Document ID: EPA-HQ-OPP-2005-0507-0009
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2006-02-01T05:00Z

1
Currently,
the
U.
S.
EPA
does
not
have
an
MCL
established
for
chlorate
nor
it
is
on
the
list
of
Drinking
Water
Contaminants,
although
chlorite/
chlorine
dioxide
are
listed
as
water
disinfection
by­
products
(
MCL
of
1.0
mgL­
1
and
a
goal
of
0.8
mgL­
1,
i.
e.,
the
MCLG
).
However,
chlorate
is
an
analyte
in
several
analytical
chemistry
methods
(
ion
chromatography)
for
water
disinfection
byproducts
Chlorate/
chlorite
detections
were
not
found
in
the
US
Geological
Survey
(
USGS)
monitoring
databases,
but
it
is
likely
that
chlorate/
chlorite
were
not
included
as
an
analyte.
Chlorate
is
not
currently
regulated
under
the
Toxic
Substance
Control
Act
(
TSCA).
UNITED
STATES
ENVIRONMENTAL
PROTECTION
AGENCY
WASHINGTON,
D.
C.
20460
OFFICE
OF
PREVENTION,
PESTICIDES
AND
TOXIC
SUBSTANCES
Date:
5th
January,
2005
MEMORANDUM
SUBJECT:
Sodium
Chlorate
(
CAS
Reg.
#
7775­
09­
9)
PC
#
073301
Reregistration
Case
#
4049
DP
Barcode
D303556
Drinking
Water
Assessment
of
Sodium
Chlorate
as
a
Desiccant/
Defoliant
on
Food/
Feed
Terrestrial
Uses
FROM:
Silvia
Carlota
Termes,
Chemist
ERB
III/
Environmental
Fate
and
Effects
Division
(
7507C)

TO:
Jacqueline
Gerry
Special
Review
and
Reregistration
Division
THRU:
Daniel
Rieder,
Acting
Branch
Chief
ERB
III/
Environmental
Fate
and
Effects
Division
Sodium
Chlorate
is
used
as
a
harvest
aid
(
defoliant/
desiccant)
on
a
variety
of
terrestrial
agricultural
commodities,
predominantly
on
cotton.
Sodium
chlorate
is
also
used
as
a
defoliant/
desiccant
in
nonfeed
non­
food
sites.
Sodium
chlorate
is
also
used
in
water
disinfection
to
generate
chlorine
dioxide
in­
situ
in
a
contained
system..
Therefore,
major
differences
are
expected
between
the
terrestrial
and
disinfectant
scenarios1.
2
Receiving
water
body,
moist
soil.

3
The
EFED,
however,
has
utilized
chemical
speciation
models
to
identify
predominant
copper
species
in
aquatic
media,
but
models
exist
to
handle
speciation
of
metals
as
a
function
of
pE­
pH
(
MINTEQ),
but
this
model
does
not
appear
to
be
suitable
for
non­
metal
species.

4
See
"
Water
Chlorination:
Chemistry,
Environmental
Impact
and
Health
Effects",
Volume
5.
Edited
by
Jolley,
R.
L.,
et
al.
Lewis
Publishers,
1985.

2
The
active
species
in
sodium
chlorate
is
the
chlorate
anion.
Chlorate
is
a
strong
oxidizing
agent
(
i.
e.,
electron
acceptor).
As
an
oxidizer,
the
reactions
of
chlorate
in
the
environment
are
dominated
by
natural
electron
donor
chemical
species
(
reductants).
Knowledge
of
the
redox
chemistry
of
chlorate
is
necessary
to
understand
its
behavior
in
the
environment,
at
least
qualitatively.
The
chlorine
system
(
See
Tables
3
and
4)
is
very
complex,
as
chlorine
can
generate
different,
often
interrelated
chemical
species
in
different
oxidation
states.
Moreover,
the
formation
and
predominance
of
these
interrelated
chemical
species
depends
on
the
pH
and
the
redox
potential
of
the
media.
Thus,
it
is
not
likely
that
chlorate
remains
as
"
chlorate"
in
an
aqueous
environment2
nor
that
there
is
going
to
be
a
one
hundred
percent
conversion
to
any
other
of
the
possible
reduced
species
(
e.
g.,
chlorite).
The
extent
and
rate
to
which
reduction
of
chlorate
occurs
will
depend
on
the
nature
and
concentration
of
reductants
(
electron
donors)
in
the
water
or
soil,
including
organic
matter.
Thus,
extensive
spatial
and
temporal
variability
is
expected
for
the
reactions
of
chlorate
in
the
environment
as
nature
and
concentration
of
reductants
can
vary
from
region
to
region
and
from
season
to
season.
It
should
also
be
noted
that,
at
least
for
cotton,
sodium
chlorate
is
used
before
harvest
(
late
Summer­
early
Fall)
and
that
time
of
the
year
receiving
surface
waters
are
likely
to
contain
a
high
amount
of
natural
reductants.
For
an
overview
of
chlorine
redox
chemistry
and
environmental
redox
process
see
Section
5.

