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This grant has funded a new research direction for the PI to investigate the mechanisms that control the removal of molybdenum (Mo) from the aqueous phase to the solid phase in marine waters. Molybdenum is present at a relatively constant concentration in oxic seawater and is removed to the solid phase under anoxic     conditions. Compelling relationships have been explored in the literature between the accumulation rates of Mo and organic carbon concentrations or burial rates, bottom water oxygen concentrations, and bottom water hydrogen sulfide concentrations.  Largely based on these relationships and Mo concentrations or accumulation rates, investigators have been encouraged to infer past changes in the conditions listed above or more general changes in reducing conditions in sediments and/or overlying waters. Difficulties in exploiting Mo as a proxy derive from a lack of mechanistic information regarding Mo accumulation. Insight on the mechanisms of Mo removal from the aqueous phase, accumulation in the solid phase, and the potential for Mo remobilization from sediments might then provide the underlying answer to why solid phase Mo accumulation correlates with bottom water oxygen or organic carbon burial. An improved understanding of Mo cycling in modern sediments should then facilitate the interpretation of Mo accumulation in ancient sediments so as to further its use as a proxy for reducing conditions or total organic carbon content in the past.

The objective of this work is to determine the role of organic molecules, either aqueous or bound to solid surfaces, in the transition of Mo between the aqueous phase and the solid phase. To provide an analogy to oxic and sulfidic conditions found in the marine environment, molybdate (MoO₄^2-) and tetrathiomolybdate (MoS₄^2-) are both investigated. Simple organic molecules are used as analogs for more complex humic material present in the environment, and oxygen-containing versus sulfide-containing organic molecules are compared. The results of this work should clarify the controls on Mo sequestration in modern sediments.

During year one, four undergraduates were involved with this project. Three students focused on this project during an intensive 10-week summer research experience, and three students concentrated on this research for their senior year Independent Research projects. Initial results regarding aqueous molybdenum-organic interactions suggest that the nature of the organic molecule is extremely important. In the aqueous phase, organic molecules with single phenolic functional groups (phenol, salicylic acid) or multiple phenolic functional roups on non-adjacent ring carbons (resorcinol) did not show any interaction with molybdate. Instead, molybdate preferentially interacts through hydrogen bonding and/or direct covalent bonds with organic molecules that have two phenolic functional groups on adjacent ring carbons, such as pyrocatechol and protocatechuic acid. These interactions are readily observed using proton NMR (nuclear magnetic resonance spectroscopy) and ATR-FTIR (attenuated total reflectance Fourier transform infrared spectroscopy). Similar results were obtained when using lactic acid and malic acid, where the flexibility of the carbon chain allowed for organic-molybdate interactions without requiring that the hydroxyl groups be present on adjacent carbons. Initial results suggest that tetrathiomolybdate does not readily interact with the abovementioned organic molecules, although future research will further investigate the possibility for these interactions. Adsorption studies were also begun with aluminum oxide (Al₂O₃) and pyrite (FeS₂) to determine the controls on molybdate and tetrathiomolybdate removal to the solid phase. Experiments, in which molybdate or tetrathiomolybdate are equilibrated with aluminum oxide or pyrite, suggest that adsorption to these solid surfaces is extremely pH dependent. Under acidic conditions, both molybdate and tetrathiomolybdate adsorb to these solids, whereas adsorption is negligible above pH 6. Further studies will investigate the effect on adsorption upon the addition of organic molecules.