Higher Dechlorination Rate Research Articles

Optimizing hydrothermal dechlorination of PVC in a SS-316 reactor: From chemistry knowledge to material considerations

PVC remains one of the most challenging wastes for environmental activists and industrial recyclers because its decomposition products cause severe corrosion to equipment and our planet. The current study was designed to systematically investigate PVC dechlorination using the well-known Response Surface Methodology (RSM) to mature the process chemistry and assess the corrosion rate in the common SS-316 alloy. Undercover non-catalytic experiments, Cl removal, reactor wall corrosion rate, and H/C molar ratio of the products made the most argument to comprehend the chemistry and find the optimum operating condition. Based on data collected from different analytic techniques, like FESEM, SEM-EDX, TG, FT-IR, CHN, and AAS, it was found that water drove Cl removal through a combination of SN2 and Elimination pathways where the in-situ production of hydrochloric acid at high Severity Factor (SF) resulted in a desirably high dechlorination rate of 89.42%. However, significant metal leakage into the aqueous phase, e.g., 278.01 mg of Fe, in the Aqueous Phase (AP) necessitated the valorization of different approaches, including multistage dechlorination and application of various hetero and homogeneous catalysts, dealing with chloride and metal salts inside the reaction chamber. Among the solutions applied, the dual-stage and Ni-assisted dechlorination showed promising performances receptively with a corrosion rate as low as 0.034 g and the highest Cl removal efficiency of 96.18%.

Cathode potential regulates the microbiome assembly and function in electrostimulated bio- dechlorination system.

Mechanism of microbiome assembly and function driven by cathode potential in electro-stimulated microbial reductive dechlorination system remain poorly understood. Here, core microbiome structure, interaction, function and assembly regulating by cathode potential were investigated in a 2,4,6-trichlorophenol bio-dechlorination system. The highest dechlorination rate (24.30μM/d) was observed under -0.36V with phenol as a major end metabolite, while, lower (-0.56V) or higher (0.04V or -0.16V) potentials resulted in 1.3-3.8 times decreased of dechlorination kinetic constant. The lower the cathode potential, the higher the generated CH4, revealing cathode participated in hydrogenotrophic methanogenesis. Taxonomic and functional structure of core microbiome significantly shifted within groups of -0.36V and -0.56V, with dechlorinators (Desulfitobacterium, Dehalobacter), fermenters (norank_f_Propionibacteriaceae, Dysgonomonas) and methanogen (Methanosarcina) highly enriched, and the more positive interactions between functional genera were found. The lowest number of nodes and links and the highest positive correlations were observed among constructed sub-networks classified by function, revealing simplified and strengthened cooperation of functional genera driven by group of -0.36V. Cathode potential plays one important driver controlling core microbiome assembly, and the low potentials drove the assembly of major dechlorinating, methanogenic and electro-active genera to be more deterministic, while, the major fermenting genera were mostly governed by stochastic processes.

HCl emission and capture characteristics during PVC and food waste combustion in CO2/O2 atmosphere

The emission and capture characteristics of HCl during PVC and food waste combustion in CO2/O2 atmospheres were studied. Replacement of N2 by CO2 decreased the dechlorination rate of limestone at 600–700 °C but increased dechlorination rate at 800–900 °C. The chlorine species and temperature highly influenced the HCl emission and capture efficiency of limestone for HCl in CO2/O2 atmospheres. Compared with inorganic chloride in food waste, organic chlorine in PVC had much greater Cl–HCl conversion percent (75.0–93.9%), and higher dechlorination rate (20.4–44.9%) with 10% limestone in 80CO2/20O2 atmosphere. The increment of O2 partial pressure in CO2/O2 atmospheres promoted Cl–HCl conversion. Sulphur in the fuel suppressed the formation of HCl but decreased the dechlorination rate at 700–1000 °C in CO2/O2 atmospheres. The dechlorination efficiency of limestone was better than magnesium based additive and could be improved by modification with NaOH. This research helps control HCl and manages MSW oxy-fuel incineration.

