Reductive Dechlorination Of Carbon Tetrachloride Research Articles

Effects of transition metal and sulfide on the reductive dechlorination of carbon tetrachloride and 1,1,1-trichloroethane by FeS

Reductive dechlorination of carbon tetrachloride (CT) and 1,1,1-trichloroethane (1,1,1-TCA) by FeS with transition metals (Cu(II), Co(II), and Ni(II)) and hydrosulfide was characterized in this study. The batch kinetic experiments were conducted by spiking each stock solution of CT and 1,1,1-TCA into 33 g/L of FeS suspensions with and without transition metals at pH 7.5. No significant enhancement was observed in the reductive dechlorination of target compounds by FeS with 1 mM transition metals. However, except the addition of Cu(II), the reduction rate of 1,1,1-TCA increased with increasing the concentration of transition metals. The rate constants with 10 mM Co(II) and Ni(II) were 0.06 and 0.11 h −1, approximately 1.3 and 3.0 times greater than those by FeS alone. The addition of 20 mM HS − also increased the rate constants of 1,1,1-TCA by FeS by one order of magnitude. SEM analysis showed that the addition of transition metal (Ni(II)) and HS − caused a noticeable morphologic change of FeS surface. The transition metal added was substituted by the structural iron resulting in the decrease of iron content of FeS (52.6–46.9%). One third of the transition metal in FeS suspension existed as zero-valent form playing a catalyst role to accelerate the reaction kinetics.

Combined Quantum Mechanical and Molecular Mechanics Studies of the Electron-Transfer Reactions Involving Carbon Tetrachloride in Solution

The reductive dechlorination of carbon tetrachloride, CCl4, by a concerted electron transfer-bond breaking mechanism was studied using combined high level quantum mechanical and molecular mechanics (QM/MM) approach. The free energy activation barriers for the first electron-transfer step were determined from the dissociation profiles of CCl4 and *CCl4(-) complexes in aqueous phase using hybrid-free energy QM/MM methodologies. Both density functional and coupled cluster perturbative triples (CCSD(T)) versions of QM/MM methods were investigated. The impact of the implicit solvent description based on continuum (COSMO) solvent models was also analyzed. QM/MM calculations at the CCSD(T)/aug-cc-pVDZ/SPCE level of theory predict that the activation barriers vary from 0.7 to 35.2 kcal/mol for -2.32 and 0.93 V reduction potentials respectively. Good agreement with experimental data for oxide-free iron electrodes (-0.6 to -1.2 V reduction potentials) is observed indicating that the measured activation barriers are consistent with the concerted electron transfer-bond-breaking mechanism.

Reductive Dechlorination of Carbon Tetrachloride in Aqueous Solutions Containing Ferrous and Copper Ions

Fe(II) associated with iron-containing minerals has been shown to be a potential reductant in natural subsurface environments. While it is known that the surface-bound iron species has the capacity to dechlorinate various chlorinated compounds, the role of transition metals to act as catalysts with these iron species is of importance. We previously observed that the reduction of Cu(II) by Fe(II) associated with goethite enhanced the dechlorination efficiency of chlorinated compound. In this study, the reductive dechlorination of carbon tetrachloride (CCl4) by dissolved Fe(II) in the presence of Cu(II) ions was investigated to understand the synergistic effect of Fe(II) and Cu(II) on the dechlorination processes in homogeneous aqueous solutions. The dechlorination efficiency of CCl4 by Fe(II) increased with increasing Cu(II) concentrations over the range of 0.2-0.5 mM and then decreased at high Cu(II) concentrations. The efficiency and rate of CCl4 dechlorination also increased with increasing dissolved Fe(II) concentration in the presence of 0.5 mM Cu(II) at neutral pH. When the Fe(II)/Cu(II) ratio varied between 1 and 10, the pseudo-first-order rate constant (k(obs)) increased 250-fold from 0.007 h(-1) at 0.5 mM Fe(II) to 1.754 h(-1) at 5 mM Fe(II). X-ray powder diffraction and scanning electron microscopy analyses showed that Cu(II) can react with Fe(II) to produce different morphologies of ferric oxides and subsequently accelerate the dechlorination rate of CCl4 at a high Fe(II) concentration. Amorphous ferrihydrite was observed when the stoichiometric Fe(II)/Cu(II) ratio was 1, while green rust, goethite, and magnetite were formed when the molar ratios of Fe(II)/Cu(II) reached 4-6. In addition, the dechlorination of CCl4 by dissolved Fe(II) is pH dependent. CCl4 can be dechlorinated by Fe(II) over a wide range of pH values in the Cu(II)-amended solutions, and the k(obs) increased from 0.0057 h(-1) at pH 4.3 to 0.856 h(-1) at pH 8.5, which was 9-25 times greater than that in the absence of Cu(II) at pH 7-8.5. The high reactivity of dissolved Fe(II) on the dechlorination of CCl4 in the presence of Cu(II) under anoxic conditions may enhance our understanding of the role of Fe(II) and the long-term reactivity of the zerovalent iron system in the dechlorination processes for chlorinated organic contaminants.

