Oxidation Of Chlorophenols Research Articles

An active, robust and transparent nanocrystalline anatase TiO2 thin film — preparation, characterisation and the kinetics of photodegradation of model pollutants

The preparation, characterisation, and photocatalytic activity of robust and transparent nanocrystalline thin films of anatase TiO2 are described. The films, which are prepared from a sol–gel can be either dip-coated or spin-coated, however, the latter have better optical and mechanical properties, since dip coating results in the retention of a significant amount of the organic stabilisers used in the sol–gel preparation.Quantum yields for the photooxidation of 4-chlorophenol (Φ4CP) on these spin-coated films are similar to those reported previously for TiO2 films and dispersions, with Φ4CP=0.5–1%. For films of different thicknesses, and therefore different absorption efficiencies, Φ4CP depends only upon the total number of photons absorbed, i.e. Φ is independent of both the absorbed photon flux and the distribution of charge carriers in the film. By way of contrast, the rate of photoreduction of methylene blue is independent of film thickness and appears to be limited by the surface area of the film. Hence, quantum yields for methylene blue reduction (ΦMBR) fall significantly from 1.7 to 0.5% as film thickness, and hence photoabsorption efficiency, is increased. These pure anatase films do not show any deleterious sodium ion effect for either 4-chlorophenol oxidation or methylene blue reduction. Quantum yields for photooxidation of stearic acid (ΦSA) deposited directly onto the films are relatively high with ΦSA=1–4%. This, combined with the excellent optical and mechanical properties of these films, suggests that they may be of particular interest for the development of ‘self-cleaning windows’.

Intermediate inhibition in the heterogeneous UV-catalysis using a TiO 2 suspension system

Langmuir–Hinshelwood (L–H) kinetic expression was used to develop a basic mathematical model, which could describe the inhibition of intermediates in the photocatalysis of 2-chlorophenol (2-CP) in a suspended TiO 2 system. Results showed that the photocatalytic oxidation of 2-chlorophenol followed the L–H type behavior and the reaction by-products displayed an inhibiting effect on the degradation rate. The inhibition was estimated by comparing to observed and estimated half-lives. The higher the initial concentration of 2-chlorophenol, the higher the inhibition of photocatalytic reaction. The L–H kinetic has been modified slightly in this study to rationalize the contrast of inhibited behavior and to improve in favor of a surface reaction. The concentrations of 2-chlorophenol were investigated ranging from 7.78×10 −5 to 7.78×10 −4 mol l −1. The degradation of 2-chlorophenol in this reaction condition approximates a first-order kinetics to near-complete degradation. Calculated kinetic profiles are in an excellent agreement with the experimental observation. The results of the theoretical analysis can be used to estimate reaction rates in different initial concentrations of target compound.

Electrochemical Oxidation of 4-Chlorophenol for Wastewater Treatment: Definition of Normalized Current Efficiency (ϕ)

The electrochemical behavior of lead dioxide and synthetic boron-doped diamond thin film electrodes (BDDs) has been studied in acid media containing 4-chlorophenol (4-CP) by bulk electrolysis under different experimental conditions. In order to quantify the electrochemical activity of a given electrode, for the electrochemical oxidation of organic compounds (4-CP), the current efficiency of the anodic oxidation has been normalized taking into consideration mass-transport limitations. The normalized current efficiency (ϕ) has been defined as the ratio between the current efficiency of the investigated anode to the current efficiency of an ideal anode which has a very fast oxidation rate, resulting in a complete combustion of organics to carbon dioxide. The results have shown that even if the complete combustion of 4-chlorophenol has been achieved at both lead dioxide and boron-doped diamond anodes, the latter give higher ϕ. The difference in reactivity of the electrogenerated hydroxyl radicals on the anode surface, has been proposed to explain the high ϕ values obtained using boron-doped diamond anodes. © 2001 The Electrochemical Society. All rights reserved.

Oxidation of 4-Chlorophenol at Boron-Doped Diamond Electrode for Wastewater Treatment

The electrochemical behavior of synthetic boron-doped diamond thin-film electrode (BDD) has been studied in acid media containing 4-chlorophenol (4-CP) by cyclic voltammetry, chronoamperometry, and bulk electrolysis. The results have shown that in the potential region of supporting electrolyte stability occur reactions involving the oxidation of 4-CP to phenoxy radical and 1,4-benzoquinone. Polymeric materials, which result in electrode fouling, are also formed in this potential region. Electrolysis at high anodic potentials, in the region of electrolyte decomposition, complex oxidation reactions can take place involving electrogenerated hydroxyl radicals, leading to the complete incineration of 4-chlorophenol. Electrode fouling is inhibited under these conditions. The experimental results have been also compared with a theoretical model. This model is based on the assumption that the rate of the anodic oxidation of 4-CP is a fast reaction. Finally, high-pressure liquid chromatographic analyses revealed that the main intermediate products of 4-CP oxidation were 1,4-benzoquinone, maleic acid, formic acid, and oxalic acid. © 2001 The Electrochemical Society. All rights reserved.

