Iron-Mediated Toluene Removal in Subsurface Soils

1. School of something
FACULTY OF OTHER
School of Civil Engineering
FACULTY OF ENGINEERING
Iron-Mediated Toluene Removal in
Subsurface Soils
Rosemary Nmavulem Orlu
Dr Douglas Stewart
Professor Simon Bottrell
cn09ro@leeds.ac.uk

2. Background
Oxidative
Iron
Bacteria
Reductive
Iron
Bacteria
Fe2+ Fe3+
Oxidation
Reduction
 Iron is ubiquitous in the
environment as the 5th most
abundant metal in the biosphere.
 The recent discovery of
microorganisms that are capable
of conserving energy for growth
from iron reduction has led to a
growing interest in the effect of
iron (iii) reduction in the
bioremediation of aquifer systems
contaminated with metals or
hydrocarbons.
 In many environments, Fe (III)
is the most abundant terminal
electron acceptor (TEA) for the
oxidation of organic matter.
 An interesting feature of Fe-
reducing conditions is the
ability to couple oxidation
and reduction of organic
contaminants.
 The process is referred to as
dissimilatory iron reduction
(DIR).
 DIR involves the transfer of
electrons from
microorganisms to external
Fe (III).
 External Fe (III) is reduced to
Fe (II), acting as an aid to the
assimilation of iron into the
organism.

3. Five common iron mineral (hydr)oxides
 Most abundant iron
oxide in nature.
 Thermodynamically
stable in
comparison to other
iron minerals.
 Common
weathering product
of siderite and
magnetite.
 Blood red in
colour.
Hematite
(α-Fe2O3)
 Most common
ingredient of iron
rust.
 Second most
abundant iron
oxide after
hematite.
 Thermodynamical
ly stable.
 Yellow in colour.
Goethite
(α-FeOOH)
 Ferro-magnetic
mixed phase
(Fe (II)/Fe(III))
iron mineral
oxide.
 Chief member
of one of the
series of the
spinel group.
 Black in
colour.
Magnetite
(Fe3O4 (FeIIFe2
IIIO4))
 Generally
poorly ordered
crystal structure.
 Commonly
termed
‘amorphous iron
oxide’ or
‘hydrous ferric
oxide’.
 Reddish-brown
in colour.
 Product of the
oxidation of
Fe2+.
 Similar
composition to
goethite but
with a different
crystal
structure.
 Orange in
colour.
Ferrihydrite
(Fe5H)8.4H2O)
Lepidocrocite
(γ-FeOOH)

4. Experimental hypotheses
Overall aim of this
study:
To assess the influence
of extraneous sources of
Fe3+ on contaminant
attenuation in subsurface
soil environments.
Hypothesis 1:
Indigenous soil
microorganisms in a
previously-
contaminated soil may
be able to respire
anaerobically and
couple the reduction of
Fe (III) to the oxidation
of a hydrocarbon
contaminant (toluene).
Hypothesis 2:
The addition of
extraneous iron
sources as terminal
electron acceptors can
increase the extent /
amount of contaminant
removal as well as the
rate of the reaction.
 The iron minerals used in this study are
commonly-occurring iron (hydr)oxides
with varying degrees of crystallinity.
 Hematite, goethite, magnetite and
lepidocrocite have a more compact
crystal structure in comparison to
ferrihydrite which is known to be an
amorphous iron mineral hydroxide.
 It is well known that amorphous sources
of iron are easily accessible to
microbes/iron reducers for biotic
reductive processes.

5. Experimental method
 Laboratory preparation of hematite, goethite and
magnetite rock samples:
Cutting with the aid of a rotating saw followed by crushing
in an automated pestle and mortar.
 Synthesis of 2-line ferrihydrite:and lepidocrocite:
-Synthesis of 2-line ferrihydrite: By titrating Fe
(NO3)3.9H2O solutions with 1N NaOH following the method
in (Parkman et al., 1999);
-Synthesis of lepidocrocite: Prepared with fresh, unoxidised
FeCl2.4H2O filtered and adjusted to pH 6.7 with freshly
prepared 1M NaOH solution as in (Yu et al., 2002).
 Mesocosm content: Live soil, iron mineral
amendment, and river water successively spiked
with toluene
 Serial sampling to monitor changes in
toluene, aqueous iron, pH
 Desired soil conditions induced by
successively spiking experimental analogues of
subsurface environments to a point at which
toluene removal occurred at suppressed levels.
 Anaerobic conditions maintained during
sampling with the use of nitrogen-filled ‘ballast
volumes’ attached to the vessels
Live
soil
Mineral
amendment
River
water
.
• Laboratory-
synthesised
2-line
ferrihydrite
.
• Laboratory-
synthesised
lepidocrocite
.
• Freeze-dried
sample