1.
Estimating
Concentrations
of
Chlorate
and
Its
Reduced
Species
in
Drinking
Water
Sources
At
the
present
time,
there
is
no
methodology
to
estimate
exposure
concentrations
in
water
for
nonmetal
inorganic
chemical
species
that
can
be
found
in
different
oxidation
states3
.
That
is,
for
inorganic
chemical
species
that
can
exhibit
an
extensive
pH­
pE
dependent
redox
chemistry,
such
as
the
chlorine
system.
Although
there
are
some
models4
available
that
are
used
in
water
chlorination,
they
are
not
suitable
for
uses
that
have
a
direct
application
to
a
terrestrial
environment,
such
as
the
use
of
sodium
chlorate
as
defoliant/
desiccant
(
terrestrial
food/
feed
and
non­
food/
non­
feed
uses).
The
environmental
conditions
for
chlorination
using
sodium
chlorate
to
generate
in­
situ
chlorine
dioxide
in
water
treatment
plants
(
close
container/
absence
of
light)
are
markedly
different
from
those
found
in
terrestrial
environments
(
open,
heterogeneous
environment/
sunlight).

Because
the
chlorate
chemistry
is
highly
dependent
on
pH­
pE
(
redox)
conditions
in
the
environment,
these
factors
ought
to
be
considered
in
modeling
the
environmental
fate
and
transport
of
chlorate.
5
See
http://
www.
epa.
gov/
oppefed1/
models/
water/
index.
htm
6
Most
of
the
kinetics
data
for
chlorine
and
its
oxyanions
is
derived
from
chemical
systems
that
do
not
have
environmental
relevance
(
e.
g.,
high
temperatures,
reductants
not
found
in
the
environment)

3
One
problem
is
that
the
environmental
fate
and
transport
models
for
pesticides
(
GENEEC,
SCIGROW
FIRST,
PRZM)
do
not
have
the
capability
to
quantitatively
assess
the
impact
of
environmental
redox
potentials
(
pH­
pE)
on
chemical
speciation
(
i.
e.,
which
chemical
species
can
form
and
their
predominance).
Although
the
EXAMS
component
of
PRZM­
EXAMS
has
the
capability
to
use
redox
potential
as
an
input
parameter
with
specific
chemical
species,
it
also
needs
kinetics
data
(
which
is
difficult
to
obtain
for
redox
systems
such
as
the
chlorine
one)
5
.
Moreover,
even
if
the
kinetics
data
were
available6,
the
nature
and
predominance
(
relative
concentrations)
of
reduction
products
still
cannot
be
obtained
from
EXAMS.

For
the
Aquatic
Exposure
Assessment,
the
EFED
used
the
Tier
I
GENEEC2
simulation
model
to
estimate
exposure
concentrations
in
aquatic
systems.
Extreme
assumptions
in
the
environmental
persistence
of
chlorate
were
made
that
resulted
in
high
exposure
concentrations
in
standard
pond
scenario.
However,
the
predicted
chlorate
concentrations
are
believed
to
be
high
because
the
chemical
speciation
of
chlorate
could
not
be
considered
in
the
assessment
This
situation
also
applies
for
estimating
concentrations
of
chlorate
in
drinking
water
derived
from
surface
and
ground
water.
Although
EFED
estimated
exposure
concentrations
of
chlorate
for
the
drinking
water
assessment
using
FIRST
,
EFED
considers
that
they
are
overly
conservative
"
theoretical
maximum
concentrations"
for
uses
of
chlorate
in
a
terrestrial
environment.