Catalytic Oxidation of 2-Chlorophenol in Confined Channels of ZSM-48

Experimentally, the formation of undesired heavy polycyclic aromatic hydrocarbons (PAHs) and chlorinated phenol byproducts in the supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) cataly...

Oxidation of chlorophenols on Pt electrode in alkaline solution studied by cyclic voltammetry, galvanostatic electrolysis, and gas chromatography­mass spectrometry

Abstract Potentiodynamic investigations on a platinum electrode show that oxidation of phenol, monochlorophenols, dichlorophenols, 2,3,6-, 2,4,5-, 2,4,6-trichlorophenols, and pentachlorophenol in 1 M NaOH solution, containing 1 mM of phenols, proceeds in the potential region of Pt oxide formation. The oxidation rate of phenols decreases with the increase in the number of chlorine atoms in the benzene ring in the row: phenol > monochlorophenols > dichlorophenols > trichlorophenols > pentachlorophenol. The electrochemical stability of phenols, as studied using a cyclic voltammetry, depends on their chlorination degree and isomerism. Galvanostatic oxidation of 1 M NaOH solutions containing 1 mM of phenol, monochlorophenols, dichlorophenols, 2,3,6-, 2,4,5-, 2,4,6-trichlorophenols, and pentachlorophenol were carried out on a platinum electrode using 30 mA cm­2 current density. The electrolysis of the solutions was performed in the course of 10 h, and concentration of phenols in the anolytes was monitored during oxidation. The concentration of phenolic compounds diminishes from 1 mM to 10­50 mM during 4­5 h of electrooxidation and does not change during further galvanostatic oxidation. A decrease in concentration of phenols during galvanostatic electrolysis weakly depends on the isomerism and a chlorination degree of the compounds. A rapid decrease in concentration of studied phenols during the first 4­5 h of electrolysis and a nonselective oxidation of different chlorophenols suggest that the oxidation proceeds via electrochemically generated oxidants. Further decrease in concentration of phenols is rather small due to deactivation of the electrode as a result of polymerization of corresponding phenols and diffusion limitations.

Supercritical Water Oxidation of 2-Chlorophenol Catalyzed by Cu2+ Cations and Copper Oxide Clusters

Experimentally, the selectivity to decomposition (S/D) (S/D = (amount of 2-chlorophenol (2CP) oxidized to CO2 and H2O)/(disappearance of 2CP)) ratio for oxidation of 2CP with Cu2+ cations in supercritical water was greater than that without any catalysts by 60%. The enhancement was due to a fast precipitation of copper chloride in the supercritical water oxidation (SCWO) process. Formation of toxic byproducts was significantly reduced. Because of a confined environment in the channels of ZSM-5, formation of undesired heavy chlorinated phenols and PAH (polycyclic aromatic hydrocarbon) byproducts in the SCWO of 2CP (catalyzed by CuO/ZSM-5) was also highly suppressed. Only trace light-PAHs with a molecule size less than 6 Å were found in the SCWO of 2CP. Carcinogenic PAHs (heavy-PAHs) were not observed. The extended X-ray absorption fine structural (EXAFS) spectra of the catalyst showed that the copper oxides in the channels of ZSM-5 may form Cu3O2 clusters with Cu−Cu and Cu−O bond distances of 2.79 and 1.91 Å, respectively. The Cu3O2 clusters were oxidized to Cu3O4 by H2O2 in the supercritical water. Since the diffusion coefficients of copper oxide clusters (Cu3O4) in the channels of ZSM-5 was greater than that of 2CP by at least 3 orders, it is possible that these clusters are relatively mobile in ZSM-5 in the SCWO process.

Formation of hydroxylated and dimeric intermediates during oxidation of chlorinated phenols in aqueous solution

The oxidation of selected chlorophenols (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol) was studied in aqueous solutions using UV/H 2O 2 and O 3 methods. The formation of oxidation intermediates was measured to elucidate their importance in the treatment of chlorophenols. Results indicated that chlorophenols can be treated efficiently by the methods studied, but the dechlorination of the compounds was insufficient. Analysis of intermediates in the acetylated extraction fractions showed that hydroxylation of chlorophenols and formation of dimeric products were involved in the oxidation of chlorophenols in both treatment processes. The majority of the intermediates detected were transient, and thus were not detectable after an extended treatment time. The presence of a complicated mixture of intermediates suggests the need for toxicity testing to confirm the detoxifying effect of the chemical oxidation of chlorophenols.

