Environmental Engineering Reference
In-Depth Information
described for the pollutants treatment at anode
of BES (Venkata Mohan and Srikanth
2011
).
Substrate degradation in the anode chamber is
mainly influenced by the oxygen in the cathode
chamber acting as terminal electron acceptor
(TEA). The strong electron acceptor conditions at
cathode enhances the electron flow in the circuit
and in turn their release from the microbial me-
tabolism of wastewater. The presence of oxidiz-
ing agents (which gain electrons) like chlorine,
bromine and ozone increases the potential dif-
ferences between electrodes and thus the redox
potential of the system which in turn favours EO,
resulting in both pollutant as well as carbon re-
moval. In general, pollutants are adsorbed on the
anode surface and get destroyed by the anodic
electron transfer reactions during DAO, while
during the IAO, these pollutants will be oxidized
by the oxidants (primary and secondary) formed
electrochemically on the anode surface under in
situ biopotential. DAO facilitates formation of
primary oxidants which further react on the anode
yielding secondary oxidants such as chlorine di-
oxide and ozone, which will have significant pos-
itive impact on treatment, especially for colour
removal efficiency. Furthermore, the reactions
between water and free radicals near the anode
yields secondary oxidants, viz. nascent oxygen,
free chlorine and hydrogen peroxide, hypochlo-
ric acid, etc. which can also help in colour/organ-
ic oxidation (Venkat Mohan and Srikanth
2011
).
On the other hand, these pollutants can also act as
mediators for electron transfer between microbes
and anode which helps in their reduction with si-
multaneous power enhancement.
Initially, the simple organic fraction of waste
will be oxidized at anode through microbial me-
tabolism releasing reducing equivalents [e
−
and
H
+
], which interact with the water molecules
under in situ biopotential forming hydroxyl radi-
cals (Israilides et al.
1997
; Venkat Mohan and
Srikanth
2011
). These hydroxyl radicals will
get adsorbed onto the active sites of anode and
initiates DAO, either alone or in combination
with free Cl
−
, (chloro hydroxyl radical) if pres-
ent in wastewater. Oxygen and water molecules
react with the radicals adsorbed on the electrode,
forming secondary oxidants (O
3
, ClO
2
and H
2
O
2
)
which initiate the IAO process. As the concen-
tration of primary oxidants increases, the forma-
tion of secondary oxidants also increases in the
electrolysed solution (Israilides et al.
1997
; Wilk
et al.
1987
). These oxidants also have a quite long
life which can also diffuse away from the elec-
trodes to the solution and enhance the IAO pro-
cess (Israilides et al.
1997
). Efficient cathodic re-
duction reaction also can influence the substrate
degradation at anode by inducing the oxidation
reaction (induced EO) at anode under in situ de-
veloped bio-potential.
General mechanism of oxidants formation:
+ₒ
(10.1)
O
e
O*
[
]
O*
+ₒ
E
[]
E O*
(10.2)
[
]
S
+ ₒ− +
E O*
S
O*
E
[]
(10.3)
where 'O' is oxidant, 'O* is excited oxidant',
'E[]' is the electrode with active site and 'S' is
the substrate.
Formation of primary oxidants:
[ ]
−
−
+
−
H O
++
E
Cl
ₒ
E ClOH
+
H
+
2e
(10.4)
−
−
H O
+ +
ₒ+++
E ClOH
Cl
2
[ ]
Cl
E
O
3H
+
+
e
−
(10.5)
2
2
[ ]
C
+ ₒ++++
E ClOH
−
E
CO
H
+
Cl
−
e
−
(10.6)
Generation of secondary oxidants:
−
+ +
ₒ++++
H O
E ClOH
Cl
2
2
(10.7)
[ ]
+
−
−
E
ClO
3H
2Cl
e
2
[ ]
−
+
−
O
+ ₒ ++
E OH
E
O
H
+
e
2
3
(10.8)
[ ]
−
+
−
HO E OH
+
ₒ
E
+
HO
+
H
+
e
2
2
2
Search WWH ::
Custom Search