The
use
of
SCI­
GROW
to
estimate
concentrations
of
chlorate
in
ground
water
is
even
more
uncertain,
as
SCI­
GROW
was
developed
as
a
regression
model
based
mostly
on
neutral
organic
pesticides.
Therefore,
the
SCI­
GROW
estimated
concentrations
in
ground
water
carry
even
a
higher
degree
of
uncertainty
that
those
determined
for
surface
water.
The
SCI­
GROW
estimated
concentrations
are
on
the
order
of
104
ppb.

3.
Concentration
Estimates
in
Drinking
Water
(
Surface
Water)

The
EFED
considered
that,
given
the
limitations
of
the
higher
tier
PRZM
and
EXAMS
simulation
models,
the
use
a
Tier
1
simulation
model
was
sufficient
for
a
screening
level
assessment
(
see
URL,
Footnote
5
for
a
description
of
FIRST)

Estimates
were
made
for
cotton
(
2
aerial
applications
at
a
total
of
15
lbs
ai/
acre
per
year)
and
for
potatoes/
chili
peppers
(
single
maximum
application
of
12.5
lbs
ai/
acre
per
year;
aerial).
No
estimates
were
made
for
non­
food/
non­
feed
terrestrial
uses.
The
application
rates
were
entered
in
terms
of
lbs
7
FIRST
assigns
a
different
Percent
of
Cropped
Area
(
PCA)
for
different
crops.
The
PCA
for
cotton
is
0.2,
but
there
are
no
PCA
for
potatoes
or
chili
peppers
("
Other
Crops").
The
PCA
for
"
Other
Crops"
is
0.8.
Thus,
the
reason
why
higher
concentrations
were
found
for
uses
on
potatoes
and
chili
peppers
than
on
cotton
fundamentally
lies
in
the
higher
PAC
assumed
for
"
Other
crops"

4
of
sodium
chlorate
per
acre.
The
estimated
concentration
of
the
chlorate
anion
in
surface
water
was
obtained
by
multiplying
by
0.78
(
molar
ratio
of
the
chlorate
anion
to
sodium
chlorate.

Table
1­
FIRST,
Tier
I
Estimated
Concentration
of
Sodium
Chlorate
in
Surface
Water
(
FIRST).
Values
in
Parentheses
are
in
Terms
of
the
Chlorate
Anion.
7
Peak
Day
(
Acute)
Annual
Average
(
Chronic)

Cotton
318
(
249)
µ
gL­
1
228
(
179)
µ
gL­
1
Potatoes/
Chili
Peppers
1,000
(
784)
µ
gL­
1
825
(
647)
µ
gL­
1
Cotton:
Two
applications
per
year
(
SRRD)
at
7.5
lb
ai/
year
at
30
days
apart
(
assumption).
Aerial
Potatoes/
Chili
peppers:
One
application
at
12.5
lb
ai/
acre.
Aerial.

These
chlorate
concentrations
are
"
theoretical
maximum"
that
do
not
take
into
account
redox
reactions
in
the
water
body.
While
chlorate
can
be
reduced
to
other
chlorine
oxyanions
(
chlorite,
hypochlorite),
chlorine
dioxide,
and
chloride,
a
one­
to­
one
molar
conversion
to
a
single
species
is
very
unlikely.
From
the
available
simulation
models,
the
predominance
(
type
and
concentration)
of
each
of
these
species
in
the
water
body
at
a
given
time
cannot
be
estimated.
In
addition,
chlorite,
hypochlorite,
and
chlorine
dioxide
can
undergo
further
oxidation­
reduction
or
other
reactions,
many
of
which
are
favored
under
environmental
conditions.
Such
an
example
is
chlorine
dioxide,
which
can
undergo
photolytic
reactions
with
hydroxy
radicals.

Even
though
EFED
is
including
in
Table
2
what
would
be
the
"
theoretical
maximum"
concentrations
of
chlorite,
hypochlorite
and
chloride
resulting
from
chlorate
in
the
water
body,
these
concentrations
must
be
used
with
caution
and
only
as
a
very
first
screening
level
as
it
is
unlikely
that
these
concentrations
will
ever
be
achieved
(
see
previous
paragraph).
Again,
these
"
theoretical
maximum"
concentrations
assume
that
none
of
these
species
undergo
any
subsequent
reactions.