Accelerated microbial reductive dechlorination of 2,4,6-trichlorophenol by weak electrical stimulation

Microbial reductive dechlorination of chlorinated aromatics frequently suffers from the long dechlorination period and the generation of toxic metabolites. Biocathode bioelectrochemical systems were verified to be effective in the degradation of various refractory pollutants. However, the electrochemical and microbial related working mechanisms for bio-dechlorination by electro-stimulation remain poorly understood. In this study, we reported the significantly improved 2,4,6-trichlorophenol dechlorination activity through the weak electro-stimulation (cathode potential of −0.36 V vs. SHE), as evidenced by the 3.1 times higher dechlorination rate and the complete dechlorination ability with phenol as the end dechlorination product. The high reductive dechlorination rate (20.8 μM/d) could be maintained by utilizing electrode as an effective electron donor (coulombic efficiency of 82.3 ± 4.8%). Cyclic voltammetry analysis of the cathodic biofilm gave the direct evidences of the cathodic respiration with the improved and positive-shifted reduction peaks of 2,4,6-TCP, 2,4-DCP and 4-CP. The optimal 2,4,6-TCP reductive dechlorination rate (24.2 μM/d) was obtained when a small amount of lactate (2 mM) was added, and the generation of H2 and CH4 were accompanied due to the biological fermentation and methanogenesis. The electrical stimulation significantly altered the cathodic biofilm structure and composition with some potential dechlorinators (like Acetobacterium) predominated. The microbial interactions in the ecological network of cathodic biofilm were more simplified than the planktonic community. However, some potential dechlorinators (Acetobacterium, Desulfovibrio, etc.) shared more positive interactions. The co-existence and possible cooperative relationships between potential dechlorinators and fermenters (Sedimentibacter, etc.) were revealed. Meanwhile, the competitive interrelations between potential dechlorinators and methanogens (Methanomassiliicoccus) were found. In the network of plankton, the fermenters and methanogens possessed the more positive interrelations. Electro-stimulation at the cathodic potential of −0.36 V selectively enhanced the dechlorination function, while it showed little influence on either fermentation or methanogenesis process. The study gave suggestions for the enhanced bioremediation of chlorinated aromatics, in views of the electro-stimulation capacity, efficiency and microbial interrelations related microbial mechanism.

Electrocatalytic dechlorination of 2,4-dichlorobenzoic acid using different carbon-supported palladium moveable catalysts: Adsorption and dechlorination activity

In the present study, carbon black (CB), multi walled carbon nanotubes (MWCNTs), and granular activated carbon (GAC) were employed to support palladium (Pd) catalyst. The prepared Pd/CB, Pd/MWCNTs and Pd/GAC moveable catalysts were aimed to address the common issues (easy loss of catalyst and poor long-term performance) of normal fixed catalysts. The results of various characterizations (e.g., TEM, XRD, and XPS) clearly show that the Pd nanoparticles were successfully loaded onto the carbon-supports, especially CB and MWCNTs. And more importantly, the morphologies, Pd distribution, and chemical structure of these moveable catalysts were almost not changed after 3 h reaction. The moveable Pd/CB, Pd/MWCNTs, and Pd/GAC catalysts had good reactivity for 2,4-dichlorobenzoic acid (2,4-DCBA) dechlorination, and Pd/CB exhibited the best performance. The Pd/CB also shows the best adsorption capacity of 2,4-DCBA and dechlorination product (benzoic acid, BA), and the adsorption of BA was significantly inhibited in the presence of current due to the repulsion between the both negatively charged compounds and adsorbents. The removal of 2,4-DCBA and the generation rate of BA was improved with a pre-adsorption process, which was a promising strategy with higher dechlorination rate and shortened electrolysis time. Moreover, the loss of Pd catalyst was negligible during the 10 consecutive cycles experiment, and the improved longevity could be expected. These results suggested the good reactivity, stability, and reusability of moveable Pd/CB catalyst.

