Degradation pathways of PCBs upon UV irradiation in hexane.

The photodegradations of eight individual PCB congeners (5, 31, 52, 77, 87, 126, 138, 169) in hexane have been investigated employing a mercury lamp. All degradation reactions of the above mentioned PCB congeners are of the pseudo first order. The principal products of PCB decomposition are the less chlorinated biphenyls, and no PCB-solvent adducts are found. Symmetrical and coplanar PCB congeners show lower photoreactivities. The reactivities of the chlorine atoms at various positions of PCB rings are generally in the order: ortho > meta > para. Photodechlorinations occur mainly on the more substituted rings, when the numbers of chlorine atoms on the two phenyl rings are unequal. During photodegradation, some coplanar PCB congeners are formed, which make the TEQ of solutions to decrease slowly or even to increase.     

Further study on the photochemistry of non-ortho substituted PCBs by UV irradiation in alkaline 2-propanol

The photochemical behaviors of six non-ortho substituted PCB congeners, i.e., 3,4-DiCB, 3,5-DiCB, 3,3',5-TriCB, 3,4,5-TriCB, 3,3',4,5-TetraCB, and 3,4,4',5-TetraCB, irradiated at 254 nm in alkaline 2-propanol were investigated. Besides the determination of the photodechlorination pathways of these compounds, the presence of photorearrangement was observed in the case of 3, 4-DiCB with its products being identified. The results indicate that dechlorination is much more important than rearrangement during the process of PCB photolysis.     

Dechlorination of polychlorinated biphenyl congeners by anaerobic microorganisms from river sediment.

The microbial dechlorination of seven kinds of polychlorinated biphenyls (PCBs) by anaerobic microorganisms from river sediment was investigated. Dechlorination rates were found to be affected by the chlorine level of PCB congeners; dechlorination rates decreased as chlorine levels increased. Dechlorination rates were fastest under methanogenic conditions and slowest under nitrate-reducing conditions. The addition of individual electron donors (acetate, pyruvate, and lactate) enhanced the dechlorination of PCB congeners under methanogenic and sulfate-reducing conditions but delayed the dechlorination of PCB congeners under nitrate-reducing conditions. PCB congener dechlorination also was delayed by the addition of various polycyclic aromatic hydrocarbons (PAHs) under three reducing conditions and by surfactants, such as brij30, triton SN70, and triton N101. The results suggest that methanogen, sulfate-reducing bacteria, and nitrate-reducing bacteria all are involved in the dechlorination of PCB congeners.     

Microbial dechlorination of polychlorinated biphenyls in anaerobic sewage sludge.

The potential of a chlorophenol (CP)-adapted consortium to dechlorinate polychlorinated biphenyls (PCBs) in sewage sludge was investigated. Results show that dechlorination rates differed significantly depending on sludge source and PCB congener. Higher total solid concentrations in sewage sludge and higher concentrations of chlorine in PCB resulted in slower dechlorination rates. No significant difference was found for 2,3,4,5-CB dechlorination from pH 6.0 to pH 8.0; however, dechlorination did not occur at pH 9.0 during a 41-day incubation period. Results show that at concentrations of 1 to 10 mg/L, the higher the PCB concentration, the faster the dechlorination rate. In addition, dechlorination rates were in the following order: methanogenic conditions > sulfate-reducing conditions > denitrifying conditions. The addition of acetate, lactate, pyruvate, and ferric chloride decreased lag times and enhanced dechlorination; however, the addition of manganese dioxide had an inhibitory effect. Dechlorination rates were also enhanced by the addition of PCB congeners, including 2,3,4-CB, 2,3,4,5-CB and 2,3,4,5,6-CB in mixture. Overall results show that the CP-adapted consortium has the potential to enhance PCB dechlorination. The optimal dechlorination conditions presented in this paper may be used as a reference for feasibility studies of PCB removal from sludge.     

