ENVIRONMENTAL
CHEMISTRY
OF
HEXACHLOROBENZENE
(
HCB)
3/
4/
05
EXECUTIVE
SUMMARY
Hexachlorobenzene
(
HCB)
is
a
stable
and
highly
persistent
molecule
and
does
not
hydrolyze
in
aqueous
medium
and
is
likely
to
become
immobile
in
soils.
It
has
large
sorption
partition
coefficients.
Aerobic
and
anaerobic
biodegradation
half
lives
are
long
and
therefore
the
main
route
of
dissipation
would
possibly
be
through
sorption
to
soils
in
the
terrestrial
settings
and
to
sediment
organic
and
inorganic
particulate
matter
in
aqueous
medium.
Because
K
OC
is
high
it
has
a
tendency
to
bind
strongly
with
soil
particles
and
therefore
less
mobile,
the
possibility
of
contamination
by
HCB
of
ground
water
does
not
seem
likely.
Because
of
high
binding
constants
with
soils,
HCB
may
possibly
accumulate
in
benthic
sediment
and
bioaccumulate
in
benthic
organisms.
Environmental
Fate
and
Effects
Division
(
EFED)
has
determined,
based
on
monitoring
data,
it
is
unlikely
that
HCB
concentration
in
surface
water
would
exceed
10
ppt
(
0.01
µ
g/
L).
1
Environmental
Fate
Assessment
In
1979
the
Agency
issued
a
document
on
Water­
Related
Environmental
Fate
of
129
Priority
Pollutants,
Volumes
I
&
II
2.
Volume
II
discussed
the
fate
of
HCB
in
the
environment.
In
the
absence
of
experimental/
measured
data
on
most
of
the
fate
studies
(
hydrolysis,
photolysis,
soil
aquatic
metabolism
etc.),
a
full
environmental
fate
assessment
could
not
be
carried
out.
The
document
recognized
a
few
important
characteristics
of
HCB
and
some
conclusions
were
drawn.
Some
of
the
conclusions
drawn
were:

"
Hexachlorobenzene
is
a
very
persistent
molecule
and
has
a
high
affinity
for
lipophilic
materials.
Sorption
and
bioaccumulation
are
expected
to
be
high.
It
is
bioaccumulative
but
does
not
appear
to
biomagnify
through
aquatic
food
chain.
Depuration
rate
is
high
although
log
K
OW
is
on
the
high
side.
HCB
found
in
the
aquatic
organisms
is
from
aqueous
medium
rather
than
dietary
sources.
Chemically
it
is
inert
at
room
temperature
and
in
the
presence
of
caustic
alkali
between
120
to
200
oC
it
produces
pentachlorophenolate."

It
should
be
noted
that
this
document
was
produced
when
HCB
was
used
as
an
agricultural
pesticide.
Subsequent
to
this
document
the
Office
of
Pesticide
Program
(
OPP)
has
issued
a
number
of
documents
which
discuss
the
presence
of
HCB
as
an
impurity
in
various
pesticide
manufacturing
processes.

Some
experimentally
measured
data
are
available
in
some
environmental
fate
studies.
Following
discussion
briefly
outlines
the
measured
and/
or
estimated
environmental
fate
and
transport
data.

1.
Abiotic
a.
Hydrolysis:

Hexachlorobenzene
is
a
highly
polar
compound
HCB
does
not
contain
any
hydrolyzable
hydrogen,
therefore
hydrolysis
is
not
likely
to
take
place.
Ellington
et
al.
,
in
19873
attempted
to
measured
K
h
(
hydrolysis
constant
for
HCB)
but
after
13
days
of
measurements
at
pHs
:
3,
7
and
11
at
85
o
C
,
K
h
was
estimated
to
be
equal
to
zero
and
hence
hydrolytic
half
life
has
not
been
determined.

b.
Photolysis:

Half­
life
for
photoxidation
of
HCB
in
air
was
measured
to
be
between
156
days
to
4.2
years
days4.
In
surface
waters,
the
photolytic
half
life
has
been
estimated
between
156
days
to
4.2
years.
However,
in
ground
water,
HCB
photolytic
half
life
has
been
estimated
between
5.3
to
11.4
years.
Photo
oxidation
half
life
of
HCB
has
been
estimated
to
be
between
2.7
to
5.7
years.
Therefore,
HCB
is
a
highly
persistent
molecule
in
aqueous
systems.

