Document jyOavG43NZGKXZ29kX2MX7JB9

.RRcT ,, J__kA-A- * ;,-W- --'f$i?.'-1- -.K&. >\iME ,.-y 'i, -*>< -: :rTrrLaTn^s'Vformations of halogenated WMdiUidnT^di^ition^bsii^onJ^ddehydrohalogenad^?wmdris occur abiodcall mn mcropuu and **-- Timothy M. \fogel Michigan State University' East lapsing, Mich. 488241 ?as cleaning sofwnts in diy-cleaning op- ______biotic chemistry, of balogenaSferatipns, andiseihiconductof manufac-.lifted aliphatic compounds^Knowledge of, Craig S. CridcQe Perry L. McCarty Stanford University ' Stanford, Calif. 94305 j4l,2-dibromo-3-chloropropane (DBCP), ;; ing biologically mediated transforma I i^S are used as pesticides (1, 2). In addi- rations. Most abiotic transformations are' tion, brominated and chlorinated meth- f slow, but they can still be significant J anes are produced as a result ofchlorin within the time scales commonly asso ation of water (,?). The production and ciated with groundwater movement. In use of halogenated aliphatic compounds -contrast, biotic transformations typi uiijoand their apparent hazard to human cally proceed much faster, provided ` Tta: health (Thble 1) (4-6) have prompted that there are sufficient substrate and Halogenated aliphatic compounds are investigations concerning their fate in nutrients and a microbial population prevalent groundwater contaminants the human body, in subsurface waters, that can mediate such transformations. and are significant components of haz and in treatment facilities. In most of Recent studies, which describe trans ardous wastes and landfill leachates. these environments, photolysis is not an formations of halogenated aliphatic Most hazardous halogenated aliphatic important transformation process, but compounds in microbial and mamma compounds released from industrial, other abiotic transformations can be lian systems, are also discussed in this commercial, and agricultural sources significant. Often, biologically medi article. These studies reveal broad pat are (nominated or chlorinated alkanes ated transformations are the most im terns of transformation in biological and alkenes that contain between one portant. systems in general. and three carbon atoms. Chlorinated This article summarizes and system All three systems (abiotic, mamma ethanes and ethenes are in common use atizes the current understanding of abi- lian, and microbial) have similarities in 722 Environ. Sd. Tachnol., Mol. 21, No. 8,1987 SL 037177 .V V/;, , v0013-^6X187/0921-0722$01.S0/0 1987 American Chemical Society v, . r* "zS8cT OKlOMsMlfMW Thmsformations of halogenated aliphatic compounds v` Oxidation, reduction, substitution, and dehydrohalogenation reactions occur abiotically or in microbial and mammalian systems Timothy M. Vogel Michigan State University East Lansing, Mich. 48824 Craig S. Criddle Perry L. McCarty Stanford University Stanford, Calif. 94305 Halogenated aliphatic compounds are prevalent groundwater contaminants and are significant components of haz ardous wastes and landfill leachates. Most hazardous halogenated aliphatic compounds released from industrial, commercial, and agricultural sources are brominated or chlorinated alkanes and alkenes that contain between one and three carbon atoms. Chlorinated ethanes and ethenes are in common use as cleaning solvents in dry-cleaning op erations and semiconductor manufac ture. Some brominated compounds, such as 1,2-dibromoethane (EDB) and l,2-dibromo-3-chloropropane (DBCP), are used as pesticides {1, 2). In addi tion, brominated and chlorinated meth anes are produced as a result of chlorin ation of water (3). The production and use of halogenated aliphatic compounds and their apparent hazard to human health (Table 1) (4-6) have prompted investigations concerning their fate in the human body, in subsurface waters, and in treatment facilities. In most of these environments, photolysis is not an important transformation process, but other abiotic transformations can be significant. Often, biologically medi ated transformations are the most im portant. This article summarizes and system atizes the current understanding of abi otic and biotic chemistry of halogena- 7 ted aliphatic compounds. Knowledge of abiotic transformations can provide a conceptual framework for understanding biologically mediated transforma- _r tions. Most abiotic transformations are % slow, but they can still be significant * within the time scales commonly asso- i dated with groundwater movement. In contrast, biotic transformations typi- ' cally proceed much faster, provided that there are sufficient substrate and nutrients and a microbial population that can mediate such transformations. Recent studies, which describe trans formations of halogenated aliphatic compounds in microbial and mamma lian systems, are also discussed in this article. These studies reveal broad pat terns of transformation in biological systems in general. All three systems (abiotic, mamma lian, and microbial) have similarities in 732 Environ. Set. Technol., Vol. 21, No. 8,1987 0013-936X/87/0921-0722S01.50/0 1987 American Chemical Society SL 037178 a- '' of ;!' j,, i' d d n *'? c ts a y reaction mechanisms and transforma tion products. They also have differ ences. For each, the transformation of halogenated aliphatic compounds can be divided into two general classes: re actions that require external electron transfer (oxidations and reductions) and those that do not (substitutions and dehydrohalogenations). External electron transfer is defined as the transfer of electrons to and from some agent other than the halogenated compound itself. Processes discussed in this article and terms that frequently appear are de fined in the side bar. Examples of trans formations are listed in Figure 1. Com mon abbreviations for the various halogenated aliphatic compounds are listed in Table 2. Substitution Halogenated aliphatic compounds undergo substitution and dehydrohalo- Coupling--a reaction in which two alkyl or aryl groups connect to gether. Dehydrohalogenation--elimina tion of HX to form an alkene. Dihalo-elimination--reductive elimination of two halide substi tuents to form an alkene. Electrophile--a reacting specie that accepts an electron pair. Elimination--a reaction in which two groups, such as hydrogen and chlorine, are lost from adjacent car bon atoms so that a double bond is formed. Epoxidation--a reaction in which an epoxide is generated. Hydrogenolysis--a reduction in which a carbon-halogen bond is broken and hydrogen replaces the halogen substituent. Hydroxylation--addition of a hy droxyl group. Ionization potential--the differ ence between the energy of ultravio let radiation used to bombard a mol ecule and the energy of th ejected electron. - * Monooxygenase--an enzyme that catalyzes reactions in which one atom of Os appears in the product and the other in HsO. Nucleophile--a reacting specie that brings an electron pair. Solvolysis--a reaction in which the solvent serves as the nu cleophile. Substitution--a reaction in which one substituent on a molecule is re placed by another. PERMISSION TO USE RACMAN WAS GIVEN BY MARI GAMES. SL 037179 genation in water in the absence of in organic or biochemical catalysts. In general, these reactions proceed slowly, with half-lives of days to centuries. However, rates can be accelerated by the activity of biologically derived en zymes such as hydrolases or glu tathione S-transferases. Solvolysis of halogenated aliphatic compounds in water (hydrolysis) leads initially to the production of alcohols (7) (Figure 1, la). If these alcohols are halogenated, then subsequent hydroly sis to acids or diols can occur. Hydroxy substitution generally occurs at the ha logenated carbon. In general, these substitution reactions are bimolecular. Nevertheless, pseudo-first-order kinet ics are observed in aqueous solutions in which water is the dominant nu cleophile. These reactions are poten tially faster at higher pH, and the hy