Chlorophenol, dechlorination

Reductive DechIorina.tion. Such reduction of chlorinated aUphatic hydrocarbons, eg, lindane, has been known since the 1960s. More recentiy, the dechlorination of aromatic pesticides, eg, 2,4,5-T, or pesticide products, eg, chlorophenols, has also been documented (eq. 10) (20). These reactions are of particular interest because chlorinated compounds are generally persistent under aerobic conditions. 

Attention is drawn to the dechlorination by anaerobic bacteria of both chlorinated ethenes and chlorophenolic compounds that serve as electron acceptors with electron donors including formate, pyruvate, and acetate. This is termed dehalorespiration and is important in the degradation of a range of halogenated compounds under anaerobic conditions, and is discussed further in Chapter 3, Part 2 and Chapter 7, Part 3. 

Utkin I, C Woese, J Wiegel (1994) Isolation and characterization of Desulfitobacterium dehalogenans gen. nov, sp. nov., an anaerobic bacterium which reductively dechlorinates chlorophenolic compounds. Int J Syst Bacteriol 44 612-619. 

Kiyohara H, T Hatta, Y Ogawa, T Kakuda, H Yokoyama, N Takizawa (1992) Isolation of Pseudomonas pick-ettii strains that degrade 2,4,6-trichlorophenol and their dechlorination of chlorophenols. Appl Environ Microbiol 58 1276-1283. 

A facultatively anaerobic organism designated Anaeromyxobacter dehalogenans (Sanford et al. 2002) was capable of dechlorinating ortho-chlorinated phenols using acetate as electron donor—2-chlorophenol was reduced to phenol and 2,6-dichlorophenol to 2-chloro-phenol (Cole et al. 1994). A strain of Desulfovibrio dechloracetivorans was also able to couple the dechlorination of ortho-substituted chlorophenols to the oxidation of acetate, fumarate, lactate, and propionate (Sun et al. 2000). 

A spore-forming strain of Desulfitobacterium chlororespirans was able to couple the dechlorination of 3-chloro-4-hydroxybenzoate to the oxidation of lactate to acetate, pyruvate, or formate (Sanford et al. 1996). Whereas 2,4,6-trichlorophenol and 2,4,6-tribro-mophenol supported growth with the production of 4-chlorophenol and 4-bromophenol, neither 2-bromophenol nor 2-iodophenol was able to do so. The membrane-bound dehalogenase contains cobalamin, iron, and acid-labile sulfur, and is apparently specific for ortho-substituted phenols (Krasotkina et al. 2001). 

Sun B, JR Cole, RA Sanford, JM Tiedje (2000) Isolation and characterization of Desulfobvibrio dechlorace-tivorans sp. nov., a marine dechlorinating hacterium growing hy coupling the oxidation of acetate to the reductive dechlorination of 2-chlorophenol. Appl Environ Microbiol 66 2408-2413. 

Dechlorination of Aromatics Investigated in Micro Reactors Cas/liquid reaction 15 [CL 15] Dechlorination of p-chlorophenol to phenol... 

JOVANOVIC, G., Sacrittichai, P., Toppinen, S., Microreactors systems for dechlorination of p-chlorophenol on palladium based metal support catalyst theory and experiment, in Proceedings of the 6th International Conference on Microreaction Technology, IMRET 6, 11-14 March 2002, pp. 314-325, AIChE Pub. No. 164, New Orleans (2002). 

Bell (1956) reported that the composition of photodegradation products formed were dependent upon the initial 2,4-D concentration and pH of the solutions. 2,4-D undergoes reductive dechlorination when various polar solvents (methanol, butanol, isobutyl alcohol, ferf-butyl alcohol, octanol, ethylene glycol) are irradiated at wavelengths between 254 to 420 nm. Photoproducts formed included 2,4-dichlorophenol, 2,4-dichloroanisole, 4-chlorophenol, 2- and 4-chlorophenoxy-acetic acid (Que Hee and Sutherland, 1981). 

Table 8.3. Bacteria that reductively dechlorinate chlorophenols...