2.
Biotic
The
half
life
HCB
in
soils
has
been
also
estimated
in
the
range
of
156
days
to
4.2
years.
The
biodegradation
half
life,
under
aerobic
conditions,
has
been
estimated
in
the
range
of
156
days
to
4.2
years
and
under
anaerobic
conditions
it
ranges
from
10.6
years
to
23
years.
HCB
has
high
binding
capacity
with
soils
and
hence
is
likely
to
be
immobile
in
soils.
However,
due
to
sorption
processes
in
soils,
HCB
migrates
into
ground
water.
Based
on
monitoring
data,
the
Agency
has
recently
estimated
that
HCB
concentration
in
surface
is
not
likely
to
exceed
10
parts
per
trillion
(
0.01
µ
g/
L).

Log
K
OW
of
HCB
has
been
estimated
to
be
between
4.5
to
6.18
and
hence
it
is
likely
bioaccumulative.
HCB
has
been
shown
to
be
bioaccumulative
in
some
aquatic
organisms.
In
rainbow
trout,
the
rate
of
bioaccumulation
is
224
days.
However,
depuration
rate
is
about
7
days.

J.
Beck
and
K.
E.
Hansen's
work5
has
shown
that
in
surface
water,
under
aerobic
conditions,
biodegradation
half
life
of
HCB
varied
between
2.7
and
5.7
years.
Under
aerobic
conditions
but
in
ground
water,
the
biodegradation
half
life
ranges
between
5.3
to
11.4
years.
A
soil
grab
sample
study,
under
aerobic
conditions
showed
that
HCB
metabolizes
in
the
2.7
to
5.7
years
range.

Although
HCB
is
not
soluble
in
aqueous
medium
but
through
aqueous
surface,
it
has
a
tendency
to
volatilize
from
the
water
column
and
from
soil
surfaces
easily
and
quickly.

Recent
experiments
on
fathead
minnows
(
Pimephales
promelas)
and
worm
(
Lumbriculus
variegatus)
showed
that
HCB
partitioned
more
efficiently
into
sediment
than
biota.
6
It
has
been
shown
that
polychlorinated
chemicals
have
a
tendency
to
dechlorinate
(
biotransform)
under
anaerobic
conditions.
HCB
goes
through
dechlorination
only
partially
in
the
presence
of
biota.
7
Bioaccumulation
values
for
some
aquatic
species:

Rainbow
trout:
>
224
days8
Subadult
rainbow
trout:
210
days
at
4
o
C
80
days
at
12
o
C
70
days
at
18
o
C
9
Worm
27
days
at
8o
C
Depuration
Rate
from
fish
~
7
days10
REFERENCES
1.
J.
Wolfe,
A
Memo
on
Drinking
Water
Assessment
of
HCB
and
PCB,
April
1998,
DP
Barocde:
D243496.

2.
Water
Related
Environmental
Fate
of
129
Priority
Pollutants,
1979.
Volumes
I
and
II,
EPA­
440/
4­
79­
29a,
029b.

3.
J.
J.
Ellington,
et
al.,
1987.
USEPA­
600/
3­
86­
043
4.
R.
Atkins,
1987,
Int.
Jour.
Chem
Kin.,
Volume
19,
pp
799­
828
5.
J.
Beck
and
K.
E.
Hansen,
1974.
Pesti.
Sci.,
Volume
5,
pp
41­
48
6.
G.
S.
Schuytema
et
al.,
1990.
Arch
Environ.
Contam.
Toxicol.,
Volume
19,
pp
1­
9
7.
B.
Z.
Fathepure
et
al.,
1988.
Appl.
Environ.
Microbiol.,
Volume
54,
pp
2976­
2980
8.
A.
J.
Niimi
and
C.
Y.
Chao,
1981.
Bull.
Environ.
Toxicol.,
Volume
24,
pp
834­
837
9.
A.
J.
Niimi
and
V.
Palazo,
1985.
Water
Re.,
volume
19(
2),
pp
205­
207
10.
W.
B.
Neely,
1980.
In
Dynamic,
Exposure
and
Hazard
Assessment
of
Toxic
Chemicals,
R.
Haque,
Editor.
Ann
Arbor
Sci.,
Ann
Arbor,
Michigan.