Understanding reduction of carbon tetrachloride at nickel surfaces.

Nickel has been found to be an effective cathode material and catalyst for reductive destruction of chlorinated solvents in contaminated water. This study investigated reductive dechlorination of carbon tetrachloride (CT) at a nickel rotating disk electrode using chronoamperometry and electrochemical impedance spectroscopy. Chronoamperometry experiments were performed to determine rates of CT reduction as a function of the electrode potential, pH, CT concentration, and temperature. The reaction products of CT dechlorination were 95 +/- 4% methane and 4.1 +/- 2.5% chloroform. Only trace levels of methylene chloride and chloromethane were produced, indicating that sequential hydrogenolysis was not the predominant pathway for methane production. Electrochemical impedance spectroscopy showed that the rate-limiting step for methane production was the transfer of the first electron to a physically adsorbed CT molecule. The temperature independence of the electron transfer coefficient and the decreasing activation energy with decreasing electrode potential indicated that the rate-limiting step involved an outer-sphere electron transfer. At neutral pH values, oxides inactivated much of the electrode surface for both CT reduction and hydrogen evolution. At lower pH values, oxide dissolution served to increase the electroactive surface area of the disk electrode. Anson analysis and kinetic modeling showed that CT adsorption to electroactive sites was a nonlinear function of the CT concentration and was in equilibrium with the bulk solution. CT dechlorination rates on nickel electrodes were 16 times slower than those on iron electrodes under similar conditions. However, CT reactions at nickel surfaces produced predominantly methane as the first detectable product, while reduction at iron surfaces produced chloroform. These results suggest that, although nickel is not a catalyst for the rate-limiting step for CT dechlorination, it may serve a catalytic role in subsequent reaction steps.

Synergistic effect of copper ion on the reductive dechlorination of carbon tetrachloride by surface-bound Fe(II) associated with goethite.

