Structural and Catalytic Differences between Two FADH(2)-Dependent Monooxygenases: 2,4,5-TCP 4-Monooxygenase (TftD) from Burkholderia cepacia AC1100 and 2,4,6-TCP 4-Monooxygenase (TcpA) from Cupriavidus necator JMP134

2,4,5-TCP 4-monooxygenase (TftD) and 2,4,6-TCP 4-monooxygenase (TcpA) have been discovered in the biodegradation of 2,4,5-trichlorophenol (2,4,5-TCP) and 2,4,6-trichlorophenol (2,4,6-TCP). TcpA and TftD belong to the reduced flavin adenine dinucleotide (FADH(2))-dependent monooxygenases and both use 2,4,6-TCP as a substrate; however, the two enzymes produce different end products. TftD catalyzes a typical monooxygenase reaction, while TcpA catalyzes a typical monooxygenase reaction followed by a hydrolytic dechlorination. We have previously reported the 3D structure of TftD and confirmed the catalytic residue, His289. Here we have determined the crystal structure of TcpA and investigated the apparent differences in specificity and catalysis between these two closely related monooxygenases through structural comparison. Our computational docking results suggest that Ala293 in TcpA (Ile292 in TftD) is possibly responsible for the differences in substrate specificity between the two monooxygenases. We have also identified that Arg101 in TcpA could provide inductive effects/charge stabilization during hydrolytic dechlorination. The collective information provides a fundamental understanding of the catalytic reaction mechanism and the parameters for substrate specificity. The information may provide guidance for designing bioremediation strategies for polychlorophenols, a major group of environmental pollutants.     

Liquid-liquid Extraction Methods to Determine Reductive Dechlorination of 2,4,6-trichlorophenol by Zero-valent Metals and Zero-valent Bimetals

Abstract

The reductive dechlorination of 2,4,6-trichlorophenol (2,4,6-TCP) was studied using zero-valent metals (ZVMs) and bimetals. A reliable extraction method was required to distinguish the removal by chemical transformation from the removal by physical adsorption on the ZVMs or bimetal. Three liquid–liquid extraction methods with ethyl acetate were evaluated by conducting mass recovery tests in the ZVM-chlorophenol systems. A typical liquid–liquid extraction showed low recoveries for 2,4,6-TCP and the reaction products while the modified liquid–liquid extraction involving acid addition gave increased recoveries. The Mg system needed an additional modification since the modified extraction method was not working on the Mg and Pd/Mg system. Pd/Fe and Pd/Zn gave the highest reaction rate for 2,4,6-TCP dechlorination, producing less or no chlorinated daughter compounds while the plain metals such as Fe and Zn showed much slower degradation rates for 2,4,6-TCP.     

2,4,6-TCP removal mechanism in the process of leaching manganese

ABSTRACT

2,4,6-trichlorophenol (2,4,6-TCP) was used in the leaching system of manganese/pyrite to achieve the simultaneous utilization and removal of 2,4,6-TCP. When the initial concentration of 2,4,6-TCP was 300 mgL^?1 and the weight percentage of pyrite was 12.8%, the removal of 2,4,6-TCP was 83.3%, meanwhile, the leaching efficiency of manganese was 97.6%. Fourier Transform Infrared Spectroscopy (FTIR) showed that the removal of 2,4,6-TCP mainly relied on the chemical oxidative degradation by Mn (IV) and the surface adsorption by leaching residue, including physical adsorption and chemical adsorption. Gas Chromatography–Mass Spectrometry analysis and Chemical Oxygen Demand (COD) indicated that 2,4,6-TCP was degraded to inorganic substances eventually.     

