We carried out pore scale visualization studies of colloid transport in unsaturated quartz sand to directly observe and characterize colloid retention phenomena. Synthetic hydrophilic (0.8, 2.6, and 4.8 microm carboxylated polystyrene latex) and relatively hydrophobic (5.2 microm polystyrene latex) colloidal microspheres were added to steady-state water flow (0.15 mm min(-1)) applied to an inclined infiltration chamber. Bright field microscopy was used to determine the positions and movement of water and colloids. Confocal laser scanning microscopy was used to determine water film geometry in an unsaturated horizontal chamber. We determined mechanisms of hydrophilic colloid retention at what is generally termed the air/water/solid (AWS) interface. Based on our observations, the AWS interface is here more accuratelytermed the air/water meniscus/solid (AWmS) interface, denoting the region where between-grain water meniscii diminish to thin water films on the grain surfaces. Colloids were retained at the AWmS interface where the film thickness approximately equaled colloid diameters. The greater retention for hydrophilic colloids at this interface (compared to elsewhere in the solid/water interface) can be explained by the additional surface tension capillary potentials exerted on colloids at the AWmS interface. While some 0.8-microm colloids were observed in thin water films, film straining played no significant role in the retention of larger colloids. Mechanisms for slightly hydrophobic colloids differed slightly. In addition to primary retention at the AWmS interface, hydrophobic colloids attached to others already present atthat interface resulting in apparent retention at the air/water (AW) interface. Attachment of hydrophobic colloids was also observed at water-solid interfaces, as hydrophobicity impelled the colloids to avoid water. Factors contributing to retention of slightly hydrophobic colloids were sand grain roughness and possibly a tendency for these colloids to flow near surfaces and interfaces, consonant with the enhanced retention of hydrophobic colloids (relative to hydrophilic colloids) observed in the literature.
长期以来，人们一直认为胶体有助于土壤中污染物的迁移，但对不饱和多孔介质中胶体的迁移和滞留进行的直接观测却很少。研究通常局限于对柱突破曲线的评估，这导致了不同的、有时甚至相互矛盾的滞留机制。我们对不饱和石英砂中胶体的迁移进行了孔隙尺度的可视化研究，以直接观察和表征胶体滞留现象。将合成的亲水性（0.8、2.6和4.8微米的羧化聚苯乙烯乳胶）和相对疏水性（5.2微米的聚苯乙烯乳胶）胶体微球添加到施加于倾斜渗透室的稳态水流（0.15毫米/分钟）中。利用明场显微镜确定水和胶体的位置及运动。利用共聚焦激光扫描显微镜确定不饱和水平室中的水膜几何形状。我们确定了亲水性胶体在通常称为气/水/固（AWS）界面处的滞留机制。基于我们的观察，这里的AWS界面更准确地称为气/水弯月面/固（AWmS）界面，表示颗粒间水弯月面减小到颗粒表面薄水膜的区域。胶体滞留在AWmS界面，此处膜厚度大约等于胶体直径。亲水性胶体在此界面处（与固/水界面的其他位置相比）具有更大的滞留率，这可以通过AWmS界面上施加在胶体上的额外表面张力毛细管势来解释。虽然在薄水膜中观察到一些0.8微米的胶体，但膜过滤在较大胶体的滞留中不起重要作用。轻度疏水性胶体的机制略有不同。除了在AWmS界面处的主要滞留外，疏水性胶体还附着在该界面处已存在的其他胶体上，导致在气/水（AW）界面处出现明显的滞留。在水-固界面处也观察到疏水性胶体的附着，因为疏水性促使胶体避开水分。导致轻度疏水性胶体滞留的因素包括砂粒粗糙度，以及这些胶体可能倾向于在表面和界面附近流动，这与文献中观察到的疏水性胶体（相对于亲水性胶体）滞留增强一致。
