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

模拟地下水流动系统中反应溶质输运的现实方法必须与溶液和表面络合反应相抗衡，这些反应会影响溶质，因为地下水组成和含水层表面的化学成分会沿着流动路径变化。因此，应用线性等温线方法（即Kd）来模拟活性溶质的平流、色散传输的局限性通常被广泛接受。为了应对这些挑战，我们试图将现场和实验室调查整合到我们对表征良好的含水层（德克萨斯州Carrizo Sand和佛罗里达州上佛罗里达）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。由于稀土元素是天然存在的，并且在环境中是稳定的，他们的研究提供了一种独特的方法来研究三价超铀元素在环境中的地球化学行为，而没有明显的安全问题和在实验室中与超铀元素相关的限制。我们之前对含水层系统中稀土元素的研究涉及研究稀土元素浓度和分异模式如何响应不断变化的地下水组成，包括沿流动路径的氧化还原条件。实验室吸附实验建立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，可以定量评估地下水系统中稀土在表面和溶液配体之间的竞争。初步观察表明，稀土在Carrizo砂上的吸附涉及到自由金属离子（Ln3+）和dicarbonato配合物Ln(CO3)2-。随着pH和碱度的增加，吸附的碳酸配合物的分数沿流道增加，说明稀土分馏模式趋于平缓。本文拟回归单矿物Carrizo砂、上佛罗里达碳酸盐含水层，开展非均质Aquia含水层研究，开展以下工作：(1)更好地表征REE、Mn、Fe、DOC、硫化物浓度及沿流道的辅助地球化学参数，更好地约束含水层中稀土元素的氧化还原相关控制和溶液组成变化；（2a）应用纳米技术（XRD、SEM、TEM、同步辐射）表征含水层沉积物的矿物学和地球化学特征，重点研究矿物表面涂层及其与稀土元素的关系；（2b）对含水层沉积物进行相关的批量吸附实验，研究其随pH、稀土元素、二氧化碳分压和溶解有机质浓度的变化规律，显著改善现有的组合溶液和稀土元素的SCM；(3)使用PHREEQC或更强大的计算机代码开发一维平流色散输运模型，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重现REE突破曲线。重点开展地下水稀土超滤研究，更好地从溶液中分离出与胶体物质、大分子量有机配体/腐殖质相关的水相稀土池。我们计划与同事（地下水流动建模师、分子地球化学家、地球微生物学家）合作，进一步了解地下水流动系统中稀土元素与含水层矿物表面、胶体和/或纳米颗粒之间的关系。

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.