Chemistry Proposal Defense – Emma Kindall

About the event

Speaker: Emma Kindall

Group: Dr. Xiaofeng Guo

Title: Thermodynamics of Rare Earth Elements Mixing in Orthophosphate Minerals

Abstract:

Rare earth element (REE) orthophosphates (REEPO4 ∙ nH2O with 0 ≤ n ≤ 2) constitute a group of mineral structures with important applications to REE resource extraction mineralogy and also to nuclear energy as a possible waste form for conditioned spent nuclear fuel (SNF) and other high-level radioactive waste. A robust characterization of the thermochemical properties of these minerals is critical to understanding their behavior (formation, stability, alterations, etc.) under mineral formation and waste storage conditions. Although, REE orthophosphate endmember thermodynamics are relatively well known, solid solution mixing behavior in this system has not been fully characterized—particularly multicomponent solutions more relevant to natural systems and mixing between non-isostructural phases. To this end the work proposed herein will fill critical gaps in orthophosphate mineral mixing systems through two primary objectives. First, anhydrous hexagonal orthophosphate mineral binary solid solutions will be thermodynamically characterized and fit to regular, sub-regular, and quasi-chemical mixing models which will be used to build multicomponent solid solutions from the constituent binary interactions and compared to Compound Energy Formalism (CEF) modeling. In the second objective, mixing between REEs with different endmember structures will be investigated through precipitation experiments, revealing appropriate synthetic routes to prepare binary monazite and xenotime solid solutions that will be calorimetrically characterized and fit to Darken’s Quadratic Formalism (DQF). Data from these experiments will be used to model aqueous/solid solution equilibria with both forward and backwards modeling yielding the possibility of self-consistency checks. This work will provide critical data for modeling the orthophosphate mineral system, which will be valuable for understanding the paragenesis of one of the primary REE extraction sources and evaluating the suitability of a candidate nuclear waste form. More broadly, it will demonstrate a process for building up to more complex mixing systems.