Download geomicrobiological interactions among iron sulfide minerals and

GEOMICROBIOLOGICAL INTERACTIONS AMONG IRON SULFIDE MINERALS AND
METHANOGENIC CONSORTIA
1
Jerry W. Gander, 2Gene F. Parkin, and 3Michelle M. Scherer
1,2,3
Department of Civil and Environmental Engineering, The University of Iowa, 2130 Seamans
Center, Iowa City, IA 52242; 1Phone: (319) 335-6164; 1Email: [email protected];
2
Phone: (319) 355-5655, 2Email: [email protected];3Phone: (319) 355-5654, 3Email:
[email protected].
Iron sulfide minerals are common soil constituents that have been identified in a number of
anaerobic aquatic systems. Iron sulfide minerals are produced primarily via anaerobic microbial
activity where hydrogen sulfide produced by sulfate-reducing bacteria reacts with various iron
species. The initial iron sulfide species formed is mackinawite, FeS1+x. Changes in the
environmental conditions, including pH, temperature and exposure to sulfate-reducing bacteria,
however, will alter the iron sulfide minerals to other crystalline structures such as greigite, Fe3S4,
and pyrrhotite, FeS1+x. Pyrite, FeS2, (i.e., fool's gold) is the most stable form of iron sulfide
mineral, and sulfate-reducing bacteria can produce pyrite from mackinawite when elemental
sulfur is present.
Iron sulfide minerals represent a potential reductant in the natural attenuation of chlorinated
aliphatic hydrocarbons (CAHs) in reduced anaerobic environments, and it may have application
as an additive in certain passive remediation applications using iron metal as a reductant. Iron
sulfide reactions with hexachloroethane have been shown to be insensitive to many organic
amendments and mild changes in ionic strength, suggesting that iron sulfide may retain its
reactivity in diverse environments. In some cases, iron sulfides may prove to be more beneficial
than iron metal in CAH degradation. This hypothesis is particularly promising, given that one
iron sulfide form, troilite, has been shown to react much faster than iron metal in trichloroethane
degradation.
Along with independent abiotic transformation processes, iron sulfide minerals could serve a
purpose in CAH degradation during biological productive dechlorination processes as well.
Methanogenic microbial populations need sources of both ferrous iron and sulfide to maintain
normal metabolic processes. This requirement for ferrous iron and sulfide implies that iron
sulfide is present in environments where methanogens thrive. Both methanogenic populations
and iron sulfide minerals have been shown to reduce CAHs through independent processes.
What has not been investigated is the relationship, if any, between the iron sulfide degradation
reactions and the reductive dechlorination processes of methanogens.
To this end, we are investigating the role of iron sulfide minerals as a source of CAH
reduction, both in the presence and absence of methanogenic populations. This will aid in
determining any relationships between iron sulfide transformation reactions and the reductive
dechlorination processes of methanogens, and it could help predict which mechanism(s)
dominate within these combined abiotic/biotic environments. To assess the interaction of iron
sulfide minerals and methanogenic cultures, a number of laboratory techniques will be used.
Preliminary laboratory work has provided a method of iron sulfide mineral synthesis that has
been confirmed with X-ray powder diffraction (XRD). Formation of iron sulfide minerals has
also been observed via the reduction of sulfate by iron metal. Due to the high sensitivity to
oxygen of both iron sulfide minerals and methanogenic cultures, synthesis and materials
preparation will be conducted in an anaerobic chamber, and all reactors will be prepared with
methods that simulate anaerobic conditions. Batch and column studies will be conducted to
investigate the reaction kinetics and product distribution of CAH reduction in the following
systems: (1) iron sulfide + CAH contaminant, (2) methanogenic culture + CAH contaminant,
and (3) iron sulfide + methanogenic culture + CAH contaminant. Conclusions drawn based on
comparisons of the performance of these various experimental systems will aid in defining the
roles of both iron sulfide minerals and methanogenic populations in natural attenuation of CAH
contaminants. Preliminary results have shown that reduction of 1,1,1-trichloroethane by iron
sulfide minerals occurs with a half-life of approximately three days. The products resulting from
this reaction have not yet been identified and are currently being investigated.
Key words: chlorinated hydrocarbons, iron reactive barriers