The dechlorination of carbon tetrachloride (CT) by Fe(II) associated with goethite in the presence of transition metal ions was investigated. X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRPD) were used to characterize the chemical states and crystal phases of transition metals on solid phases, respectively. CT was dechlorinated to chloroform (CF) by 3 mM Fe(II) in 10 mM goethite (25.6 m2 L(-1)) suspensions. The dechlorination followed pseudo-first-order kinetics, and a rate constant (k(obs)) of 0.036 h(-1) was observed. Transition metal ions have different effects on CT dechlorination. The addition of Ni(II), Co(II), and Zn(II) lowered the k(obs) for CT dechlorination, whereas the amendment of 0.5 mM Cu(II) into the Fe(II)-Fe(III) system significantly enhanced the efficiency and the rate of CT dechlorination. The k(obs) for CT dechlorination with 0.5 mM Cu(II) was 1.175 h(-1), which was 33 times greater than that without Cu(II). Also, the dechlorination of CT by surface-bound iron species is pH-dependent, and the rate constants increased from 0.008 h(-1) at pH 4.0 to 1.175 h(-1) at pH 7.0. When the solution contained Cu(II) and Fe(II) without goethite, a reddish-yellow precipitate was formed, and the concentration of Fe(ll) decreased with the increase in Cu(II) concentration. XPS and XRPD analyses suggested the possible presence of Cu2O and ferrihydrite in the precipitate. Small amounts of aqueous Cu(I) were also detected, reflecting the fact that Cu(II) was reduced to Cu(I) by Fe(II). A linear relationship between k(obs) for CT dechlorination and the concentration of Cu(II) was observed when the amended Cu(II) concentration was lower than 0.5 mM. Moreover, the k(obs) for CT dechlorination was dependent on the Fe(II) concentration in the 0.5 mM Cu(II)-amended goethite system and followed a Langmuir-Hinshelwood relationship. These results clearly indicate that Fe(II) serves as the bulk reductant to reduce both CT and Cu(II). The resulting Cull) can further act as a catalyst to enhance the dechlorination rate of chlorinated hydrocarbons in iron-reducing environments.

Effects of AgI, AuIII, and CuIIon the Reductive Dechlorination of Carbon Tetrachloride by Green Rust

Green rusts (GRs), mixed iron(II)/iron(III) hydroxide minerals found in many suboxic environments, have been shown to reduce a range of organic and inorganic contaminants, including several chlorinated hydrocarbons. Many studies have demonstrated the catalytic activity of transition metal species in the reduction of chlorinated hydrocarbons, suggesting the potential for enhanced reduction by GR in the presence of an appropriate transition metal catalyst. Reductive dechlorination of carbon tetrachloride (CT) was examined in aqueous suspensions of GR amended with Ag(I), Au(III), or Cu(II). The CT reduction rates were greatly increased for systems amended with Cu(II), Au(III), and Ag(I) (listed in order of increasing rates) relative to GR alone. Observed intermediates and products included chloroform, dichloromethane, chloromethane, methane, acetylene, ethene, ethane, carbon monoxide, tetrachloroethene, and various nonchlorinated C3 and C4 compounds. Product distributions for the reductive dechlorination of CT were highly dependent on the transition metal used. A reaction pathway scheme is proposed in which CT is reduced primarily to methane and other nonchlorinated end products, largely through a series of one-electron reductions forming radicals and carbenes/carbenoids. Recently, X-ray absorption fine structure analysis of aqueous GR suspensions amended with Ag(I), Au(III), or Cu(II) showed that the metals were reduced to their zerovalent forms. A possible mechanism for CT reduction is the formation of a galvanic couple involving the zerovalent metal and GR, with reduction of CT occurring on the surface of the metal and GR serving as the bulk electron source. The enhanced reduction of CT by GR suspensions amended with Ag(I), Au(III), or Cu(II) may prove useful in the development of improved materials for remediation of chlorinated organic contaminants.

Reductive dechlorination of carbon tetrachloride and tetrachloroethylene by zerovalent silicon-iron reductants.

Reductive dechlorination of carbon tetrachloride (CT) and tetrachloroethylene (PCE) by zerovalent silicon (ZVS, Si0) and the combination of Si0 with metal iron (Fe0) was investigated as potential reductants for chlorinated hydrocarbons. The X-ray photoelectron spectroscopy (XPS) was used to identify the surface characteristics of Si0. CT and PCE can be completely degraded via sequential reductive dechlorination to form lesser chlorinated homologues by Si0. Productions of chloroform (CF) and trichloroethylene (TCE) accounted for 80% of CT and 65% of PCE dechlorination, respectively. The degradation of CT and PCE by Si0 at pH 8.3 followed pseudo-first-order kinetics, and the normalized surface rate constants (k(sa)) were 0.288 and 0.003 L m(-2) h(-1), respectively, which react more efficiently than zerovalent iron in CT and PCE dechlorination. A linear relationship was also established between pH and the k(sa) value. The XPS results showed that the hydrogenated silicon surface and silicon oxides on the silicon surface were removed during the dechlorination processes, thus providing a relatively clean silicon surface for dechlorination reactions. The combination of zerovalent silicon with iron influences both the dechlorination rate and the distribution of products. Sequential reductive dechlorination was still the main reaction for CT dechlorination by Si0/Fe0, while reductive dechlorination and beta-elimination were the dominant reaction pathways for PCE dechlorination with ethane and ethene as the major end products. Also, the combination of silicon and iron constitutes a buffer system to maintain the pH at a stable value. A 0.3 unit of pH changed upon increasing the amount of Fe by a factor of 35 was observed, depicting that Si0 serves as a pH buffer in Si0/Fe0 system during dechlorination processes.

