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WjilARCH N1WI DDT: An Unrecognized Source of Polychlorinated Biphenyls Polychlorinated biphenyls (POB's) arc disturbingly widespread in the earth's biosphere. They have been found in newly developed real estate lakes, at isolated geological Held stAttont. And in polar hears and seals north of the Arc tic Circle--at a Urge number of sites, in short, where a relatively involatilc industrial chemical shouldn t appear. Their ubiquity, if not their concen trations. resembles that of the much more widely used, more volatile pesti cide. DDT. Now it appears that this similarity of distribution is far from coincidental. Kenneth W. Moilancn and Donald G. Crosby of the University of California at Duvis last month (old (he 105th na tional meeting of the American Chem ical Society that DDT vapor can be converted to PCB's by irradiaiion with ultraviolet liyht of the same wavelengths present in sunlight in the lower atmo sphere. This finding, if confirmed, promises to change much scientific thought about the fate of pesticides in the environment. DDT Wn Thuufihl Inert DDT 11,1.1 -trichloro-2,2-bis(/7-chlorophco'lKMlumcl in solution or us a thin Him i> essentially inert to sunlight, al though it is readily decomposed by ul traviolet light of shorter wavelengths. Many scientists have assumed that the vapor-phase photochemistry of DDT would he similar to that in solution, so that atmospheric degradation of DDT by sunlight would be unimportant in comparison to transport, even though the mean resilience time of DDT in the atmosphere is estimated to be approx imately 4 )cars, DDT in water is also inert, but in soil it is converted by some microor ganisms to the more volatile and more persistent pesticide relative DDE (l.l-dichloro-2.2-bisl/-chlorophcnyl)cih>lcnc). Since DDE is even more inert thun DDT. large quantities of both chemicals should he accumulating in ihc biosphere. Vet ulob.d models of pesticide transport and monitoring of pesticide residues can atcoum for only a small Irw'.ton of the apptoximatcly 2.N billion kilograms ol 1>DI that have been dispensed since the early 1940 s, and sciennsis have been unable to ex plant this discrepancy. Moilanen and Vi Crosby conclude that much of the DDT and DDE may have been converted to other chemicals whose presence in the environment would not necessarily be suspicious because there are other sources. The two workers used a special re actor incorporating a collimated beam of light designed to minimize reactions occurring on the reactor walls. Con trol experiments were performed with both a dark reactor and one in which a convex mirror permitted light to strike the reactor walls, so that the ef fects of wall reactions and reactions oc curring in the absence of light could be recognized and tuken into account. When DDT vapor in the presence of a large excess of air wax irradiated with light in the region 290 to 310 nm. they found thul.the DDT was converted very slowly to DDD [l.l-dichloro-2.2bis(p-chlorophenyi)ethane] and about 15 times more rapidly to DDE. After 4 days of irradiation, as much as 34 percent of the DDT was converted to DDE and about 2 percent was con verted to DDD- Further experiments showed that DDD is inert to ultraviolet radiation, hut that under the same con ditions DDE is converted to at least nine other products, including 4,4'-dichlorohcnzophcnunc and several chlor inated biphenyls. The photodegradation of DDE is, however, much slower than its formation from DDT. The conversion of DDT to DDD re quires the presence of a hydrogen donor because it involves replacement of a chlorine atom with a hydrogen atom: this conversion might thus be a surface reaction occurring on the reactor walk, since the extent of the reaction is prob ably limited not by the amount of light that reaches the reactor wulls, but by the very limited number of hydrogen donors present in the reactor. In the atmosphere, then, this transformation could be enhanced by the presence of particulates and of other hydrocarbons. Apparently DDD is not degraded fur ther in the atmosphere. The conversion of DDT to DDE, in contrast, is probably a urtimolccular re action in which a molecule of hydro gen chloride is expelled. The DDE it self breaks down by cither of two path ways. In the least important route, two molecules of DDE apparently interact directly to form small qnanntics of Ji , iri*. and tctruchiorobiphenyls and even smaller quantities of other, as yet un identified, chlorinated compounds. In the more important pathway, DDE is converted at a moderate rate to DDMU 11 -chloro-2.2-his(/-chlorophenyDcthylenc). This reaction, like the formation of DDD, requires a hydro gen-donor, and (here is some possibil ity that it is also a surface reaction. In the photolysis of thin films of DDE, however, a major product is 3,hdichlorofluorenone. which is inert to further irradiation. Moilanen and Crosby found the dichlorolluorenonc onlv when light was allowed to reach the reactor walls, so there is a strong possibility that con version of DDE to DDMU is, in fact, a vapor-phase reaction. In either ease, the