droxide ion acts as the nucleophile. However, below pH 11, a pH depen dence for substitution reactions is gen erally not observed (S, 9). High ionic strength may increase the likelihood of unimolecular reactions as a result of in creased stability of charged intermedi ates. A summary of half-lives for several chlorinated and brominated aliphatic compounds in aqueous solution is shown in Table 3 (8-20). In general, monochloro- and monobromoalkanes hydrolyze with half-lives of about one month at 25 C; polychlorinated spe cies hydrolyze less readily. For dichlorinated alkanes, substitution (hy drolysis) is more likely than dehydrohalogenation, which also is possible. The nature of the halogen substituents and the degree of halogenation influ ence substitution rates (Figure 2). Bro mine, a better leaving group than chlo rine (7). is lost from haloaliphatic compounds more readily than is chlo rine. Increased halogenation leads to slower substitution reactions and longer half-lives (8, 9). Abiotic substitution reactions might also be enhanced by catalysts such as clays (20). Sulfides also react with halogenated aliphatics via substitution to produce mercaptans (21) (Figure 1, lb). Chemi cally, the sulfhydryl group (SH-) is more reactive than the hydroxyl group (OH-) or water (7), although concentra tions in natural waters are usually not high enough for sulfhydryl-related re actions to dominate. Substitution reactions also can occur in mammalian systems (Table 4) (2229). Glutathione reacts with halogena ted aliphatic compounds to produce sul fur-containing compounds. Under proper conditions, these compounds are further transformed into mercapturic acids (28). Some chlorinated al kanes are transformed into alcohols in FIGURE 1 ' - Abiotic and biotic reactions f halogenated aliphatic compounds `'5f ' 'V; '1 Reactions' : (.Substitution , #. (a) soiwtysis, hydrolysis < RX * HO -----RON* MX t - (b) conjugation and other nucleophilic reactions RX * N~ -- RN * X" II. Dehydrohatogenaiion *1 ,*a6- \\ XH \ / CC 4 MX tN III. Oxidation (a) <*-hydroxytation >, \ l: -- C -X * H 0 - -C -X + 2H*> 2- H OH (b) haiosyt oxidation 1 ^ i *_ - e-x+H,o -- -c-x o * 1 + 2H++ 2~ (c) epoxidation X Ox \/ . s/ V 4 CC + H O -- c - C * 2H + 2" / \z /\ (d) biohaiogenation (alkenes) .. rf . cc x%h,,o-- -- c -- c -- \ 2 II + H*. 2*~ IV Reduction (a) hydrogenolysis RX + H% a" -- RH * x~ (b) dihalo-eiimination 11 * \s -- c-- C -- + .2*'-- c-c + 2X XX (c) coupling 2RX+2*--- R-R +2X" 'Examples ' CH,CH- CH 8r -H O----- CH-CM-OCOI HBr ' 3 2 2 **;.. V-' m ... 4Jy ; CH CH Br HS"-----* CH,CH,SH - Br" * -32 * - -~`*t - "'Sf',' > J COaCH, ----- Cd,CH? . HO . * " . "' r * vo:-. . 'i ^ '/ . -*_ . v ,' ; K*, CHgCHOj- H20 CHjCdjOH +S2H** 2*~ V: .. ' , V | CHgCHClj-HjO -- CH3CHaab~* 2H*. H~ / "' . ,';*'**" 1' ' r '' ' CHOCCJj * HjO -- CMOOCCtj- 2H* . 2t~ - '' 'j'-'* --. ."-v*i v * '"-'vf '* ' 'A *1 "i " '. ' ^ CHjCHj + crv HjO WjOHCHjO -H`-2- ' . 'l V ' * . . ',r- . , 7-:- ca3cci3 + a*~~l** caaca2* icr- ' ' ,'rT W,,`*V :::T '' 44 \ v'"`^ . 2Cd4. 2`--* COjCdj 20" FIGURE 2 ' -.' -v.ijy. 40^ ' ' Estimated half-lives for some halogenated aliphatic compounds Bremmauon compounds (A) ChkvinMed compounds (+) 72* Environ. Sci. Teehnol, Vbl. 21,-No. 8. 1987 SL 037180 live mice, but the responsible agents are unknown. Such reactions may be simple substitution reactions--the oxi dation states of carbon in both reactant and product are the same. Examples are the conversion of tetrachloromethane to carbon dioxide or dichloromethane to carbon monoxide. These reac tions might in reality be a sequential combination of reductions and oxida tions. For example, tetrachloromethane can be reduced to trichloromethane and subsequently oxidized to carbon diox ide via phosgene (30). However, for many of the transformations in mam malian systems, complete mechanistic interpretations are not yet possible be cause there is insufficient information. Dehydrohalogenation Halogenated alkanes in water also can undergo elimination reactions that produce alkenes (Figure 1, Ua). Elimi nation of HX is termed dehydrohalo genation or, less frequently, dehydrodehalogenation. These reactions consist of removal of a halogen front one car bon atom and concomitant (E2) or sub sequent (El) removal of a hydrogen atom from an adjacent carbon. Al though the formal oxidation state of a halogenated aliphatic compound de creases as a result of the loss of a halo gen, it increases with loss of hydrogen. Dehydrohalogenation reactions thus do not include external electron trans fer, and no net change occurs in the oxidation state of the reacting mole cule. Monohalogenated aliphatics appar ently do not undergo dehydrohalogena tion in water under normal environ mental conditions (see Table 3), and such reactions are unlikely to occur (7). Polychlorinated alkanes undergo dehydrohalogenation under extreme basic conditions (17), and at pH 7 (12). These reactions generally follow bimolecular kinetics, depending on hydrox ide ion concentration. Under normal pH conditions (near pH 7), dehydroha logenation by interaction with weaker bases (e.g., water) might be important. The number and kind of halogen substi tuents have a strong influence on dehy drohalogenation rates. When more chlorine substituents are attached to the carbon atom that loses a chlorine sub stituent, faster rates are observed, Brominated compounds tend to undergo dehydrohalogenation with fewer halo gen substituents and more rapidly than their chlorinated analogues (Figure 2). Dehydrohalogenation reactions do not appear to occur with vicinal dichloroalkanes (chlorine on adjacent carbon atoms) (12), but they do occur, at least partially, with vicinal dibromoalkanes (9). This increased rate of dehydrohalo genation from compounds that contain SL 037181 TABLE 2 tjjiv tea.--. Abbreviations used for chemical sp cies 0.' - X., j?di..*3; ..S-liUT'*. Abbreviation Chemical name - ": ' Alternative chemical name A v' Ethane BCM Bromochloromethane a SiSS^a.. ; >;;-f ' *! BDCM ;BF r-: Bromodichloromethane Tribromomethane :`Vr Bromoform'X' -f BM Bromomethane if5*1 ,: BTCM 'v'* "'`J' ' Bromotrichloromethane CA Chtoroethane -i*'- wsiprxftfci t J. .) CF ibc.iij'tv Trichloromethane ' dj-d Chloroform CM \ ,i -- 1 Chloromethane CT . DBCM DBCP DBDCM DBM DCA DCE tane Dichloroethene <'*' 7^ >?,. ... ,*v E. Ethene .' ?!' - r.i; Ethylene EDB Dibromoethane X'Ar... . , K Ethylene dibromide HCE Hexachloroethane - i};iW iis\ ?-< -J M Methane MC Dichloromethane . vfo ; Methylene chloride PCA Pentachloroethane . , PCE Tetrachloroethene X , 'i.Perchloroethylene TCBM Tribromochloromethane \ TCA 1,1,1-Trichloroethane Xi ?? * Methyl chloroform TCE TEBM Trichloroethene Tetrabromomethane . ... ,> -<,j i> -J .r (Carbon tetrabromide TECA Tetrachloroethene VC TABLE 3 1 II..... --I*" Environmental half-lives and products from abiotic hydr lysis or dehydrohalogenation of halogenated aliphatic compounds at 20 C - Compound ' v, ' 1* ' J*- IV HftIMIf* year* (reference) Product(a) (reference) Methanes <. : ... ' Dichloromethane 1.5 (10), 704 (8) Trichloromethane 1.3 (IQ), 3500 (8) Tetrachloromethane 7000(8) ..w'r Bromomethane - * ! .0.10(8) ' ' Dibromomethane Tribromomethane Bromochloromethane 183(8) .j' 686(8) v-- '.' xtv'i'v''S\ * 4 1 ' ' *"y t 44(8) r Bromodichloromethane 137 (8) v;?;..,V7 v s' , t* Dibromochloromethane Ethanes 274(8) A ; M **''.***.'* - '- aW'r7. -?y Xt - . r' , . ' Chloroethane .' 1,2-Dichloroethane 0.12(H)* , 50(12) v Ethanol (11)* 1 rv . 