Electrooxidation of chlorophenols at a glassy carbon electrode in a pH 11 buffer

The electrochemical oxidation of three chlorophenols (CPs), namely 2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP), and 2,4,6-trichlorophenol (2,4,6-TCP) at a glassy carbon electrode in a pH 11 carbonate/hydrogen carbonate buffer electrolyte has been studied by cyclic voltammetry (CV) and impedance measurements. The three CPs show in CV an irreversible oxidation peak whose potential decreased linearly with increasing pK_a of the CPs, showing that the oxidation is the more facile the less acidic is the CP. Although the oxidation is not under diffusion control, i.e., the peak potential was not directly related to the rate constant, the same slope was obtained when the potential at which the integrated charge began to increase significantly was plotted instead of the peak potential. A reversible process near −0.45 V is observed for 2,4,6-TCP from the second positive scan, which was attributed to the formation of a soluble (since this couple nearly disappeared upon solution stirring) benzoquinone. Impedance data showed that for 2-CP and 2,4-DCP the formation of the polyphenolic film begins already at 0.0-0.1 V, and that the film is non-porous and non-conducting. On the contrary, with 2,4,6-TCP polyphenol formation began near 0.15 V, and the film is so porous that it allows the oxidation of 2,4,6-TCP to continue.     

Hydrodechlorination of 2,4,6-trichlorophenol for a permeable reactive barrier using zero-valent iron and catalyzed iron

Dehalogenation of toxic organic compounds has been intensively studied during the last decade by using zero-valent iron (ZVI). However, the reactivity of iron is compound specific and very low reactivities were reported for aromatic compounds including chlorophenols. In this study, hydrodechlorination of 2,4,6-trichlorophenol (2,4,6-TCP) was conducted in a batch system by using ZVI and catalyzed iron. No degradation was observed with ZVI over the 40 days experiments. Catalyzed ZVIs removed 2,4,6-TCP and palladium-coated iron (Pd/Fe) and nickel-coated iron (Ni/Fe) showed relatively enhanced reactivity while copper-coated iron (Cu/Fe) and platinum-coated iron (Pt/Fe) showed lower reactivities. The surface area normalized kinetic constants (k _SA ) of Pd/Fe, Ni/Fe, Cu/Fe, Pt/Fe are 2.54×10⁻⁴, 1.01 × 10⁻⁴, 2.24×10⁻⁵, 2.56×10⁻⁵ L m⁻² h⁻¹, respectively. The identification of less chlorinated phenols and phenol confirmed that the removal is dechlorination. Pd/Fe system exerts relatively low pH compared with the ZVI system, and the low pH is favorable for the dechlorination. The reactivity enhancement of catalyzed iron was discussed in terms of catalytic effects and the corrosion potential by the bimetal coupling. Variable Pd content on the Pd/Fe was tested, and the degradation rate of 2,4,6-TCP increased in proportion to the increase of Pd content.     

Electrooxidation of 2-chlorophenol and 2,4,6-chlorophenol on glassy carbon electrodes modified with graphite–zeolite mixtures

This work reports the behavior of zeolite–graphite electrodes for the oxidation of 2,4,6-trichlorophenol (2,4,6-TCP) and 2-chlorophenol (2-CP) in a pH 3, 0.5 M Na₂SO₄ electrolyte using cyclic voltammetry, chronoamperometry, electrochemical impedance and galvanostatic electrolysis. Three zeolites with different hydrophobic/hydrophilic balance were used, in order to determine the possible effect of this parameter. On glassy carbon (GC) modified with 80–20 % graphite–zeolite mixtures the voltammetric peak charge of 2-CP oxidation was higher than that on GC modified with graphite alone, clearly showing a catalytic effect of the zeolites. ZSM-5 was the more active zeolite, probably due to its hydrophobic character, which should favor its interaction with the also hydrophobic 2-CP. In agreement with this, the charge-transfer resistance of GC modified with a graphite-ZSM-5 mixture was about half that of GC modified with graphite alone, and its double-layer capacity about 50 % higher, indicating a modification of the interface which favours the reaction. On the contrary, for 2,4,6-TCP both the oxidation peak charge, and the charge-transfer resistance, for GC electrodes modified with graphite–zeolite mixtures was about the same as that of GC modified with graphite alone. This lack of catalytic action could be due to the fact that the oxidation of 2,4,6-TCP requires the release of at least one chlorine atom, this release being so difficult that it cannot be promoted by any of the zeolites studied. However, in prolonged galvanostatic electrolysis practically all the current went to the oxidation of 2,4,6-TCP, while in the case of 2-CP only a small fraction was oxidized. Probably with 2-CP at long times surface poisoning blocked the reaction, as reflected by the fact that the electrolysis potential rose to a value 0.2 V higher than that reached by TCP, and so high that most of the current went to oxygen evolution. The zeolites ZA and ZY decreased the mineralization of 2,4,6-TCP, since they promoted a one-electron oxidation reaction, so that its concentration at 80 min was about 80 % of that observed with graphite alone. The oxidation of 2-CP was not significantly affected by any zeolite as compared with that on graphite.     