There
are
two
things
that
EFED
can
say
with
confidence:
(
1)
Perchlorate
is
not
likely
to
form
from
chlorate
in
the
environment
and
(
2)
The
end
reduced
product
is
chloride.
However,
how
fast
all
of
the
chlorate
converts
to
all
chloride
cannot
be
estimated
and
will
be
highly
dependent
on
the
characteristics
of
the
water
body.
8
"
Standard
Potentials
in
Aqueous
Solutions",
Edited
by
A.
J.
Bard,
R.
Parsons,
and
J.
Jordan
for
the
International
Union
of
Pure
and
Applied
Chemistry,
Physical
and
Analytical
Chemistry
Divisions,
Commissions
on
Electrochemistry
and
Electroanalytical
Chemistry.
Published
by
Marcel
Dekker,
New
York,
1985.

5
Table
2.
Theoretical
Maximum
Concentrations
of
Chlorite,
Hypochlorite,
and
Chlorine
from
Chlorate
in
Surface
Water
(
FIRST)

Peak
Day
(
Acute)
µ
gL­
1
Annual
Average
(
Chronic)
µ
gL­
1
Cotton
202
(
Chlorite)
154
(
Hypochlorite)
105
(
Chloride)
145
(
Chlorite)
111
(
Hypochlorite)
75
(
Chloride)

Potatoes/
Chili
Peppers
635
(
Chlorite)
486
(
Hypochlorite)
329
(
Chloride)
524
(
Chlorite)
401
(
Hypochlorite)
272
(
Chloride)

Theoretical
Maximum
Concentrations
for
chlorite,
hypochlorite,
and
chloride
were
obtained
by
multiplying
the
concentration
of
chlorate
(
Table
1)
by
the
molar
ratios:
0.81
(
chlorite/
chlorate);
0.62
(
hypochlorite/
chlorate);
0.42
(
chloride/
chlorate).

4.
Attempts
to
Refine
the
Estimated
Concentrations
in
Water
Resources.

Further
refinement
was
attempted
to
investigate
the
impact
of
redox
conditions
on
the
distribution
of
chemical
speciation
of
chlorine
including:
(
1))
Chlorate,
ClO
3
­;
Cl(
V);
(
2)
Chlorite,
ClO
2
­;
Cl
(
III);
(
3)
Hypochlorite,
ClO­
;
Cl
(
I);
and
(
4)
Chloride,
ClO­;
Cl
(­
I).

A
chemical
equilibrium
modeling
approach
was
used
as
a
first
approximation
on
the
distribution
of
chemical
species
of
chlorine
as
function
redox
potential.
This
approach,
however,
does
not
consider
reaction
kinetics.
Reaction
kinetics
have
been
shown
to
be
an
important
consideration
in
determining
the
stability
of
chlorate
(
ClO
3
­)
in
soil
and
aquatic
environments.
The
reaction
kinetics
of
the
chlorine
system
is
extremely
complex
and
most
of
data
comes
from
study
conditions
that
are
not
relevant
to
conditions
found
in
the
environment.
However,
thermodynamic
data
is
readily
available
and
was
obtained
from
peer­
reviewed
data
included
in
publications
widely
used
as
reference8
.

A
mole
fraction
diagram
for
chlorate
and
redox
species
for
the
reactions
was
constructed
to
show
the
relative
predominance
of
chlorine
species
as
function
of
redox
potential
(
Figure
1).
This
exercise
demonstrated
that
chlorate
(
ClO
3
­)
and
chlorite
(
ClO
2
­)
under
chemical
equilibrium
conditions
are
not
expected
to
be
predominant
chlorine
species
under
normal
environmental
redox
potentials
(
pe+
pH<
17).
Hypochlorite
was
the
predominant
chlorine
species
pe+
pH
>
~
17,
which
is
outside
the
environmentally
significant
pe
+
pH
range.
As
expected,
the
chloride
ion
(
Cl­)
was
the
predominant
Cl
species
under
normal
environmental
redox
potentials
(
pe+
pH
<
17)
These
data
suggest
that
chlorate
and
chlorite
are
not
expected
to
be
stable
under
normal
environmental
redox
conditions
in
surface
water.
This
assessment,
however,
assumes
the
kinetics
of
reactions
do
not
control
the
rates
9
It
is
important
to
remember
that
thermodynamics
only
indicates
that
a
reaction
can
occur,
but
it
does
not
indicate
that
it
will
occur
(
i.
e.,
the
kinetics
component).

