Disposal and Decontamination of Pesticides - ACS Publications

1 Conquering the Monster—The Photochemical Destruction of Chlorodioxins D. G. CROSBY

Downloaded by KAOHSIUNG MEDICAL UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: June 30, 1978 | doi: 10.1021/bk-1978-0073.ch001

Department of Environmental Toxicology, University of California, Davis, CA 95616

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has become recognized as probably the most poisonous o fallsynthetic chemicals. In additiontohaving a high acute toxicity, it is mutagenic, embryotoxic, and causes skin disorders at very low exposure levels (1). I t also has been considered to be very persistent (2) and to bioconcentrate i n animals. No wonder that a 1976 newspaper article referred toitas "the monsterous chemical" and that its detectable presence i n Vietnam War d e f o l i a n t s , commercial herbicides, and elsewhere i n the environment has caused so much apprehension and controversy. The use o f products which could contain chlorinated dibenzo-p-dioxins has been remarkably widespread (3). Pesticides such as 2,4,5-T, chlorophenols used f o r slime- and algae-control, and the common bactericide hexachlorophene--in f a c t , any chemical made from 2,4,5-trichlorophenol--might contain traces of TCDD. Commercial pentachlorophenol (PCP), used primarily as a preservative and i n s e c t i c i d e , contains hexa-, hepta-, and octachlorodioxins which can be reduced b i o l o g i c a l l y to less-chlorinated homologs; f o r example "chick-edema" disease has been traced to dioxins formed i n PCP-treated hides from which a poultry-feed supplement was derived (4). The manufacture of chlorophenols, e s p e c i a l l y , has led on occasion t o severe toxicological problems such as those encountered i n a 1976 accident at Seveso, Italy. E a r l i e r work indicated that pure TCDD was almost inert toward attack by microorganisms, other biological breakdown, and environmental forces (2). This is not s t r i c t l y true; under certain conditions, i t Ts very unstable to u l t r a v i o l e t (UV) l i g h t (5). The purpose of the present paper i s to define the photochemical c r i t e r i a f o r that i n s t a b i l i t y , describe laboratory experiments concerned with i t , and indicate how i t might be applied f o r the intentional destruction o f "the monstrous chemical" and i t s r e l a tives.

0-8412-0433-0/78/47-073-001$05.00/0 © 1978 A m e r i c a n C h e m i c a l Society

Kennedy; Disposal and Decontamination of Pesticides ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

2

DISPOSAL AND DECONTAMINATION OF PESTICIDES


Background

Photochemistry

The photochemical replacement of halogen atoms on aromatic rings by hydrogen has been known for some time. For example, the i r r a d i a t i o n of pentachlorophenol in hexane with UV l i g h t produced tetrachlorophenols, pentachloronitrobenzene (PCNB) was reduced to tetrachloronitrobenzenes and pentachlorobenzene, and polychlorobenzenes were further dechlorinated (6) ( F i g . 1). However, thin films of the pure compounds were not appreciably affected; some donor of hydrogen atoms also was necessary for the reaction. Organic solvents serve this purpose admirably, although photoreduction experiments with chlorobenzoic acids revealed that the chemical nature of that solvent could strongly influence the rate of reaction (7). Although photonucleophilic displacement reactions of aromatic halides have been shown to be commonplace in aqueous solution (8), most evidence for the mechanism of reductive dechlorination in organic solvents supports an i n i t i a l homolysis of the r e l a t i v e l y weak C-Cl bond followed by abstraction of the hydrogen atom from solvent by the resulting phenyl radical ( F i g . 2). For example, when the solvent i s benzene, phenylation rather than hydrogénation often predominates (9), and the above-mentioned photoreduction of PCNB i n hexane also produced the three isomeric chlorohexanes by transfer of chlorine atoms (6). This mechanistic problem i s by no means solved, but Nordblom and M i l l e r (7) confirmed that the chlorine-replacing hydrogen indeed i s derived by breaking C-H bonds of the solvent. As might be expected, this reductive dechlorination of polyhalogenated compounds takes place stepwise (Fig. 3). UV i r r a d i a tion of tetrachlorobiphenyl s in organic solvents produced corresponding t r i - , d i - , and monochlorobi phenyl s, but the monochloro compound proved to be e s s e n t i a l l y stable toward further reaction due to n e g l i g i b l e absorption of the l i g h t energy provided (10). The same phenomenon has been observed in the photoreduction of chlorinated benzonitriles (11); the rate of the primary homolytic (bond-breaking) process in We halide i s dependent upon the degree of l i g h t absorption as measured by the compound's molar extinction c o e f f i c i e n t , ε . Consequently, the relationship of the incident l i g h t to the UV absorption spectrum of the halide as well as to that of any other strongly-absorbing compound present becomes extremely important. It i s apparent that three conditions must exist in order f o r photoreduction of chlorinated aromatic com pounds to take place: a hydrogen-donating solvent must be pre sent, UV l i g h t of appropriate wavelength must impinge on the solution, and that l i g h t must be absorbed. The energy available in natural sunlight l i m i t s the photo reduction of many chemicals in the environment. Sunlight inten s i t y drops o f f abruptly below about 310 nm (Fig. 4) and becomes n e g l i g i b l e below about 290 nm. Consequently, i t i s not surprising that 3,3* ,4,4*-tetrachlorobiphenyl (ε = 6740 at 290 nm) i s readily