Characterization and high-throughput sequencing of a trichlorophenol-dechlorinating microbial community acclimated from sewage sludge

Abstract This study reports the dechlorination characteristics of acclimated biomass, high-throughput sequencing of the 16Sribosomal RNA (rRNA) gene harbored by the microorganisms, and analysis of the relation between the dechlorination function and community structure. Anaerobic 2,4,6-trichlorophenol (TCP)-dechlorinating biomass was acclimated from active sludge, and it showed a high TCP-dechlorination rate. It dechlorinated TCP only at the ortho-position, with 4-chlorophenol as the terminal product. Illumina Hiseq sequencing analyses of a 16SrRNA gene amplicon indicated that the abundance of the phyla Bacteroidetes and Firmicutes doubled in the acclimation process. Compared with that in raw sludge, Betaproteobacteria disappeared from acclimated biomass, and Deltaproteobacteria was replaced by sulfate-reducing bacteria (SRB) including Desulfobulbus, Desulfovibrio, Desulfomicrobium, and Syntrophus. Syntrophomonas of the phylum Firmicutes predominated (abundance 6.7%), and it was responsible for the high efficacy of dechlorination. Dechlorinators included Dechloromonas, Clostridium, SRB and Geobacter. The emergence of SRB provided microbiological evidence of the sulfate-reducing ability of the dechlorinating biomass. Propionicimonas was discovered during this study, and it is known to produce vitamin B12, which is an essential factor for reductive dehalogenase enzymes. The core microbiome resulted in the high dechlorination rate.

Thermal dechlorination of heavily PCB-contaminated soils from a sealed site of PCB-containing electrical equipment.

A large amount of soils are contaminated by leakage of polychlorinated biphenyls (PCBs) from sealed-up PCB-containing electrical equipment in China. Thermal dechlorination of soils contaminated with PCBs at a level of 108mgg(-1) and PCB77 (3,3',4,4'-tetrachlorobiphenyl) as a model isomer in conjunction with calcium oxide was investigated in this study. The PCB dechlorination rate improved with increased temperature and time. The highest dechlorination rate was 85.3%, and temperature was the main influencing factor. Pentachlorobiphenyl and tetrachlorobiphenyl in soils decreased or disappeared in response to treatment at 350 and 400°C for 4h, while monochlorinated biphenyl and biphenyl were detected after the reaction, indicating the presence of a dechlorination/hydrogenation pathway. Discrepancy in chlorine balance was observed after low-temperature thermal dechlorination. The species of dechlorination products were identified as amorphous carbon containing a crystalline graphite plane structure and a carbonyl group-containing polymerized product, demonstrating the existence of a dechlorination/polymerization pathway. The yield of amorphous carbon and high-molecular-weight intermediates increased with heating time. The results showed that the discrepancy in chlorine balance was because of the generation of polymerized products and undetected intermediates.

Enhancing the reactivity of bimetallic Bi/Fe0 by citric acid for remediation of polluted water

In this study, the environmentally benign citric acid (CA) was utilized to improve the aerobic degradation of 4-chlorophenol (4-CP) over bismuth modified nanoscale zero-valent iron (Bi/Fe0). The characterization results revealed the existence of bismuth covering on the Fe0 surface under zero-valent state. And, the Bi/Fe0-CA+O2 system performed excellent reactivity in degradation of 4-CP due to the generation of reactive oxygen species (ROS), which was confirmed by electron spin resonance (ESR) spectroscopy. After 30min of reaction, 80% of 4-CP was removed using Bi/Fe0-CA+O2 accompanying with high dechlorination rate. The oxidative degradation intermediates were analyzed by HPLC and LC-MS. We found that CA could promote the bismuth-iron system to produce much reactive oxygen species ROS under both aerobic and anaerobic conditions due to its ligand function, which could react with Fe3+ to form a ligand complex (Fe(III)Cit), accompanying with a considerable production of Fe2+ and H2O2. This study provides a new strategy for generating ROS on nZVI and suggests its application for the mineralization of many recalcitrant pollutants.

