An Experimental Investigation of U(IV) Organic Aqueous Complexation

知识优点：与有机配体的水络合强烈影响U（VI）在自然界中的化学形态，并且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。很难定量评价有机络合对UO2稳定性的重要性，因为对于潜在重要的U(IV)-有机络合反应，很少有可用的热力学值。UO2是铀的主要矿物，铀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，也是去除石油中硫污染物的潜在催化剂。因此，了解铀（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将研究合成无定形U（IV）氧化物（UO2(am)）在柠檬酸作为U（IV）络合配体存在下的溶解度，柠檬酸被选为自然沉积物中遇到的模型三羧酸有机酸，这是由于原位真菌和微生物活动的结果。溶解度数据将主要从最初的U（IV）欠饱和（UO2（am）溶解）实验体系中获得，作为pH、配体活性和离子强度（NaCl）的函数。在溶解的柠檬酸盐存在下，U（IV）的溶解度数据将用于回归U(IV)-有机络合反应的化学计量和平衡稳定常数的最佳模型依赖值。络合相关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开始）来测试。更广泛的影响：拟议的研究将根据U（IV）在柠檬酸盐作为螯合剂存在下的溶解度，对U(IV)-柠檬酸盐络合反应的性质进行估计。铀是沉积岩、变质岩和火成岩中重要的微量地球化学指示物，其在水合体系中的分布受络合作用和氧化还原作用双重控制。UO2是一种重要的战略矿物，是核反应堆燃料和乏燃料废弃物的主要成分。由于这些原因，有必要获得描述控制铀地球化学的主要反应的精确热力学值。本研究获得的实验数据将提高对地下水中还原条件下铀（IV）地球化学性质的认识。西密歇根大学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，学生的研究为未来的专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，这将通过这里所要求的本科生和研究生支持进一步促进。WMU鼓励少数民族和弱势群体的学生入学并取得成功，该项目将积极吸引大学为该项目招募少数民族和女性学生。透过这项计划所产生的教育机会，将有助于促进学生富有成效和创造性的研究和训练活动，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.