Myeloperoxidase-catalyzed formation of PCDD/F from chlorophenols

PCDD/F determination in sewage sludge composting. Influence of aeration and the presence of PCP

Composting of sewage sludge is a common practice for sludge disposal. Some previous studies found high levels of polychorodibenzo-p-dioxins and polychorodibenzofurans (PCDD/Fs) after composting, especially octachlorodibenzo-p-dioxin (OCDD) but also 1234678-heptachlorodibenzo-p-dioxin (1234678-HpCDD) to a lesser extent. In this work, the concentrations of OCDD, 1234678-HpCDD and the rest of the 17 toxic congeners of PCDD/Fs were determined in compost obtained under different conditions. Although the toxicity of the two compounds mentioned above is small, their generation may reach undesirable levels. The PCDD/F content was analyzed in a composting plant and in a laboratory test. In both cases, the composted material was a mixture of sewage sludge, straw and sawdust. The composting plant was a tunnel with air turbine aeration and with a turner to homogenize and move the mixture upwards. The laboratory tests were carried out with Dewar vessels (with air dispersion at the bottom and controlled temperature) and with small vessels inside a controlled oven with non-forced aeration. The laboratory runs were also carried out with the addition of pentachlorophenol in some runs, as a dioxin precursor. The highest OCDD levels were found in three samples of the composting plant (30000-90000pg/g dry matter or dm), with toxicity values surpassing the limit level for soil amendment (17pgI-TEQ/gdm). Their formation was analyzed considering their concentration vs. that of octachorodibenzofuran (OCDF), which is not formed during composting. In the laboratory, in experiments carried out in a vessel with non-forced aeration conditions and with the addition of pentachlorophenol, the formation of OCDD was significant (e.g. from 80 to 1500pg/gdm). That means that these two factors, non-forced aeration and the presence of pentachlorophenol, can cause the OCDD formation.

The role of CuCl on the mechanism of dibenzo-p-dioxin formation from poly-chlorophenol precursors: A computational study

A computational study is performed for the elucidation of the role played by CuCl in the condensation of two polychlorophenol molecules to yield PCDDs. The mechanism found consists of six sequential steps, which allow the final recuperation of the CuCl molecule, and applies for phenol molecules with an ortho chlorine. In the temperature range of 453-473 K (previously reported as adequate to diminish PCDDs formation in the post-combustion area), CuCl is able to softly retain chlorophenol molecules, mainly those less chlorinated. After a first HCl release, Cu(I) remains bonded to phenol oxygen atom, thus avoiding the formation of phenoxy radicals and the subsequent radical processes. A temperature raise up to 1200 K destabilizes the initial CuCl-chlorophenol complexes and causes that the rate limiting step change from the formation of the first oxygen bridge to HCl elimination. It has been checked that tetra and penta-chlorophenols undergo essentially the same reaction process of 2-chlorophenol. In view of our results and trying to arrive at a practical way to diminish the rate of formation of PCDDs, we propose that an extra addition of powdered CuCl to the post-combustion zone, cooled down to temperatures lower than 473 K, could act as an inhibitor in the formation of these pollutants.

PCDD/F formation from chlorophenols by lignin and manganese peroxidases

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) formation was studied, in vitro, with two different chlorophenol mixtures (group "di+tri" 2,4-dichlorophenol; 2,3,4-, 2,3,5-, and 3,4,5-trichlorophenols and group "tri+tetra+penta" with 2,4,5-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol) and two different lignolytic enzymes, lignin and manganese peroxidase (LiP and MnP respectively), which can be found during the composting process of sewage sludge. The concentrations of PCDD/F in final samples are compared to the PCDD/F content of the control samples containing the chlorophenols. High increases were observed for experiments with MnP and phosphate buffer. Experiments that contained tri-, tetra- and pentachlorophenol with MnP resulted in more than 8·10(8)ng of OCDD kg(-1) chlorophenol which was much higher than the initial amount (1·10(7)ng OCDD kg(-1) chlorophenol). In relation to LiP experiments, only those at 37°C showed a moderate increase (from 1.3·10(7) to 2.6·10(7)ng of OCDD kg(-1) chlorophenol). The results agree with the literature in which high amounts of HpCDD and OCDD were found after a composting process and could explain the biogenic formation suggested by others, but the incidence on the total toxicity is less than that expected.