degradation of DDE would be ac celerated by the presence of other hy drocarbons in the atmosphere. DDMU. in turn, is converted at a moderate rote to 4.4'-dichlorobcn/ophenone by interaction with oxygen. The dichlorobenzophcnonc is relatively stable to irradiaiion. hut it is converted at a finite rale to 4,4/-di:hlorohiphcny! All of the biphenyls .lie themselves re sistant to further irradiation in the vapor phase, and would thus be ex pected to accumulate in the biosphere. Replace One Contaminant with Another Extrapolated to the atmosphere, then, Moilanen and Crosby \ oscrall scheme (Fig, I) predicts that DDT vapor is converted to small amounts ol DDD. which should accumulate in the bio sphere, and to a great ileal more DDE. which should degrade more slowly than DDT. DDE vapor, in turn, is converted at a moderate rate to dichlornbcnzophenone (via DDMU). and then at a much slower rate to dichiorobiphcnyl. Some of the DDE is also converted to 3,6-dichlorofluorcnonc. whose cm ironmental fate is unknown, ami to small amounts of dk tri*. and ictrachlorobiphenyls. The principal result is thus re placement of one environmental con taminant (DDT) with another fPCBM. Part of this scheme resembles the photochemical decomposition of solid DDT or of DDT in hexane as described in 1969 by \V D. Guen/.i of the L'.S. Department of Agriculture m Tort foi ling, C olorado; but Guenzi did not oh- HONS 086607 SClCNff:. VOI 1 HO Pig. I. Proposed scheme for the degradation of DDT vapor in sunlight. [Source: K. W. Moilanen and D. G. Crosby, 1,'ni'erUty of California at Davis) serve the formation of DDMU or of PCBV Most important, his work was performed at 253 nm, a wavelength thut is not present in sunlight in the lower atmosphere, and there has been little previous evidence of photodecomposilion of DDT and DDR at the wave* lengths used by Moilanen and Crosby. Evidence supporting the existence of Ihc DDT and DDE reaction pathways in lha atmosphere may. moreover, be difficult to acquire because of the pres ence of competing puthways to some of the decomposition products. DDT in the soil, for example, is con verted to DDE and DDD by several microorganisms, although at a very slow rate; since DDE and DDD have higher vapor pressures than DDT. how ever, they arc preferentially released from the soil, according to Mark M. Cliath and William F. Spencer of the University of California, Riverside. They reported last fall that DDL: com prises as much ns NO percent of the organochlorinc chemicals in the air above freshly plowed Helds ihai had previously been sprayed with DDT. It h.o> recently been recognized, in fact, : >i DDE is now among the most mlant organochlorinc contaminants he biosphere. Even if only a small don of this DDE were produced may ivm photochemically, however, all of it in the atmosphere would still be suscep tible to further photochemical degra dation. Some Products Not Detected Dichlorobenzophenone and DDMU have apparently not been detected in atmospheric samples, but that may be simply because no one is looking for them. Dichlorobenzophenone. more over. is a common metabolite of many plants and animals, Crosby says, and Us presence in tissues would not be suspicious. And finally, most investigators who hgve detected PCD's in the environ ment have measured only the total amount of PCB's present and have not attempted the more difficult task of assaying the relative amounts of high ly chlorinated and partially chlorinated btphcnvls. There is thus little evidence to indicate whether environmental PCB's arc enriched in the less highly chlorinated isomers, as would be ex pected if phnioilcgmlaiion of DDT is a major source. Much work remains, then, before ihr full significance of Moilanen and Ci< by's results can he assessed. As a t; stop, they will soon begin i. study effects of particulates and other hydio. MONS carbons on the photochemistry of DDT and DDE. In association with J M. Seibcr, they also plan this coming win ter to analyze air samples trom test fields where DDT has been fresh!'1 ap plied (some experimental applications of DDT are still permitted, even thou-h its use as a pesticide was banned at the end of 1972) to look for the presence of DDMU and dichlcrohenzophew-nc. They do sum expect lo limi P( IF- m the air samples because the concentra tions should be too low. hut the' hope that other groups will bemn lo examine oil slicks, biota, and other sites where PCB's arc concentrated m the environ ment to determine whether the POB\ arc. in fact, enriched in the lex- hi-'hlv chlcrinatcd compounds Only vsiu-u all of these results have been obtained, Crosby says, will they be cnmidciii that their laboratory work can he ex trapolated lo reactions in the atmo sphere.--Thomas H. Mamui li AMWmU Rradini I A. R. MnjJi'f. W D. limiut, L l. M ile- IM HOl .......... 2. <1 \1. H.UI. P. IV i ./../ 174, II' I ' l vT | , II .,, J. M. M. Clulli .<ml \V. I XrcfK.-r. / S<>. r.-.)ml *. >M0 4. Drvtn-loioor of Siinhvr.t <</./. .`.I,-It. . . > ihr ttitHfherr, Pr.-n ulitvs <.l i .. ............ s.in I l.tlUAC*. C.iHI.. I? to M iSVnnn.il At.d.ntv nl X.U.UA-- VS..I.01 > > n o. ivtji 086608