Carbon Tetrachloride Reduction by Fe2+,S2-, and FeS with vitamin B12 as Organic Amendment

The reductive dechlorination of carbon tetrachloride (CT) was examined in the presence of free iron ions (Fe2+), sulfide ions (S2-), and freshly precipitated ferrous sulfide (FeS) as reducing agents, and vitamin B12 as organic amendment. The reductive dechlorination of CT by the reducing ions Fe2+ and S2- in homogeneous phase resulted in the formation of variable amounts of the mono- and di-dechlorination products chloroform (CF) and dichloromethane (DCM). In the ferrous chloride (FeCl2) solutions (200 mM) where Fe2+ was the only electron source, 76% of the original CT was depleted within 1/2 h and about 28% and 8% went to CF and DCM, respectively, at a pH of 3.1 and with no buffer present. These same dechlorination products were observed in unbuffered sodium sulfide (Na2S) solutions (110 and 200 mM) with S2- as the electron source at pH 13.4. Dechlorination products were also observed in heterogeneous systems where FeS (75–200 mM) acted as the bulk reductant and the addition of vitamin B12 (0–4 mM) resulted in an enhancement of the dechlorination reaction. When 4 mM of vitamin B12 were added to 200 mM FeS, CT was removed continuously and the amount of CF and DCM formed increased significantly over time, yielding a mass recovery of 40% and higher after 1 h and a pseudofirst-order rate constant of 1.91 h−1. The reductive dechlorination of CT in the absence of vitamin B12, resulted in a slower disappearance of CT and the formation of smaller amounts of CF and DCM accounting for only 1% of the mass loss in the 75 mM FeS suspensions to 15% in the 200 mM FeS suspensions after 2 h.

Electrochemical investigation of the rate-limiting mechanisms for trichloroethylene and carbon tetrachloride reduction at iron surfaces.

The mechanisms involved in reductive dechlorination of carbon tetrachloride (CT) and trichloroethylene (TCE) at iron surfaces were studied to determine if their reaction rates were limited by rates of electron transfer. Chronoamperometry and chronopotentiometry analyses were used to determine the kinetics of CT and TCE reduction by a rotating disk electrode in solutions of constant halocarbon concentration. Rate constants for CT and TCE dechlorination were measured as a function of the electrode potential over a temperature range from 2 to 42 degrees C. Changes in dechlorination rate constants with electrode potential were used to determine the apparent electron-transfer coefficients at each temperature. The transfer coefficient for CT dechlorination was 0.22 +/- 0.02 and was independent of temperature. The temperature independence of the CT transfer coefficient is consistent with a rate-limiting mechanism involving an outer-sphere electron-transfer step. Conversely, the transfer coefficient for TCE was temperature dependent and ranged from 0.06 +/- 0.01 at 2 degrees C to 0.21 +/- 0.02 at 42 degrees C. The temperature-dependent TCE transfer coefficient indicated that its reduction rate was limited by chemical dependent factors and not exclusively by the rate of electron transfer. In accord with a rate-limiting mechanism involving an electron-transfer step, the apparent activation energy (Ea) for CT reduction decreased with decreasing electrode potential and ranged from 33.0 +/- 1.6 to 47.8 +/- 2.0 kJ/mol. In contrast, the Ea for TCE reduction did not decline with decreasing electrode potential and ranged from 29.4 +/- 3.4 to 40.3 +/- 3.9. The absence of a potential dependence for the TCE Ea supports the conclusion that its reaction rate was not limited by an electron-transfer step. The small potential dependence of TCE reaction rates can be explained by a reaction mechanism in which TCE reacts with atomic hydrogen produced from reduction of water.

