Collaborative Research: The Advance of Colloid Mobilization and Transport Fronts

9418172 Elimelech地下水中的放射性核素、金属和非极性有机化合物由于吸附在固定的含水层沉积物上而受到严重限制。然而，在胶体存在的情况下，这些低溶解度污染物迁移的距离远远大于仅考虑污染物在溶解相和吸附相之间分布的模型所预测的距离。胶体的存在要求在输运模型中包含污染物的吸附流动相。这一要求将我们的注意力转移到含水层中胶体的动员、运输和沉积上。胶体的形成可由含水层中的化学或物理扰动引起的原位沉淀或动员而发生。当化学扰动增加胶体和颗粒表面之间的排斥力时，胶体被动员并随地下水运输。如果导致这种扰动的溶质的运输相对于地下水被延迟，那么被动员的胶体最终将通过溶质前沿，遇到尚未受到溶质或胶体动员剂影响的沉积物。在这些条件下，我们预计胶体将重新沉积在颗粒上，并将留在那里，直到溶质“赶上”。我们假设，由化学扰动动员的胶体的运输将永远不会超过胶体动员剂的运输。为了验证这一假设，我们建议：(1)建立一个同时考虑胶体和胶体动员剂运输的模型；(2)进行一系列模拟和测试胶体动员和运输的小规模、中等规模和现场实验。该模型将由加州大学洛杉矶分校的研究人员开发和严格测试。该模型将考虑胶体的沉积和释放、胶体和颗粒表面微观和宏观电荷的非均质性、保留胶体对胶体沉积和释放的影响、溶质的吸附和解吸以及“宏观”非均质性（即在含水层沉积物中沉积）。它将在实验室实验中用于模拟胶体和溶质的一维和二维传输，并将在现场实验中扩展到三维。小规模和中等规模的实验将在科罗拉多大学水资源实验室进行。实验材料将包括赤铁矿和高岭石胶体、石英和氧化铁包覆石英多孔介质、磷酸十二烷酸（一种表面活性剂），并从现场分离出NOM作为胶体动员剂。为确定中尺度和田间试验的参数，将进行小柱试验。中等规模的实验将在一个长10米、高2米、宽5厘米的二维容器中进行，容器中填充均质和非均质（层状）多孔介质。罐体实验将直接检验胶体运输与胶体动员剂运输之间的关系。现场实验将在南卡罗来纳州乔治城的Barouch森林科学研究所进行。BFSI地点的浅层含水层主要由石英砂、氧化铁和层状非均质组成。提出了一项实地实验，将检查合成高岭石的沉积和动员碰撞标记稳定同位素（氘或18O）或钛作为硅的同构替代品。分别注入将测试来自附近池塘的十二烷酸和富无机水在含氧和缺氧含水层中作为胶体动员剂的效果。

9418172 Elimelech The transport of radionuclides, metals, and non-polar organic compounds in groundwater is severely restricted by adsorption to immobile aquifer sediments. In the presence of colloids, however, these low-solubility contaminants migrate over distances much greater than those predicted by models that consider only the distribution of the contaminant between the dissolved and the adsorbed, immobile phase. The presence of colloids requires inclusion of an adsorbed, mobile phase of the contaminant in transport models. This requirement has shifted our attention to the mobilization, transport, and deposition colloids in aquifers. Colloid formation may occur by in situ precipitation or mobilization caused by chemical or physical perturbations in the aquifer. When a chemical perturbation increases the repulsive forces between the colloid and grain surfaces, colloids are mobilized and transported with the groundwater. If the transport of the solute causing that perturbation is retarded relative to the groundwater, the mobilized colloid will eventually pass the solute front and encounter sediments that have not yet been affected by the solute, or colloid-mobilizing agent. Under these conditions, we expect that the colloids will be re-deposited on grains and will remain there until the solute "catches up." We hypothesize that the transport of colloids mobilized by a chemical perturbation will never exceed the transport of the colloid-mobilizing agent. To test this hypothesis, we propose to (1) develop a model that will simultaneously account for the transport of the colloids and the colloid-mobilizing agent and (2) conduct a series of small-scale, intermediate-scale, and field experiments simulating and testing colloid mobilization and transport. The model will be developed and rigorously tested by the UCLA researchers. The model will account for colloid deposition and release, microscopic and macroscopic charge heterogeneity of colloid and grain surfaces, the effect of retained colloids on the deposition and release of colloids, solute adsorption and desorption, and "megascopic" heterogeneities (i.e., laying in aquifer sediments). It will be formulated to model colloid and solute transport in one and two dimensions for the laboratory experiments and it will be extended to three dimensions for the field experiment. The small-and intermediate-scale experiments will be conducted at the University of Colorado's Water Resources laboratory. The materials used in the experiments will include hematite and kaolinite colloids, quartz and ferric oxyhydroxide-coated quartz porous media, and phosphate dodecanoic acid (a surfactant), and isolate NOM from the field site as colloid-mobilizing agents. Small-scale column experiments will be conducted to identify parameters for the intermediate-scale and field experiments. The intermediate-scale experiments will be conducted in a two-dimensional tank of 10 m length 2 m height, and 5 cm width filled with homogeneous and heterogeneous (layered) porous media. The tank experiments will directly test the hypothesis relating colloid transport to the transport of the colloid-mobilizing agent. The field experiment will be conducted at the Barouch Forest Science Institute site in Georgetown, S.C. The surficial aquifer at the BFSI site is composed primarily of quartz sand, ferric oxyhydroxides, and layered heterogeneity. A field experiment is proposed that will examine the deposition and mobilization of synthesied kaolinite collids labeled with a stable isotope (deuterium or 18O) or titanium as an isomorphous substitute for silicon. Separate injections will test the effects of dodecanoic acid and NOM-rich water from a nearby pond as the colloid-mobilizing agent in both oxic and suboxic portions of the aquifer.