The roles of methanogens and acetogens in dechlorination of trichloroethene using different electron donors.

We evaluated the effects of methanogens and acetogens on the function and structure of microbial communities doing reductive dechlorination of trichloroethene (TCE) by adding four distinct electron donors: lactate, a fermentable organic; acetate, a non-fermentable organic; methanol, a fermentable 1-C (carbon) organic; and hydrogen gas (H2), the direct electron donor for reductive dechlorination by Dehalococcoides. The fermentable electron donors had faster dechlorination rates, more complete dechlorination, and higher bacterial abundances than the non-fermentable electron donors during short-term tests. Phylotypes of Dehalococcoides were relatively abundant (≥9%) for the cultures fed with fermentable electron donors but accounted for only ~1-2% of the reads for the cultures fed by the non-fermentable electron donors. Routing electrons to methanogenesis and a low ratio of Dehalococcoides/methanogenesis (Dhc/mcrA) were associated with slow and incomplete reductive dechlorination with methanol and H2. When fermentable substrates were applied as electron donors, a Dhc/mcrA ratio ≥6.4 was essential to achieve fast and complete dechlorination of TCE to ethene. When methanogenesis was suppressed using 2-bromoethanesulfonate (BES), achieving complete dechlorination of TCE to ethane required a minimum abundance of the mcrA gene. Methanobacterium appeared to be important for maintaining a high dechlorination rate, probably by providing Dehalococcoides with cofactors other than vitamin B12. Furthermore, the presence of homoacetogens also was important to maintain a high dechlorination rate, because they provided acetate as Dehalococcoides's obligatory carbon source and possibly cofactors.

Degradation of dichloromethane by an isolated strain Pandoraea pnomenusa and its performance in a biotrickling filter

A strain Pandoraea pnomenusa LX-1 that uses dichloromethane (DCM) as sole carbon and energy source has been isolated and identified in our laboratory. The optimum aerobic biodegradation of DCM in batch culture was evaluated by response surface methodology. Maximum biodegradation (5.35 mg/(L·hr)) was achieved under cultivation at 32.8°C, pH 7.3, and 0.66% NaCl. The growth and biodegradation processes were well fitted by Haldane's kinetic model, yielding maximum specific growth and degradation rates of 0.133 hr−1 and 0.856 hr−1, respectively. The microorganism efficiently degraded a mixture of DCM and coexisting components (benzene, toluene and chlorobenzene). The carbon recovery (52.80%–94.59%) indicated that the targets were predominantly mineralized and incorporated into cell materials. Electron acceptors increased the DCM biodegradation rate in the following order: mixed > oxygen > iron > sulfate > nitrate. The highest dechlorination rate was 0.365 mg Cl−/(hr·mg biomass), obtained in the presence of mixed electron acceptors. Removal was achieved in a continuous biotrickling filter at 56%–85% efficiency, with a mineralization rate of 75.2%. Molecular biology techniques revealed the predominant strain as P. pnomenusa LX-1. These results clearly demonstrated the effectiveness of strain LX-1 in treating DCM-containing industrial effluents. As such, the strain is a strong candidate for remediation of DCM coexisting with other organic compounds.

Guided cobalamin biosynthesis supports Dehalococcoides mccartyi reductive dechlorination activity