Microbial transformation and degradation of polychlorinated biphenyls

This paper reviews the potential of microorganisms to transform polychlorinated biphenyls (PCBs). In anaerobic environments, higher chlorinated biphenyls can undergo reductive dehalogenation. Meta- and para-chlorines in PCB congeners are more susceptible to dechlorination than ortho-chlorines. Anaerobes catalyzing PCB dechlorination have not been isolated in pure culture but there is strong evidence from enrichment cultures that some Dehalococcoides spp. and other microorganisms within the Chloroflexi phylum can grow by linking the oxidation of H(2) to the reductive dechlorination of PCBs. Lower chlorinated biphenyls can be co-metabolized aerobically. Some aerobes can also grow by utilizing PCB congeners containing only one or two chlorines as sole carbon/energy source. An example is the growth of Burkholderia cepacia by transformation of 4-chlorobiphenyl to chlorobenzoates. The latter compounds are susceptible to aerobic mineralization. Higher chlorinated biphenyls therefore are potentially fully biodegradable in a sequence of reductive dechlorination followed by aerobic mineralization of the lower chlorinated products.     

Polychlorinated biphenyls and dibenzofurans retained in the tissues of a deceased patient with Yucheng in Taiwan

Various tissues from a deceased Yucheng (oil disease) patient were analyzed for individual polychlorinated biphenyl (PCB) and dibenzofuran (PCDF) congeners. With a few exceptions, PCBs with 5 or less chlorine atoms were almost completely excreted from the patient's tissues about two years after the ingestion of the toxic oil. In contrast, most PCB congeners with 6 or more chlorine atoms were retained in the tissues. The intestinal fat contains the highest level of PCBs while the liver contains the highest concentration of PCDFs. The major PCDF congeners retained in the tissues were 1,2,3,4,7,8-hexaCDF, 2,3,4,7,8-pentaCDF, and 1,2,4,7,8-pentaCDF. The former two congeners, especially 2,3,4,7,8-pentaCDF, are very toxic PCDFs; they may play important roles in the etiology of Yucheng.     

The study on the dechlorination of OCDD with Pd/C catalyst in ethanol-water solution under mild conditions

Dechlorination of octachlorodibenzo-p-dioxin (OCDD) was carried out in ethanol-water (v/v=1:1) solution of NaOH in the presence of Pd/C catalysts with the use of H(2). The substrate was dechlorinated with Pd/C under mild conditions (atmospheric pressure and <100 degrees C) to give a chlorine-free product, dibenzo-p-dioxin (DD), in high yields. After reaction of 3h at 50 degrees C, 95.9% OCDD was degraded to low dechlorinated congeners and the yield of DD was 77.4%. We have also studied the dechlorination selectivity of chlorine atoms on the different substituted positions and postulated the dechlorination pathway of OCDD. For OCDD, the 2-position has higher reactivity than 1-position, but the difference is very small. From the distribution statistics of the intermediates during the reaction, we postulate that the steric effect plays an important role during the reaction and affect the dechlorination pathway of OCDD.     

Reductive dechlorination of non-ortho substituted polychlorinated biphenyls by ultraviolet irradiation in alkaline 2-propanol

The photodechlorination pathways of 3,3′,4,5,5′-pentachlorobiphenyl (3,3′,4,5,5′-PentaCB) and the related lower polychlorinated biphenyl congeners irradiated at 254 nm in alkaline 2-propanol were elucidated. Steric effect is the most important factor for controlling the dechlorination pattern of these substrates. Electronic effect also influences the photoreactivity of chlorine substituents.     

