Emissions of PCDDs and PCDFs from the pulp industry

Chemosphere, Vol.17, No.4, Printed in Great Britain

pp 681-691,

1988

0045-6535/88 $3.OO + .OO Pergamon Press plc

EMISSIONS OF PCDDs AND PCDFs FROMTHE PULP INDUSTRY Stephen E. Swanson, C h r i s t o f f e r Rappex Department of Organic Chemistry U n i v e r s i t y of UmeA S-901 87

UMEA, Sweden and

Jan Malmstr~m 430 20 VEDDIGE, Sweden

and

Knut P. Kringstad ~ STFI S-II4 86

STOCKHOLM,Sweden

ABSTRACT Samples of pulp and process l i q u o r s collected at various sites in a m i l l producing bleached softwood k r a f t pulp were analyzed with respect to PCDDs and PCDFs. The r e s u l t s comprised 22 d i f f e r e n t isomers i n c l u d i n g a l l 12 known as p a r t i c u l a r l y t o x i c . The r e s u l t s show that minor amounts of PCDDs and PCDFs indeed are formed during the bleaching of pulp. The isomer pattern, being dominated by 2,3,7,8-TCDF and 2,3,7,8-TCDD is character$stic and e a s i l y d i s t i n g u i s h a b l e from the isomer pattern normally found for i n c i neration samples. Compared to other sources for PCDDs and PCDFs in Sweden the amounts formed in pulp bleaching are small. INTRODUCTION In 1974, the f i r s t

discussions occurred regarding the p o s s i b i l i t y that the pulp industry

might be a source of p o l y c h l o r i n a t e d dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzo furans (PCDFs) ( I ) . Quite recently, concern has been expressed in the U.S. regarding the Dossi. bility x

that 2,3,7,8-TCDD might be present in the pulp and paper m i l l sludges, and the possible To whom correspondence should be addressed.

Extended version of Paper presented at Dioxin 87, Seventh I n t e r n a t i o n a l Symnosium on C h l o r i nated Dioxins and Related Compounds. Las Vegas, Oct. 4-9, 1987.

681

682

impact of t h i s s i t u a t i o n on the i n d u s t r y ' s land a p p l i c a t i o n programs (2). At Dioxin

86,

results were presented from analysis of crab samples and sediments taken at the e f f l u e n t o u t l e t and in the sedimentation lagoon of a m i l l producing bleached softwood k r a f t pulp ( 3 ) The crab samples showed elevated levels of TCDDs and TCDFs with a c h a r a c t e r i s t i c isomer Drof i l e f o r these compounds in the samples taken d i r e c t l y at m i l l e f f l u e n t o u t l e t . Several processes in a pulp m i l l may be considered to be potential sources f o r PCDD and PCDF. One area f o r consideration is the recovery b o i l e r where large q u a n t i t i e s of organic material in the spent pulping l i q u o r are burned to recover chemicals and energy. In a first

study conducted p a r t l y in these laboratories and reported by SSVL, the number and

level of PCDD and PCDF isomers were found to be very low in the stack gases sampled before scrubbing from a recovery b o i l e r of a m i l l producing some I000 tons/day of bleached s o f t wood and hardwood k r a f t pulp

(4). None of the stack gas samples contained detectable quan-

t i t i e s of the t o x i c 2,3,7,8-TCDD. Assuming that the inspected recovery b o i l e r is represent a t i v e the level found suggests a t o t a l emission into the a i r of about 2 g TEF (Eadon) per year f o r a l l recovery b o i l e r s in Sweden. The annual production of bleached pulp in Sweden is about 4 m i l l i o n tons. Another p o te n t i a l source f o r PCDD and PCDF in a pulp m i l l is the bleach plant. I t is well known that during bleaching, a large v a r i e t y of chlorinated, aromatic compounds of varying r e l a t i v e molecular mass are formed and exposed to s h i f t i n g pH and

temperature con-

d i t i o n s (5). As a part of i t s survey of the environmental impact of the Swedish pulp and pa. per industry (Milj~ 90), SSVL has therefore also included an i n v e s t i g a t i o n into the format i o n and occurrence of PCDDs and PCDFs in the bleach plant. This i n v e s t i g a t i o n includes both a survey of pulp m i l l s using d i f f e r e n t bleaching sequences as well as laboratory bleaching studies to examine the effects of various types of bleaching and exploring the mechanisms of formation of PCDD and PCDF. The present report is the f i r s t

report from

this program. Various systems have been used to coP~vert a multitude of PCDD and PCDF levels into a more simple expression such as TEF ( t o x i c equivalent f a c t o r ) .