Dehalococcoides mccartyi strains are corrinoid-auxotrophic Bacteria and axenic cultures that require vitamin B12 (CN-Cbl) to conserve energy via organohalide respiration. Cultures of D. mccartyi strains BAV1, GT and FL2 grown with limiting amounts of 1 µg l(-1) CN-Cbl quickly depleted CN-Cbl, and reductive dechlorination of polychlorinated ethenes was incomplete leading to vinyl chloride (VC) accumulation. In contrast, the same cultures amended with 25 µg l(-1) CN-Cbl exhibited up to 2.3-fold higher dechlorination rates, 2.8-9.1-fold increased growth yields, and completely consumed growth-supporting chlorinated ethenes. To explore whether known cobamide-producing microbes supply Dehalococcoides with the required corrinoid cofactor, co-culture experiments were performed with the methanogen Methanosarcina barkeri strain Fusaro and two acetogens, Sporomusa ovata and Sporomusa sp. strain KB-1, as Dehalococcoides partner populations. During growth with H2/CO2, M. barkeri axenic cultures produced 4.2 ± 0.1 µg l(-1) extracellular cobamide (factor III), whereas the Sporomusa cultures produced phenolyl- and p-cresolyl-cobamides. Neither factor III nor the phenolic cobamides supported Dehalococcoides reductive dechlorination activity suggesting that M. barkeri and the Sporomusa sp. cannot fulfil Dehalococcoides' nutritional requirements. Dehalococcoides dechlorination activity and growth occurred in M. barkeri and Sporomusa sp. co-cultures amended with 10 µM 5',6'-dimethylbenzimidazole (DMB), indicating that a cobalamin is a preferred corrinoid cofactor of strains BAV1, GT and FL2 when grown with chlorinated ethenes as electron acceptors. Even though the methanogen and acetogen populations tested did not produce cobalamin, the addition of DMB enabled guided biosynthesis and generated a cobalamin that supported Dehalococcoides' activity and growth. Guided cobalamin biosynthesis may offer opportunities to sustain and enhance Dehalococcoides activity in contaminated subsurface environments.

Increasing the activity and stability of chemi-deposited palladium catalysts on nickel foam substrate by electrochemical deposition of a middle coating of silver

The objective of this study was to enhance the activity and stability of the chemi-deposited palladium/nickel foam (Pd/Ni foam) electrode used in the electrochemical dechlorination process. The electrochemical deposition of silver onto a nickel foam substrate (Ag/Ni foam) was applied before the chemical deposition of palladium. The physicochemical properties of the resultant material (Pd/Ag/Ni foam) were characterized by X-ray diffraction and scanning electron microscopy. The results showed that the silver coating was in the form of spherical apophyses, and the palladium was dispersed finely on the surface of the Ag/Ni foam. Dichloroacetic acid was selected as an indicator to evaluate the activity and stability of the palladium catalyst. Pd/Ag/Ni foam electrode consistently had relatively higher dechlorination rate and current efficiency (CE) in the current density range of 1.5–10.5 mA cm −2 compared to the Pd/Ni foam electrode. Quantitative analysis of the main intermediates and final products involving monochloroacetic acid, acetic acid and the chloride ion further demonstrated the enhancement of electrochemical dechlorination by the presence of silver. Moreover, after five cycles of dechlorination, the CE for the Pd/Ni foam electrode decreased dramatically from 67% to 13%, whereas that for the Pd/Ag/Ni foam electrode was decreased by much less, from 78% to 62%, and the palladium loss was clearly reduced by the addition of a silver coating between the Ni foam and the Pd outer layer.

Interactively interfacial reaction of iron-reducing bacterium and goethite for reductive dechlorination of chlorinated organic compounds

The interactively interfacial reactions between the iron-reducing bacterium (Shewanella decolorationis, S12) and iron oxide (α-FeOOH) were investigated to determine reductive dechlorination transformation of chlorinated organic compounds (chloroform and pentachlorophenol). The results showed that the interactive system of S12+ α-FeOOH exhibited relatively high dechlorination rate. By comparison, the S12 biotic system alone had no obvious dechlorination, and the α-FeOOH abiotic system showed low dechlorination rate. The enhanced dechlorination of chloroform and pentachlorophenol in the interactive system of S12+α-FeOOH was derived from the promoted generation of adsorbed Fe(II) by S12. A decrease in redox potential of the Fe(III)/Fe(II) couple in the interactive reaction system was determined by cyclic voltammetry. Our results will give new insight into interactively interfacial reaction between iron-reducing bacterium and iron oxides for degradation of chlorinated organic compounds under anaerobic condition.