Reductive Dechlorination of Carbon Tetrachloride Using Iron(II) Iron(III) Hydroxide Sulfate (Green Rust)

The reductive dechlorination of CCl4 and CHCl3 in the presence of the synthetic sulfate form of green rust (GRSO4), FeII4FeIII2(OH)12SO4yH2O, at pH ∼ 8 and room temperature was investigated. Reduction of CCl4 produces CHCl3 and C2Cl6 as main chloroaliphatic products, while GRSO4 is oxidized to magnetite (Fe3O4). The formation of C2Cl6 indicates a coupling reaction between trichloromethyl radicals in the suspension. Chloroform was much less susceptible than CCl4 to reductive dechlorination by GRSO4 showing reduction rates approximately 100 times less than for reduction of CCl4. The transformation of CCl4 by GRSO4 can be described by pseudo-first-order reaction kinetics with respect to formation of chloride. At room temperature the rate expression is given as: d[Cl-]/dt ≅ −d[CCl4]/dt = r·kobs[Fe(II)]GR, where kobs is in the range (0.47 × 10-5)−(2.18 × 10-5) s-1 for CCl4 concentrations above its aqueous solubility. This narrow range may be due to the constant CCl4(aq) concentration owing to buffering of the...

Soil- and surfactant-enhanced reductive dechlorination of carbon tetrachloride in the presence of Shewanella putrefaciens 200

Sorption of organic contaminants to soils has been shown to limit bioavailability and biodegradation in some systems. Use of surfactants has been proposed to reverse this effect. In this study, the effects of a high organic carbon content soil and a nonionic surfactant (Triton X-100) on the reductive dechlorination of carbon tetrachloride (CCl 4) were examined in anaerobic systems containing Shewanella putrefaciens. Although more than 70% of the added CCl 4 was sorbed to the soil phase in these systems, the reductive dechlorination of CCl 4 was not diminished. Rather, rates of CCl 4 dechlorination in systems containing soil were enhanced relative to systems containing non-sorptive sand slurries. This enhancement was also observed in sterile soil slurries to which a chemical reductant, dithiothreitol was added. It appears that the organic soil used in these experiments contains some catalytic factor capable of transforming CCl 4 in the presence of an appropriate chemical or microbial reductant. The addition of Triton X-100 to sand and soil slurries containing S. putrefaciens resulted in increased CCl 4 degradation in both systems. The effect of Triton could not be explained by: (i) surfactant induced changes in the distribution of CCl 4, (i.e. decreased sorption) or the rate of CCl 4 desorption; (ii) a direct reaction between Triton and CCl 4; or (iii) increased cell numbers resulting from use of the surfactant as a substrate. Rather, it appears that Triton X-100 addition resulted in lysis of bacterial cells, a release of biochemical reductant, and enhanced reductive transformation of CCl 4. These results provide insights to guide the development of more effective direct or indirect bioremediation strategies.

Transformation of carbon tetrachloride under sulfate reducing conditions.

The removal of carbon tetrachloride under sulfate reducing conditions was studied in an anaerobic packed-bed reactor. Carbon tetrachloride, up to a concentration of 30 microM, was completely converted. Chloroform and dichloromethane were the main transformation products, but part of the carbon tetrachloride was also completely dechlorinated to unknown products. Gram-positive sulfate-reducing bacteria were involved in the reductive dechlorination of carbon tetrachloride to chloroform and dichloromethane since both molybdate, an inhibitor of sulfate reduction, and vancomycin, an inhibitor of gram-positive bacteria completely inhibited carbon tetrachloride transformation. Carbon tetrachloride transformation by these bacteria was a cometabolic process and depended on the input of an electron donor and electron acceptor (sulfate). The rate of carbon tetrachloride transformation by sulfate reducing bacteria depended on the type of electron donor present. A transformation rate of 5.1 nmol x ml(-1) x h(-1) was found with ethanol as electron donor. At carbon tetrachloride concentrations higher than 18 microM, sulfate reduction and reductive dechlorination of carbon tetrachloride decreased and complete inhibition was observed at a carbon tetrachloride concentration of 56.6 microM. It is not clear what type of microorganisms were involved in the observed partial complete dechlorination of carbon tetrachloride. Sulfate reducing bacteria probably did not play a role since inhibition of these bacteria with molybdate had no effect on the complete dechlorination of carbon tetrachloride.