Photochemical Destruction of Chlorodioxins


CROSBY

Figure 1.

h v

ArCl

Ar- + C H g

C1

. C H +

Photodecomposition products from PCNB in hexane

6

1 4

1 3

Ar* + Α γ · C

H

6 13-

+

C

>• Ar- + CI-

Primary Process

>- ArH + CgH^*~

C H C1 6

Reactions

13

>- Af£ H

6 13

C

H

12 26

SideReactions

Etc. Figure 2.

Proposed mechanism of photochemical reduction of aromatic halides in an organic solvent (e.g., hexane)


4



Wavelength, nm Figure 4.

Spectral energy distribution of sunlight


1.

CROSBY

5


reduced at sunlight wavelengths ( 7 J 0 ) , while 4-chlorobiphenyl i s r e l a t i v e l y stable (ε = 10 at 310 nmTT only substances which absorb UV l i g h t above 290 nm can be expected to react. Chlorodioxins Like other chlorinated aromatic compounds, chlorodioxins are reductively dechlorinated when t h e i r solutions in organic solvents are irradiated with appropriate wavelengths of UV l i g h t (12). Exposure of a d i l u t e methanol solution of TCDD ( X 306 nm, ε = 6000) to l i g h t from a fluorescent UV lamp caused rapid degradation (Fig. 5); the i n i t i a l product, 2,3,7-trichlorodibenzo-p-dioxin, which exhibits a very similar absorption spectrum but has only about half the molar extinction (Xm 304 nm, ε = 3500), slowly accumulated and then i t s e l f disappeared. 1,2,3,4,6,7,8,9-Octachlorodibenzo-p-dioxin (0CDD), with X 318 nm and ε 4000, reacted even more slowly (Fig. 6). Less-chlorinated photoreduc tion products were readily detected and i d e n t i f i e d by gas chroma tography and mass spectrometry in each instance. Pure, c r y s t a l l i n e TCDD was stable to sunlight wavelengths when applied as thin films to glass (5) or leaves (12) or suspend ed in water (3). Apparently, the c r y s t a l l i n e state prohibits TCDD molecules from losing s i g n i f i c a n t chlorine or from abstracting hydrogen atoms from each other. However, when dissolved in methanol (5), diesel o i l (13), or l i q u i d phenoxy ester (13), photoreduction was complete a f t e r a few hours exposure to outdoor sunlight. Again, trichloro-and dichloro-homologs were detected during photolysis, but they in turn eventually were dechlorinated to nontoxic dibenzo-p-dioxin which i t s e l f underwent further photodegradation (.3). This suggests that TCDD may decompose under many practical application conditions and that i t even could be intentionally destroyed as long as the three photochemical c r i t e r i a c i t e d above were s a t i s f i e d .


m a x

ax

m a x

1

Practical Applications Continuing concern has been expressed over the presence of small but detectable levels of TCDD in commercial herbicide formu l a t i o n s . T y p i c a l l y , such formulations are composed of roughly equal amounts of l i q u i d esters of 2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-tri chlorophenoxyaceti c a c i d ) , usually dissolved in o i l or emulsified in water (Table I ) . The fear i s that the contained dioxin--currently less than 0.1 ppm but o r i g i n a l l y as much as 100 times more than that—might p e r s i s t and bioconcentrate following f i e l d application of herbicides.



6


Figure 5.

Stepwise photoreduction of

TCDD


1.

CROSBY

7


Table I. Composition of Herbicide Sprays


Esteron Brush-killer Water 2,4-D propylene glycol ether esters 2,4,5-T propylene glycol ether esters Organic solvent and emulsifier

Lbs/Acre 100

Agent Orange 2,4-D η-butyl ester 2,4,5-T η-butyl ester Alcohols, acids, etc. equivalent to 1 lb/acre 2,4,5-T (

Disposal and Decontamination of Pesticides - ACS Publications

Recommend Documents