Effect of reduction temperature on the preparation of zero-valent iron aerogels for trichloroethylene dechlorination

Zero-valent iron (ZVI) aerogels have been synthesized by sol-gel method and supercritical CO2 drying, followed by H2 reduction in the temperature range of 350–500 °C. When applied to trichloroethylene (TCE) dechlorination, the ZVI aerogel reduced at 370 °C showed the highest performance in the conditions employed in this study. Thus, the effect of reduction temperature in preparing ZVI aerogels has been investigated by several characterizations such as BET, XRD, TPR, and TEM analyses. As the reduction temperature decreased from 500 to 350 °C, the BET surface area of the resulting aerogels increased from 6 to 30 m2/g, whereas their Fe0 content decreased up to 64%. It was also found that H2 reduction at low temperatures such as 350 and 370 °C leads to the formation of ZVI aerogel particles consisting of both Fe0 and FeOx in the particle cores with a different amount ratio, where FeOx is a mixture of maghemite and magnetite. It is, therefore, suggested that reduction at 370 °C for ZVI aerogel preparation yielded particles homogeneously composed of Fe0 and FeOx in the amount ratio of 87/13, resulting in high TCE dechlorination rate. On the other hand, when Pd- and Ni-ZVI aerogels were prepared via cogellation and then applied for TCE dechlorination, we also observed a similar effect of reduction temperature. However, the reduction at 350 or 370 °C produced Pd- or Ni-ZVI aerogel particles in which Fe0 and Fe3O4 co-exist homogeneously. Since both Fe0 and Fe3O4 are advantageous in TCE dechlorination, the activities of Pd- and Ni-ZVI aerogels reduced at 350 °C were comparable to those of both aerogels reduced at 370 °C, although the former aerogels have less Fe0 content.

Kinetics of trichloroethene dechlorination and methane formation by a mixed anaerobic culture in a bio-electrochemical system

In the present study, we investigated the use of a lab-scale, bio-electrochemical system to generate H 2 from protons reduction at controlled cathodic potentials, in support of the microbial reductive dechlorination of trichloroethene (TCE). Several batch potentiostatic experiments were performed on graphite cathodes at different potentials ranging from −0.700 to −1.000 V vs. Ag/AgCl. By appropriately changing the applied cathode potential it was possible to finely control the liquid phase H 2 concentration, which resulted from a balance between H 2 generation (from protons reduction) and consumption (from dechlorination and methanogenesis). Microbial TCE dechlorination was stimulated when the potential applied to the graphite cathode was lower than −0.800 V vs. Ag/AgCl. However, a combination of high dechlorination rate and high current efficiency was achieved only in a very narrow range of cathode potentials (i.e., −0.850 to −0.875 V vs. Ag/AgCl). Methane formation was significantly slower than TCE dechlorination, probably due to the presence in the mixed culture of a lower number of methanogens compared to dechlorinators. In spite of this fact, these two competing metabolisms were stimulated in a similar way by the application of an external potential, thereby indicating a similar affinity for H 2. Indeed, calculated half-velocity coefficients for H 2 for dechlorination and methanogenesis were 20.1 ± 7.6 and 17.9 ± 8.5 nM, respectively.

Reductive dechlorination of tetrachloroethene to cis-1, 2-dichloroethene by a thermophilic anaerobic enrichment culture.

