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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 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 and tetrathiomolybdate are both investigated. Simple organic molecules are used as analogs for more complex humic material present in the environment. Organic molecules with oxygen-containing functional 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 and pyrite, are used to isolate the factors that influence Mo removal from the aqueous phase. Pyrite was chosen due to its prevalence under sulfidic conditions.

Our 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 catechol and protocatechuic acid. Similar results were obtained when using lactic acid and malic acid, where the flexibility of the carbon chain allowed for hydroxide-molybdate interactions whether the hydroxyl groups are present on adjacent or next-neighbor carbons. These interactions are readily observed using proton NMR (nuclear magnetic resonance spectroscopy), and ATR-FTIR (attenuated total reflectance Fourier transform infrared spectroscopy). To determine the stochiometry of the aqueous metal:organic complex a Job’s plot, or method of continuous variation, was used at pH 5 and 7 in conjunction with UV/Vis (ultraviolet/visible) spectroscopy. The Job’s plot supported the formation of a 1:2 Mo:catechol complex.  The concentration of this molybdate-catechol complex did decrease as the pH decreased, which may indicate that the equilibrium is either pH dependant or that there is a competitive reaction that is more efficient at lower pH. Initial results from the LC/MS (liquid chromatography/mass spectrometry) support the formation of a 1:2 Mo:catechol complex.  Initial experiments with 2-mercaptopropionic acid and molybdate also suggest the formation of a Mo:organic complex with a 1:2 stochiometry at pH 5. In this case, Mo is likely interacting with hydroxide and thiol groups on the carbon chain, rather than just hydroxyl groups. This result is promising for Mo:organic interactions under sulfidic conditions in the environment.   

Multiple analyses of the aqueous phase of Mo and the solid over time were used to establish the amount of time required to reach equilibrium. Subsequent adsorption experiments were then completed with Mo, organic molecules, and single solid phases over a range in pH. The aqueous phase was analyzed for the final Mo concentration, with the difference between the initial and final concentrations suggesting the amount adsorbed to the solid surface. When molybdate or tetrathiomolybdate was equilibrated with aluminum oxide or pyrite, the adsorption to these solid surfaces was found to be extremely pH dependent. Consistent with the literature, under acidic conditions both molybdate and tetrathiomolybdate adsorb to these solids whereas adsorption is much less under more basic conditions above pH 7. All adsorption experiments with molybdate and aluminum oxide or pyrite have been consistent with a one-site adsorption surface as modeled with a Langmuir isotherm. When studying the adsorption of molybdate to pyrite, it was determined from the Langmuir model that both the adsorption equilibrium constants and the maximum adsorption capacities were inversely proportional to the change in pH. The addition of 2-mercaptopropionic acid appeared to inhibit the adsorption of molybdate to pyrite, although it is still uncertain whether the thiol is competitively occupying pyrite adsorption sites or if the thiol covalently binds to molybdate making it incapable of adsorbing to the solid surface. Ultimately, the results of this research should clarify the controls on Mo sequestration in modern sediments.