Formation of environmentally relevant brominated dioxins from 2,4,6,-tribromophenol via bromoperoxidase-catalyzed dimerization

Polybrominated dibenzo-p-dioxins (PBDD) are emerging environmental pollutants with structural similarities to the highly characterized toxicants polychlorinated dibenzo-p-dioxins. The geographical and temporal variations of PBDD in biota samples from the Baltic Sea do not display features that are normally related to anthropogenic sources such as incineration, and therefore the natural formation of PBDDs has been suggested. This study of the bromoperoxidase mediated oxidative coupling of 2,4,6-tribromophenol (an abundant substance that is naturally formed in marine systems) identified the formation of ppb-level yields of 1,3,6,8-tetrabromodibenzo-p-dioxin (1,3,6,8-TeBDD) through direct condensation. Additional TeBDDs (1,3,7,9-TeBDD, 1,2,4,7-TeBDD, and/or 1,2,4,8-TeBDD) and tri-BDDs (1,3,7-TrBDD and 1,3,8-TrBDD) were frequently formed but at lower yields. The formation of these TeBDDs probably proceeds via bromine shifts or Smiles rearrangements, whereas the TrBDDs may result from subsequent debromination processes. Because all of the congeners formed by oxidative coupling and subsequent reactions are also found in Baltic Sea biota, the results support the theory that PBDDs are formed from natural precursors.

Combined toxicity of three chlorophenols 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol to Daphnia magna

The toxicity of single and combined mixtures of 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and pentachlorophenol (PCP) to Daphnia magna was studied. The toxicity ranking of these three single chlorophenols (CPs) to Daphnia magna was PCP > 2,4-DCP > 2,4,6-TCP. The toxic units (TU) approach was used to estimate the combined effects in experiments, the median effective concentration (EC(50)) values were 0.87-1.21 and 0.46-0.59 for binary and ternary mixtures, respectively. Response surface models of General Linear Models (R(2) > 0.90, residual deviation < 3.25) were established for all three binary mixtures. The toxicity for ternary mixtures based on the EC(50)-value and 10% effective concentration (EC(10))-value fixed mixture ratio presented a synergism. The risk based on the single CP's toxicity test may be underestimated. In addition, four approaches (concentration addition, toxicity equivalency factors, effect summation, and independent action) were used for the calculation of combined effects of the mixture. The experimental results showed that concentration addition and toxicity equivalency factor approaches were effective methods for calculation of additive effects of mixtures from binary systems of CPs; while independent action and effect summation (low simulated tail) predicted lower toxicity than experimental results. Limitations of the traditional focus on the effects of single agents were highlighted; hazard assessments ignoring the possibility of joint action of CPs will almost certainly lead to significant underestimations of risk.

Degradation of pentachlorophenol and 2,4-dichlorophenol by sequential visible-light driven photocatalysis and laccase catalysis

Chlorophenols (CPs) can be degraded by visible-light driven photocatalysis or laccase catalysis. However, previous and present studies have shown that neither of the two methods was efficient when being used individually. Low degradation rates were observed for the degradation of pentachlorophenol (PCP) by laccase-catalysis and that of 2,4-dichlorophenol (2,4-DCP) by photocatalysis. To remove CPs more completely, a sequential photolaccase catalytic system was designed to degrade PCP and 2,4-DCP mixture in water at the optimal pH value. The results showed that photocatalysis prior to laccase-catalysis (PPL) is a better approach than laccase-catalysis prior to photocatalysis (LPP), eliminating CPs more efficiently and generating lower toxic products. The identified intermediate products consisted of adipic acid, hexanediol, glycol, propylene glycol, hydroquinol, and phthalandione. Based on the products identified, the sequential degradation process was proposed, including the interlace reactions involving quinoid oxidation, reductive dechlorination, and no-enzyme polymerization. Upon reaction optimization, a piston flow reactor (PFR) was designed to treat the continuous feeding of simulated wastewater containing PCP and 2,4-DCP. After a 128 h period of treatment, 87.4-99.5% total concentration of CPs were removed (PPL removed 99.7% PCP and 99.2% 2,4-DCP; LPP removed 95.9% PCP and 78.9% 2,4-DCP).

Theoretical targets for TCDD: a bioinformatics approach

Dioxins are a group of highly toxic molecules that exert their toxicity through the activation of the aryl hydrocarbon receptor (AhR). The most important agonist of the AhR, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a highly toxic compound. Although most of the effects related to TCDD exposure have been linked to the activation of AhR, the objective of this work was to use a bioinformatics approach to identify possible new targets for TCDD. The Target Fishing Docking (TarFisDock) Server was used to find target proteins for TCDD. This virtual screening allowed the identification of binding sites with high affinity for TCDD in diverse proteins, such as metallopeptidases 8 and 3, oxidosqualene cyclase, and myeloperoxidase. Some of these proteins are well known for their biochemical role in some pathological effects of dioxin exposure, including endometriosis, diabetes, inflammation and liver damage. These results suggest that TCDD could also be interacting with cellular targets though AhR-independent pathways.

