Electrocatalytic dechlorination of atrazine

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), a photosynthetic inhibitor that is used in large quantities for weed control in corn and sorghum, is dechlorinated in aqueous solution upon electrolysis at a reticulated vitreous carbon cathode in the presence of noble-metal catalysts. Electrocatalytic hydrogenolysis to 2-ethylamino-4-isopropylamino-s-triazine occurs in quantitative yield, and is most efficient with palladium-based catalysts. Current efficiency increases with atrazine and catalyst concentration, and decreasing current density. A previously unobserved phenomenon with Pd catalysts is that current must be passed for a certain time before dechlorination commences. This lag time is explained in terms of the palladium lattice absorbing a finite amount of hydrogen before catalytically active hydrogen atoms appear on the catalyst surface.Key words: electrocatalytic hydrogenoloysis, dechlorination, atrazine, palladium catalysts.

Electrochemical treatment of acidic aqueous ferrous sulfate and copper sulfate as models for acid mine drainage

Acid mine drainage (AMD) is a serious environmental problem in the mining industry. The present work describes electrolytic reduction of solutions of synthetic AMD, comprising FeSO4/H2SO4 and CuSO4/H2SO4, in flow-through cells whose anode and cathode compartments were separated using ion exchange membranes. In the case of FeSO4/H2SO4 at constant flow rate, the pH of the effluent from the catholyte increased progressively with current at a variety of cathodes, due to electrolytic reduction of H+ ions to elemental hydrogen. Near-quantitative removal of iron was achieved by sparging air into the catholyte effluent, thereby precipitating iron outside the electrochemical cell, and avoiding fouling of the electrodes. The anode reaction was the oxidation of water to O2, a proton-releasing process. Using cation exchange membranes and sodium sulfate as the supporting electrolyte in the anode compartment, the efficiency of the process was compromised at high currents by transport of H+ competitively with Na+ from the anode to the cathode compartments. Higher efficiencies were obtained when anion exchange membranes were used, and in this case no additional supporting electrolyte other than dilute H2SO4 was needed, the net reaction being the electrochemically driven transfer of the elements of H2SO4 from the cathode to the anode compartments. Current efficiencies approximately 50% were achieved, the loss of efficiency being accounted for by ohmic heating of the solutions. In the case of CuSO4/H2SO4 and anion exchange membranes at high currents, reduction of Cu2+ and H+ ions and transport of SO4(2-) ions out of the catholyte caused unacceptably high potentials to be generated.

Synthesis of polybrominated diphenyl ethers and their capacity to induce CYP1A by the Ah receptor mediated pathway

Polybrominated diphenyl ethers (PBDEs) have become widely distributed as environmental contaminants due to their use as flame retardants. Their structural similarity to other halogenated aromatic pollutants has led to speculation that they might share toxicological properties such as hepatic enzyme induction. In this work we synthesized a number of PBDE congeners, studied their affinity for rat hepatic Ah receptor through competitive binding assays, and determined their ability to induce hepatic cytochrome P-450 enzymes by means of EROD (ethoxyresorufin-O-deethylase) assays in human, rat, chick, and rainbow trout cells. Both pure PBDE congeners and commercial PBDE mixtures had Ah receptor binding affinities 10(-2)-10(-5) times that of 2,3,7,8-tetrachlorodibenzo-p-dioxin. In contrast with polychlorinated biphenyls, Ah receptor binding affinities of PBDEs could not be related to the planarity of the molecule, possibly because the large size of the bromine atoms expands the Ah receptor's binding site. EROD activities of the PBDE congeners followed a similar rank order in all cells. Some congeners, notably PBDE 85, did not follow the usual trend in which strength of Ah receptor binding affinity paralleled P-450 induction potency. Use of the gel retardation assay with a synthetic oligonucleotide indicated that in these cases the liganded Ah receptor failed to bind to the DNA recognition sequence.

Treatment methods for the remediation of nitroaromatic explosives

The production and use of nitroaromatic explosives for military operations have resulted in their dissemination into the environment, where their presence in waterways and soil poses an ecological and health hazard. This paper reviews technologies that are available or under investigation to remediate areas contaminated with these compounds.

Preliminary risk assessment of the wet landscape option for reclamation of oil sands mine tailings: bioassays with mature fine tailings pore water

Chemical and biological assays have been carried out on the "pore water" that results from the settling of the tailings that accompany bitumen recovery from the Athabasca oil sands. Examination of the nonacidic extracts of pore water by gas chromatography-mass spectroscopy allowed the identification of numerous two- to three-ring polycyclic aromatic compounds (PACs), to a total concentration of 2.6 micrograms/L of pore water. The PACs were biodegraded by microflora naturally present in the pore water. Acute toxicity was associated principally with the acidic fraction (naphthenic acids) of pore water extracts according to the Microtox assay; other work has shown that acute toxicity dissipates fairly rapidly. Both individual PACs and concentrated pore water extracts showed minimal levels of binding to the rat Ah receptor and induced minimal ethoxyresorufin-O-deethylase activity in primary rat hepatocytes, showing an insignificant risk of inducing monooxygenase activity. Taken together with previous work showing negligible mutagenic activity of these extracts, we conclude that it should be possible to develop tailing slurries into biologically productive artificial lakes.