Pathways and products of the degradation of PCBs by the sodium dispersion method

Nine polychlorinated biphenyl (PCB) congeners (2-chlorobiphenyl, 3-chlorobiphenyl, 4-chlorobiphenyl, 2,3,4-trichlorobiphenyl, 2,2',5,5'-tetrachlorobiphenyl, 2,3',4,4',5-pentachlorobiphenyl, 3,3',4,4',5-pentachlorobiphenyl, 2,2',4,4',5,5'-hexachlorobiphenyl, and decachlorobiphenyl) were dechlorinated by the sodium dispersion method (SD) at low temperature (60 degrees C). The dechlorination of 4-chlorobiphenyl was the fastest among the three monochlorobiphenyls. As for the other six congeners, we investigated the major dechlorination pathways. Although reaction selectivity was not very sensitive to the position of the chlorine substituent, the chlorines at the para position were slightly easier to dechlorinate than those at the ortho or meta positions. The decomposition rate increased with the total numbers of chlorine substituents. A chlorine situated between two other chlorines showed a high reactivity. When the numbers of chlorines on each of the phenyl rings were different, the reactions occurred on the more substituted ring. In the degradation of 4-chlorobiphenyl at elevated temperature (160 degrees C), we investigated the structures of the polymerized products and whether all the organic chlorinated compounds degraded finally or not. As for the dimers, p-quarterphenyl (QP) and m,p-QP were detected but not o-QP, m-QP, o,p-QP, o,m-QP, or the mono- to tetra-chlorinated QPs. Compounds with a molecular weight of 534.4183 or 758.6713 were detected. They were considered to have C40H54 or C56H86 as their molecular formula. The compounds were most probably the polymerized products resulting from coupling of hexadecane or two hexadecanes and two phenylcyclohexadienes. It was thought the dechlorination and the polymerization were the main reactions. All of many detected compounds were hydrocarbons without chlorines, and no peaks originating from organic chlorinated compounds were observed by mass spectroscopic (MS) methods.     

Photodechlorination of octachlorodibenzothiophene and octachlorodibenzofuran: comparison of experimental degradation pathways with degradation pathways predicted by DFT

Polychlorodibenzothiophenes (PCDTs) are sulfur analogues of polychlorodibenzofurans (PCDFs) and have been detected in environmental samples. We used density functional theory calculations (Gaussian 98W) to predict the photodechlorination pathways of octachlorodibenzothiophene (OCDT) and octachlorodibenzofuran (OCDF) in hexane, and we compared the predicted pathways with those observed during UV irradiation experiments. OCDT and OCDF were observed to degrade through first-order dechlorination processes, and the rate constant for OCDT was less than one-third that for OCDF. The main experimental photodechlorination pathways of OCDT and OCDF led to hexachlorinated and tetrachlorinated congeners, respectively; that is, the photodechlorination pathway of OCDT differed from that of OCDF. On the assumption that the dechlorination mechanisms involved radical reactions, we used DFT calculations to estimate bond-dissociation energies and single-point energies of OCDT and OCDF and their dechlorinated congeners, and we used the resulting information, along with hypotheses regarding the rate-controlling step of the degradations, to predict theoretical degradation pathways. We propose that reaction of dechlorinated radicals with a hydrogen donor was the rate-controlling step for OCDT and that C-Cl bond dissociation by UV light was the rate-controlling step for OCDF.     

Electron transfer sensitized photodechlorination of surfactant solubilized PCB 138

Sensitized photodechlorination of polychlorinated biphenyl, PCB 138, in three different surfactant solutions was studied. The sensitizer of choice was leuco-methylene blue, which was produced in situ from methylene blue using either triethylamine or sodium borohydride. Three types of surfactants, anionic (SDS), neutral (TWEEN 80), and cationic (CTAB) at different concentrations were investigated. The neutral and cationic surfactants were found to be more effective than anionic. In each case the surfactant concentration was found to play a significant role in the rate of dechlorination. For different sensitized systems (triethylamine or sodium borohydride), a different product distribution and a different pathway of dechlorination was observed.     

Correlation of gas chromatographic properties of chlorobenzenes and polychlorinated biphenyls with the occurrence of reductive dechlorination by untamed microorganisms

To understand the dechlorination ability of chlorobenzenes (CBs) and polychlorinated biphenyls (PCBs) by untamed microorganisms under anaerobic condition and to correlate gas chromatographic properties with the occurrence of reductive dechlorination, introduction of CBs and PCBs in the culture medium inoculated with microorganisms from sludge and sediment, respectively, were performed. Three kinds of culture media preparing from sludge, river water and a synthetic medium were used in the experiments. HCB was degraded to 1,3,5-trichlorobenzene (1,3,5-TCB) and 1,3-dichlorobenzene (1,3-DCB) in both sludge medium and synthetic medium with inoculated microorganisms. Three PCB congeners including 2,3,4-, 3,4,5- and 2,3,4,5-CBp (chlorinated biphenyl) were not found to be dechlorinated in the river water medium with inoculation culture but to be dechlorinated in the synthetic medium. MNDO methodology was used to compute theoretical dechlorination reaction heats and GC-ECD techniques were used to estimate chromatographic data of CB and PCB congeners. Both CB and PCB congeners showed that dechlorination by untamed microorganisms under anaerobic mixed cultures were more likely to occur when larger amounts of energy were released and greater deltaln RRT value between the parent congener and the daughter product was observed. Deltaln RRT provided a more precise information on the singularity of PCBs ortho-dechlorination in an aspect of thermodynamic favorable rule.     

