Pitfalls in the aquatic photochemistry testing of chlorinated aromatic compounds

Chemosphere, Vol°lT, No.2, Printed in G r e a t B r i t a i n

pp

395-398,

1988

0045-6535/88 $ 3 . 0 0 + .OO Pergamon Journals Ltd.

PITFALLS IN THE AQUATIC PHOTOCHEMISTRY TESTING OF CHLORINATED AROMATIC COMPOUNDS

Paul van Noort *'t, Ren4 Smit, Erik Zwaan, and Jelle Zijlstra

Laboratory for Ecotoxicology,

Environmental Chemistry,

and Drinking Water,

National Institute for Public Health and Environmental Hygiene, P.O.Box i, 3720 BA

Bilthoven,

The Netherlands

ABSTRACT

Eventual phototransformation of ehloroaromatics in water containing humie acid is proposed to result from unidentified trace impurities in the water introduced by the addition of concentrated solutions of humic acid to the irradiation mixtures, Humie acid itself does not seem to sensitise the phototransformation of these type of compounds. INTRODUCTION

The aquatic photochemistry of chlorinated aromatic compounds has been the subject of many investigations because of the adverse biological properties of compounds of In

the

aquatic

environment

many

of

these

compounds

this

type.

may in principle be transformed by

sunlight as a result of either direct light absorption by these compounds or by reaction with reactive

intermediates

direct photolysis reported

formed from e,g. humic materials on irradiation

is the incident light being absorbed by the compound.

combination

of

. A prerequisite for In

I

for direct photolysis although

transformation

was

reported

accelerate

phototransformation is

not

photochemically

yet

cases

the

2

'

Humic

material 3

mechanism

some

light source and molecular structure did not suggest possibilities

clear.

incorporated

of

chlorinated

Choudry into

humic

et

aromatic

compounds

4

5

seems

to

6

' ' ' ,

but

the

al.

reported that polychlorobenzenes were 7 model monomers , but it did not appear from their

paper where the light was absorbed: by the humic model monomers or by the polychlorobenzenes. Therefore,

we

now

report

our

results

disappearance of pentachlorobenzene

on

measurements

and methoxychlor

of

the

rate

of

photo-induced

in various types of purified water

or without added humic acid as a model for aquatic humic material.

t Present adress:

Stork Screens B.V., P.O. Box 67, 5830 AB

395

Boxmeer,

The Netherlands.

with

396

MATERIALS AND METHODS

Humic

acid

was

a

obtained

from

Fluka.

Aqueous humic acid solutions were prepared

according to Zepp . Irradiations were carried out in a Rayonet Photochemical Reactor equiped with eight RUL-3500A lamps and a RMA-400 Merry-Go-Round.

RPR 208

Deionized water was purified

by two series of oxidation with KMnO 4 and distillation. Quantitative analysis of penta-, hexachlorobenzene,

and

methoxychlor

in

water

was

containing trichlorobenzene as internal standard

done

followed

by by

extraction injection

and

with cyclohexane on

a

Carlo

Erba

gaschromatograph equiped with an ECD.

RESULTS

The

irradiation of 12.6 ~g/l of pentachlorobenzene in deionized water resulted in a

decrease of the concentration with a rate constant of 0.032 h -I (Fig.la). Irradiation in

the

presence of humic acid (DOC- I0 mg/l) afforded a rate constant of 0.14 h "I (Fig. ib). In (Co/C) 10

In (Co/C) 1.0

Fig. la

x

xJ

/

Fig.x x

0.5

.S

0.5

jJ x

0

5

10

15

20

25 h

0

1

3

5

7h

Fig.l. Pseudo first-order plot for disappearance of pentachlorobenzene upon A 350 nm ation in (a) deionised water,

The presence of oxygen, KNO s (IM), KCI (IM), or Zn(NOs) ~ (0.5 M) had almost no the

rate

constants:

hexachlorobenzene

irradi-

(b) deionised water containing humic acid at i0 mg C/I.

they were 0.13, 0.17, 0.13, and 0.18 h "I respectively.

influence

on

Disappearance of

(7.8 ~g/l) upon irradiation in the presence of humic acid (i0

mg

C/I)

in

deionised water proceeded with a rate constant of 0.i0 h -I. When the irradiation of pentachlorobenzene was carried water