OPTIMIZATION OF PHOTOCATALYTIC PROCESS PARAMETERS FOR THE DEGRADATION OF 2,4,6-TRICHLOROPHENOL IN AQUEOUS SOLUTIONS

In the present study, the photocatalytic degradation of 2,4,6-trichlorophenol (2,4,6-TCP) was carried out in a batch reactor under UV light in aqueous solution for 5 h using titania P-25 (surface area 50 m²/g) as a photocatalyst and sodium hypochlorite as an oxidant. Variables studied include catalyst dose (0.25–1.25 g/L), pH (2–6), and concentration of the oxidant (3.06 × 10^?6M–15.3 × 10^?6M). The rate of degradation was studied in terms of changes in concentration of the pollutant and reduction in chemical oxygen demand. The optimal values of operational parameters leading to 2,4,6-TCP abatement were obtained by using response surface methodology. The percent degradation and COD reduction of 2,4,6-TCP was found to increase with increases in the catalyst dose up to the dose of 1.1 g/L, pH in the range of 4–4.5, and oxidant concentration of 9.95 × 10^?6M.     

Studies in Batch and Continuous Solvent Sublation. IV. Continuous Countercurrent Solvent Sublation and Bubble Fractionation of Hydrophobic Organics from Aqueous Solutions

Abstract

Experimental results on the solvent sublation of four hydrophobic compounds [pentachlorophenol (PCP), 1,2,4-trichlorobenzene (TCB), 2,3,6-trichloroanisole (TCA), and 2,4,6-trichlorophenol (TCP)] from the aqueous phase tc organic solvents are reported. The experiments were conducted with the aqueous and air phases in continuous countercurrent modes and the organic solvent as a stagnant layer. TCB, TCA, and TCP were sublated into mineral oil. PCP at pH 2.9 as neutral molecules were sublated into mineral oil and decyl alcohol while ionic PCP at pH 8.9 were sublated as a complex with hexadecyltrimethyl ammonium bromide into decyl alcohol. The effect of the two organic solvents were compared for neutral PCP sublation from the aqueous phase. The effects of air flow rates, influent feed rates, and the volume of organic solvent were studied. Comparisons were made between the bubble fractionation and solvent sublation of neutral PCP and TCP into mineral oil. The efficiency of sublation was largest for TCB, smallest for TCP, and followed the relative magnitudes of “effective” partition constants for the solutes between the air bubble and aqueous phase.     

Synthesis of amino-modified ordered mesoporous silica as a new nano sorbent for the removal of chlorophenols from aqueous media

Contamination of natural water with organic pollutants is a problem of major concern and the demand for effective adsorbents for the removal of toxic compounds is increasing. Present work deals with the adsorption of 2-chlorpphenol (2-CP) and 2,4,6-trichlorophenol (2,4,6-TCP) from aqueous solutions on a new nano sorbent; amino-modified ordered mesoporous silica (APS-MCM-41) material. It was found that APS-MCM-41 shows significant adsorption for 2-CP and 2,4,6-TCP. This might be due to the acid and alkaline interactions among the amino functional groups and chlorophenols. Batch adsorption studies were carried out to study the effect of various parameters like adsorbent dose, pH, initial concentration and contact time. It was found that adsorption of 2-CP and 2,4,6-TCP depends upon the solution pH. From the sorption studies it was observed that the uptake of 2,4,6-TCP was higher than that of 2-CP. Freundlich and Langmuir adsorption isotherms were used to model the equilibrium adsorption data for 2-CP and 2,4,6-TCP.     

