Environmental Engineering Reference
In-Depth Information
Fig. 3
Generalized pathway
for the anaerobic oxidation
of organic matter to carbon
dioxide with Fe
3
+
oxide
serving as an electron
acceptor in temperate,
freshwater and sedimentary
environments. The process is
mediated by a consortium of
fermentative microorganisms
and Geobacter species (ca.
Geobactter metallireducens
).
Data source
Lovley (
2006
)
Fe
2
+
+
HCO
3
+
OH
−
→
FeCO
3
+
H
2
O
(2.26)
The ferrous iron, hydroxyl ions and bicarbonate, produced together in (Eqs.
2.24
,
2.25
) can combine to form siderite in aquatic sediment porewaters (Eq.
2.26
)
(Coleman et al.
1993
). Evidences from that research study show that two genera of
sulphate-reducing bacteria,
Desulfobacter
and
Desulfovibrio
, can oxidize H
2
and
acetate in aquatic sediment waters (Coleman et al.
1993
). It is shown that H
2
is the
most important electron donor for
Desulfovibrio
(Eq.
2.27
), and acetate is the most
environmentally significant electron donor for
Desulfobacter
(Eq.
2.28
) sulphate
reducing bacteria (Coleman et al.
1993
):
4H
2
+
SO
4
2
−
→
S
2
−
+
4H
2
O
(2.27)
CH
3
COO
−
+
SO
4
2
−
→
S
2
−
+
2HCO
3
+
H
+
(2.28)
The study shows that
Desulfobulbus propionicus
can oxidize S to SO
4
2
−
with an
electrode serving as the electron acceptor (Lovley
2006
). This is an important reac-
tion at the anode surface in sediments, where high concentrations of sulphide can
abiotically react with electrodes producing Sº (Fig.
4
) (Lovley
2006
). This abiotic
reaction merely yields two out of eight electrons potentially available from sulphide
(S
2
−
) oxidation (Fig.
4
). Oxidation of Sº to SO
4
2
−
extracts six electrons and regener-
ates SO
4
2
−
as an electron acceptor for further microbial reduction by microorgan-
isms in the family
Desulfobulbaceae
(Lovley
2006
; Holmes et al.
2004
).
