Microbial metabolisms can impact abiotic mineral-promoted
trichloroethene
(TCE) reduction in groundwater environments, but mechanistic understanding
of these coupled processes is limited. Here, we explore how sulfate-reducing
bacteria (SRB) enhance TCE reactivity of iron sulfide minerals, specifically
addressing how SRB maintain reactive iron sulfide surfaces after biogenic
mineral formation. Iron sulfides were formed either abiotically (ferrous
iron and sulfide) or biotically (ferrous iron and sulfate reduction
by Desulfovibrio vulgaris) in batch
systems. TCE was added, and reaction products were monitored under
different ferrous iron:sulfur (Fe:S) ratios. With D.
vulgaris present, higher Fe:S ratios showed over an
order of magnitude increase in TCE transformation rates. These rates
increased with lower reduction potentials (R
2 = 0.66, p = 0.0014), as potentials decreased
below −150 mV vs SHE. Mineral precipitate characterization
indicated the presence of mackinawite (FeS), and pH and redox potentials
confirmed experimental conditions in the FeS stability range. Filtered D. vulgaris media (SRB removed) showed similarly
high rates to biotic experiments, implying the role of biogenic redox-active
soluble microbial products (SMPs) in maintaining reducing conditions.
From these results, we propose a reaction scheme, where iron sulfide
surfaces reduce TCE, oxidizing mineral surface species, which are
then “re-reduced” by SMPs from D. vulgaris.
Reactive
oxygen species generated during the oxygenation of different
ferrous species have been documented at groundwater field sites, but
their effect on pollutant destruction remains an open question. To
address this knowledge gap, a kinetic model was developed to probe
mechanisms of •OH production and reactivity with trichloroethene
(TCE) and competing species in the presence of reduced iron minerals
(RIM) and oxygen in batch experiments. RIM slurries were formed by
combining different amounts of Fe(II) and sulfide (with Fe(II):S ratios
from 1:1 to 50:1) or Fe(II) and sulfate with sulfate reducing bacteria
(SRB) added. Extents of TCE oxidation and •OH production were
both greater with RIM prepared under more reducing conditions (more
added Fe(II)) and then amended with O2. Kinetic rate constants
from modeling indicate that •OH production from free Fe(II)
dominates •OH production from solid Fe(II) and that TCE competes
for •OH with Fe(II) and organic matter (OM). Competition with
OM only occurs in experiments with SRB, which include cells and their
exudates. Experimental results indicate that cells and/or exudates
also provide electron equivalents to reform Fe(II) from oxidized RIM.
Our work provides new insights into mechanisms and environmental significance
of TCE oxidation by •OH produced from oxygenation of RIM. However,
further work is necessary to confirm the relative importance of reaction
pathways identified here and to probe potentially unaccounted for
mechanisms that affect abiotic TCE oxidation in natural systems.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.