6
pe
+
pH
2
4
6
8
10
12
14
16
18
20
Mole
Fraction
0
20
40
60
80
100
CL02
­

CL0­
Cl­
NO3
­
­
N02
­

Fe
+
3
­
Fe
+
2
SO4
+
2
­
vs
S
­
2
of
reduction
and
oxidation
of
the
various
chlorine
species9.
As
previously
discussed,
the
nature
and
concentration
of
redox
species
in
natural
water
should
also
be
taken
into
account.
However,
given
the
extensive
spatial
and
temporal
variability
of
redox
species
in
natural
water,
a
quantitative
assessment
cannot
be
performed
Figure
1.

Figure
1­
Mole
Fraction
Diagram
Showing
that
Chloride
is
the
Predominant
Chemical­
Equilibrium
Species
in
the
Environment.

5.
A
General
Overview
of
the
Chlorine
System
and
of
Redox
Processes
in
the
Environment
This
section
has
been
included
to
familiarize
readers
with
the
complexity
and
nomenclature
of
the
chlorine
system
.
The
scope
of
this
overview
is
limited,
but
provides
some
basis
to
envision
how
complex
the
environmental
fate
of
chemicals
can
become
when
oxidation­
reduction
reactions
control
the
transformation
and
transport
of
chemicals.
The
Section
also
includes
examples
of
natural
chemical
10
The
most
important
oxides
of
chlorine
are
"
Chlorine
Monoxide,
Cl2O"
and
"
Chlorine
Dioxide,
ClO2".
Both
are
gases
at
room
temperature.
"
Cl2O"
is
the
anhydride
of
hypochlorous
acid.
Chlorine
dioxide
is
explosive
as
a
liquid
or
concentrated
gas
and,
for
this
reason
it
is
generated
in­
situ
when
it
is
used
as
antimicrobial
or
wood­
pulp
bleaching
agents.
It
is
usually
prepared
by
reducing
sodium
chlorate.
The
oxidation
state
of
chlorine
in
chlorine
dioxide,
ClO2,
is
IV
(+
4).

7
species
can
be
involved
in
redox
chemistry
in
the
environment.
In
addition,
this
section
attempts
to
distinguish
between
what
is
meant
by
"
thermodynamically
favored
reactions"
and
"
kinetically
feasible
reactions".

Table
3­
Chlorine
and
Chlorine
Species
in
Aqueous
Media10.

Oxidation
State
Name
of
the
Acid
Form
Chemical
Representation
the
Acid
Form
Chemical
Name
of
the
Anion/
Salts
Chemical
Representation
of
the
Anion
0
Not
Applicable
Elemental
chlorine,
Cl2
(
gas)
Not
Applicable
Not
Applicable
­
I
(­
1)
Hydrochloric
acid
HCl
Chloride/
Chlorides
Cl­

I
(+
1)
Hypochlorous
acida
HOCl
Hypochloriteb/
Hypochlorites
"
ClO­"

III
(+
3)
Chlorous
acida
HOClO
Chloriteb/
Chlorites
ClO2
­

V
(+
5)
Chloric
Acida
HOClO2
Chlorateb/
Chlorates
ClO3
­

VII
(+
7)
Perchloric
Acid
HOClO3
Perchlorateb/
Perchlorates
ClO4
­

a
Stable
only
in
aqueous
solutions;
b
Oxyanion.
The
oxidation
state
of
chlorine
in
chlorine
dioxide
(
ClO2)
is
IV
Table
4
show
half­
cell
reactions
involving
chlorate
as
an
electron
acceptor
(
oxidizer).
Any
redox
reaction
involves
an
oxidizer
(
electron
acceptor)
and
a
reductant
(
electron
donor).
In
the
environment,
there
are
a
wide
variety
of
chemical
species
that
can
act
as
electron
donors
(
reductants).
Therefore,
the
reactions
appearing
in
Table
4
show
the
chlorine
species
that
may
form
as
chlorate
gets
reduced.
Note
the
pH
and
electrode
potential
dependence
of
these
reactions.