1,1,1 -Trichloroelhane 0.5(10), 1.7(12) Acetic acid (12-14) 0.8 (15p. 2.5 (16)* 1,1-Dichloroethylene (14-16) 1.1,2-Trichloroethane 170 (12) 1.1-Dichloroethene (17) 1,1,1,2-Tetrachloroethane 384 (12) Trichloroethene (72) 1,1,2,2-Tetrachioroethane 1,1,2,2,2-Pentachloroethane Bromoethane 0.8(12) V 0.01 (12) 0.08 (8) Trichloroethene (72) Tetrachloroethene (72) 1,2-Dibromoethane 2.5(9) Bromoethene (9) 2.5 (18) Ethylene glycol (78) Ethanes Trichloroethene 0.9 (10), 2,5 (ISf Tetrachloroethene Propanes 0.7(10), 6(15)* '1 1-Bromopropane 0.07 (8) 1,2-Dibromopropane 0.88(9) Bromopropene (9) 1,3-Dibromopropane 0.13(9) Bromopropanol (9) 1,2-Dibromo-3-chloropropane 35 (19) Bromochloropropene (79) '( Extrapolated by 2 from Reference 11. "At 10 "C in sea water. *At 20 "C. Envtfon. Sci Technol., Vol. 21, No- 8, 1987 725 bromine substituents is consistent with the findings of Burlinson et al., who noted that bromine is eliminated from DBCP six times faster than is chlorine (19). Few halogenated aliphatic com pounds undergo dehydrohalogenation in mammalian systems, but there are some. 1,1,2,2-Tetrachloroethane and 1,1,1,2,2-pentachloroethane undergo dehydrohalogenation to trichloroethene and tetrachloroethene, respectively, in live mice and under reducing condi tions in hepatic microsomes (Table 4). Reaction rates. Most experiments that are used to determine reaction rates in water are performed at elevated tem peratures, because reaction rates are too slow at environmental tempera tures. Rates measured at higher temper atures may then be extrapolated to lower temperatures, using the Arrhe nius equation (7): of halogenated aliphatic compounds in aqueous solutions are about 100 10 kJ/mol (8, 9). This translates to a 3.5fold decrease in reaction rate for each 10 C decrease in temperature. Many of the half-lives listed in Table 3 were extrapolated in this manner from stud ies conducted at higher temperatures. Rates are often determined by use of aqueous solutions that contain high concentrations of organic solvents (e.g., 10% dioxane) (8). However, dif ferent investigators often report differ ent rates, mechanisms, and products for the same compound (Table 3). In some cases, substitution may prevail at one temperature and dehydrohalogena tion at another. These difficulties are at least partially responsible for the differ ences in reported rates (Table 3). Therefore, caution is required in using extrapolated rates, and more research is needed. With the caution noted above, re- ^2 _ a T* -T, t r ( -i?fr-) in which k is a rate constant, Ea is the activation energy (kJ/mol), R is die uni versal gas constant (8,314 J/mol-K), and T is the temperature (K), Activa tion energies for abiotic transformation FIGURE 4 Estimated relative half-reaction reduction potentials* WT v-t&S ------- Hydrogenotysis 3.0 ------ Dihalo-elimination 0.66 \ Z0 H\ /Cl TECA " 10-51 \n.56 TECA 0.94 Standard state: aqueous solution, pH 7, infinite dilution, 25 C, chloride activity 0.001 tHXE 1,1-dfchlotiethane M12-TECA m l,1.i^4etracMoroethane USA - trtohtoipethene tf-OCA - 1,1-dichtaroelhane HC - hexacMoroethane . PCA -oenteehtoroethane 1122-TtCA m 1,t,2,24etncMeroeihane 7/2-7CA t.f^tticNoreathene 72-OCA 1.2rchloroethnne . C*-**ioethane ,. A-ethane 1 :" : PCS - pentachtoroethene ' ICC - tetrechtoreethene ", cOC - otodtehtarealhene tCDE m eanedtohtoroelhene 7OCF - 1,1-cHehtoroethene VC - vinyl chloride -ethylene, ,t ,<3 1.0 0.51 0.0 W' A h volt*, tor dechlorination ol chlorinated alkanes and afkenee. Relative potentiate are illustrated by the vertical position tne compounds Bos deacribae Die calculations ot reduction potentials. Scale ot reduction potential with respect to ethane. 7!B Environ. Sci. Technol., Vbl. 21, No. 8, 1987 SL 037182 t i 'i ported reaction rates from Table 3 are plotted as a function of the number of halogen substituents per carbon atom in Figure 2. With the possible exception of tetrachloromethane, compounds that are susceptible to substitution reactions, such as monohaloalkanes or 1,3-dihaloalkanes, and those that are susceptible to dehydrohalogenation, such as polyhaloalkanes, behave similarly in water and mammalian systems. Increased halogenation tends to decrease substitu tion reaction rates and increase dehy drohalogenation rates. Consequently, most highly halogenated aliphatic com pounds, with the exception of C| com pounds, mainly undergo dehydrohalo genation (Figure 2), although some undergo both reactions (14). Rates of dehydrohalogenation and substitution in mammalian systems generally have not been reported and are no doubt quite complex because of the many po tential facilitating enzymes present. Hence, comparison of these rates with rates in water cannot be made. Oxidations and reductions Unlike substitutions and dehydrohalogenations, oxidations and reductions require external electron acceptors and donors, respectively (Figure 1, III and IV). Generally, organic compounds act ing as electron donors undergo oxida tion reactions. However, because of the electronegative character of halogen substituents on aliphatic compounds, polyhalogenated aliphatic compounds often behave as electron acceptors or oxidants and are reduced in the proc ess. Thus, halogenated aliphatic com pounds may be either oxidized or re duced, depending on their structure and environmental conditions. In most re actions described to date, the electron acceptors and donors used to oxidize and reduce halogenated aliphatic com pounds are derived from biological sys tems. Oxidations. Biologically mediated oxidation of organic compounds has been studied extensively. Organic com pounds generally represent reduced forms of carbon, and, as such, oxida tion is energetically favorable. In con trast, halogenated aliphatic compounds are relatively oxidized by the presence of halogen substituents: the more halo gen substituents, the more oxidized the compound, and the more susceptible it is to reduction. Thus, with increased halogenation, reduction becomes more likely than does oxidation. Most oxidations observed in mam malian systems (Table 5) (23-26, 3143) involve monooxygenase that con tains cytochrome P450. Cytochrome P450 is a heme-containing protein (with iron-porphyrin active sites) that can mediate both oxidation and reduc- Sl* 037183 Calculations for reduction potentials The procedure for estimating half-reaction reduction potentials contained in Figures 4-6 is as follows. Three steps were involved in the calculations for T = 25 C (298 K). The first was estimation of the aqueous-phase free energy of formation for the compound, the second was computation of free energy changes for a given half-reaction, and the third was calculation of reduction potentials using the Nernst relationship. 1. Aqueous-phase free energy of formation values were estimated from published gas-phase free energy of formation [A G (g)] data (52) and pub lished (71) and estimated (68) values of Henry's constant (H), which related activity in the gas phase to activity in the aqueous phase at equilibrium. A Gt (aq) * A Gt (g) + RTInH 2. Free energy changes for a given half-reaction were computed to be consistent with general principles and with Figure 5 based on the equation ^RX + r&H+ + e" -- 'teRH + ifeX" as follows: A G (aq) = G( (aq) for products - Ga( (aq) for reactants A G (aq) = standard free energy change for half-reaction, and values thus calculated were adjusted to the desired reference state ([H+] - 10~7, [Cl-l - 10-3, [Br] - 10-% using: A G0' = AG + ifeRTln((X-]/[H+J) 3. Reduction potentials were obtained from the Nernst relationship: E' = A G'/nF or pE0' - nFE'/2.3RT E0' - reduction potential (volts) R - universal gas constant (8.314 J/K-mol) n - electron equivalents transferred (52) F = Faraday's constant (96,487 J/volt-equivalent) T - temperature (K) TABLE 4 Substitution and dehydrohalogenation reactions of halogenated aliphatics in mammalian systems Compound Methanes Dichloromethane Tetrachloromethane Dibromomethane Ethanes 1,1,2-Trichloroethane 1,1,2,2-Tetrachloroethane 1,1,1,2-Tetrachloroethane Pentachloroethane Ethenes Chloroethene 1,1-Dichloroethene Product(s) Carbon monoxide Carbon dioxide Carbon monoxide Dichloroethanol Triehloroethanol Trichloroethene Triehloroethanol Tetrachloroethene Glutathionedependent sulfurcontaining organic compound Glutathionedependent sulfurcontaining organic compound Systems* hm/N h/GO hm/NO