Electrochemical treatment of 2,4,6-trinitrotoluene and related compounds

This work involves electrolysis of nitrotoluene congeners, which are persistent pollutants that enter the environment as a consequence of their manufacture and use as explosives. Reduction to aminotoluenes occurred with high current efficiency at a variety of cathodes, at potentials -0.5 to -1 V vs SCE. The products were formed in high chemical yield and with excellent mass balance. Preliminary experiments were also carried out to find methods of removing the electrolysis products from solution by oxidative oligomerization. The most satisfactory method was partial reoxidation at a Ti/IrO2 anode, suggesting an overall remediation technology in which reduction is followed by reoxidation of the spent catholyte in the anode compartment of the same electrolytic cell.

Application of the ethoxyresorufin-O-deethylase (EROD) assay to mixtures of halogenated aromatic compounds

The ethoxyresorufin-O-deethylase (EROD) assay monitors the induction of the xenobiotic-metabolizing enzyme cytochrome P-450 (CYP) 1A1 and is a widely used biomarker for exposure of wildlife to substances that bind the aryl hydrocarbon (Ah) receptor. In this work the induction of EROD activity by single compounds and binary mixtures in primary rat hepatocytes was compared with the predictions of a kinetic model involving mixtures of inducers. The inducing agents were also examined for their ability to activate the Ah receptor to its DNA-binding form and for their ability to act as competitive inhibitors for CYP 1A1. Xenobiotics that failed to activate the Ah receptor did not induce EROD activity. Competitive inhibition for CYP 1A1 between the xenobiotic and 7-ethoxyresorufin caused EROD activity to fall at high xenobiotic concentrations. Competition for a limited number of Ah receptor sites depressed the EROD activity of a strong inducer such as 2,3,7,8-tetrachlorodibenzo-p-dioxin at high concentrations of a weak inducer. Application of the kinetic model to the example of a mixture of low concentrations of dibenzo-p-dioxins and much higher concentrations of polychlorinated biphenyls indicated that EROD assays often seriously underestimate the true potency of an environmental sample. Hence the EROD assay cannot be used for determining dioxin equivalent concentrations using the toxic equivalence factor approach.

Metabolism of polychlorinated dibenzo-p-dioxins by rat liver microsomes

The in vitro metabolism of several chlorinated dibenzo-p-dioxin congeners (PCDDs) was studied using rat liver microsomes as a source of CYP 1 enzymes. The reactions were kinetically first order in both enzyme and substrate and showed a general trend toward decreasing reactivity with increasing chlorination. Michaelis-Menten kinetics were followed for 1-chlorodibenzo-p-dioxin (1-CDD); the reactivity of the enzyme preparation toward 1-CDD exactly paralleled its activity toward 7-ethoxyresorufin. The unreactive congeners 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD) and 2,2'-dichlorobiphenyl (2,2'-DCB) acted as competitive inhibitors toward 1-CDD, with inhibition constants in the micromolar range, similar to the value of the Michaelis constant of 1-CDD. The inhibitory potency of furafylline, a mechanism-based inhibitor that is selective for CYP 1A2, declined in the order acetanilide (standard) > 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) > 1-CDD. We conclude that 1-CDD and 1,2,3,4-TCDD are oxidized almost exclusively by CYP 1A1, whereas 2,3,7,8-TCDD and 1,2,4,7,8-PeCDD are oxidized mainly by CYP 1A2. 1,2,3,7,8-PeCDD was oxidized too slowly for us to reach any conclusion about the P450 isozyme responsible.

Competitive behavior in the interactive toxicology of halogenated aromatic compounds

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that binds and mediates responses to many halogenated aromatic compounds (HACs). Exposure to mixtures of HACs frequently results in nonadditive behavior in both in vivo and in vitro assays. One cause is antagonism, which results when two or more ligands compete for a limited supply of the AhR; one interacts agonistically to induce a strong response, and the other interacts unproductively, eliciting little or no response. This study involves the mechanism by which HACs induce CYP 1A1. Agonistic (e.g., TCDD) and unproductive (e.g., PCB 153) HACs behaved similarly through the stages of initial AhR binding and conversion of the initial AhR-ligand complex to the form that possesses increased affinity for the bound ligand. They diverged in the ability of the AhR-HAC complex to bind to a synthetic oligonucleotide containing the consensus dioxin response enhancer sequence, as studied by the gel retardation assay. Competition for the Ah receptor was used to explain antagonistic behavior between TCDD and other HACs in both the gel retardation assay and the downstream response of CYP 1A1 induction in primary rat hepatocytes.