In this report, the TEF cal-

c u l a t i o n formula suggested by Eadon has been used (6). EXPERIMENTAL

~!~_~9!!~£~i~ Samples were taken from a pulp mill oroducing about 850 tons/day bleached softwood (pine) kraft pulp. The pulp is semi-bleached or f u l l y bleached using aconventional bleaching sequences including the following

stages: C (chlorine), E (alkaline extraction),

H (hypochlorite), D (chlorine dioxide) and (C+D)EOHDIED2 respectively. The following samples were collected: l ) The suspension of unbleached pulp entering the bleach plant 2) 3)

The pulp suspension at f i l t e r after the (C+D)-stage The pulp suspension at f i l t e r after the EO-stage

683

4)

The pulp suspension at f i l t e r

a f t e r the H-stage

5)

The pulp suspension at f i l t e r

a f t e r the D~-stage

6)

The f i n a l , dried bleached pulp and

7)

The t o t a l e f f l u e n t taken at the m i l l o u t l e t

The samples were collected during production of f u l l y bleached pulp. All samples, with the exception of the bleached pulp sample were composite samples collected over a period of three days. The bleached pulp sample was a grab sample. The charge of chemicals during the sampling period was C+D-stage:

56 kg Cl2/tgo%pulp, 3 kg CI02/~o% pulp

EO -stage:

32 kg NaOH/tg0 % pulp, 4 kg 02/tg0%pulp

H -stage: D~ -stage:

2 kg NaOCl/tgo% pulp 15 kg CIO 2 / t g 0 % p u l p

E -stage:

5 kg NaOH/tgo %pulp

D2 -stage:

4 kg CI02 / t 9 o % pulp

The kappa number of the unbleached pulp was 29. In a d d i t i o n , samples of the t o t a l m i l l e f f l u e n t were taken on two other occasions when the m i l l was producing semibleached pulp. The charge of chemicals was 65 kq C12, 35 kq NAOCI, 1.5 kg NaOCl, and 11.5 kg C102/tongo% pulp respectively. The kappa number of the unbleached pulp was 30.

Each o f the suspensions (approximately 1 L) was divided into three sub-samples, as follows. F i r s t , the pulp was removed using a coarse Buchner funnel. The pulp was then washed twice with d i s t i l l e d

water using approximately 1 L of water each time. The washings were

saved. The washed pulp was a i r dried for at least 48 hours and an a l i q u o t extracted as described below for s o l i d samples. The f i l t r a t e

was weighed, adjusted to DH 12 to minimize ex-

t r a c t i o n of chlorinated phenolic material and the remaining fine material removed by f i l t e ring through a pair of pre-weighed f i l t e r s The f i l t e r s

(glass f i b e r and 0.45 ~m c e l l u l o s e f i b e r f i l t e r s ) .

were a i r dried for at least 16 hours, weighed, then extracted as described below

for s o l i d samples. F i n a l l y , the f i n a l f i l t r a t e

was extracted as described below for l i q u i d

samples. The t o t a l e f f l u e n t was treated as the f i l t r a t e

from the pulp f i l t r a t i o n

as described

above.

Prior to e x t r a c t i o n , each sample was spiked with 500 pg of the surrogate spiking sol u t i o n shown in Table I . Burdick and Jackson " D i s t i l l e d in Glass" solvents were used in a l l e x t r a c t i o n and clean-up procedures. The e x t r a c t i o n techniques which were used for l i q u i d samples were based on methods described e a r l i e r (7). In t h i s procedure, the samples are adjusted to pH greater than 12 then extracted in a separatory funnel with dichloromethane.

684

Table I .

Compounds in Surrogate Spike (Carbon 13 l a b e l l e d ) .