M M M M M PIG RIG Reference (22) (23) (22) (24) (25) (25) (26) (27) (28) (29) Slashes separate conditions for an individual experiment, hm hepatic (rat liver) microsomes; FI - live rats; M live mice; N - NADH dependence; Q glutathione dependence; 0 - presence of oxygen. Environ. Sci. Technol., Vbl. 21, No. 8, 1987 727 tion reactions. Cytochrome P450-containing monooxygenase mediates oxi dation of halogenated aliphatic compounds by three general mecha nisms: by incorporation of oxygen in the carbon-hydrogen bond (a-hydroxylation) (30, 44) (Figure 1, Ilia); by oxidation of a halogen substituent (44, 45) (Figure 1, mb); and by oxidation of a carbon-carbon double bond via epoxidation (30, 44, 46-48) (Figure 1, Hie). The first mechanism leads to a sub stitution product, an alcohol. However, mechanistic studies have shown that atomic oxygen, derived from molecular oxygen, is inserted into the carbon-hy drogen bond (22, 33). Subsequently, for halogenated alcohols, the hydrogen of the hydroxy group and a halogen substituent leave as the hydrogen ion (H+) and the halide ion (X-), resulting in an aldehyde. Halogenated formalde hyde can be chemically transformed into carbon monoxide. The second mechanism, halogen oxi dation, proceeds via an unstable halosyl intermediate (CH2X+=0"). Presum ably, this species hydrolyzes rapidly to produce an alcohol and hypochlorite ion (49). Because an alcohol is pro duced, the overall reaction resembles a substitution reaction. No net oxidation of the organic compound occurs (30). Also, because halosoaliphatic com pounds have not yet been detected, this mechanism might not actually occur with halogenated aliphatic compounds. a-Hydroxylation of halogenated aliphatic compounds would result in the formation of acids and alcohols as de scribed above. In contrast, /3-elimina tion of a halosyl intermediate could result in an alkene. Therefore, the ob servation of alkene formation from halogenated propanes was reported by Tachizawa et al. to be indicative of a halosyl mechanism (45). They docu mented propene formation and noted that brominated alkanes undergo trans formation to alkenes more readily than do chlorinated alkanes. In mammalian systems, dihalogenated aliphatic compounds are susceptible to oxidation by either of the above mechanisms. For example, transforma tion of dihalomethanes to carbon mon oxide under aerobic conditions has been reported (Table 4) (22). Overall, this is a substitution reaction, but it in volves an oxidation step. Oxidation proceeds sequentially from halogenated alcohol to halogenated aldehyde before final reduction to carbon monoxide. Yllner observed that mice transformed polychlorinated ethanes to chlorinated aliphatic acids (24-27, 34). The following characteristics of the third mechanism, epoxidation or the oxidation of a carbon-carbon double bond, are well documented for mam malian systems. Epoxidation is the first step in alkene oxidation that is pro- 728 Environ. Sci. Technol., Vol. 21, No. 8, 1987 SL 037184 nslan . nable >ve las ill, inon ed ire le. ed ed he he ile nrst o- moted by cytochrome P450-containing monooxygenase. The oxygen that is in corporated into halogenated alkenes is derived from molecular oxygen (22, 33, 48). The epoxide is normally short lived and might undergo one of several different reactions. Halogenated alde hydes or acyl chlorides are common in termediates and are often subsequently transformed into acids (oxidation), al cohols (reduction), or hydrolyzed to acids or carbon monoxide (via acyl chlorides) (30, 44, 48). Epoxide chemistry in aqueous solu tions partially explains the fate of epox ides in mammals. In aqueous solution, for example, trichloroethene epoxide decomposes to formyl chloride and dichloromethanol. Subsequently, for myl chloride eliminates hydrogen chlo ride (HC1) to form carbon monoxide, and dichloromethanol hydrolyzes to form formate (49). These two products predominate at pH > 9. At lower pH, other products dominate: at pH 7, dichloroacetic acid is produced; and at pH 2, glyoxylic acid is produced. All the products shown in Figure 3 have been observed in mammalian sys tems. In addition, transformation of chlorinated ethenes proceeds via chlo rine migration to produce chloral (2,2,2-trichloroacetaldehyde) (JO) (Ta ble 5). The production of compounds such as chloral in mammalian systems, but not in aqueous solutions, may be the result of the interaction between the chlorinated epoxide and the cyto chrome P450-based monooxygenase (47). It also may be the result of the formation of an intermediate other than the epoxide (incorporation of oxygen into the halogenated alkene) and the concomitant formation of a cationic or radical intermediate (JO). Ozonation of halogenated alkenes could serve as a chemical model for the epoxidation of these compounds in mammalian systems. The reaction is largely an electrophilic attack at the double bond by ozone. Ozonation rates of chlorinated ethenes decrease as the number of chlorine substituents in creases (51). In addition, the inductive effect of additional chlorine substi tuents is more important in the reduc tion of reaction rates than the increased steric hindrance of these substituents. Ionization potential (IP) is an adequate predictor of relative ozonation rates (Table 6) (52). Mesomeric delocaliza tion of positive charge by chlorine sub stituents decreases both IP and ozona tion rates. Therefore, IP might be a reasonable predictor of relative rates of epoxidations of halogenated alkenes in biological systems. Another substance that can be in volved in the oxidation of halogenated aliphatic compounds by biological sys- TABLE 5 Oxidati n of halogenated aliphatic compounds in mammalian systems CompoundProducts)System*Reference Methanes Dichloromethane Trichloromethane Formaldehyde Carbon dioxide Carbon dioxide Carbon dioxide via phosgene hm/N/O hm/O h/G/O hm/P/O (37) (32) (23) (33) Ethanes t ,2-Dichloroethane 1.1.2-Trichloroethane 1.1.2.2-Tetrachloroethane 1.1.1.2-Tetrachloroethane Pentachloroethane 1.2- Dibromoethane Chloroacetic acid, carbon dioxide Dichloroacetic acid, chloroacetic acid, carbon dioxide Trichloroacetic acid Dichloroacetic acid, carbon dioxide Trichloroacetic acid Trichloroacetic acid Bromoacetaldehyde M M M M M M M R/G (34) (24) (25) (25, 35) (25) (26) (27) (36) Ethenes Chloroethene 1.1-Dichloroethene 1.2-Dichloroethene Trichloroethene Tetrachloroethene Bromoethene Chloroacetic acid 2-Chloroacetaldehyde, glycolaldehyde, 2-chloroethanol Chloroacetic acid Dichloroacetic acid Dichloroacetic acid Trichloroethanol, Trichloroacetic acid Carbon monoxide Carbon dioxide Glyoxylic acid Trichloroacetic acid Bromoacetaldehyde hm/O hm/P/N hm/O hm/O R. M hm/O hm/P, R hm/P hm/P hm/P hm/O, hm/P/N hm/P/N (37) (38) (37) (37) (39, 40) (25, 39,41) (41) (41) (41) (37, 42) (43) Commas separate different experimental systems that result in similar products. Slashes separate conditions for an individual experiment, hm = hepatic (rat liver) microsomes; R = live rats: M - live mice. N - NADH dependence; P = cytochrome P450 dependence; G > glutathione dependence; 0 - presence of oxygen. TABLE 6 Ionization potentials (IP) and ozonation rates (k) for some halogenated aliphatic compounds Compound Tetrachloroethene Trichloroethene 1,2-Dichloroethene Chloroethene Ethene Reference 52. Reference 51. No. of chlorines 4 3 2 1 0 IP* 9.32 9.45 9.6 9.99 10.51 k" (L/mol s) 1.0 3.6 591 1180 >20,000 terns is glutathione. Glutathione-medi ated oxidations generally occur in the soluble or cytosolic fraction of cells (30). Oxidation is initiated by nucleo philic attack of glutathione on the elec trophilic carbon (generally one bound to a halogen) (31). Dihalomethanes are oxidized to formaldehyde and formic acid following conjugation with cytoso lic glutathione (Table 5), One final category of oxidation in volving halogenated compounds is the phenomenon of biohalogenation. This process is widespread