On the Correlation between ΔH‡ vs ΔS‡ in the thermal cis-to-trans isomerization of azobenzenes

Variable-temperature kinetics allow the determination of Δ H‡ vs Δ S‡ in a single experiment. Multiple variable-temperature data sets for the thermal isomerization of cis-azobenzene show an apparent isokinetic relationship, but this is due only to the mathematical correlation between Δ H‡ vs Δ S‡; the magnitude of the effect is almost identical to that observed for the thermal isomerization of a series of ortho-alkylated azobenzenes.

Successive photocyanation of highly chlorinated aromatic compounds

Successive photocyanation was found to be a general reaction when chlorinated aromatic compounds were photolyzed with sodium cyanide. The products were polycyanated hydroxychloro compounds with various degrees of chlorine replacement. Although the products from some substrates could be isolated, identified, and characterized, most reactions proceeded with low regioselectivity, which limits their synthetic potential. Quantum yields of substrate disappearance increased with the number of chlorine substituents on a substrate, and followed the expected relationship ϕ-1 is proportional to [CN-]-1. In some cases, ϕ depended also on the concentration of the chloro compound, indicating the involvement of excimers, although the major reaction channel appears to be SN2Ar*. Sensitization and quenching experiments established the triplet excited state to be reactive for all substrates tested.Key words: photocyanation, chlorinated aromatic compounds.

Electrochemical reduction and oxidation of DDT

Electrolysis has been studied as a possible method to treat DDT wastes. In methanol, the major process was dehydrochlorination to DDE followed by further reduction. In an aqueous emulsion containing 1% heptane and 0.1% Triton SP-175®, DDT was reduced at a deposited lead electrode with sodium sulphate as the supporting electrolyte by sequential hydrodechlorination of the aliphatic chlorine atoms. An excellent material balance was achieved, but the current efficiency was poor, even at low current densities. Electrooxidation of DDT was also investigated; in aqueous solutions or emulsion, little oxidation occurred because of competing oxidation of water at the highly positive potentials needed to oxidize DDT. In acetonitrile, electrooxidation occurred with high current efficiency by way of "electrochemical combustion" of DDT and its intermediate oxidation products to CO2. We conclude that development of an electrolytic technology for destroying DDT wastes is unlikely.Key words: electroreduction, electrooxidation, voltammetry, surfactant media.

Competitive inhibition by inducer as a confounding factor in the use of the ethoxyresorufin-O-deethylase (EROD) assay to estimate exposure to dioxin-like compounds

The ethoxyresorufin-O-deethylase (EROD) assay has been extensively used in whole animals and in cell culture as a biomarker of exposure to environmental contaminants such as dioxin-like compounds (DLCs). This paper addresses two controversial phenomena that arise when DLCs are examined by the EROD assay. Firstly, the maximum level of induced EROD activity varies with the identity of the inducing compound; secondly, the induced EROD activity reaches a concentration-dependent maximum level that is followed by an apparent reduction in activity when the concentration of inducer is further increased. These phenomena are completely explained by competitive inhibition of the EROD enzyme-substrate reaction by the dioxin-like compound. A kinetic model explains the biphasic appearance of EROD induction curves as a function of a compound's binding affinity with the Ah receptor (Kd) and its binding affinity to CYP 1A1 (Ki) which results in inhibition of the EROD enzyme-substrate reaction. These results limit the reliability of the information obtained from calibration curves of EROD activity versus concentration of a standard DLC such as 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Metabolism of polychlorinated dibenzo-p-dioxins and related dioxin-like compounds

Recent work on the metabolism of polychlorinated dibenzo-p-dioxins and related compounds such as polychlorinated dibenzofurans and coplanar polychlorinated biphenyls is reviewed. Most in vivo studies and epidemiological studies on humans have been concerned with rates of elimination of these compounds; in vitro work has shown that a wide variety of metabolites can be found, either in hydroxylated format (typically in feces) or as conjugates (in urine). Metabolism of this group of compounds seems always to represent a detoxification process rather than one of bioactivation.

Gel-filtration chromatographic method for determining relative binding affinities: rat hepatic estrogen receptor as an example system

We report a gel-filtration-based chromatographic method for separation of specific, nonspecific, and free radioligand in a protein receptor-ligand binding assay for the example of the estrogen receptor ERalpha. This assay affords relative binding affinities (RBAs) without the need for a separate determination of nonspecific binding. The probit method is recommended as the most satisfactory method of evaluating the data. The assay responds to both estrogen agonists and antagonists, mixtures respond additively, and the slopes of the probit plots indicate that all ligands bind to the same site on the estrogen receptor. RBAs obtained with rat and rainbow trout ERalpha were in good agreement, and also with those from other reported assays, consistent with the interspecies conservation of key regions of the ligand binding domain among estrogen receptors.