Towards a Hydrochemical Transport Model for Rare Earth Elements in Groundwater Flow Systems: Coupling Field, Laboratory, and Computational Techniques

在地下水流动系统中模拟反应物溶质运移的实际方法必须与溶液和表面络合反应作斗争，这些反应会随着地下水成分和含水层表面化学成分沿流动路径的变化而影响溶质。因此，应用线性等温线方法(即Kd)来模拟平流的、弥散的反应性溶质输运的局限性被普遍接受。为了应对这些挑战，我们试图在我们对特征良好的含水层(德克萨斯州卡里佐·桑德和佛罗里达州上弗洛里丹)中的稀土元素(REE)的研究中整合野外和实验室调查。稀土是自然存在的非放射性元素，通常是放射性超铀元素的化学类似物，如Pu(III)、Am(III)、Cm(III)和Cf(III)。由于稀土元素在环境中自然存在且稳定，他们的研究提供了一种独特的方法来研究环境中三价超铀化合物的地球化学行为，而没有明显的安全问题和与实验室中使用超铀化合物相关的限制。我们之前对含水层系统中稀土元素的研究涉及到稀土元素浓度和分馏模式如何响应地下水成分的变化，包括沿流动路径的氧化还原条件。实验室吸附实验得到了一个初步的表面络合模型(SCM)，该模型与现有的溶液络合模型相关联，并允许定量评估地下水系统中表面和溶液配体之间对稀土的竞争。初步观察表明，稀土在Carrizo砂上的吸附涉及自由金属离子(Ln3)和二碳络合物Ln(CO3)2-。随着pH值和碱度的增加，吸附的二碳络合物的比例沿流动路径增加，这解释了稀土分馏模式的平坦化。本文建议回归单矿物Carrizo砂岩，即上佛罗里达碳酸盐含水层，并开始研究非均质含水层，以便进行以下工作：(1)更好地表征流动路径上的稀土元素、锰、铁、DOC、硫化物浓度和辅助地球化学参数，以更好地约束与氧化还原相关的控制和改变含水层中稀土的溶液成分；(2a)应用纳米尺度技术(X射线衍射仪、扫描电子显微镜、透射电子显微镜、同步辐射)来表征含水层沉积物的矿物学和地球化学性质，重点是矿物表面涂层以及稀土与这种涂层的关联；(2b)对含水层沉积物进行联合批量吸附实验，作为pH、REES、PCO2和溶解有机质浓度的函数，以显著改善现有的稀土组合溶液和SCM；以及(3)使用PHREEQC或更强大的计算机代码来开发一维平流、弥散传输模型，该模型与改进的溶液和SCM相关联，可以在拟议的实验室柱实验中再现REE突破曲线。还将侧重于对地下水稀土进行超滤研究，以便更好地将与胶体物质、大分子有机配体/腐殖质有关的稀土水池部分与真正存在于溶液中的部分区分开来。计划与同事(地下水流动模型师、分子地球化学家、地质微生物学家)开展合作，以加深我们对稀土元素与地下水流动系统中含水层矿物表面、胶体和/或纳米颗粒之间关系的理解。

0538084Johannesson Realistic approaches to modeling transport of reactant solutes in groundwater flow systems must contend with solution and surface complexation reactions that affect solutes as groundwater composition and aquifer surface site chemistry change along flow paths. Consequently, the limitations of applying the linear isotherm approach (i.e., Kd) to modeling advective, dispersive transport of reactive solutes is generally well accepted. To meet these challenges, we have sought to integrate field and laboratory investigations in our studies of the rare earth elements (REE) in well characterized aquifers (Carrizo Sand, Texas, and Upper Floridan, Florida). The REEs are naturally occurring, generally non-radioactive elements that are chemical analogs of radioactive transuranic elements such as Pu(III), Am(III), Cm(III), and Cf(III). Because the REEs occur naturally and are stable in the environment, their study provides a unique way to investigate the geochemical behavior of trivalent transuranics in the environment without the obvious safety concerns and restrictions associated with working with transuranics in the laboratory. Our previous studies of REEs in aquifer systems involved investigations of how REE concentrations and fractionation patterns respond to changing groundwater compositions, including redox conditions, along flow paths. Laboratory adsorption experiments led to a preliminary surface complexation model (SCM), which was linked to an existing solution complexation model, and which allows for quantitative assessments of competition between surface and solution ligands for REEs in groundwater systems. Preliminary observations indicate that adsorption of REEs onto Carrizo sand involves free metal ions (Ln3+) and the dicarbonato complex, Ln(CO3)2-. The fraction of adsorbed dicarbonato complex increased along the flow path as pH and alkalinity increased, explaining the flattening of REE fractionation patterns. Proposed herein is a return to the monomineralic Carrizo Sand, the carbonate Upper Floridan aquifer, and initiation of study of the heterogeneous Aquia aquifer in order to conduct the following work: (1) better characterize REE, Mn, Fe, DOC, sulfide concentrations, and ancillary geochemical parameters along flow paths, to better constrain redox related controls and changing solution composition on REEs in aquifers; (2a) apply nanoscale techniques (XRD, SEM, TEM, synchrotron radiation) to characterize the mineralogy and geochemistry of aquifer sediments with emphasis on mineral surface coatings and the association of REEs with such coatings and (2b) conduct allied batch adsorption experiments of aquifer sediments as a function of pH, REEs, PCO2, and dissolved organic matter concentrations to significantly improve the existing combined solution and SCM for REEs; and (3) develop a 1-D advective, dispersive transport model using PHREEQC, or a more robust computer code, linked to the improved solution and SCM that can reproduce REE breakthrough curves in proposed laboratory column experiments. Emphasis will also be placed on conducting ultrafiltration studies of groundwater REEs in order to better sort out the fraction of the aqueous REE pool that is associated with colloidal materials, large-molecular weight organic ligands/humics, from that which is more truly in solution. Collaborative efforts with colleagues (groundwater flow modelers, molecular geochemists, geomicrobiologists) are planned to further our understanding REE association with aquifer mineral surfaces, colloids, and/or nanoparticles within groundwater flow systems.