2,3,7,8- ~C-Tetra-CDF 2,3,7,8-13C-Tetra-CDD 1,2,3,7,8-13C- Penta-CDD 1,2,3,4,7,8-I~C-Hexa-CDF 1,2,3,6,7,8-13C-Hexa-CDD 1,2,3,4,6,7,8-13C-Hepta-CDD 13 C-Octa-CDD Solid samples (dried pulp and dried f i l t e r e d suspended solids) were Soxhlet extracted with hot toluene f o r 24 to 48 hours. Each e x t r a c t was reduced in volume to less than 1 ml usinq r o t a r y evaporation followed by evaporation under a gentle stream of dry nitrogen usinq 50 ~I of tetradecane as a "keeper". Each e x t r a c t was solvent exchanged into n-hexane and taken through the three-column clean-up procedure also described previously (8,9). This procedure involves the use of a s u l f u r i c a c i d / s i l i c a gel column (40% w/w conc. s u l f u r i c acid on kieselgel) followed by a basic alumina column (Woelm B akt. I) and, f i n a l l y ,

a charcoal column (18% w/w Carbopak C

on C e l i t e 545). The r e s u l t i n g e x t r a c t was concentrated and analyzed by HRGC/MS as described below.

~D~!~i~ All extracts were analyzed by High Resolution ( c a p i l l a r y ) Gas Chromatography with Mass Spectrometry detection (HRGC/MS) using a VG model 70-250 double focusinq mass spectrometer. The f o l l o w i n g parameters were used: 60 m x 0.25 mm fused s i l i c a wall-coated with SP-2330 (Supelco)

Column:

Column temp:

lO0°C (2 min hold) to 180°C at 20°C/min then to 260°C at 3°C/min (20 min hold)

Injector:

Grob-type s p l i t l e s s

I n j e c t o r temp:

260°C

Carrier gas:

He at 40 psig

Nominal Mass Resolution:

5000 r e s o l u t i o n

l o n i z a t i o n Mode:

Electron impact

The actual gas chromatographic r e t e n t i o n indices f o r each of the t e t r a - through octa-CDDs and CDFs were determined using a mixed standard which contained a l l of the t e t r a - through octa-isomers.

The q u a n t i t a t i o n standard contained the isomers l i s t e d

in Table 2. A high degree of s e n s i t i v i t y was achieved due to the high r e s o l u t i o n of the MS-instrument combined with Selected lon Response scanning. This resulted in detect i o n l i m i t s f o r the PCDDs and PCDFs in the range of sub-picograms per i n j e c t i o n leading to detection l i m i t s (signal to noise >3) of less than 1 ppt for pulp samples and less than I0 ppq for l i q u i d samples. The i n d i v i d u a l PCDD and PCDF isomers were i d e n t i f i e d

685

Table 2.

PCDF/PCDDQuantitation Standard.

Isomer

Surrogate

1,3,6,8-Tet ra-CDF 2,3,7,8-Tet ra- CDFx

2,3,7,8-13C-Tetra-CDF

3,4,6,7-Tetra-CDF 2,3,7,8-Tetra-CDD X

2,3,7,8-13C-Tetra-CDD

1,3,4,6,8-Penta-CDF 1,2,3,7,8-Penta-CDF X 2,3,4,6,8-/ 1 , 2 , 4 , 6 , 9 - Penta-CDF 2,3,4,6,7-Penta-CDF 1,2,3,7,8-Penta-CDD X

1 , 2 , 3 , 7 , 8 - i ~-Penta-CDD

1,2,3,4,6,8-Hexa-CDF 1,2,3,4,7,8-/ 1,2,3,4,7,9-Hexa-CDF

1,2,3,4,7,8-13C-Hexa-CDF

1,2,3,6,7,8-Hexa-CDF ~ 1,2,3,7,8,9-Hexa-CDF 2,3,4,6,7,8-Hexa-CDF 1,2,3,4,7,8-Hexa-CDD X

1,2,3,6,7,8-13C-Hexa-CDD

1,2,3,4,6,7,8-Hepta-CDF 1,2,3,4,7,8,9-Hepta-CDF X 1 , 2 , 3 , 4 , 6 , 7 , 8 - Hepta-CDD~

1,2,3,4,6,7,8-13C-Hepta-CDD

Octa-CDFx Octa_CDDx

i 3C_Octa_CDD 1,2,3,7,8 - I 3C-Penta-CDFX ( I n t e r n a l Standard)

x

Isomers used f o r q u a n t i t a t i o n

by the match of chromatographic retention times and by the correct r a t i o for the i n t e n s i t i e s of the two ions monitored in the molecular ion c l u s t e r , as shown in Table 3.

686

Table 3.

Masses Monitored for PCDD and PCDF Analysis.

