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

0538084 Johannesson模拟地下水流系统中反应物溶质运输的现实方法必须与影响溶质的溶液和表面络合反应相抗衡，因为地下水成分和含水层表面化学性质沿着流动路径变化。因此，应用线性等温线方法的局限性（即，Kd）来模拟反应性溶质的平流、弥散输运，这一点得到了广泛的认可。为了应对这些挑战，我们试图将实地和实验室调查整合到我们对特征良好的含水层（德克萨斯州卡里索沙和佛罗里达上佛罗里达）中稀土元素（REE）的研究中。稀土元素是天然存在的，通常是非放射性元素，是放射性超铀元素如Pu（III），Am（III），Cm（III）和Cf（III）的化学类似物。由于稀土元素天然存在，在环境中稳定，因此它们的研究提供了一种独特的方法来研究环境中三价超铀元素的地球化学行为，而没有明显的安全问题和与实验室中使用超铀元素相关的限制。我们以前的研究稀土在含水层系统涉及调查如何稀土浓度和分馏模式响应不断变化的地下水成分，包括氧化还原条件，沿着流动路径。 实验室吸附实验导致了初步的表面络合模型（SCM），这是连接到现有的溶液络合模型，并允许定量评估的表面和溶液配体之间的竞争，稀土在地下水系统。初步观察表明，吸附到卡里佐砂稀土涉及自由金属离子（Ln 3+）和碳酸氢根络合物，Ln（CO 3）2-。吸附的碳酸氢根络合物的分数增加沿着流动路径的pH值和碱度的增加，解释了平坦的稀土分馏模式。本文建议回到单矿物Carrizo砂，碳酸盐岩Upper Floridan含水层，并启动非均质Aquia含水层的研究，以进行以下工作：（1）更好地表征REE，Mn，Fe，DOC，硫化物浓度和辅助地球化学参数沿着流动路径，以更好地限制与氧化还原有关的控制和改变含水层中REE的溶液组成;（2a）应用纳米技术（XRD，SEM，TEM，同步辐射），以表征含水层沉积物的矿物学和地球化学，重点是矿物表面涂层和稀土元素与此类涂层的关联，以及（2b）进行含水层沉积物作为pH值，稀土元素，PCO 2，和溶解有机物浓度，以显着提高现有的组合解决方案和SCM的稀土;以及（3）使用PHREEQC或更鲁棒的计算机代码开发一维平流、色散传输模式，链接到改进的解决方案和SCM，可以重现稀土穿透曲线在拟议的实验室柱实验。重点还将放在进行地下水稀土元素的超滤研究，以更好地从更真实的溶液中分离出与胶体物质、大分子量有机配体/腐殖酸相关的含水稀土元素池部分。计划与同事（地下水流建模，分子地球化学家，地质微生物学家）的合作努力，以进一步了解地下水流系统内的含水层矿物表面，胶体和/或纳米颗粒与REE的关系。

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.