Iron sulfur molecular clusters, FeS_(aq), have been found in an increasing range and number of environments, highlighting their importance in iron and sulfur chemistry. 

We have investigated how these molecules are associated with solid Fe-S minerals of varying structures in both a series of controlled laboratory experiments and in a number of natural environments.  Major findings of this research indicate that FeS(aq) is a critical intermediate form for both the formation of Fe-S minerals and the dissolution of Fe-S minerals, affecting the pathways and kinetics of formation and transformation, dissolution, and oxidation reactions governing the Fe-S system.

As part of this project we have investigated FeS_(aq) in abandoned mines near Butte, MT –with Dr. Chris Gammons at Montana Tech University.  Samples shipped overnight in sealed bottles on ice and voltammetric field analyses utilizing a flow-through cell in line with a continuously pumping wellhead completed at 150 meters depth showed significant FeS_(aq), accounting for a significant fraction of the iron and sulfur budget of this water (Figure 1).  We have concluded a series of experiments showing, for the first time, that a

Figure 1 – Solid state Au-amalgam microelectrode voltammetric scan collected on site in a sealed flow-through chamber.  Note the predominance of the FeS_(aq) peak at -1.15 V vs. Ag/AgCl, very low amounts of free Fe²⁺ at -1.4 V vs. Ag/AgCl, and the absence of Fe³⁺ at -0.3 V vs. Ag/AgCl in this voltammogram.

commonly used method of analyzing hydrogen sulfide in waters (the methylene blue method) does not recover all of the sulfide in FeS_(aq).  We believe this may account for a significant part of the confusion and disagreement in the literature surrounding accurate determinations of iron sulfide mineral solubility products (exemplified in Chen and Liu, 2005).  We also determined that methods to recover dissolved sulfide commonly used for isotopic analyses using Ag(NO₃)₂ addition and AgS recovery do recover all of the sulfide in FeS_(aq).  These results helped describe iron and sulfur cycling at these flooded, anoxic mine sites; further sulfur isotopic work at this site demonstrated the clusters are most likely formed as a result of iron and sulfide generated through bacterial activity (sulfate reducing bacteria).  These results are part of a paper currently in press with Aquatic Geochemistry. 

To test our central hypothesis on the role of FeS_(aq) in the dissolution and oxidation of Fe-S minerals and the role of these FeS_(aq) in Fe-S cycling through time and in a host of environments (including petroleum-related environments past and present) we have now completed a large number of laboratory experiments utilizing anaerobic and aerobic agarose gradient tubes.  These experiments have been conducted at pH 7 and show distinctly Fe²⁺(H₂O)₆ and FeS_(aq) diffusion from solid nanocrystalline mackinawite over time (Figure 2A-2D).  Our experiments additionally have shown that FeS_mackinawite and other Fe-S minerals dissolve non-stoichiometrically over these time scales, likely related to preferential dissolution of more Fe-rich crystallographic faces.  These experiments have not shown any evidence for H₂S_(aq) formation from mackinawite, a species for which the detection limit is several of orders of magnitude more sensitive then Fe²⁺(H₂O)₆.  This is supported by significant differences in oxidation kinetics for Fe²⁺ and H₂S (i.e. the Fe²⁺ oxidation occurs many orders of magnitude faster, therefore the absence of H₂S in our gradient experiments is not due to oxidation).  Our set of gradient profiles in time (Figure 2A-2D) additionally indicate that the FeS_(aq) and Fe²⁺ come to an equilibrium with the FeS_mackinawite, interpreted from the diffusion profiles of these species where the reaction became diffusion limited (i.e. the reaction came to equilibrium lower in the column).  These results are in good general agreement with the results of Rickard (2006) who suggested an equilibrium between mackinawite and FeS_(aq) could be reached based on the reactions:

FeS_mack = FeS_(aq)                                                                                                          (1)

FeS_mack + H⁺ = Fe²⁺ + HS^-                                                                                           (2)

However, our observations of non-stoichiometric dissolution and the presence of only FeS_(aq) and Fe²⁺ in these systems, coupled with the possibility that the FeS(aq) molecule is not of 1:1 stoichiometry indicate that these reactions may not represent at least initial FeS_mack dissolution and equilibrium.  Laboratory difficulties with the agarose matrix and electrochemical scans at pH 4 and 10 precluded additional efforts to investigate the pH effects.  Use of organic modifiers, focused on humic acid as an environmentally relevant analogue, showed significant differences in how mackinawite dissolved, resulting in no FeS_(aq) and lower levels of iron in general.  Oxidation experiments suggest that there is a significant change in the reaction pathways through which these minerals are oxidized in non-oxidative surface dissolution occurs vs. surface oxidation.

Figure 4.  Weekly gradient profiles monitoring redox species diffusivity and equilibrium for the non-oxidative dissolution of mackinawite, measured with Au-Hg microelectrode in gradient culture tube containing a mixture of 0.2% agarose and 0.01 M PIPES/0.01 M NaCl  buffer solution.