Effects of electron donor and microbial concentration on the enhanced dechlorination of carbon tetrachloride by anaerobic consortia

Reductive dechlorination of carbon tetra-chloride (CCl4) by anaerobic bacterial communities from anaerobic digester sludge with the amendment of low concentrations of electron donors and microorganisms was undertaken to evaluate the influence of electron donors and microbial concentration on the rate of dechlorination of CCl4. Humic acid, acetate, and glucose were selected to examine the feasibility of the electron donor with respect to the remediation of a contaminated subsurface. The addition of an electron donor and microorganisms significantly enhanced the dechlorination rate of carbon tetrachloride. The addition of an electron donor increased the cell numbers of anaerobic consortia, thereby increasing the rate of dechlorination. Glucose was a better electron donor than acetate and humic acid under reducing environments. The pseudo-first-order degradation rate constants of CCl4 ranged from 0.0057 day-1 to 0.135 day-1, depending on the conditions of the electron donor and biomass supplemented. Furthermore, the addition of the electron donor in the batches amended with 0.56 mg volatile suspended solids (VSS)/l biomass had a higher enhanced efficiency than those with 1.7 mg VSS/l biomass. These results suggest that there is a potential for stimulating the dechlorinating capability of anaerobic consortia to remedy the chlorinated hydrocarbons in the oligotrophic environment if the conditions of the supplementing electron donor are properly selected.

Reductive dechlorination of carbon tetrachloride with elemental iron

Carbon tetrachloride (CCl 4) was abiotically dechlorinated to chloroform (CHCl 3) and methylene chloride (CH 2Cl 2) by zero valent iron (Fe 0). Dechlorination of CCl 4 was rapid, and approximately followed first-order kinetics in the range of concentrations tested (CCl 4: 1.5–5.5 μ M; Fe 0 powder: 1–10 g per 265 ml distilled water). Initial dechlorination rate coefficients for anoxic batch reactors (0.290 ± 0.009 h −1 for 1 g Fe 0; 1.723 ± 0.078 h −1 for 10 g Fe 0) increased with iron surface area (initially 2.4 ± 0.2 m 2/g). Rate coefficients also increased with time, probably because of an increase in reactive surface area due to cathodic depolarization and pitting of the iron surface. Dechlorination also occurred under oxic conditions, although the rates were significantly slower (e.g., 0.085 ± 0.041 h −1 for 1 g Fe 0 and initial dissolved oxygen 7.4 mg/l). A rapid pH increase was synchronous to dissolved oxygen consumption, and the pH remained constant after the oxygen was depleted. This was attributed to the proton and oxygen consuming aerobic corrosion of the Fe 0 surface. The potential for developing remediation technology with Fe 0 is discussed.

Reductive Dechlorination of Carbon Tetrachloride In Water Catalyzed by Mineral-Supported Biomimetic Cobalt Macrocycles

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTReductive Dechlorination of Carbon Tetrachloride In Water Catalyzed by Mineral-Supported Biomimetic Cobalt MacrocyclesLjerka. Ukrainczyk, Malama. Chibwe, Thomas J. Pinnavaia, and Stephen A. BoydCite this: Environ. Sci. Technol. 1995, 29, 2, 439–445Publication Date (Print):February 1, 1995Publication History Published online1 May 2002Published inissue 1 February 1995https://doi.org/10.1021/es00002a021RIGHTS & PERMISSIONSArticle Views326Altmetric-Citations75LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (771 KB) Get e-Alerts Get e-Alerts