Homolog

Masses Monitored (Theoretical r a t i o )

Tetra-CDF

303.902 '305.899

(76/100)

Tetra-CDD

319.896 1321.894

(76/100)

Penta-CDF

337.863 /339.860

(61/100)

Penta-CDD

353.858 1355.855

(61/100)

Hexa-CDF

371.824 1373.821

(51/100)

Hexa-CDD

387.819 1389.816

(51/100)

Hepta-CDF

405.785 ~407.782

(43/100)

Hepta-CDD

421.799 ~423.777

(43/100)

Octa-CDF

441.746 1443.740

(87/100)

Octa-CDD

457.738 1459.735

(87/100)

Prior to e x t r a c t i o n , each sample was f o r t i f i e d

as described above with 500 pg of

13-carbon-labelled surrogate compounds l i s t e d in Table I . The recovery of these compounds was measured in each sample to monitor the e f f i c i e n c y of e x t r a c t i o n and the possible loss of analyte during clean-up. Since the 13-carbon-labelled compounds were spiked into each sample, each sample became both an unknown and a q u a l i t y assurance sample. In a d d i t i o n , for each sample, a laboratory blank sample was prepared and analyzed using the same sample preparation and e x t r a c t i o n procedures and glassware as for the samples. The r e s u l t s showed comparable surrogate recoveries to the samples assuring t h a t the detected l e vels in the samples were a c t u a l l y present in the samples and were not due to laboratory contamination. The r e p r o d u c i b i l i t y in analysis of l i q u i d and s o l i d samples (blood, human milk, l i q u i d and s o l i d samples from i n c i n e r a t o r s ) using appropriate1~C-surrogates was previously found to be approximately 20-30% for t e t r a - and penta-CDDs and CDFs ( I 0 ) . RESULTS The t o t a l pulp m i l l e f f l u e n t has been examined on three d i f f e r e n t occasions. On the first

two occasions the m i l l was producing semibleached pulp. On the l a s t occasion f u l l y

bleached pulp was produced. The r e s u l t s , expressed as TEF (Eadon)/year, are summarized in Table 4. The i n i t i a l

r e s u l t (< 2 g TEF (Eadon)/year) was determined during the method deve-

lopment stage of t h i s project. At that stage, suspended p a r t i c u l a t e matter was not separat e l y analyzed, very l i k e l y preventing the e x t r a c t i o n of a s i g n i f i c a n t portion of the PCDDs and PCDFs. The f o l l o w i n g two r e s u l t s were obtained using the complete method as described

687

Table 4.

Results o f Analysis o f Total M i l l Ef f luent .

Date S a m p l e d

Production

Quantity ~rams TEF(EADON)/wea~ Liquid Fine material Total

27

May, 1 9 8 6

Semi-bleached

19

Sept., 1986

Semi-bleached

0.8

1.2

2.0 (62%)x

Nov.,

F u l l y bleached

3.9

1.9

5.8 (33%)~

5

x

1986

< 2

NA

<2

Value given in parentheses is )ercent of t o t a l found in f i n e ma t e r ia l.

above and f o r samples taken approximately two months apart. I t may be seen that the levels d i f f e r considerably. The difference is greater than what which may be expected as a r e s u l t of v a r i a t i o n in the a n a l y t i c a l method and may therefore r e f l e c t e i t h e r v a r i a t i o n s in the m i l l operations and or inhomogeneity in the investigated samples. As discussed above the level o f PCDDs and PCDFs found in the e f f l u e n t from the i n vestigated pulp m i l l corresponds to a release into the receiving waters o f 2 g respectively 5-6 g TEF(Eadon)/year. As part of the complete program s i m i l a r surveys o f other Swedish pulp m i l l s are now underway. The results are not yet completed and w i l l be presented in detail

at a l a t e r date. However, a

preliminary evaluation shows that the majority of these

m i l l s , which a l l are using more modern bleaching processes than the conventional one described here, has lower or much lower emissions of PCDDs and PCDFs. Currently, f or the Swedish pulp industry as a whole, the t o t a l release of TEF(Eadon) into receiving waters may be e s t i mated to 5-15 g/year. Recently, the Swedish Environmental Agency has estimated that the t o t a l release o f PCDDs and PCDFs from a l l known sources in Sweden amounts to up to 1 kg/year exDressed in the same units ( I I ) .