Table
4
Standard
Electrode
Potentials
for
Redox
Reactions
of
Chlorate
(
Half­
cell
Potentials)

Redox
Couplea
E
°
/
E
°
b,
volts
(
V)
d
E
°
(
E
°
b)/
dT,
mVK
­
1
(
K=
°
Kelvin)

ClO3
­
+
6
3H2O
+
6e­
6
Cl­
+
6
OH­
(
1)
E
°
b,
=
0.622
dE
°
b)/
dT
=
­
1.333
Redox
Couplea
E
°
/
E
°
b,
volts
(
V)
d
E
°
(
E
°
b)/
dT,
mVK
­
1
(
K=
°
Kelvin)

11
Pourbaix,
M.
Atlas
d'équilibres
èlectrochimiques
à
25
°
C.
Gauthier­
Villars,
Paris,
1963.
Pourbaix
diagrams
are
routinely
used
in
corrosion
and
minerals
processing
work.

8
2
ClO3
­
+
12
H+
+
10e­
6
Cl2
(
g)
+
6H2O
(
2)
E
°
=
1.468
dE
°
/
dT
=
­
0.347
ClO3
­
+
2H2O
+
4e­
6
ClO­
+
4
OH­
(
3)
E
°
b,=
0.488
dE
°
b/
dT
=
­
1.467
ClO3
­
+
3H+
+
3e­
6
HClO2
+
H2O
(
4)
E
°
=
1.181
dE
°
/
dT
=
­
0.180
ClO3
­
+
H2O
+
2e­
1
6
ClO2
­
+
2OH­
(
5)
E
°
b,
=
0.295
dE
°
b/
dT=
­
1.467
ClO3
­
+
2
H+
+
e­
6
ClO2
(
g)
+
H2O
(
6)
E
°
=
1.175
dE
°
/
dT
=
1.026
ClO3
­
+
H2O
+
e­
1
6
ClO2
(
g)
+
2OH­
(
7)
E
°
b,
=
­
0.481
dE
°
b/
dT
=
­
0.646
ClO4
­
+
2
H++
2e­
1
6
ClO3
­
+
H2O
(
8)
E
°
=
1.201
dE
°
/
dT
=
­
0.416
ClO4
­
+
H2O
+
2e­
1
6
ClO3
­
+
2OH­
(
9)
E
°
b,
=
0.374
dE
°
b/
dT
=
­
1.252
a
Defined
as
any
pair
of
the
same
element
in
different
oxidation
states.
Source:
Standard
Potentials
in
Aqueous
Solutions.
Edited
by
Bard,
A.
J.,
Parsons,
R.,
and
Jordan,
J.
IUPAC,
Commission
on
Electrochemistry
and
Electroanalytical
Chemistry,
Published
by
Marcel
Dekker,
Inc.,
New
York,
1985.
Most
of
these
redox
couples
were
experimentally
determined
Predominance
of
Chemical
Species
in
the
Environment
The
effects
of
pH
on
the
form
in
which
an
element
in
a
given
oxidation
state
exists
in
natural
waters
(
or
soils)
can
be
summarized
with
chemical
species
predominance
diagrams.
Knowledge
of
the
environmental
pH
is
not
sufficient
for
predicting
the
form
(
i.
e.,
oxidation
state;
chemical
species;
phase)
in
which
an
element,
the
chlorine
system
in
this
case,
will
exist
in
natural
waters.
When
a
chemical
element
can
exist
in
two
or
more
oxidation
states,
it
must
also
be
taken
into
consideration
whether
the
aqueous
environment
is
well
aerated
(
oxidizing)
or
polluted
with
organic
wastes
(
reducing).
Therefore,
it
becomes
necessary
to
add
the
redox
variable
to
expand
the
predominance
diagram
to
include
the
reduction
potential
of
the
environment
as
well
as
the
pH.