Chloroperoxidase-mediated transformation of highly halogenated monoaromatic compounds

Peroxidase transformations of widely distributed pollutants, tetra- and penta-chlorinated phenols and anilines, were studied using different peroxidases. Chloroperoxidase from Caldariomyces fumago was able to transform tetra- and penta-chlorinated phenols and anilines, while horseradish peroxidase, lignin peroxidase from Phanerochaete chrysosporium and versatile peroxidase from Bjerkandera adusta were able only to transform the halogenated phenols. Chloroperoxidase showed a specific activity on pentachlorophenol two orders of magnitude higher than lignin peroxidase and horseradish peroxidase, and one order of magnitude higher than versatile peroxidase. The main product from peroxidase oxidation in all cases was a polymeric and insoluble material. The insolubilization of halogenated phenols and anilines permits their removal, reduces their bioavailability, and thus reduces their environmental impact. The other minor products from the enzymatic transformation of highly chlorinated compounds were determined by mass spectrometry. Tetrachloroquinone, dimers and trimers of halogenated compounds were also identified. Chloroperoxidase was able to halogenate tetrachloroaniline to form pentachloroaniline.

Microbial degradation of chlorinated dioxins

Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) were introduced into the biosphere on a large scale as by-products from the manufacture of chlorinated phenols and the incineration of wastes. Due to their high toxicity they have been the subject of great public and scientific scrutiny. The evidence in the literature suggests that PCDD/F compounds are subject to biodegradation in the environment as part of the natural chlorine cycle. Lower chlorinated dioxins can be degraded by aerobic bacteria from the genera of Sphingomonas, Pseudomonas and Burkholderia. Most studies have evaluated the cometabolism of monochlorinated dioxins with unsubstituted dioxin as the primary substrate. The degradation is usually initiated by unique angular dioxygenases that attack the ring adjacent to the ether oxygen. Chlorinated dioxins can also be attacked cometabolically under aerobic conditions by white-rot fungi that utilize extracellular lignin degrading peroxidases. Recently, bacteria that can grow on monochlorinated dibenzo-p-dioxins as a sole source of carbon and energy have also been characterized (Pseudomonas veronii). Higher chlorinated dioxins are known to be reductively dechlorinated in anaerobic sediments. Similar to PCB and chlorinated benzenes, halorespiring bacteria from the genus Dehalococcoides are implicated in the dechlorination reactions. Anaerobic sediments have been shown to convert tetrachloro- to octachlorodibenzo-p-dioxins to lower chlorinated dioxins including monochlorinated congeners. Taken as a whole, these findings indicate that biodegradation is likely to contribute to the natural attenuation processes affecting PCDD/F compounds.

Metabolic fate of [(14)C]-2,4-dichlorophenol in tobacco cell suspension cultures

In plant tissues, xenobiotics often are conjugated with natural constituents such as sugars, amino acids, glutathione, and malonic acid. Usually, conjugation processes result in a decrease in the reactivity and toxicity of xenobiotics by increasing the water solubility and polarity of conjugates, and reducing their mobility. Due to their lack of an efficient excretory system, xenobiotic conjugates finally are sequestered in plant storage compartments or cell vacuoles, or are integrated as bound residues in cell walls. Chlorophenols are potentially harmful pollutants that are found in numerous natural and agricultural systems. We studied the metabolic fate of 2,4-dichlorophenol (DCP) in cell-suspension cultures of tobacco (Nicotiana tabacum L.). After a standard metabolism experiment, 48 h of incubation with a [U-phenyl-(14)C]-DCP solution, aqueous extracts of cell suspension cultures were analyzed by high-performance liquid chromatography (HPLC). Metabolites then were isolated and their chemical structures determined by enzymatic and chemical hydrolyses, electrospray ionization-mass spectrometry in negative mode (ESI-NI), and (1)H nuclear magnetic resonance analyses. The main terminal metabolites identified were DCP-glycoside conjugates, DCP-(6-O-malonyl)-glucoside, DCP-(6-O-acetyl)-glucoside, and their precursor, DCP-glucoside. More unusual and complex DCP conjugates such as an alpha(1-->6)-glucosyl-pentose and a triglycoside containing a glucuronic acid were further characterized. All the metabolites identified were complex glycoside conjugates. However, these conjugates still may be a source of DCP in hydrolysis reactions caused by microorganisms in the environment or in the digestive tract of animals and humans. Removal of xenobiotics by glycoside conjugation thus may result in underestimation of the risk associated with toxic compounds like DCP in the environment or in the food chain.

