59th Annual Report on Research 2014

Project Objectives:  The overarching purpose of this project is to define better the chemical pathway leading to Re deposition within anoxic waters.  Recent evidence from the Black Sea suggests that Re removal may be linked to Mo deposition via co-precipitation of Re with a posited FeMoS solid phase (i.e., Fe₅Mo₃S₁₄).  This potential relationship has led the project to include batch experiments involving formation of FeMoS solid(s) in the presence of trace concentrations of ReO₄^-.  These experiments aim to determine thermodynamic constants for the FeMoS solid(s) as well as quantify any fractionation occurring between Re and Mo during their possible co-precipitation.  The project also seeks to develop a reverse phase ion pair chromatography (RP-IPC) method for separating mixtures of all oxythiomolybdates (MoO₄^2-, MoO₃S^2-, MoO₂S₂^2-, MoOS₃^2-, and MoS₄^2-) and all oxythioperrhenates (ReO₄^-, ReO₃S^-, and ReS₄^-; ReO₂S₂^- and ReOS₃^- are kinetically unstable.).  Presently, Mo and Re speciation can only be predicted from thermodynamic data.  Eventual coupling of RP-IPC to ICP-MS may allow for quantifying actual Mo and Re speciation within natural sulfidic waters.  Because Mo and Re have stable isotopes, RP-IPC with MS may be used in the future to quantify isotopic fractionation occurring as MoO₄^2- or ReO₄^- transforms into MoS₄^2- or ReS₄^-. 

Research Progress:

a.)    Co-precipitation of Re with an FeMoS solid phase 

Figure 1 shows the reaction progress upon addition of 70 mM MoS₄^2- to a solution containing initially equimolar Fe²⁺ and 10 mM SS^2- at pH = 8.5.   At ~6 days, 0.1 M MgCl₂ was added to the reaction mixture to cause flocculation of initially formed colloids that would otherwise escape filtering.  The main graph in the figure displays an initial rapid loss of MoS₄^2- followed by a sluggish decay until equilibrium is achieved after ~85 days. Concomitantly, total dissolved Fe begins to accumulate in solution until it appears to reach equilibrium with MoS₄^2-. Total dissolved Mo remains constant upon MgCl₂ addition. These results imply the formation of FeMoS solid and aqueous phases.  In the aqueous phase, the stoichiometry of the FeMoS complex appears to be 1:1 in Fe:Mo, consistent with 1:1 FeMoS aqueous complexes (e.g., [(Fe₂S₂)(MoS₄)₂]^4-) known to form in the presence of pyrrhotite surfaces.

The inset in the figure shows data related to the FeMoS solid phase. The y-axis in the inset is calculated from the difference between initial total Mo and Fe and equilibrium values of dissolved Mo and Fe.  Total Fe and total Mo associated with the FeMoS solid yield an Fe:Mo ratio of 1.56 ± 0.07. This ratio is in very good agreement with that of Fe₅Mo₃S₁₄ (Fe:Mo = 1.7 ± 0.7), the solid posited to account for Mo removal (and possibly Re) in euxinic water columns.

Figure 1:  Reaction progress upon addition of 70 mM MoS₄^2- to a solution containing equimolar Fe²⁺ and 10 mM SS^2- at pH = 8.5.   Although the solution is saturated WRT FeS_mackinawite, the system becomes dominated by FeMoS_(aq,s) chemistry at equlibrium (~85 days).  If mackinawite controlled Fe solubility, SFe_dissolved ~ 0.1 nM.  Note that FeMoS_(aq) is ~1:1 Fe:Mo at equilibrium (Mo in FeMoS_(aq) = dashed blue minus solid black line).  FeMoS_(s) equilibrium data shown in inset graph.  Total metal concentrations quantified by AAS; MoS₄^2- quantified by UV-vis.

The solid formed after 112 days in the solution described in Fig. 1 was isolated under an N₂ atmosphere and subjected to X-ray absorbance spectroscopic (XAS) analyses at the Advanced Photon Source associated with Argonne National Laboratory.  The XAS work was performed by Dr. Anthony Chappaz with whom I collaborate on research related to trace metal geochemistry. Figure 2 shows XANES and EXAFS spectra of the FeMoS solid as well as those of various standards.  XANES results suggest that the FeMoS solid contains Mo^IV. Modeled interatomic distances for the FeMoS solid from EXAFS spectra are consistent with those of a FeMoS cubane cluster. Most intriguing, the EXAFS spectra of the synthesized FeMoS solid agree with major aspects of spectra previously taken of the form of Mo found within 7000 to 8000 year old Lake Cadagno sediments (black vs. green or pink spectra).  Taken together, these data suggest that we have reproduced in the lab a naturally occurring pathway to Mo burial. At present, Re:Mo ratios within the synthesized FeMoS solid and equilibrated aqueous phase have not been quantified for characterizing any Re and Mo fractionation during possible co-precipitation. However, several batch experiments over a range of solution conditions await ICP-MS analyses over the coming year.   

Figure 2:  XANES spectra and Fourier Transform of EXAFS spectra for the FeMoS solid formed after 112 days in the solution described in Fig. 1.  For comparison, blue and red XANES spectra are those of MoS₄^2- and MoS₂. Pink and green EXAFS Fourier Transforms are those of the form of Mo in 7000 or 8000 year old Lake Cadagno sediments.

b.)   Separation of all oxythiomolybdate and all oxythioperrhenate anions

Figure 3 displays a chromatogram demonstrating the successful separation of a mixture containing all eight oxythiomolybdate and oxythioperrhenate anions at mM concentration. The anions within a given series emerge from the column at substantially increasing times with increased sulfidation.  As a whole, the oxythioperrhenates are much more strongly retained relative to the oxythiomolybdates during RP-IPC elution. Isocratic elutions demonstrated that retention of especially oxythiomolybdates is greatly diminished by either addition of p-cyanophenol (p-CP) or enhancement of acetonitrile (ACN) in the eluent.  These observations led to the gradient elution conditions shown above the chromatogram.     

Figure 3:  Chromatogram showing separation of all stable oxythiomolybdates and oxythioperrhenate anions at mM concentrations using RP-IPC and conductivity detection.  TBA⁺ = tetrabutylammonium cation; ACN = acetonitrile; p-CP = p-cyanophenol (added to promote elution of strongly retained MoS₄^2-, ReOS₃^-, and ReS₄^-).

Student participation, publications, and presentations:  One publication in Metallomics and three presentations were supported by PRF during 2013-2014.  A total of five undergraduates participated in the project.  Two students presented their results at the Spring 2014 ACS national meeting.  The PI presented results at the Fall 2013 ACS national meeting as well as the Goldschmidt Conference in Spring 2014.