Chemistry PhD Final Defense — Megan Hawkins, Graduate Student

About the event

Speaker: Megan Hawkins

Group: Di Wu

Title: Thermodynamics of Layered Double Hydroxides for Removal of Iodine Species from Aqueous Media

Abstract: Nuclear energy is a cleaner energy form that can reduce the consumption of fossil fuels and minimize the emission of carbon dioxide. The major cost is that the nuclear wastes generated in the fission process and spent fuel reprocessing must be properly treated and converted to stable waste forms for long-term storage in geological repositories. Due to their high mobility in the aqueous media of the earth’s critical zone, radioactive iodine (¹²⁹I and ¹³¹I), particularly, in anionic forms (iodide, I^–, and iodate, IO₃^–) causes negative environmental impacts and potentially leads to health issues. This thesis is on the separation and electrochemical sensing of anionic iodine species from aqueous media using nickel (Ni)-based layered double hydroxides (LDHs). LDHs are brucite-like layered materials with positively charged layers, open interlayer space, electrochemically sensitive and redox-active surfaces, and intercalated charge-balancing anions. Therefore, they have great potential for the separation and detection of I^– and IO₃^–. Here, I document a set of systematic studies, including (i) the energetic, thermal, and structural stability of NiAl-LDH-CO₃^2–, NiAl-LDH-I^–, and NiAl-LDH-IO₃^–, (ii) the electrochemical sensing of I^– and IO₃^– employing NiM-LDH-CO₃^2– (M³⁺ = Al³⁺, Cr³⁺ and Fe³⁺). The results suggest that the formation of NiAl-LDH-I^– is thermodynamically more favorable than NiAl-LDH-CO₃^2–, more exothermic by ~30 kJ/mol LDH, while intercalation of IO₃^– results in significantly decreased energetic stability by ~100 kJ/mol LDH. On the other hand, when M³⁺ = Al³⁺ and Cr³⁺, the specific capacity are significantly modified by I^– and IO₃^– exchange suggesting high sensitivity suitable for anionic iodine sensing. The magnitudes of specific capacities for NiFe-LDH-CO₃^2–, NiFe-LDH-I^– and NiFe-LDH-IO₃^– are low all below 1.0 mAh/g. Therefore, according to the data in this thesis NiFe-LDH-CO₃^2– may not be sensitive enough to detect I^– and IO₃^–. The fundamental calorimetric and electrochemical insights of this thesis may benefit iodine waste form design and anionic iodine sensing.