Metabolism of [14C]-2,4-dichlorophenol in edible plants

Several 2,4-dichlorophenoxyacetic acid (2,4-D)-sensitive plants have been modified by genetic engineering with tfdA gene to acquire 2,4-D tolerance. The expression product of this gene degrades 2,4-D to 2,4-dichlorophenol (DCP), which is less phytotoxic but could cause a problem of food safety. After a comparison of 2,4-D and DCP metabolism in transgenic 2,4-D-tolerant and wild cotton (Gossypium hirsutum L.), a direct study of DCP metabolism in edible plants was performed. After petiolar uptake of a [U-phenyl-(14)C]-DCP solution followed by a 48 h water chase, aqueous extracts were analysed by high-performance liquid chromatography. Metabolites were thereafter isolated and their structural identities were determined by enzymatic and chemical hydrolyses and mass spectrometry analyses. The metabolic fate of DCP was equivalent to 2,4-D metabolism in transgenic 2,4-D-tolerant cotton. In addition, DCP metabolism was similar in transgenic and wild cotton. The major terminal metabolites were DCP-saccharide conjugates in all species, essentially DCP-(6-O-malonyl)-glucoside or its precursor DCP-glucose. The significance of this metabolic pathway with regard to food safety is discussed.

Metabolic fate of [14C]-2,4-dichlorophenol in macrophytes

The metabolic fate of 2,4-dichlorophenol (DCP) was investigated in six macrophytes representing different life forms. Salvinia natans and Lemna minor were chosen as surface-floating plants, Glyceria maxima and Mentha aquatica as emergent species and Myriophyllum spicatum and Hippuris vulgaris as submerged aquatic plants. After uptake of a [U-phenyl-14C]-DCP solution followed by a 48 h water chase, whole plants (L. minor, S. natans) or excised shoots were harvested and aqueous extracts were analysed by high performance liquid chromatography (HPLC). Metabolites were then isolated, submitted to enzymatic or chemical hydrolyses and characterised by electrospray ionisation-mass spectrometric analyses. Whereas DCP monoglucosides or more complex monoglucoside esters, either malonyl or acetyl, were found in most species, an unusual glucosyl-pentose conjugate was identified as the DCP major metabolite in L. minor and G. maxima. Our results showed for the first time the ability of five macrophytes to uptake and metabolise DCP and the characterisation of their metabolic pathways of DCP biotransformation.

The diversity of naturally produced organohalogens

More than 3800 organohalogen compounds, mainly containing chlorine or bromine but a few with iodine and fluorine, are produced by living organisms or are formed during natural abiogenic processes, such as volcanoes, forest fires, and other geothermal processes. The oceans are the single largest source of biogenic organohalogens, which are biosynthesized by myriad seaweeds, sponges, corals, tunicates, bacteria, and other marine life. Terrestrial plants, fungi, lichen, bacteria, insects, some higher animals, and even humans also account for a diverse collection of organohalogens.

Half-lives of tetra-, penta-, hexa-, hepta-, and octachlorodibenzo-p-dioxin in rats, monkeys, and humans--a critical review

The elimination half-lives (t1/2) in Sprague-Dawley rats for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2, 3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD), 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) and 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin (OCDD) were estimated in long-term studies by Schlatter, Poiger and others. Furthermore, there are some published half-lives of TCDD in adult humans. The average half-life of TCDD in adult humans is approximately 2840 days, while in Sprague-Dawley rats the average t1/2 of TCDD is 19 days. The t1/2 of TCDD in humans is about 150 times that of rats. This factor was used to calculate the t1/2 values of the other polychlorinated dibenzo-p-dioxins (PCDDs) in humans from the rat data. Furthermore, the terminal t1/2 values of PCDDs in adult humans were calculated from the regression equation: logt1/2H = 1.34 logt1/2R + 1.25 which was recently established for 50 xenobiotics (t1/2H = terminal half-lives in days for humans, t1/2R = terminal half-lives in days for rats). The following terminal half-lives in adult humans were obtained: 12.6 years for 1,2,3,7,8-PeCDD, 26-45 years for 1,2,3,4,7,8-HxCDD, 80-102 years for 1,2,3,4,6,7,8-HpCDD and ca. 112-132 years for OCDD. These half-lives of PCDDs are critically compared with measured t1/2 values of PCDDs and other persistent organic pollutants in rats, monkeys and humans.