Electro-oxidation of chlorophenols at glassy carbon electrodes modified with polyNi(II)complexes

The effect of the ligand macrocycle (phenylporphyrin (PP) or phthalocyanine (Pc)) and of the ligand substituent (NH₂ or SO₃⁻) on the catalytic activity for the electro-oxidation in a pH 11 buffer electrolyte of 2- and 4-chlorophenol (2-CP and 4-CP), 2,4- and 2,6-dichlorophenol (2,4-DCP and 2,6-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and pentachlorophenol (PCP) at glassy carbon electrodes modified with electropolymerized Ni(II) macrocycles was studied. The polyphenolic residue deposited at the electrode surface was characterized by cyclic voltammetry, impedance measurements, ex situ Fourier transform infrared spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). A band of aliphatic CO stretching in the IR spectrum of the fouling film produced by potential cycling in 2,4,6-TCP indicated that the aromatic ring had been broken, yielding ketones, aldehydes and/or carboxylic acids. The sulphonated Ni(II) polymers, which showed the Ni(III)/Ni(II) process in the CV, had XP spectra typical of paramagnetic Ni(II), indicating that they contained Ni(OH)₂ clusters. On the contrary, the CVs of the amino Ni(II) did not show the Ni(III)/Ni(II) process at all, this process appearing only after previous activation by potential cycling, and only to a small extent. As was to be expected, the XP spectra of activated amino films corresponded to diamagnetic Ni(II), showing that the concentration of Ni(OH)₂ clusters was very small. The amino films were less active than the sulpho films for the oxidation of chlorophenols, in agreement with the lower concentration of Ni(OH)₂ clusters in the former films. For all electrodes the highest activity was observed for 2,4,6-TCP, since its oxidation yields a phenolic residue which is much more porous than those produced by the other CPs.     

OPTIMIZATION OF PHOTOCATALYTIC PROCESS PARAMETERS FOR THE DEGRADATION OF 2,4,6-TRICHLOROPHENOL IN AQUEOUS SOLUTIONS

In the present study, the photocatalytic degradation of 2,4,6-trichlorophenol (2,4,6-TCP) was carried out in a batch reactor under UV light in aqueous solution for 5 h using titania P-25 (surface area 50 m²/g) as a photocatalyst and sodium hypochlorite as an oxidant. Variables studied include catalyst dose (0.25-1.25 g/L), pH (2-6), and concentration of the oxidant (3.06 × 10^-6M-15.3 × 10^-6M). The rate of degradation was studied in terms of changes in concentration of the pollutant and reduction in chemical oxygen demand. The optimal values of operational parameters leading to 2,4,6-TCP abatement were obtained by using response surface methodology. The percent degradation and COD reduction of 2,4,6-TCP was found to increase with increases in the catalyst dose up to the dose of 1.1 g/L, pH in the range of 4-4.5, and oxidant concentration of 9.95 × 10^-6M.     

Design,Synthesis and Biological Evaluation of Novel 1,3,5-Triazines: Effect of Aromatic Ring Decoration on Affinity to 5-HT7 Receptor

Considering the key functions of the 5-HT₇ receptor, especially in psychiatry, and the fact that effective and selective 5-HT₇ receptor ligands are yet to be available, in this work, we designed and synthesized novel 1,3,5-triazine derivatives particularly based on the evaluation of the effect of substituents at aromatic rings on biological activity. The tested compounds showed high affinity to the 5-HT₇ receptor, particularly ligands N²-(2-(5-fluoro-1H-indol-3-yl)ethyl)-N⁴-phenethyl-1,3,5-triazine-2,4,6-triamine 2 (K_i = 8 nM) and N²-(2-(1H-indol-3-yl)ethyl)-N⁴-(2-((4-fluorophenyl)amino)ethyl)-1,3,5-triazine-2,4,6-triamine 12 (K_i = 18 nM) which showed moderate metabolic stability, and affinity to the CYP3A4 isoenzyme. As for the hepatotoxicity evaluation, the tested compounds showed moderate cytotoxicity only at concentrations above 50 µM. Compound 12 exhibited less cardiotoxic effect than 2 on Danio rerio in vivo model.     