in nature, occur ring in certain species of bacteria, fungi, algae, higher plants, and ani mals. It is brought about by the activity of haloperoxidases in the presence of hydrogen peroxide (Figure 1, Illd). The chemistry of these transformations is similar to that of hypohalous acids; unsaturated substrates such as ethene may be converted to halohydrins or di halides by this process (53). Reductions. Certain transition metals and transition metal complexes Si. 037185 Environ. Sci. Technol.. Vol. 21, No. 8, 1987 729 reduce halogenated aliphatic com pounds (Table 7) {54-61). As a result, these metals and metal complexes are themselves oxidized. Because transi tion metal complexes are frequently lo cated at the active sites of the macromolecules that are used for electron transfer in living organisms, reactions between metal complexes and halo genated aliphatic compounds are useful models that simulate transformations in living organisms. Transition metals may also play a role in the abiotic re duction of certain halogenated aliphatic compounds in groundwater. Initially, most reductions by transi tion metal complexes involve the trans fer of a single electron and the forma tion of an alkyl radical. This occurs with a variety of transition metals, in cluding nickel (67), iron (62), chro mium {63) and cobalt {64), although some two-electron reductions also oc cur with cobalt {54, 65). Formation of an alkyl radical upon removal of a halo gen substituent is the first and, in most cases, the rate-limiting step in the twostep reduction of halogenated aliphatic compounds: A final possibility is dimerization of the radicals: 2 [ 7C~C7 ] " \I I / --C--C--C--C-- (coupling) /I I \ Reduction of polyhalogenated al kanes can result in the production of both alkanes and alkenes, as illustrated in Figure 1. Formation of the alkyl radical ap pears to involve either the transfer of an electron to the reduced transition metal, with a standard reduction potential as the driving force, or the transfer of a halogen atom, a process governed by \ / reduced slow --C--C-- + transition -- / \ X metal \/ oxidized --C--C-- -I- transition + X' / metal The alkyl radical that results from the above step can undergo several reac tions. The simplest of these involves scavenging a hydrogen atom from the immediate surrounding matrix--possi bly from the complex itself (55): \/ --C--C-- + H+ + e/ -- \/ --C--C-- (hydrogenolysis) / \H Another involves the loss of a second halogen substituent from a carbon atom adjacent to the radical carbon to form an alkene (66). The aikene that results from this last step is more stable and has two fewer halogen substituents, thus decreasing the likelihood of fur ther reduction: X\ / \/ --c--c-- -- c=c + / /\ H+ (dihalo-elimination) 730 Environ. Sci. Technol., Vol 21. No. 8, 1987 SL 037186 carbon-halogen bond energies. Re gardless of the exact mechanism, how ever, relative rates for reduction of halogenated aliphatic compounds should follow certain patterns (<57). In general, smaller carbon-halogen bond energies are conducive to faster oneelectron (two-step) reductions. An ex ample is the reduction of halogenated aliphatics by iron(II) porphyrins. Be cause the rate-limiting step in the twostep reduction is probably the forma tion of the carbon radical (62), the heat of formation of the alkyl radicals is one measure of the carbon-halogen bond strength and should be inversely pro portional to the rate of reduction. This is shown in Table 8 for pseudofirst-order reduction rates derived from data from Klecka and Gonsior (55). As another measure, Eberson (67) sug gested that although rates might differ with different reductants (iron porphy rin and the like), for a given reductant, the relative rates of reduction of halo genated aliphatic compounds should correlate with their relative standard re duction potentials (assuming no change in mechanism). Relative standard re duction potentials can be estimated for many halogenated aliphatic compounds using the values as shown in Figures 4 and 5. The calculation method is de scribed in the sidebar onp. 727 (68-71). Highly halogenated aliphatic com pounds have higher relative standard potentials than do their less halogenated counterparts, as indicated by the verti cal positioning of compounds in Fig ures 4 and 5. Thus more energy is re leased upon reduction of the more highly halogenated compounds. Another type of comparison between reduction potentials is illustrated in Fig ure 6. Here, biologically relevant re ductants, as well as oxidants, are in cluded. Note that hexachloroethane (HCE) is a stronger electron acceptor than is oxygen, and several halogenated compounds, such as tetrachloromethane, tetrachloroethene, and trichloromethane, are stronger acceptors than nitrate. This suggests environmental conditions under which their reduction is likely. Also, as illustrated in the lower portion of Figure 6, several bio logically active donors, and even fer rous ion, have lower reduction poten tials than do most of the halogenated aliphatic compounds, and they could be involved in halogen removal by reduc tions. Thermodynamic considerations can help indicate which halogenated aliphatic compounds might be coupled by reduction with the oxidation of indi vidual electron donors typically found in biological systems. Many of the reductions indicated above occur in mammalian systems (30, 44). All three kinds of reduction-- TABLE 7 Reduction of halogenat d aliphatic compounds by transition metal c mpl xes Compound Methanes Products Rsductant Reference Chloromethane Dichloromethane Trichloromethane Tetrachloromethane Bromomethane Dibromomethane Tribromomethane Ethanes Chloroethane 1,1-Dichloroethane 1,1,1 -Trichloraethane Hexachloroethane Bromoethane 1,1-Dibromoethane 1,2-Dibromoethane Propanes 1-Chloropropane 1,1-Dichloropropane 1-Bromopropane 1,2-Dibromo-3chloropropane Alkylated co-complex Methane Cl-alkylated B12 Methane Dichloromethane Dichloromethane Cl-alkylated B12 Methane Chloroform Cl-alkylated B,2 Methane Alkylated co-complex Methane, ethene Brj-alkylated B12 Alkylated co-complex Ethane, ethanol Ethane, ethanol, ethene, chloroethene 1,1-Dichloroethane 1,1-Dichloroethane Tetrachloroethene Tetrachloroethene Ethane Ethane, ethanol Ethene Ethene Alkylated co-complex Propane, propanol, propene Alkylated co-complex Propene, allyl chloride Co(l) chelates Cr(ll)S04 B,rCo(lll) (methylcobalamine) Cr(ll)SO* Fe(ll)P Fe(ll)P B,j-Co(lll) Cr(ll)SO* Fe(ll)P B,rCo(lll) Cr(ll)S04 Co(l)-complex Fe(l!)P BirCo(lll) Co(l)-complex Cr(ll)SO,, Cr(ll)SO* Fe(ll) Fe(ll)P Fe(ll)P Cr(ll)SO* Ni(l) Cr(ll)S04 Fe(ll) Fe(ll)P Co(l)-complex Cr(ll)S04 Co(l)-compiex Cr(ll)S04 (54) (55) (56) (55) (57) (58) (56) (55) (57, 58) (56) (55) (54) (57) (56) (54) (55) (55) (57) (58) (57, 59) (60) (60) (55) (57) (58) (54) (55) (54) (60) Note: The homolytic cleavage of cobalt-carbon bonds requires 15-30 kcal/mol and can occur at relatively low temperatures. TABLE 8 Pseudo-first-order reduction rates and heat of formation f car bon radicals for some chlorinated aliphatic compounds Compound Dichloropropane Trichloromethane 1,1,1-Trichloroethane Tetrachloromethane Reference 58. "References 52 and 67. k (d-') <0.001 0.00165 0.058 115 Heat of radical formation'' (kj/mol) 117.6 100.8 92.0 78.2 hydrogenolysis, in which a hydrogen atom replaces a halogen substituent (Figure l, IVa); dihalo-elimination, in which two halogens are removed from adjacent carbons (Figure 1, IVb); and coupling (Figure 1, IVc)--have been observed in mammalian systems (Table 9) (27, 72, 81). Hexachloroethane, pentachloroethane, and tetrachloromethane are reductively dehalogenated (by hydrogenolyis) to pentachloroethane, tetrachloroethane, and trichloromethane, respectively. Also, hexachloroe thane and pentachloroethane are re duced (by dihalo-elimination) to the alkenes, tetrachloroethene and trichloroethene. Most research in mammalian systems implicates