Dechlorination of PCBs by electrochemical reduction with aromatic radical anion as mediator

Mediated electrochemical reduction was applied to the dechlorination of polychlorinated biphenyls (PCBs) in tetra-n-butylammonium perchlorate/dimethylformamide solution. Rapid and complete dechlorination was possible with biphenyl or naphthalene as the mediator, whereas the reaction was much slower with anthracene or 9,10-diphenylanthracene. The reaction rate was so high with naphthalene or biphenyl radical anion that differences in reactivity could not be observed between congeners. Side reactions, other than biphenyl formation, could occur depending upon the mediators and the substrates, but contributed less than 10% of the total products in the case of naphthalene-mediated dechlorination. Almost all chloride ion formed in the dechlorination remained in the cathode solution. The order of the reaction was determined to be 0.5 for the substrate and 1 for the mediator (naphthalene) in the dechlorination of 2-chlorobiphenyl; identical to results for the mediated dechlorination of 1-chloronaphthalene. The reaction rate in practical PCB dechlorination could be estimated with the use of the initial concentration of the mediator and chlorine content of the solution provided that the problem of the deactivation of the electrode surface could be solved.     

Dynamics of PCB removal and detoxification in historically contaminated soils amended with activated carbon

Activated carbon (AC) can help overcome toxicity of pollutants to microbes and facilitate soil bioremediation. We used this approach to treat a Histosol and an Alluvial soil historically contaminated with PCB (4190 and 1585 mg kg⁻¹, respectively; primarily tri-, tetra- and pentachlorinated congeners). Results confirmed PCB persistence; reductions in PCB extractable from control and AC-amended soils were mostly due to a decrease in tri- and to some extent tetrachlorinated congeners as well as formation of a bound fraction. Mechanisms of PCB binding by soil and AC were different. In addition to microbial degradation of less chlorinated congeners, we postulate AC catalyzed dechlorination of higher chlorinated congeners. A large decrease in bioavailable PCB in AC-amended soils was demonstrated by greater clover germination and biomass. Phytotoxicity was low in treated soils but remained high in untreated soils for the duration of a 39-month experiment. These observations indicate the utility of AC for remediation of soils historically contaminated with PCB.     

Theoretical study of the dechlorination reaction pathways of octachlorodibenzo-p-dioxin

The dechlorination reaction pathways of 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin (OCDD) by the hydrogen atom are investigated by the density-functional theory B3PW91 method. The dechlorination reactions have large exothermicity and small activation energies. The activation energies (approximately 5 kcal/mol) of the sigma-complex formation due to the hydrogen addition are lower than those (approximately 9 kcal/mol) of the direct chlorine abstraction. It is suggested that the sigma-complex plays an important role in the reactions, although it has scarcely been shown in previous studies of the dechlorination of dioxins. The sigma-complex formation is favored at low temperatures and the chlorine abstraction is favored at high temperatures. Furthermore, it is found that the lateral positions have a marginal preference over the longitudinal positions. The dechlorination of OCDD by the hydrogen atom is thus not likely to result in a dominant formation of the laterally substituted toxic congeners.     

Distribution characteristics of PCBs in the sediments of the lower Nakdong River, Korea

The objective of this study is to investigate the distribution characteristics of polychlorinated biphenyl (PCB) congeners in sediments of the lower Nakdong River of Korea. The sediments were collected using a grab sampler in May 1999. PCBs were extracted in the Soxhlet extractor and cleaned by using adsorption chromatographic techniques and concentrated sulfuric acid. A HP6890 gas chromatograph equipped with an electron capture detector was used for quantification. The results of congener specific analysis showed that the total concentration is ranged over 1.1-141 ng/g dry weight. The levels of PCB congeners are significantly correlated with the total organic carbons (TOC) in the sediments. The major fractions of PCBs in these sediments are congeners containing 4, 5 or 6 chlorine atoms per biphenyl molecule.     