钴铈复合氧化物催化降解2,4,6-三氯酚

采用溶胶-凝胶法制备了一种疏松多孔的钴铈复合氧化物,该复合氧化物具有较大的比表面积和孔体积。并以废水中一定浓度的2,4,6-三氯酚作为模型化合物,研究自制的钴铈复合氧化物对水中的2,4,6-三氯酚的催化降解效果。研究结果表明,自制的钴铈复合氧化物所具有的氧化还原性能以及协同催化作用使其对废水中的2,4,6-三氯酚具有一定的催化降解活性。初始溶液的pH值对降解影响较大,酸性条件更有利于2,4,6-三氯酚的降解。100 mg自制的钴铈复合氧化物与10 mL 100 mg/L的2,4,6-三氯酚在25℃下恒温振荡30 h,2,4,6-三氯酚的降解率可达到86.3%,溶液中可被氧化的有机物含量下降了36.7%。     

Continuous-mode photocatalytic degradation of chlorinated phenols and pesticides in water using a bench-scale TiO2 rotating disk reactor

Photocatalytic degradation of phenol, chlorinated phenols, and lindane was evaluated in a continuous flow TiO₂ rotating disk photocatalytic reactor (RDPR). The RDPR operated at a hydraulic residence time of 0.25 day and at a disk angular velocity of 12 rpm. At low molar feed concentrations (0.038 mmol/l), the removal efficiencies for phenol and chlorinated phenols were in the order of 86% or higher, whereas the removal efficiency for lindane at a feed concentration of 0.016 mmol/l was in the order of 63%.

For 2,4,6-trichlorophenol (TCP), an increase in the molar influent concentration resulted in a decrease in removal efficiency but in an increase in removal rate. The degradation rate of 2,4,6-TCP followed a saturation type dependancy with the effluent concentration, suggesting a Langmuir–Hinshelwood (L–H) reaction rate equation. A L–H equation was employed to determine the reaction rate constant and the adsorption coefficient for 2,4,6-TCP. The photonic efficiency increased from 0.68% at an influent concentration of 0.13 mmol/l to 2.06% at an influent concentration of 1.0 mmol/l.     

Mechanism of removing chlorophenolic compounds from solution by a water-insoluble cationic starch

The removal of three chlorophenolic compounds from solutions using highly crosslinked cationic starch containing a tertiary amine group was investigated. The three chlorophenolic compounds were 2-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol. The adsorption process predominated over the initial pH = 2∼6 for 2-chlorophenol, and predominated over the initial pH = 2 to 4 range for 2,4-dichlorophenol and 2,4,6-trichlorophenol. The ion-exchange process predominated at initial pH = 12 for 2-chlorophenol, and predominated over the initial pH = 10 to 12 range for 2,4-dichlorophenol and 2,4,6-trichlorophenol. Both processes coexist over the initial pH=8 to 10 range for 2-chlorophenol, and coexist over the initial pH = 6∼8 for 2,4-dichlorophenol and 2,4,6-trichlorophenol. Experimental results indicate that the adsorption process is initially pH-independent, and the ion-exchange process is initially pH-dependent. The adsorption and ion-exchange processes are concentration-dependent and correspond to the Langmuir isothermal adsorption. They are endothermic for 2-chlorophenol and 2,4-chlorophenol, and are exothermic for 2,4,6-chlorophenol. The ion-exchange capacity is larger than the adsorption capacity. Moreover, the order of the amount of three chlorophenolic compounds removed by the cationic starch is 2,4,6-chlorophenol > 2,4-dichlorophenol > 2-chlorophenol.     