cytochrome P450 as the ac tive reducing agent. Reduction results when halogenated aliphatics outcompete with oxygen (when present) for the electrons supplied by nicotinamide ade nine dinucleotide phosphate (NADPH) (44). The mechanism involves forma tion of either a carbon radical or a di- St 037187 Environ. Sci. Technol., Vol. 21, No. 8, 1987 731 halocarbene complex with cytochrome P450. Evidence for a carbon radical follows from two general observations: the ob served production of hexachloroethane from carbon tetrachloride (Figure 1, IVc and Table 9), apparently as a result of trichloromethyl radical dimerization, and indications of radical formation in chemical studies that involve iron(II) porphyrin. Formation of a carbon radi cal depends on the strength of the car bon-halogen bond (and standard reduc tion potential). This bond is weaker for brominated than for chlorinated com pounds. Thus brominated aliphatics should be more susceptible to reduction by cytochrome P450 than are chlorin ated compounds. The number of halo gen substituents also is an important factor in determining the feasibility of radical formation. Evidence for the production of a dihalogenated carbene intermediate comes from observations of the reduc tion of polyhalogenated methanes. A dihalogenated carbene complex would be expected to hydrolyze to carbon monoxide, and carbon monoxide has been produced from trichloromethane and tetrachloromethane (Table 9). However, compounds having less than three halogen substituents apparently do not undergo reduction to carbon monoxide by cytochrome P450 ( 73). Halogenated aliphatic compounds with few halogen substituents generally are not reduced or are reduced rela tively slowly in mammalian systems. For example, 1,2-dichloroethane is not significantly reduced under anaerobic conditions by cytochrome P450 (82). However, both 1,2-dichloroethane and 1,2-dibromoethane are transformed to ethene in mammalian systems (Table 9). Livesey and Anders indicated that this reduction requires the presence of reduced glutathione in hepatic microsomes (80). They also postulated the following two-step process, involving nucleophilic attack by glutathione (GSH) and a thiol sulfur (RSH): monas putida closely resembles micro somal P450 (84), and whole cells can use it to mediate the reduction of car bon tetrachloride and bromotrichloromethane to chloroform (85, 86). Glu tathione also is widely distributed among gram-negative bacteria (87), and it is used by Hyphomicrobium sp. in the metabolism of dichloromethane (89). In general, however, microorga- FIGURE 7 Relative rates of reduction and oxidation* VC 12-OCE 11-DCE ICE PCE Increasing extent of halogenation - 4Aa a function of the extent of halogenation. FIGURE. 8 Pathways for the transformation of TCA under methanogenic conditions* Ethenes CHjCCIj GSH + Cl--CH2--CH2--Cl -- GS--CH2--CH2--Cl + HC1 RSH + GS--CH2--CH2--Cl -- GSSR + HC1 + CH2=CH2 Microbially mediated reactions Some of the dehalogenating agents found in mammalian systems are mani fest in microbial populations as well. Many microorganisms contain cyto chrome P450 that is similar to mamma lian P450 even in regard to halogenated compound transformation (83). For ex ample, cytochrome P450 from Pseudo 731 Environ. Sci. Technol., Vol. 21, No. 8. 1987 A CH3CHjOH Ethanol B CO, CO, "A indicates abiotic reactions; B indicates biotic reactions. Source: Reference 97 SL 037188 CHjCOOH \ Acetic acid sn: --- -is COj nisms are capable of more diverse bio chemical reactions than are mammalian systems. Microorganisms obtain energy from a wide variety of electron donors and acceptors under different redox condi tions (Table 10). Aerobic metabolism dominates where sufficient oxygen is present. Where oxygen is depleted, however, other electron acceptors, such as nitrate, sulfate, and carbon dioxide, are used. Differences in available elec tron acceptors and in the resulting re dox conditions also appear to affect the potential and pathways for transforma tion of different halogenated aliphatic compounds. Another factor affecting the potential for transformation is the reactivity of enzymes and coenzymes associated with different microorga nisms. Regardless of the metabolic differ ences between microorganisms and mammalian systems, the types of reac tions mediated by bacteria and mamma lian systems are similar. Like mamma lian systems, bacteria mediate substitution reactions with monohalogenated or dihalogenated aliphatic com pounds (Table 11) (18, 88-111). For example, aerobic bacteria isolated from a contaminated soil transform 1,2-dichloroethane to chloroethanol, which is subsequently mineralized to carbon di oxide (100). However, the pathways for the observed mineralization of many halogenated organics are unknown. A compound such as tetrachloromethane, which is mineralized to carbon dioxide (Table 11), may undergo reduction to dichloromethane and subsequent oxida tion to carbon dioxide, as previously described for mammalian systems. In addition, different organisms may par ticipate in different steps of the overall mineralization of a compound. Bacteria also oxidize chlorinated alkenes, presumably via epoxidation (Ta ble 11). Because many halogenated aliphatic compounds are eventually mineralized to carbon dioxide, other oxidation pathways may be involved. Some net oxidations, such as the miner alization of dichloromethane to carbon dioxide, could proceed either by oxida tion via phosgene (COCL) or by substi tution via alcohols. In general, oxida tion pathways are not well known. Ionization potentials, which are used to predict ozonation rates for halogenated alkenes, might also be used to model oxidations in microbial systems. How ever, the number of halogen substi tuents may be a more reasonable pre dictor of oxidations--the more halogen substituents, the less susceptible the compound is to oxidation. This is con sistent with Figure 6. Polychlorinated methanes, ethanes, and ethenes are reduced by microbial TABLE 9 Reducti ns of halogenated aliphatic comp unds in mammalian systems Compound Methanes Trichloromethane Tetrachloromethane Bromotrichloromethane Tribromoethane Tetrabromomethane Ethanes 1,2-Dichloroethane Pentachloroethane Hexachloroethane 1,2-Oibromoethane Product(s) Carbon monoxide Dichloromethane Carbon monoxide Trichloromethane Hexachloroethane Trichloromethane Hexachloroethane Bromodichloromethane Carbon monoxide Carbon monoxide Carbon monoxide Ethene Trichloroethanol Trichloroethene 1,1,2,2-Tetrachloroethane Pentachloroethane, Tetrachloroethane Ethene System* hm/P/r hm/P/N R, hm/P r R, B R hm/P/r hm/P/r hm/P/r hm/G/O M M, hm/r hm/r, hm P/N/r hm/r, S, hm/P/N hm/G/O Reference (72, 73) (74) (75) (73, 75-77) (74, 78, 79) (77. 78) (77) (73) (73) (73) (80) (27) (27, 79) (79) (79, 81) (80) Commas separate different experimental systems that result in similar products. Slashes separate information regarding each experiment, hm - hepatic (rat liver) microsomes; R live rats: B live rabbits; M - live mice; S - live sheep; N * NADH dependence; P cytochrome P450 dependence; G - glutathione dependence; O - presence of oxygen; r reduced or anaerobic conditions reported. TABLE 10 Electron acceptors in microbial processes Environment Aerobic Anaerobic Electron acceptor Oj no3scvC02 Process Aerobic metabolism Denitrification Sulfate reduction Methanogenesis Order of preference 1 2 3 4 consortia or mixed cultures (Table 11). Generally, reduction entails the replace ment of halogen substituents by hydro gen (hydrogenolysis) (vertical arrows in Figure 4). However, reduction can also involve the loss of two halogens (dihalo-elimination) (wide arrows in Figure 4), as in the case of hexachloroethane transformation to pentachloroethane and of 1,2-dibromoethane transformation to ethylene (Table 11). These two dihalo-eliminations can oc cur under aerobic conditions, whereas hydrogenolysis has only been observed under anaerobic