An Experimental Investigation of U(IV) Organic Aqueous Complexation

EAR-0545104HAS智能优点：与有机配体的水络合强烈影响自然界中U(VI)的化学形态，在缺氧条件下，也可能影响U(IV)的分布和U(IV)矿物相(如UO2)的稳定性。要定量评价有机络合对UO2稳定性的重要性是困难的，因为对于潜在重要的U(IV)-有机络合反应的热力学数值很少。UO2是铀的主要矿石矿物，是核反应堆燃料和乏核燃料中最常见的铀形式，是军事弹道技术中广泛使用的成分，也是从石油中去除硫污染物的潜在催化剂。因此，了解U(IV)氧化物的地球化学稳定性是一个重要的研究课题。在拟议的研究中，PI将研究合成的无定形U(IV)氧化物(UO2(Am))在柠檬酸存在下的溶解度，柠檬酸作为U(IV)的络合配体，由于原位真菌和微生物的活性，被选为天然沉积物中遇到的模型三羧酸。溶解度数据将主要从最初的U(IV)不饱和(UO2(Am)溶解)实验体系中获得，作为pH、配体活性和离子强度(氯化钠)的函数。U(IV)在溶解柠檬酸盐存在下的溶解度数据将被用来回归U(IV)-有机络合反应的化学计量和平衡稳定常数的最优模型依赖值。与络合相关的溶解度关系的可逆性将通过在柠檬酸盐存在下进行U(IV)氧化物沉淀实验(从最初的过饱和条件)来测试。广泛影响：拟议的研究将基于U(IV)在柠檬酸盐存在下的溶解度来评估U(IV)-柠檬酸盐络合反应的性质。铀是沉积岩、变质岩和火成岩中重要的痕量地球化学指示剂，其在水化体系中的分布受络合作用和氧化还原作用的双重控制。UO2是一种重要的战略矿石矿物，也是核反应堆燃料和乏燃料废物的主要成分。由于这些原因，有必要获得准确的热力学数值来描述控制铀地球化学的主要反应。从这项拟议的调查中获得的实验数据将有助于加深对地下水还原条件下U(IV)的地球化学性质的理解。西密歇根大学(WMU)是一所以学生为中心的研究型大学。WMU和PI致力于促进一个富有成效和有意义的研究生和本科生研究环境，在这个环境中，学生调查为未来的职业发展提供坚实的基础。PI在学生研究指导方面有着强大而富有成效的记录，将通过这里要求的本科生和研究生支持进一步促进这一点。WMU鼓励少数群体和代表不足群体的学生入学并取得成功，该项目将积极吸引该大学招收少数群体和女性学生参加该项目。该项目带来的教育机会将有助于推动富有成效和创造性的学生研究和培训活动，丰富和促进年轻地球科学专业人员的发展。

EAR-0545104HAASIntellectual Merit: Aqueous complexation with organic ligands strongly influences the chemical speciation of U(VI) in nature, and under anoxic conditions is likely to also affect the distribution of U(IV) and the stability of U(IV) mineral phases, such as UO2. It is difficult to quantitatively evaluate the importance of organic complexation on UO2 stability, because there are few available thermodynamic values for potentially important U(IV)-organic complexation reactions. UO2 is the predominant ore mineral for uranium, the most common form of U in nuclear reactor fuel and spent nuclear fuel, a widely-used component in military ballistic technology, and a potential catalyst for removing sulfur contaminants from petroleum. Thus, understanding the geochemical stability of U(IV) oxides is a significant research concern. In the proposed study, the PI will investigate the solubility of synthetic amorphous U(IV) oxide (UO2(am)) in the presence of citric acid as a U(IV)-complexing ligand, which was selected as a model tricarboxylic organic acid encountered in natural sediments as a result of in situ fungal and microbial activity. Solubility data will be obtained primarily from initially U(IV) undersaturated (UO2(am) dissolution) experimental systems, as a function of pH, ligand activity and ionic strength (NaCl). Solubility data for U(IV) in the presence of dissolved citrate will be used to regress optimal model-dependent values for stoichiometric and equilibrium stability constants for U(IV)-organic complexation reactions. Reversibility of complexation-related solubility relations will be tested by conducting U(IV) oxide precipitation experiments (from initially supersaturated conditions) in the presence of citrate.Broader Impacts: The proposed study will develop estimates for the properties of U(IV)-citrate complexation reactions, based on U(IV) solubility in the presence of citrate as a chelating agent. Uranium is an important trace geochemical indicator in sedimentary, metamorphic and igneous rocks, and its distribution in hydrated systems is controlled by both complexation and redox processes. UO2 is an important strategic ore mineral and the primary constituent of nuclear reactor fuel and spent fuel waste. For these reasons it is necessary to obtain accurate thermodynamic values describing the major reactions governing U geochemistry. The experimental data obtained from this proposed investigation will improve understanding of the geochemical properties of U(IV) under reducing conditions in ground waters.Western Michigan University (WMU) is a student-centered research university. WMU and the PI are committed to promoting a productive and meaningful graduate and undergraduate research environment, where student investigations provide a sound footing for future professional advancement. The PI has a strong and productive record of student research mentoring, which will be further promoted through the undergraduate and graduate support requested here. WMU encourages the enrollment and success of minorities and students from under-represented groups, and this project will actively engage the university in recruiting minority and female students to this project. The educational opportunities engendered through this project will help to advance productive and creative student research and training activities, enriching and advancing the development of young earth science professionals.