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Probiotics
for Aquaculture - Use
of Thiobacillus ferrooxides
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How T. ferroxidans works?s
T ferroxidans is applied to Aqua pond and upon exposure to the air, pyrite undergoes oxidation as follows: |
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| 01 |
FeS2(s) + (7/2)O2 + H2O = Fe2+ + 2SO42- + 2H+
The ferrous iron is further oxidized to ferric iron. This is precipitated if the environmental pH is higher than about 3. |
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| 02 |
Fe2+ + (1/4)O2 + H+ = Fe3+ + (1/2)H2O |
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| 03 |
Fe3+
+
3H2O
=
Fe(OH)3(s)
+
3H+
Thus,
the
overall
reaction
is: |
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| 04 |
FeS2(s)
+
(15/4)O2
+
(7/2)H2O
=
Fe(OH)3(s)
+
2SO42-
+
4H+
producing
four
equivalents
of
acidity
from
the
oxidation
of
one
mole
of
pyrite.
It
is
known
that
the
reaction
(2)
is a
very
slow
process
if
it
proceeds
in a
purely
chemical
fashion.
This
the
reason
why
T
ferroxiidans
is
needed
to
be
applied
to
Aqua
pond
to
hasten
the
process
of
oxidation.
In
the
absense
of
treatment
with
T
ferroxidans,
at a
pH
of
3,
the
half
life
of
this
reaction
is
around
1000
days
(Stumm
and
Morgan
1970).,
However,
this
reaction
is
hastened
by
autotrophic
iron
bacteria,
Thiobacillus
ferrooxidans
which
is
explained
by
the
following
reaction: |
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| 05 |
FeS2
+ 14Fe3+ + 8H2O
= 15Fe2+ + 16H+
+ 2SO42-
This produces
even more
acidity. The
reaction runs
quite rapidly.
Due to treatment
of Aqua Pond
with T
ferroxidans, the
half life of
this reaction is
reduced to 20 to
1000 minutes
(from 1000
days). The
oxidation of
sulfur in this
reaction is thus
facilitated by
autotrophic
bacterium,
Thiobacillus
thiooxidans.
Stumm and Morgan
(1970) provided
the following
schematic
drawing of the
overall process
of pyrite
oxidation
They stated
that:
"To
initiate the
sequence, pyrite
is oxidized
directly by
oxygen (a) or is
dissolved and
then oxidized
(a'). The
ferrous iron
formed is
oxygenated
extremely slowly
(b) and the
resultant ferric
iron is rapidly
reduced by
pyrite (c),
releasing
additional
acidity and new
Fe (2) to enter
the cycle via
(b). Once this
sequence has
been started,
oxygen is
involved only
indirectly in
the reoxidation
of ferrous iron
(b), the
oxygenation of
FeS2 (a) being
no longer of
significance.
Precipitated
ferric hydroxide
serves as a
reservoir for
soluble Fe (2)
(d). If the
regeneration of
Fe (2)
decreases, it
will be
replenished by
dissolution of
solid
Fe(OH)3".
In Aqua Pond,
the reaction (b)
is mediated by
iron bacteria T
ferrooxidans ,
and thus it may
not be seriously
rate limiting.
But it has to be
noted that to
make smooth
pyrite oxidation
possible, it is
necessary to
keep the ferric
iron activity
high by
treatment with T
ferroxidans. In
this relation,
Murakami (1965)
remarked that it
is noteworthy
that liming at
an early stage
can retard the
oxidation of
pyrite. |
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