A survey of PCB congeners in U.K. cows'' milk

Samples of unpasteurised bulked milk, taken directly from ten herds of dairy cattle on rural and urban farms in the north west of England on five separate sampling occasions, were analysed for a range of PCB congeners. ΣPCB concentrations (sum of 37 congeners) ranged from 3.4–16.4 ng/g milk fat with a mean ΣPCB concentration of 8.4 ng/g milk fat. The dominating congeners were 118, 153, 138 and 180, which contributed 15%, 20%, 17% and 9% of the ΣPCB load respectively. The chlorine pattern of the congeners which made moderate or major contributions to the ΣPCB concentration were typically substituted at both para positions (4, 4′), while the PCB congeners not detected in the milk had at least one ring that was not 4-substituted. These results indicate the 4,4′ substitution pattern as being the key to PCB persistence in cows. It is estimated that consumption of typical daily intakes of milk with the PCB concentrations measured in this study would contribute 11 % of the average daily ΣPCB intake for individuals in the UK. This contribution would increase to 30% when exposure through the consumption of dairy products prepared from such milk (e.g. cheese, butter) is taken into account. It is estimated that the inclusion of the TEF assigned PCBs would typically increase the TEQ rating of cows' milk by approximately 40% over that attributed to PCDD/Fs alone.     

Ability of bacterial biphenyl dioxygenases from Burkholderia sp. LB400 and Comamonas testosteroni B-356 to catalyse oxygenation of ortho-hydroxychlorobiphenyls formed from PCBs by plants

Capacity of enzymes of the biphenyl/chlorobiphenyl pathway, especially biphenyl dioxygenase (BPDO) of two polychlorinated biphenyls (PCB) degrading bacteria, Burkholderia sp. LB400 and Comamonas testosteroni B-356, to metabolize ortho-substituted hydroxybiphenyls was tested.,These compounds found among plant products of PCB metabolism, are carrying chlorine atoms on the hydroxyl-substituted ring. The abilities of His-tagged purified LB400 and B-356 BPDOs to catalyze the oxygenation of 2-hydroxy-3-chlorobiphenyl, 2-hydroxy-5-chlorobiphenyl and 2-hydroxy-3,5-dichlorobiphenyl were compared. Both enzyme preparations catalyzed the hydroxylation of the three chloro-hydroxybiphenyls on the non-substituted ring. Neither LB400 BPDO nor B-356 BPDO oxygenated the substituted ring of the ortho-hydroxylated biphenyl. The fact that metabolites generated by both enzymes were identical for all three hydroxychlorobiphenyls tested; exclude any other mode of attack of these compounds by LB400 BPDOs than the ortho-meta oxygenation.     

Prediction of the bioconcentration factor of PCBs in fish using the molecular connectivity index and fragment constant models.

Models based on molecular connectivity index (MCI) and fragment constant (FC) method were developed for prediction of the bioconcentration factor (BCF) for polychlorobiphenyls (PCBs) in fish. The mean residuals for the MCI and FC models were 0.195 and 0.223 log units, respectively. The two models were then compared in terms of their mean residuals. In addition to the chlorine atom substitution number, other important structural features exhibiting a significant influence on the BCFs of PCB congeners were discussed and incorporated to the models. These features include the degree of the ortho-substitution, the presence of chlorine pairs in the three- and five- positions, and the crowding of chlorine atoms on the phenyl ring.     

Catalyzed dehalogenation of Delor 103 by sodium hydridoaluminate

The complete dechlorination of PCB liquid Delor 103 (42.6% Cl) to biphenyl has been effected in toluene with sodium dihydridobis(2-methoxyethoxo)aluminate (SDMA) in the presence of catalytic amounts of Ni(II) and Co(II) 2,4-pentanedionates at elevated temperatures. Changes in PCB congeners distribution during dechlorination were monitored by GC(ECD) and GC/MS, and the scheme of their transformation was proposed on the basis of site selectivity observed. A kinetic analysis of the reaction is presented.