This
type
of
predominance
diagram
is
known
as
a
Pourbaix­
diagram
or
E­
pH
diagram11,
which
can
also
be
expressed
in
a
pE
scale.
The
pE
(
pE=
­
log
E)
scale
is
analogous
to
that
of
pH.
It
represents
the
concentration
of
the
standard
reducing
agent
and
is
obtained
from
the
reduction
potential
by
dividing
by
0.059.
Low
E
(
pE)
values
represent
a
reducing
environment,
while
high
E
(
pE)
values
represent
an
oxidizing
environment.

Thermodynamics
versus
Kinetics
The
Standard
Redox
Potentials,
as
well
as
the
predominance
diagrams
are
based
on
equilibria
relationships
(
i.
e.,
thermodynamics).
In
the
environment,
particularly
in
heterogeneous
media
such
12
For
example,
the
conversion
of
diamond
to
the
more
thermodynamically
stable
graphite.

13
These
environmental
redox
species
are
also
pH­
pE
dependent.
Therefore,
the
nature
and
concentration
(
predominance)
of
each
individual
species
will
vary
depending
on
the
pH­
pE
conditions
9
as
soils,
suspended
particulates
in
natural
water,
redox
species
are
not
generally
in
equilibrium
and
the
concentrations
of
ionic
species
deviate
from
unit
activity.
In
addition,
redox
potentials
do
not
tell
how
fast
the
redox
reactions
take
place
(
i.
e.,
the
kinetics
of
the
redox
reactions).
Thus,
a
redox
reaction
that
may
be
thermodynamically
favored
might
be
kinetically
very
slow12.
In
summary,
the
redox
reactions
based
on
thermodynamic
equilibria
indicates
that
the
reaction
can
occur,
but
it
does
not
mean
that
they
will
occur
and
how
fast.

Natural
Redox
Environments:
Water
and
Soil
A
major
factor
controlling
the
redox
environment
is
the
nature
and
concentration
of
redox
species.
Major
chemical
species
associated
with
reducing
environments
are
transition
metals
in
low
oxidation
states
(
e.
g.,
Fe(
II),
Mn(
II)),
N­
species
in
low
oxidation
states
(
NO
2
­
;
NH
4
+);
S(­
II)
(
e.
g
HS­,
S
2­;
polysulfido
species),
and
organic
matter.
Major
chemical
species
associated
with
oxidizing
environments
are
dissolved
dioxygen
(
O
2;
molecular
oxygen),
transition
metals
in
high
oxidation
states
such
as
Fe(
III);
Mn(
III,
IV),
sulfate,
and
nitrate13.
In
addition
to
"
straight"
redox
reactions,
many
of
the
redox
species
in
natural
waters
may
also
act
as
photosensitizers.
Other
factors
controlling
the
redox
chemistry
of
a
natural
environment
are
the
population
of
aerobic
and
anaerobic
microorganisms.
In
addition,
many
of
the
transition
metals
may
be
present
as
mineral
phases
that
could
be
involved
in
surface­
catalyzed
reactions
The
reactions
below
are
examples
of
how
different
functional
groups
undergo
redox
reactions.
Many
of
these
functional
groups
are
present
in
natural
organic
matter,
for
example,
hydroxyl
groups
in
humic
acids,
but
other
simpler
organic
species
such
as
oxalate
may
also
be
present
in
natural
water.

Table
5
Redox
Reactions
with
Organic
Functional
Groups
Type
of
Oxidation
Reactions
Type
of
Reduction
Reactions
Alkanes
to
alcohols
(
R­
H
6
R­
OH)
Reductive
dehalogenation
(
R­
X
6
R­
H)

Alcohols
to
aldehydes
(
R­
OH
6
RCHO)
Vicinal
dehalogenation
(
X­
R­
R­
X6
R=
R)

Aldehydes
to
acids
(
RCHO
6
RCOOH)
Nitro
reduction
(
R­
NO2
6
R­
NH2)

Dehydrogenation
(­
CH­
CH­
6
­
C=
C­)
Azo
reduction
(
Ar­
N=
N­
Ar
6
2Ar­
NH2)

Oxidative
coupling
(
2R­
OH6
R­
O­
R)
Disulfide
to
thiol
(
R­
S­
S­
R6
2R­
SH)