The major part of t h i s is released into the a i r but very l i k e l y with

f i n a l deposition to a large part as bottom sediments. Compared to this the combined emissions to a i r and receiving waters from the pulp industry is minor. In spite of t h i s , attempts to find ways o f reducing the formation of these compounds in pulp bleaching to even lower levels c o n s t i t u t e an important part of the continued SSVL d i o x i n project. To obtain an understanding of the stage or stages in which the PCDDs and PCDFs are formed, pulp suspensions taken from the various bleachinq staqes during production of f u l l y bleached pulp were analyzed. Figures I-3 show the obtained results expressed as Dg/g f or the pulp and pg/kg f o r the aqueous f r a c t i o n and f i n e ma t e r ia l. As may be seen, even the suspension of unbleached pulp entering the bleach plant contains detectable levels of TCDD and TCDF. The o r i g i n of these is not clear at present. However, i t may f u r t h e r be seen that each of the four pulp suspensions sampled from the bleaching stages contains considerably higher levels of PCDDs and PCDFs showing that these indeed are p r i m a r i l y formed during the bleaching process. Unfortunately, the data are inconclusive with regard to the p a r t i c u l a r stage at which t h i s occurs. This is due to possible carryovers between stages, countercurrent

688

~

130 120 iiO

130 Pulp

÷

~2o

[ ~ ] Filtrate * fines

ioo p-//.

a_ 90 o_ 80 c" 7O o S c

~

110

o.

i00

g

90 ~÷ 8O N 7O ~ 60

60 50

~

50 ._~

"~.~

40 3O 20

30 2O I0

10 C*D stage

EO stage

H stage

D stage

&

130 Pulp II0}~00 a_ 90 '5 80

"~ ~J

3~ .2

Unbl. stage

~

~ oc O

--] Filtrate ÷ fines

o~

12o 110

;2

~oo ~ c 90

~÷

8O N O

~o

7O ~

6O

60

~

5O 4o

50

.2

30 20 10

3O

~

20

~o

g=

U ~

10 ,4

Unbl. stage

C.D stage

EO stage

H stage

130

Pulp 120[c~ 110[-

g

D1 stage

120

--1 Filtrate * fines

110

o~ o.

IO0

c

c

I a_ 90 f

90 8O

"~

5o

70 60 50

o

30 2O

3O 20

8 co

10'

10

~ 8o S 70

+

.o. 0

o

~

Unbl. stage

C°D stage

EO stage

H stage

D stage

3:-

689

recycling of spent l i q u o r s between stages, and to u n c e r t a i n t i e s regarding the influence of pH on the s o l u b i l i t y and a d s o r p t i v i t y of d i o x i n s . Nevertheless, i t is somewhat s u r p r i s i n g t h a t r e l a t i v e l y high levels of PCDDs and PCDFs were found in the suspension taken a f t e r the c h l o r i n e dioxide stage (DI). However, t h i s can not be interpreted to suggest that c h l o r i n e dioxide promotes the formation of PCDDs and PCDFs. In separate laboratory studies, we have found that using c h l o r i n e dioxide to bleach k r a f t DulD results in the formation of only very minor levels of PCDDs and PCDFs. However, t h i s question is under f u r t h e r study in l a boratory and m i l l scale bleaching experiments. Isomeric patterns of PCDDs and PCDFs have been found to be quite useful to i d e n t i f y sources for PCDDs and PCDFs. Thus, i t has been reported t h a t various i n c i n e r a t i o n sourcesall y i e l d a t y p i c a l " i n c i n e r a t i o n " pattern. Figure 4 shows an example of such a pattern (9). Also

in Figure 4 the TCDD and TCDF patterns obtained for the t o t a l m i l l e f f l u e n t sample are