conditions. Thus re ductions proceed by the same general pathways, regardless of whether transi tion metals, mammalian systems, or microorganisms provide the reductant. Methanogens, which grow under some of the most severely reducing conditions (Table 10), do not have the cytochrome systems that are present in aerobic organisms. However, methano gens do have nickel-containing en zymes or cofactors such as F430 (112). Reduced nickel complexes can reduce halogenated aliphatic compounds (Ta ble 7). Indeed, several of the microbial reductants have much greater reducing potentials than the mammalian enzymes do (see Table 12) (54, 67, 84,113), and this may lead to a broader reductive dehalogenation ability. For comparison. Figure 7 illustrates the half-reaction reduction potentials for halogenated aliphatic compounds that function as electron acceptors. Pos sible electron donors are shown as well. Common electron acceptors, such as oxygen, nitrate, sulfate, and carbon dioxide, also are plotted in Figure 6. Here, the various couples are arranged according to their standard half-reac tion reduction potentials at pH 7. Each arrow points in the direction for which the stated transformation is thermo dynamically possible. In the direction opposite to the arrow, the reverse reac tion is thermodynamically favorable. Thus a direct, graphic reading can be obtained for the free-energy change as sociated with the coupling of two half reactions. For example, combining the SL 037189 Environ. Sci. Technol., Vol. 21, No. 8, 1987 733 reduction of hexachloroethane to pentachloroethane with the oxidation of Fefll) to Fe(OH)3 yields a favorable ffee-energy change of about 140 kJ per mole of electrons transferred. The re duction of hexachloroethane to pentachloroethane and 1,2-dibromoethane to ethene have higher reduction poten tials than those associated with oxygen reduction to water (Figure 6), and these dihalo-eliminations are energetically fa vored in aerobic systems. This is con sistent with the observed transforma tions of hexachloroethane and 1,2-dibromoethane under aerobic con ditions. In addition, the low potential for re duction of carbon dioxide to methane, compared with the reduction potential for hydrogenolysis of polyhalogenated aliphatic compounds (Figure 6) is con sistent with their reported reductive transformations under methanogenic conditions (Table 11). For example, the sequential reduction of tetrachloroethene to trichloroethene, to dichloroethene, and finally to chloroethene (vinyl chloride) occurs under methano genic conditions (106), as does the se quential reduction of tetrachloroethane to 1,1-dichloroethane, and finally to chloroethane (16, 96,102). These path ways follow the hydrogenolysis se quences illustrated by the narrow ar rows in Figure 4 for these compounds. Microbial and mammalian systems follow the same general trends with re gard to the oxidation and reduction of halogenated aliphatic compounds: The more halogenated the aliphatic com pound, the faster the relative rate of re duction; the less halogenated the com pound, the faster the rate of oxidation. Substitution reactions follow the same general trends. Many useful generaliza tions can be drawn from oxidation-re duction potentials of halogenated aliphatic compounds. Dehydrohalogenation has not yet been reported for microbial systems. Environmental applications Application of these general princi ples to environmental problems is com plex . For example. Figure 8 illustrates the different pathways possible for the transformation of 1,1,1-trichloroethane (97), which can undergo two abiotic transformations as well as reductive dehalogenation by anaerobic microorga nisms. Abiotically, the half-life for 1,1,1-trichloroethane at 25 C is about two years (Table 3), and biologically it could be much less. The abiotic proc esses are dehydrohalogenation (14, 15, 99), as well as hydrolysis (12, 13, 14). Acetic acid, the product of hydrolysis, is fairly inert chemically, but it can be mineralized rapidly by microorga nisms. Dehydrohalogenation occurs at TABLE 11 Biotransf rmations of halogenat d aliphatic comp unds by microorganisms CompoundProduct(a)System*Reference Methanes Chloromethane Dichloromethane Trichloromethane Tetrachloromethane Bromomethane Ethanes 1.1-Dichloroethane 1.2-Dichloroethane 1,1,1 -Trichloroethane Formaldehyde Carbon dioxide Carbon dioxide Carbon dioxide Carbon dioxide Formaldehyde Carbon dioxide Carbon dioxide Dichloromethane Carbon dioxide Chloroform Carbon dioxide Chloroform Formaldehyde Chloroethane Carbon dioxide Chloroethanol Carbon dioxide 1,1-Dichloroethane 1.1.2.2-Tetrachloroethane Hexachloroethane Bromoethane 1.2- Dibromoethane Ethenes Chloroethene Dichloroethene Trichloroethene Tetrachloroethene Not identified 1,1,2-Trichloroethane Tetrachloroethane Ethene Ethene Carbon dioxide Carbon dioxide Carbon dioxide Chloroethene Chloroethene Carbon dioxide Dichloroethene Dichloroethene Dichloroethene Carbon dioxide Carbon dioxide Trichloroethene Propanes 1-Chloropropane -- 1.2-Dichloropropane -- 1.2-Dibromo-3-chloropropane Propanol E O/M O/P O/M O/P E A/M/m O/S A/M A/M/m A/M/n A/S E/mo A/M/m O/P/p O/P/x A/M/m A/S A/M/m A/M A/M/m A/M/m A/M O/M/s O/P/x O/S A/M/m/r O/S A/M/m O/P/mb A/M/m A/S, A/M A/M/m A/S A/M O/P O/S A/M/m A/M O/P/x O/P/x O/S (88) (89) (90) (97) (92) (93) (94) (95) (96) (94) (97) (98) (89) (95) (96) (97) (94) (98) (99) (702) (94) (94) (702) (103) (101) (104) (78) (705) (706) (709) (99, 106) (702, 707) (706) (98, 108) (102) (110) (HI) (94, 106) (102) (101) (101) (104) 'Comma separates different experimental systems that result in similar products. Slashes separate information regarding each experiment. O aerobic: A - anaerobic, which often Ib methanogenic (m). but often is not explicitly stated; M = mixed culture; P pure culture; S soil or aquifer used as biological seed; E enzyme derived from microorganism; m methanogenic culture; x Xanthobacter; mo - monooxygenase; mb Mycobacterium: p Pseudomonas. about one-fifth the rate of hydrolysis at 40 C (19). The product of dehydroha logenation, 1,1-dichloroethene, can be transformed further by reductive dehalogenation to chloroethene (vinyl chlo ride) under methanogenic conditions (Table 11, Figure 8). Under the biologi cal transformation route, 1,1,1-trichlo roethane is reduced to 1,1-dichloroe thane and then transformed abiotically by hydrolysis to ethanol (Table 3), which can in turn be rapidly minera lized by microorganisms. The products and complex pathways shown in Figure 7 are consistent with field observations of products consistently found present in groundwaters contaminated with 1,1,1-trichloroethane (114). Other ha logenated aliphatic compounds are also likely to undergo complex transforma- 734 Environ. Sci, Technol,, Vol, 21, No. 8,1987 SL 037190 lions under natural environmental con ditions. Conclusions The fate of halogenated aliphatic compounds in the environment is de pendent on their particular chemical properties and potential chemical and biological transformations. The most likely transformations to occur under given environmental conditions are controlled mainly by the number and type of halogen substituents. Increased halogenation or substitution of bromine for chlorine substituents increases the electrophilicity and oxidation state of the compound, making it more suscep tible to dehydrohalogenation and reduc tion and less susceptible to substitution and oxidation (Figure 7). Oxidations and reductions are more common reac tions in mammalian and microbiologi cal systems, where they are mediated by enzymes or coenzymes. For oxida tions, initial products are generally al cohols or epoxides. For reductions, products are generally less halogenated than were their precursors. A variety of transition metal complexes, including iron porphyrins, are potential media tors of these reactions. Acknowledgments This material is based on work supported in part by the National Science Foundation (Grant ECE 8519243) and in part by the Environmental Protection Agency (Grant EPA CR-81220). It has not been subject to EPA's peer and administrative review and therefore should not be assumed to reflect the views or bear the endorsement of that agency. This article has been reviewed for suit ability as an ES&T feature by J. M. Wood, University of Minnesota, Navarre, Minn. 55392; and by B. Rittmann, University of Illinois at Urbana-Champaign, Urbana, 111. 