Dolfing, J. (1995). Letter to the editor Regiospecificity of chlorophenol reductive dechlorination by Vitamin B12s. Applied and Environmental Microbiology, 6l, 2450-1. 

It is important to note that both H202 and Fe2+ have to be overdosed to maintain a steady-state concentration of hydroxyl radical and to obtain a satisfactory approximation of the mathematical model with the experimental data. When H202 and Fe2+ concentrations are 5 x 10-3 M and 2 x 1th4 M, respectively, the relative rate constants of 2-chlorophenol (2-CP) and 2,4,6-TCP with respect to 2,4-DCP can be calculated. The oxidation and dechlorination constants of 2,4-DCP were found to be 0.995 1/min (fc4) and 0.092 1/min (k2), as reported in a previous study (Tang and Fluang, 1996). For comparison, Table 6.1 summarizes all the kinetic constants as determined in this study and in the related literature. 

To evaluate the effect of the number of chlorines on the degradation rate constants of different chlorophenols, Table 6.2 shows the rate constants of elementary, oxidation, and dechlorination for the ratios of k2 CP/k2/l DcP and 2,4,6-tcp/ 2,4-dcp The relative rate constants are plotted against the number of sites unoccupied by chlorine atoms on the chlorinated phenols in Figure 6.3.A linear correlation between the rate constants and the number of sites available is found with a standard deviation of 0.132. Clearly, the more chlorine atoms the aromatic rings contain, the fewer sites are available for hydroxyl radical attack however, the correlation should not be used for... 

The kinetics show two regimes a low-concentration regime that is zero order in [CPOH], and a second regime at higher concentrations where the rate displays saturation-type kinetics reminiscent of Langmuir-type behavior in solid/gas systems. It suggests that the reaction takes place in the solution bulk at low concentrations of chlorophenol, while at the higher concentrations the reaction occurs predominantly at the gas bubble-liquid interface. Chlorophenols are decomposed and dechlorinated almost quantitatively to form hydroxylated aromatic intermediate products subsequently, species with fewer carbon atoms remained undetectable under these conditions. 

Ukrainczyk, L. and M.B. McBride. 1993. Oxidation and dechlorination of chlorophenols in dilute aqueous suspensions of manganese oxides - Reaction products. Environ. Toxicol. Chem. 12, 2015-2022. 

A glassy C electrode, which was immobilized with tyrosinase, was used for amperometric detection of phenol in a polyimide chip. Phenol was enzymatically converted to catechol, which was then oxidized to quinone during detection. Chlorophenol can also be detected, but this is achieved after a dechlorination step (to phenol) using a Mg/Pd metal catalyst [229]. 

Cheng F, Fernando Q, Korte N. Electrochemical dechlorination of 4-chlorophenol to phenol. Environ Sci Technol 1997 31 1074-1078. 

Graham LJ, Jovanovic G. Dechlorination of p-chlorophenol on a Pd/Fe catalyst in a magnetically stabilized fluidized bed Implications for sludge and liquid remediation. Chem Eng Sci 1999 54 3085-3093. 

Lin CH, Tseng SK. Electrochemically reductive dechlorination of penta-chlorophenol using a high overpotential zinc cathode. Chemosphere 1999 39 2375-2389. 

Lin, C.H. and Tseng, S.K. (2000) Electrochemically reductive dechlorination of chlorophenol using nickel and zinc electrodes. Water Sci. Technol. 42, 167-172. 

Hart and Cassis, Jr. utilized the dechlorination with Raney alloy and alkali in the synthesis of 2,6-di-r-butylphenol from 4-bromo- or 4-chlorophenol as starting mate-... 

The transformation of chlorophenols upon irradiation of nitrate mainly yields hydroxylated derivatives, while no nitration was observed. The absence of nitro derivatives can be attributed to the electron-withdrawing character of the chlorine atoms, inhibiting nitration. Dihydroxybenzenes and benzoquinone were also observed implying that a dechlorination process, induced by either hydroxyl or light, takes place in the system [123]. 

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