(without

out

in

purified

process (Fig.2a).The addition of pentachlorobenzene to the irradiated mixture of not,

upon

deionized

humic acid) the kinetic of the disappearance rather suggested a second order

prolonged

irradiation,

result

in

Fig.2a

did

a further decrease of the pentachlorobenzene

concentration. This demonstrates that the disappearance

of

pentachlorobenzene

is

not

the

The effect of humic acid on the aquatic photochemistry of pentachlorobenzene in

the

result of the addition of pentachlorobenzene. purified

water

was

determined

by

measuring

the

decrease

concentration upon irradiation of a solution of pentachlorobenzene

of

the

pentachlorobenzene

(12.6 #g/l) and humic acid

(DOC- 20 mg/l) in purified deionized water containing pentachlorobenzene at 13 ~g/l which was

397

pre-irradiated for 24 h (Fig.2b). Also in this case pentachlorobenzene

the

kinetic

of

decrease

of

the

C/Co 1.0

CICo 1.0 Fig, 2a

Fig. 2b

~

0.8

x x

0.5 -

-f

-

X - - X ~

x

x

x

.

1'o

1's

2'o

o

s

20

Fig.2. The change of the relative concentration of pentachlorobenzene ation

the

concentration was apparently second order.

in

(a) purified deionised water,

with pentachlorobenzene present,

2Sh

upon A 350

nm

irradi-

(b) purified deionised water pre-irradiated for 24 h

followed by humic acid addition prior to the

final

irradi-

ation.

Since methoxychlor was reported by Zepp to disappear very rapidly

on

irradiation

in

humic

s

waters

,

we studied the photochemical behaviour of methoxychlor under the same conditions as

for Fig.2b.

In contrast to Zepp,

we

found

the

concentration

of

methoxychlor

to

remain

constant upon irradiation for 6 h.

DISCUSSION

Our

results

from

the

irradiation in the not-purified deionised water demonstrate

that the addition of humic acid affords an acceleration of the disappearance by a

factor

of

about 4. This acceleration is of the same magnitude as found for the irradiation of 2-chloro3

biphenyl in humic acid containing water

4

, of chlorophenols

in estuarine water

, and of the

2-

5

butoxyethyl

ester of 2,4-D in a natural water

, but is much less than found for methoxychlor

e

in natural waters In expected

the

since

disappearance trace

. purified

prior

impurity

water

pentachlorobenzene

pentachlorobenzene

of

to

does

not

absorb

does at

not disappear. the

This is of course

wavelengths

employed.

Its

the pre-irradiation demonstrates the presence of a photosensitising

unknown

identity

in

the

water

which

is

completely

consumed

upon

irradiation. In the presence of humic acid, pentachlorobenzene water

was

consumed

to some extent.

in

ance, this may also be the result of the presence of (unknown) addition

of

a

concentrated

humic

the

pre-irradiated,

purified

Because of the second order character of the disappear-

acid

solution

impurities introduced

by

the

to the pre-irradiated water. We have no

analytical data on the impurity since we do not know what

to

look

for.

The

stability

of

398

methoxychlor

also

points

to

the

photochemical

irreactivity

of

humic

acid

towards

chloroaromatics. Recently,

the near-surface

steady-state

concentration

of the photoproduced

hydrated

9

electron was determined by us to be i.I×i0 -17 M per mg DOC/I in natural waters

and

by

Zepp

Io

et

al. to be I.2xlO-17M

Zepp, but on different material

may

for the Swiss Greifensee

grounds,

undergo

photoreductions

hydrated electron steady-state steady-state

concentration

the chloroaromatics second

order

conversion of with

at

associated

a

concentrations.

dissolved

organic

aquatic

rate higher than expected on the basis of the Under our

in humic water for 6 h (in other words,

acid,

acceleration

with

conditions

the

hydrated

is 3xI0-17M 9. The pseudo first order rate constant

much less than l~lO'2h-l). humic

We pointed out, and so did

electron

for reaction of

with the hydrated electron can be estimated from literature data for the 1, constants to be about IxlO'3h -I, For methoxychlor we found no

rate

it

pentachlorobenzene

with

that substrates

(DOC= 4 mg/1)

will

some

This demonstrates be

by

less

association

than

any disappearance

a

factor

assisted acceleration

will not be of several orders of magnitude.

humic

acid had no detectable 12 excited humic acid to substrates

would proceed at a

rate

that if there is acceleration by association of

I0

for

methoxychlor.

could have occurred.

Earlier we

found

For

Anyhow,

that

this

association

effect on the rate of triplet energy transfer from photo13

Finally,

our results demonstrate

that on performing

purity of the water should be subject to attention,

tests on photodegradation

especially when the test

the

solution

is

weak light absorber.

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a