61801. 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Pre sc TABLE 12 Standard potentials (E) of bi or reductants Reductants Vitamin B12 Co(l) tetraphenylporphin Ferrodoxin (reduced) h2 Cr(ll) NADH + H* Cytochrome P450 (unactivated) Glutathione (reduced) Cytochrome P450 (activated) Fe(ll) deuteroporphin IX Ubiquinone (reduced) Cytochrome c (+ 2) Fe(ll) HjO gically rel vant electron donors E (volta) -0.59 to -0.8 -0.56 -0.43 -0.42 -0.41 -0.32 -0.30 -0.23 -0.17 0.00 0.10 0.22 0.77 0.82 Reference (54) (67) (77) (84) (67) (64) (777) (84) (777) (67) (84) (84) (84) (84) sented at the Annual Meeting of the Ameri can Chemical Society, Division of Environ mental Chemistry, September 1983. (13) Britton, E. C,; Reed, W. R, Chem. Abstr. 1932, 26, 5578. (14) Haag, W. R.; Mill, T.; Richardson, A. Extended Abstract. Annual Meeting of the American Chemical Society, 1986. (15) Pearson, C. R.; McConnell, G. Proc. R. Soc. London Ser. B. 1975, 189. 305-32. (16) Vogel, T. M.; McCarty, P. L. J. Contam inant Hydrology 1987, 1, 299-308. 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Technol., Vtol. 21, No. 8, 1987 735 Light-Activated Pesticides he pressures on world agricultural pro Tductivity caused by pests is reaching a critical stage. At the same time, there is an increasing concern about pesticide use. Researchers are hard pressed to meet the future demand for safe, effective pesti cides. A promising area of pesticide re search is in the catalytic action of light on certain chemicals in biological systems. This new book explores the rapid exploitation of this mechanism over the past two decades and charts the possible courses this re search will take in the future. Primary con sideration has been given to insecticides, followed by herbicides and fungicides. This volume focuses on four main areas of re search and development: development of light as pen of the toxicological action of pesticides development of the various classes of lightactivated pesticides underlying mechanisms of light activation specific studies of applied areas of light activation Research programs are examined from such diverse areas as: synthetic dyes natural products chemical intermediates that lead to photodynami- tally active chlorophyll derivatives This work serves as a single source for any one interested in obtaining current knowl edge of light-activated pesticides. Special ists in the fields of photochemistry, photobiology, agriculture, chemical ecology, or plant pathology will find this book a val uable addition to the literature. Developed from a symposium sponsored by the Divi sion of Agrochemicals of the American Chemical Society James R. Hertz. Editor. Mississippi State University Kelsey R. Downum. Editor. Florida Institute of Technology ACS Symposium Series No. 339 355 pages (1987) Clothbound LC 87-1342 ISBN 0-8412-1026-8 US & Canada $69.95 Export $83.95 Order from: American Chemical Society Distnbution Office Dept, 56 1155 Sixteenth St. N.W Washington, DC 20036 or CALL TOLL FREE 800-227-5558 and use your credit card! (66) Wade, R. S.; Havlin. R.; Castro. C. E. J. Amer. Chem. Soc. 1969, 91, 7530. (67) Eberson, L, Ada Chem, Scan. Ser, B 1982, 36, 533-43. (68) Lyman, W, J.; Reehi. W. E, Rosenblatt, D. H. Handbook of Chemical Property Esti mation Methods; McGraw-Hill: New York, 1982. (69) Smith, B. D.: Srivastava. R. Thermody namic Data for Pure Compounds. Physical Sciences Data. Vol. 25 (Part B); Elsevier Press: New York, 1986; p. 109. (70) Horvath, A. L. Halogenated Hydrocar bons: Solubility-Miscibility with Water; Mar cel Dekker: New York, 1982, p. 478. (71) MacKay, D.; Shin, W. Y. J. Phys. Chem. Ref. Data 1981, 10. 1175-99. (72) Stevens, J. L.; Anders, M. W. Chem. Biol. 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Harrison, P., Ed.; Verlag-Chemie, Weinheimf Deerfield Beach, Fla., 1985, pp. 157-206 (85) Lam, T.; Vilker, V. L. Biotechnol. and Bioeng. 1987,29, 151-59. (86) Castro, C. E.; Wade. R. S.; Belser, N. O. Biochemistry, 1985, 24, 204-10. (87) Fahey, R. C. et al. J. Bacteriol. 1978, 133 (3), 1126-29. (88) Stirling, D. L: Dalton, H. FEMS Micro biol. Lett. 1979,5, 315-18. (89) Rtttmann. B. E.; McCarty, R L. Appl. Environ. Microbiol. 1980, 39, 1225-26. (90) Brunner, W.; Staub, D.; Leisinger, T. Appl. Environ. Microbiol. 1980, 40, 95058. (91) Stucki. G. et al. Arch. Microbiol. 1981, 130, 366-71. (92) Lapat-Polasko. L. T.; McCarty, P. L.; Zehnder, A.J.B. Appl. Environ. Microbiol. 1984, 47. 825-30. (93) Patel. R. N. etal. Appl. Environ. Micro biol. 1982, 44. 1130-37. (94) Bouwer, E. L; McCarty, P. L. Appl. En viron. Microhtol. 1983, 45. 1286-94. (95) Strand, S. E.; Shipper!, L. Appl. Envi ron. Microbiol. 1986, 52, 203-05. (96) Gossett, J. M. "Anaerobic Degradation of Cl and C2 Chlorinated Hydrocarbons"; final report ESL-TR-85-88; Air Force Engi neering and Services Center, Tyndall Air Force Base, Florida, 1985. (97) Bouwer. E. J.; McCarty, P L. Appl. En viron. Microbiol. 1983, 45, 1295-99. (98) Parsons. F.; Barno-Lage, G.; Rice. R. Environ. Toxicol. Chem. 198$, 4. 739-42. (99) Vogel, T. M.; McCarty, R L., submitted for publication in Environ. Sci. Technol. (100) Stucki, G.; Krebser, U.; Leisinger, T. Experiemia 1983,39. 1271-73. (101) Janssen. D. B. et al. Appl. Environ. Mi crobiol. 1985, 49, 673-77. (102) Hallen, R. T.; Pyne, J. W., Jr.; Molton. P. M. Extended Abstract. Annual Meeting of the American Chemical Society, 1986. (103) Criddle, C. S. et al. J. Contaminant Hy drology 1986, 1. 133-42. (104) Castro, C_ E.; Belser, N. O. Environ. Sci. Technol. 1968, 2, 779-83. (105) Pignatello, J. J. Appl. Environ, Micro biol. 1986, 51. 588-92. (106) Vogel, T. M.: McCarty, R L. Appl. En viron. Microbiol. 1985, 49, 1080-83. (107) Parsons, F.; Wood, R R.; DeMarco, J. J. Amer. Hitter Works Assoc. 1984, 76 (5), 56-59. (108) Kleopfer. R. D. et al. Environ. Sci. Technol. 1985, 19, 277-80. (109) Hartmans, S.; de Bont, J.A.M.; Tramper, J.; Luyben, K. Ch. A. M. Biotech nol. Lett. 1985, 7. 383-88. (110) Nelson, M.J.K. et al. Appl, Environ. Microbiol. 1986, 52, 383-84. (111) Wilson, J. T.; Wilson, B. H. Appl. Envi ron. Microbiol. 1985, 49, 242-43. (112) Schonheit, P.; Thauer, R. K. In Biotech nological Advances in Processing Municipal Wastes for Fuels and Chemicals. Antonopoulos, A.A.. Ed.; ANL/CNSV-TM-167, Argonne National Laboratory; Argonne, 1985,41-51. (113) Stryer. L. Biochemistry, 2nd ed,; W, H. Freeman; San Francisco, 1981. (114) Cline, P. V.; Viste, D.R. Waste Manage. Res. 1985, 3, 351-60. Perry L. McCarty is a professor of envi ronmental engineering and science in the civil engineering department of Stanford University. Since 1962 he has taught at Stanford and conducted research on bio logical processesfor water quality control. Timothy M. Vogel (l.) is an assistant pro fessor of civil and environmental engineer ing at Michigan State University. When this article was written, he was a doctoral candidate at Stanford University, Previ ously, he was a geochemical oceanogra pher with the U.S. Geological Survey. His current research focuses on biological and chemical transformations of pollutants in aquatic environments. Craig S. Criddle (r.) is a doctoral candi date in environmental engineering and sci ence at Stanford University. He holds a B.S. and an M.S. in civil and environmen tal engineering from Utah State University. His research interests are in biological processes for water quality control. 736 Environ. Sci. Technol , Vol. 21, No. 8. 1987 ,, SL 037192