氯酚化合物在活性炭上的吸附

氟酚化合物由于其对人类身体健康的危害和对环境所造成的污染，尤其是对水体和土壤的污染，被许多国家列为环境优先控制污染物。文中选取环境中常见的三种氯酚化合物对氟酚（p-CP）、2，4，6-三氯酚（2，4，6-TCP）和五氟酚（PCP）作为代表，研究了它们在水溶液中被活性炭吸附的情况，阐述了影响吸附的可能性因素，并取得了良好的吸附去除效果。     

Electrochemically reductive dechlorination of micro amounts of 2,4,6-trichlorophenol in aqueous medium on molybdenum oxide containing supported palladium

An electrodepositing method is used to prepare a Pd/MoO_x/GC (glass carbon) composite electrode that provides a surface modification with catalytic properties for reductive dechlorination of 2,4,6-trichlorophenol in aqueous medium at ambient temperature. The palladium particles are uniformly dispersed on a MoO_x film that is previously electrodeposited on a GC electrode. XPS (X-ray photoelectron spectroscopy) of this composite electrode presents a broad peak in the Mo (3d) region revealing the existence of Mo⁶⁺ species as well as lower valence states such as Mo⁵⁺, Mo⁴⁺. Compared with the Pd/GC electrode surface, the Pd (3d) region reveals another peak whose binding energy value is higher than in Pd/GC. This peak suggests a strong interaction between the palladium particles and the MoO_x film. This composite electrode shows better performance for electrocatalytic reductive dechlorination of 2,4,6-trichlorophenol than Pd/GC. Totally dechlorinated product (phenol) is observed at the early stage of electrolysis, no intermediate products are observed besides trace amounts of 2,4-dichlorophenol. The formation of trace amounts 2,4-dichlorophenol may occur by direct electronation on the cathode. The influence of current density and substrate concentration are also investigated.     

Synthesis and crystal structure determination of 2,6-di-tert-butyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate and its corresponding perchlorate salt

2,6-Di-tert-butyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate belongs to the class of negatively solvatochromic dyes commonly used for the empirical determination of solvent polarities. By way of exception, it crystallizes without any solvent of crystallization due to the presence of two bulky tert-butyl groups in the phenolate moiety. The synthesis of this hydrophobic betaine dye has been improved and an X-ray crystal structure analysis of this solventless zwitterionic dye and its protonated form (as a perchlorate salt) has been carried out.     

Towards an improved understanding of the β-TCP crystal structure by means of “checkerboard” atomistic simulations

The large variability of biological responses to β-TCP implants reported in the literature could possibly be related to subtle differences in the β-TCP crystal structure. The structure contains one partially occupied site Ca(4). In order to better understand the ordering of this site, 12 pairs of unit cells with different Ca(4) site occupancies were combined in different checkerboard patterns with an average occupancy of 3. Atomistic simulations were conducted to identify the lowest energy configurations. The previously published low energy configuration is not the most stable one when considering a larger supercell. Plotting the 662 simulation outputs by lattice parameters a or c versus relative lattice energy revealed clusters of high density which are composed of configurations with predominant motifs of Ca(4) occupancy. The tools introduced in this study can be applied in future simulation studies to better explain the Ca(4) site occupancy in the β-TCP crystal structure.     

Ozonation,Ozone/UV and UV/H2O2 Degradation of Chlorophenols

The performance of the O₃, O₃/UV and UV/H₂O₂ processes for degradation of six chlorophenols (4-chlorophenol, 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol) were studied in laboratory reactors. Comparative study showed that chlorophenols can be degraded successfully by all of the methods studied, whilst traditional ozonation at high pH was determined to be the most effective method to treat chlorophenols. Even though the molar absorptivity of chlorophenols is known to be relatively high in the UV-region, the combination of UV-radiation with ozone did not accelerate the degradation of chlorophenols further. The toxicity of degradation products formed during ozonation of chlorophenols has been compared with the toxicity of pure chlorophenols utilizing Daphnia magna 24 hours test. Ozonation of chlorophenols yielded less toxic or even nontoxic products for Daphnia magna compared with parent compounds.