shown. As may be seen the two patterns are e a s i l y distinguished. Thus, the i n c i n e r a t i o n pattern contain the 1,3,6,8- and 1,3,7,9-TCDFs as p a r t i c u l a r c h a r a c t e r i s t i c whereas the pulp m i l l e f f l u e n t pattern is dominated by the 2,3,7,8-TCDF and 2,3,7,8-TCDD isomers. As shown in Figure 5, the pattern in the samples from the various bleaching stages are s i m i l a r to the one found in the pulp m i l l e f f l u e n t thus c o n s t i t u t i n g a c h a r a c t e r i s t i c pulp bleach pattern. Also, t h i s pattern corresponds to the patterns found in the environmental samples such as crabs and sediment collected in receiving waters close to pulp m i l l e f f l u e n t o u t l e t s as discussed above. At a distance f u r t h e r away from the o u t l e t , the pattern chanqes into a " i n c i n e r a t i o n " pattern witnessing of a general, large scale contamination from other sources (3,12,13). As described above, Figures I-3 show that in the various pulp suspensions sampled w i t h in the investigated m i l l , a part of the PCDDs and PCDFs is associated with the oulp f r a c t i o n of the suspensions. This i n i t i a t e d the question of the presence of these compounds in the f i n a l bleached, dried pulp. A sample of the pulp was therefore also analyzed for PCDDs and PCDFs. The sample contained 25-30 pg TEF(Eadon)/g of dried pulp and the concentration of 2,3,7,8-TCDD was I I pg/g of dried pulp. This is comparable to levels recently reported in bleach pulps in the United States (14). In i n v e s t i g a t i o n s not yet completed and to be reported in d e t a i l l a t e r , bleached pulps from other Swedish m i l l s have also been analyzed f o r PCDDs and PCDFs. The r e s u l t s show that the TEF content may vary considerably. Thus, in some samples contents were below the l i m i t detection (< 1 pg/g). Further studies are therefore necessary to obtain an understanding in t h i s area. Such studies are also underway at these laboratories. ACKNOWLEDGEMENT This i n v e s t i g a t i o n has been carried out as part of the SSVL (The Swedish Forestry Industries Water and A i r P o l l u t i o n Research Foundation) Projekt M i l j ~ 90, delprojekt Blekning.

690

100"

D

100.

Bteoching

Incineration

2,3,7,8 -TCDF

50'

50

0

0

10o-

I

100-

BLeaching

. 2,3,7,.~j-TCDF

Incinerotion

o

2,3,7,8 -TCDD 50.

50-

c

D

loo.

100-

C- stoge

H -stage

2.3,7.8-

2.318TCDF

/

1.2.7,8 - TC~

50-

50-

o

too-

El-stage

1.2.3.811.4.6.7/ 2.L.6.8/1.2.3.6- TCDF 50-

.

:r

0

"~"

'; -- ;"

~

2.3.?,8-

~COF

100- D1-stage 1,2,7,8-TCDF

\

50-

2.3.7. 8 TCDF

/

691

REFERENCES I.

Sandermann,W. Natuwissenschaften 61:207 (1974).

2.

Blossner, R.O., and Miner, R.A. Proceed. EUCEPASymp., Helsinki, Fin., p. 164 (May, 1986).

3.

Rappe, C. presented at Dioxin 86. Fukuoka, Japan, (September 1986). Chemosphere (in press).

4.

Bostr~m, C-A. SSVL, Milj5 90, Report I, Stockholm (1986).

5.

Kringstad, K.P., and Lindstr~m, K. Environ. Sci. Technol. 18 (8): 236 A (1984).

6.

Eadon, G., et al. as discussed by B e l l i n i , J.S. and Barnes, D.G. Toxicol. and Indust. Health 1 (4): 235 (1985).

7.

U.S. Environmental Protection Agency waste water analysis method: 2,3,7,8-tetrachlorodibenzo-p-dioxin. Method 613 Fed Reg 44:69464.

8.

Smith, L.M., Stalling, D.L., and Johnson, J.L. Anal. Chem. 56:1830-1842 (1984).

9.

Marklund, S., Kjeller, L.-O., Hansson, M., Tysklind, M., RaDpe, C., Ryan, C., Collazo, H., and Dougherty, R. in Chlorinated Dioxins in Perspective, RaDpe, C., Choudhary, G., and Keith, L.H., (eds.) Lewis Pub., Chelsea, Mich., p. 79-92 (1986).

I0.

Nygren, M., Hansson, M., and Rappe, C. (October 1987).

II.

Swedish Environmental Protection Agency. Report "Dioxin", May, 1987.

12.

Aslander, O. Swedish Ministry of Environment Dioxin Project Report "Dioxins in Crabs from the Swedish West Coast" (24 November, 1986).

13.

Rappe, C., Kjeller, L.-O., Jonsson, P., Jonsson, B., H~kanson, L. Swedish Environmental Protection Agency, Report "Chlorinated Dibenzodioxins and Dibenzofurans and Extractable Organic Chlorine: Studies of Ocean Sediment Outside Forest Industries". March 1987.

14.

American Paper Institute/U.S.E.P.A. Joint Dioxin Screening Study (30 March, 1987).

(Received in Germany

21 N o v e m b e r

1987;

presented at Dioxin 87, Las Vegas, USA

accepted

15 December

1987)