Thermophilic anaerobic biodegradation of tetrachloroethene (PCE) was investigated with various inocula from geothermal and nongeothermal areas. Only polluted harbor sediment resulted in a stable enrichment culture that converted PCE via trichloroethene to cis-1, 2-dichloroethene at the optimum temperature of 60 to 65 degrees C. After several transfers, methanogens were eliminated from the culture. Dechlorination was supported by lactate, pyruvate, fructose, fumarate, and malate as electron donor but not by H2, formate, or acetate. Fumarate and L-malate led to the highest dechlorination rate. In the absence of PCE, fumarate was fermented to acetate, H2, CO2, and succinate. With PCE, less H2 was formed, suggesting that PCE competed for the reducing equivalents leading to H2. PCE dechlorination, apparently, was not outcompeted by fumarate as electron acceptor. At the optimum dissolved PCE concentration of approximately 60 microM, a high dechlorination rate of 1.1 micromol h-1 mg-1 (dry weight) was found, which indicates that the dechlorination is not a cometabolic activity. Microscopic analysis of the fumarate-grown culture showed the dominance of a long thin rod. Molecular analysis, however, indicated the presence of two dominant species, both belonging to the low-G+C gram positives. The highest similarity was found with the genus Dehalobacter (90%), represented by the halorespiring organism Dehalobacter restrictus, and with the genus Desulfotomaculum (86%).

Specificity of reductive dehalogenation of substituted ortho-chlorophenols by Desulfitobacterium dehalogenans JW/IU-DC1.

Resting cells of Desulfitobacterium dehalogenans JW/IU-DC1 growth with pyruvate and 3-chloro-4-hydroxyphenylacetate (3-Cl-4-OHPA) as the electron acceptor and inducer of dehalogenation reductively ortho-dehalogenate pentachlorophenol (PCP); tetrachlorophenols (TeCPs); the trichlorophenols 2,3,4-TCP, 2,3,6-TCP, and 2,4,6-TCP; the dichlorophenols 2,3-DCP, 2,4-DCP, and 2,6-DCP; 2,6-dichloro-4-R-phenols (2,6-DCl-4-RPs, where R is -H, -F, -Cl, -NO2, -CO2, or -COOCH3; 2-chloro-4-R-phenols (2-Cl-4-RPs, where R is -H, -F, -Cl, -Br, -NO2, -CO2-, -CH2CO2, or -COOCH3); and the bromophenols 2-BrP, 2,6-DBrP, and 2-Br-4ClP [corrected]. Monochlorophenols, the dichlorophenols 2,5-DCP, 3,4-DCP, and 3,5-DCP, the trichlorophenols 2,3,5-TCP, 2,4,5-TCP, and 3,4,5-TCP, and the fluorinated analog of 3-Cl-4-OHPA, 3-F-4-OHPA ("2-F-4-CH2CO2- P"), are not dehalogenated. A chlorine substituent in position 3 (meta), 4 (para), or 6 (second ortho) of the phenolic moiety facilitates ortho dehalogenation in position 2. Chlorine in the 5 (second meta) position has a negative effect on the dehalogenation rate or even prevents dechlorination in the 2 position. In general, 2,6-DCl-4-RPs are dechlorinated faster than the corresponding 2-Cl-4-RPs with the same substituent R in the 4 position. The highest dechlorination rate, however, was found for dechlorination of 2,3-DCP, with a maximal observed first-order rate constant of 19.4 h-1 g (dry weight) of biomass-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete biological reductive transformation of tetrachloroethene to ethane.

Reductive dechlorination of tetrachloroethene (perchloroethylene; PCE) was observed at 20 degrees C in a fixed-bed column, filled with a mixture (3:1) of anaerobic sediment from the Rhine river and anaerobic granular sludge. In the presence of lactate (1 mM) as an electron donor, 9 microM PCE was dechlorinated to ethene. Ethene was further reduced to ethane. Mass balances demonstrated an almost complete conversion (95 to 98%), with no chlorinated compounds remaining (less than 0.5 micrograms/liter). When the temperature was lowered to 10 degrees C, an adaptation of 2 weeks was necessary to obtain the same performance as at 20 degrees C. Dechlorination by column material to ethene, followed by a slow ethane production, could also be achieved in batch cultures. Ethane was not formed in the presence of bromoethanesulfonic acid, an inhibitor of methanogenesis. The high dechlorination rate (3.7 mumol.l-1.h-1), even at low temperatures and considerable PCE concentrations, together with the absence of chlorinated end products, makes reductive dechlorination an attractive method for removal of PCE in bioremediation processes.