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This grant has funded a new research direction for the PI to investigate the controls on 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 as molybdate (MoO₄^2-) and is removed to the solid phase under reducing and sulfidic conditions. Based on the response of Mo to the extent of reducing conditions, investigators have been encouraged to use Mo solid phase concentrations or accumulation rates to infer past changes in reducing conditions in sediments and/or overlying waters. However, difficulties in exploiting Mo as a proxy derive from a lack of mechanistic information regarding Mo accumulation – exactly what are the controlling factors for the removal of Mo from the aqueous phase to the solid phase? For example, compelling relationships have been explored in the literature between the accumulation rates of Mo and organic carbon concentrations or burial rates.  Therefore, insight on what controls 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 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. Organic molecules with oxygen-containing function groups are used to mimic the functional groups found in fulvic and humic acids under oxic conditions, whereas sulfide-containing organic molecules are used to mimic the functional groups present under sulfidic conditions. For example, glutathione, 3-mercaptopropanoic acid, and mercaptosuccinic acid are often present in sulfidic marine pore waters, and their thiol functionality is common on more complex organic molecules found in the environment. Single minerals, such as aluminum oxide (Al₂O₃) and pyrite (FeS₂), are used to isolate the factors that influence Mo removal from the aqueous phase. Pyrite was chosen due to its prevalence under sulfidic conditions, and this solid was carefully cleaned using hydrochloric acid and sodium sulfide solutions to ensure that the surface is relatively free of oxidized sites.

Initial results suggest that the nature of the organic molecule and the type of functional group are extremely important for aqueous molybdenum-organic interactions. In the aqueous phase, organic molecules with single phenolic functional groups (phenol, salicylic acid) or multiple phenolic functional groups 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.

Multiple analyses of the aqueous phase of Mo and the solid over time are used to establish the amount of time required to reach equilibrium. Subsequent adsorption experiments are then done with Mo, solid phase, and organic molecules over a range in pH. The aqueous phase is analyzed for both final Mo and organic concentrations; the difference between the initial and final concentrations denotes the amount adsorbed to the solid surface. Early adsorption experiments, in which molybdate or tetrathiomolybdate are equilibrated with aluminum oxide or pyrite, suggest that adsorption to these solid surfaces is extremely pH dependent. Consistent with the literature, under acidic conditions, both MoO₄^2- and MoS₄^2-  adsorb to these solids whereas adsorption is negligible above pH 6. Research during this past year has focused on thiols. Based on just the initial and final aqueous Mo concentrations, the addition of 2-mercaptopropionic acid appears to inhibit the adsorption of MoO₄^2- to pyrite, although it is uncertain whether the thiol is competitively occupying pyrite adsorption sites or if the thiol covalently binds to MoO₄^2- making it incapable of adsorbing to the solid surface. Although the oxygen-containing organic compounds were easily quantified using UV/Vis (ultraviolet/visible spectroscopy), the quantification of the sulfide-containing molecules requires a different approach. Thiols are quantified using a derivatization reaction with monobromobimane, which fluoresces upon bonding with a thiol. Samples are monitored using LC/MS (liquid chromatography-mass spectrometry), which provides separation and identification for the various solution components. We are also investigating the utility of LC/MS for verifying the formation of aqueous Mo-organic complexes. Ultimately, the results of this work should clarify the controls on Mo sequestration in modern sediments.