Reductive Dechlorination of Carbon Tetrachloride Mediated by Cationic Water‐Soluble Metalloporphyrins

AbstractMicrobial degradation of halogenated pollutants in wastewaters may be limited by toxic contaminant concentrations or poor membrane permeability of the target compound. Extracellular catalysts can be used in wastewater treatment schemes to overcome these limitations. Water soluble porphyrins with functional groups suitable for immobilization were synthesized as catalysts for reductive dehalogenation of organic contaminants. Synthesized mesosubstituted porphyrins contained (with one exception) three N‐methyl‐4‐pyridinium substituents and a phenyl substituent having the ‐OH, ‐OCH3, ‐NH2, or ‐C(O)OCH2C6H4‐CH=CH2 group located in the p‐position. Catalytic activity was assessed by measuring carbon tetrachloride disappearance in reactions containing a synthetic porphyrin, dithiothreitol, and tris buffer (pH 8.2). Porphyrins metallated with Co or Ni had higher catalytic activities than those metallated with Fe. Changes in meso substituents also altered catalytic activity, with aminophenyl substitution resulting in porphyrins catalyzing the highest rates of carbon tetrachloride disappearance. Although chloroform, dichloromethane, and carbon monoxide were found to be breakdown products, no methane was detected and <40% of the carbon tetrachloride could be recovered in identified volatile products. When 14C carbon tetrachloride was used as substrate, approximately 80% of the label was found in the aqueous phase of the reaction mixture. Synthetic porphyrins have potential utility in wastewater treatment as catalysts for reductive dehalogenation, but further identification of degradative products is necessary.

Reduction dechlorination of carbon tetrachloride by cobalamin(II) in the presence of dithiothreitol: mechanistic study, effect of redox potential and pH

A mechanistic study of the reductive dechlorination of carbon tetrachloride by vitamin B 12 (cyanocobalamin) in the presence of dithiothreitol was conducted as a function of redor potential and pH. The solution redor potential decreased both with an increase in the total concentration of dithiothreitol present and with an increase in pH. The pseudo-first-order rate constant of carbon tetrachloride disappearance increased with decreasing redor potential. The predominant cobalt species present under the reaction conditions was cobalamin(II) (vitamin B 12r ), as confirmed by spectrophotometric analysis, suggesting a one-electron reduction of vitamin B 12 and the involvement of two vitamin B 12 molecules per reacting carbon tetrachloride molecule

Ethanol Oxidation and Toxicity: Role of Alcohol P‐450 Oxygenase

The isolation and characterization of ethanol-inducible rabbit liver microsomal cytochrome P-450, termed P-450 3a or P-450ALC, has provided definitive evidence for the role of this enzyme in alcohol oxidation. From findings on the distribution, substrate specificity, and mechanism of action of P-450ALC we have suggested "alcohol P-450 oxygenase" as a more biochemically accurate name than "microsomal ethanol-oxidizing system." The present review is concerned with studies in this and other laboratories on activities and inducers associated with this versatile enzyme. Numerous xenobiotics, including alcohols and ketones, nitrosamines, aromatic compounds, and halogenated alkanes, alkenes, and ethers, are known to undergo increased microsomal metabolism after chronic exposure of various species to ethanol. Diverse compounds and treatments may induce P-450ALC, including the administration of ten or more chemically different compounds, fasting, or the diabetic state. Whether a common mechanism of induction is involved is unknown at this time. As direct evidence that P-450ALC catalyzes numerous metabolic reactions, the purified rabbit enzyme has been used in a reconstituted system to demonstrate various metabolic transformations, including the oxidation of various alcohols, acetone, acetol, p-nitrophenol, and aniline, the dealkylation of substituted nitrosamines, the reductive dechlorination of carbon tetrachloride, carbon tetrachloride-induced lipid peroxidation, and acetaminophen activation to form the glutathione conjugate.