PhD Oral Exam - Milad Ezzati, Chemistry

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The preservation of organic matter (OC) and phosphorous in aquatic sediments is strongly governed by interactions with reactive iron minerals. While Fe(III) (oxyhydr)oxides are recognized as important sinks for OC and PO43- under oxic conditions, their reductive dissolution may remobilize these compounds. This thesis investigates how oxidized and reduced iron minerals regulate carbon and phosphorus cycling across redox transitions and reassesses the analytical framework used to quantify iron-associated OC. The citrate–bicarbonate–dithionite extraction method was evaluated using synthetic lepidocrocite, clay minerals, and iron–organic complexes. The non-reductive control treatment mobilized a substantial fraction of OC associated with iron oxides, resulting in an underestimation of iron-associated OC by approximately 22–45%. Stable carbon isotope measurements indicated that citrate contamination was negligible. The interactions of mackinawite (FeS) with OC were examined through adsorption, coprecipitation, molecular characterization, and oxidation experiments. FeS selectively retained distinct organic fractions, with coprecipitation generally promoting greater carbon retention and molecular fractionation than adsorption. OC also enhanced FeS stability during oxidation, while part of the associated carbon remained in the solid phase and was transferred to newly formed Fe(III) minerals. FeS further immobilized PO43-under anoxic conditions, with a sorption capacity comparable to that of moderately crystalline Fe(III) minerals. Although sorbed OC reduced PO43- affinity by approximately 75%, PO43-remained retained during short-term oxidation through recapture by iron oxides or formation of vivianite-like phases. Overall, these findings identify iron minerals as dynamic biogeochemical shuttles that regulate the retention, transformation, and transfer of OC and PO43- across redox boundaries.