Hydroquinones
to
quinones
(
HO­
Phenyl­
OH6
O=
Phenyl=
O)
Deoxygenation
of
sulfoxo
and
sulfoxides
(
R­
SO2­
R6R­
SO­
R6R­
S­
R)

Thiooxidation
(
R­
S­
R6
R­
SO­
R;
R­
SO2­
R)
Nitrosamine
Reduction
(
R2N­
N=
O6R2N­
H
+
HNO)
Type
of
Oxidation
Reactions
Type
of
Reduction
Reactions
10
Formation
of
disulfide
linkages
(
2R­
SH
6
R­
S­
S­
R)
Quinones
to
hydroquinones
(
O=
Phenyl=
O6HO­
Phenyl­
OH)

­
Dealkylation
(
R­
Y­
R'
6
R­
YH
+
R'H)

Table
6
Input
Parameters­
Information
on
Application
Rates
and
PCA
Appear
in
Table
1.
The
only
environmental
fate
parameters
for
SCI­
GROW
are
Aerobic
Soil
Metabolism
(
which
is
meaningless
for
chlorate)
and
Koc
(
also
meaningless
for
chlorate)

Information
Needed
by
FIRST
Input
Parameter
Comment
Method
of
application
Maximum
application
rate
(
lbs
ai/
A)
Aerial
All
labels
allow
for
aerial
applications
for
agricultural
uses.

Kd
0
Chlorate
is
an
anion.
Thus,
it
is
expected
to
be
very
mobile
in
soils
(
high
leaching
and
runoff
potential)
In
addition,
it
has
a
very
low
potential
to
volatilize
from
soils
and
water
(
very
low
vapor
pressure
and
extremely
high
solubility
in
water
Aerobic
Soil
Metabolism
0
Persistence
in
soil
is
highly
dependent
on
type
of
soil,
pH,
other
chemical
species
present
in
soil,
soil
moisture
temperature,
precipitation
(
i.
e.
high
spatial
and
temporal
variability).
The
only
persistence
information
comes
from
a
USDA
report,
which
is
not
reported
in
terms
of
half­
lives.
Because
persistence
was
expressed
in
terms
of
"
toxic
persistence"
and
that
this
ranged
from
6
to
12
months
ad
minimum
the
half­
life
was
assumed
to
be
zero.
In
addition,
sodium
chlorate
is
a
soil
sterilant.
Therefore,
"
metabolic"
processes
are
meningless.

Aerobic
Aquatic
Metabolism
0
See
comment
under
"
Hydrolysis";
Aerobic
Soil
Metabolism
[
Direct]
Photolysis
in
Water
0
The
chlorate
anion
does
not
absorb
energy
in
the
wavelength
range
of
sunlight.
Therefore,
it
lacks
the
necessary
condition
for
a
chemical
to
undergo
direct
photolysis.
See
comment
under
"
Hydrolysis".

Hydrolysis
(
abiotic)
0
The
chlorate
anion
is
not
expected
to
react
with
water.
a
Solubility
in
water
(
mgL­
1;
ppm)
1
x
106
at
25
°
C
Sodium
chlorate
is
a
fully
ionized
salt
in
water
Information
Needed
by
FIRST
Input
Parameter
Comment
11
a
Theoretically,
it
may
be
possible
to
estimate
the
redox
potential(
s)
conditions
at
which
formation
chlorite
may
be
most
favored.
The
"
chlorine­
chlorine
anions­
oxyanions"
redox
chemistry
is
well
known.
GENEEC,
FIRST
and
SCI­
GROW
do
not
have
the
capability
to
handle
redox
data
and
to
predict
the
distribution
and
predominance
of
reduction
products
of
chlorate.
Even
EXAMS
cannot
provide
such
information.

The
behavior
of
chlorate
(
an
oxidant)
is
controlled
by
the
nature
and
concentration
of
reducing
(
i.
e.,
electron
donors)
chemical
species
in
water
and
other
environmental
media.
A
major
chemical
species
that
control
the
redox
behavior
of
chlorate
in
aqueous
media
is
the
concentration
of
nitrate.
An
important
reduced
species
of
chlorate
is
chlorite
(
ClO
2
­).
In
addition,
some
of
the
constituents
of
natural
waters
have
the
potential
to
act
as
photosensitizers