Electron-Transfer Oxidation of Chlorophenols by Uranyl Ion Excited State in Aqueous Solution. Steady-State and Nanosecond Flash Photolysis Studies

The oxidation of chlorophenols by photoexcited uranyl ion was studied in aqueous solution at concentrations where the ground-state interactions were negligible. Nanosecond flash photolysis showed that a clean electron-transfer process from the chlorophenols to the excited uranyl ion is involved. This is suggested to lead to the formation of a U(V)/chlorophenoxyl radical pair complex. The efficiency of this charge-transfer process is unity for the three chlorophenols. However, low product yields suggest that in the absence of oxygen, back electron transfer, both within the radical pair and from separated uranium(V) to phenoxyl radicals, appears to be the major reaction pathway. In the presence of oxygen the quantum yields of disappearance of chlorophenol and of photoproduct formation increased. This leads to the conclusion that oxygen favors reaction with uranium(V) and/or the uranium(V)-phenoxyl radical pair, leading to the formation of the superoxide anion and its conjugate acid, HO{sub 2}{sup {sm_bullet}}, which then regenerate UO{sub 2}{sup 2+}. Based on this, a catalytic cycle for chlorophenol photooxidation involving uranyl ion and molecular oxygen is proposed.

Byproduct Shape Selectivity in Supercritical Water Oxidation of 2-Chlorophenol Effected by CuO/ZSM-5

Reduction of trace polycyclic aromatic hydrocarbon (PAH) byproducts in supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) effected by CuO/ZSM-5 has been investigated in the present work. In the channels of ZSM-5, due to a restricted environment, formation of undesired PAH byproducts in the SCWO of 2CP was significantly suppressed. Only trace light-PAHs with a molecular size less than 6 A were formed in the SCWO of 2CP. Carcinogenic PAHs (heavy-PAHs) were not found. Structures of CuO and Cu2O in the channels of ZSM-5 that involved in the catalytic SCWO of 2CP were studied by X-ray photoelectron spectroscopy (XPS). The Cu2O-to-CuO ratio was approximately 0.15. Existence of the Cu(I) and Cu(II) was also confirmed by the preedge X-ray absorption near edge structural (XANES) spectroscopy. The copper oxides with a square-plane structure were observed by electron paramagnetic resonance (EPR) spectra. Furthermore, the extended X-ray absorption fine structural (EXAFS) spectra suggest that the copper oxid...

Oxidation of 2-Chlorophenol in Water by Ultrasound/Fenton Method

One of the toxic and refractory pollutants formed during the color removal of fertilizer, chemical, and petroleum industrial effluents by chlorine is 2-chlorophenol (2-cp). The objective of this study is to inves- tigate the effect of H2O2 and Fe 21 dosages on the decomposition of 2-cp using a coupled ultrasound/Fe 21 /H 2O2 process. The extent of 2-cp decomposition and mineralization depends on the dosages of H2O2 and Fe 21 . More than 99% of 2-cp was decomposed and 86% of 2-cp was mineralized using the ultrasound/Fe 21 /H 2O2 process at Fe 21 of 10 mg/L and H 2O2 of 500 mg/L. Oxidation-reduction potential (ORP) monitoring is a useful method for determining the decomposition efficiency of the target compound. There was a slight increase in the ORP values with increasing Fe 21 dosages, and an apparent increase with increasing H 2O2 dosages was observed. The major intermediate formed during the decomposition of 2-cp was 2-chloro-p-benzoquinone. It was also readily decomposed using the coupled ultrasound/Fe 21 /H 2O2 process.

Oxidation of chlorophenols with hydrogen peroxide in the presence of goethite

The use of goethite (α-FeOOH) and hydrogen peroxide was recently found that they could effectively oxidize organic compounds. The study was to investigate the effect of goethite particle size, goethite concentration, Fe 2+ and Fe 3+ on the 2-chlorophenol oxidation. Results indicated that 2-chlorophenol can be decomposed with hydrogen peroxide catalyzed by goethite and the oxidation rate increased with decreasing goethite particle size. 2-Chlorophenol degradation was almost retarded with 0.8 g/l of goethite because ferrous ions could not be produced at this condition. Addition of Fe 2+ and Fe 3+ can enhance the catalytic oxidation rate of 2-chlorophenol very efficiently. In conclusion, the main mechanism of goethite catalyzing hydrogen peroxide to oxidize 2-chlorophenol may be due to the catalysis of ferrous ions and goethite surface.

Supercritical water oxidation of 2-chlorophenol effected by Li + and CuO/zeolites

Catalytic oxidation of 2-chlorophenol (2CP) in supercritical water was investigated. Experimentally, conversion of 2CP in supercritical water oxidation (SCWO) process is effectively enhanced in the presence of Li + that also reduces the formation of higher chlorinated phenols and PAHs. The global reaction rate of SCWO of 2CP in the presence of Li + is expressed as: 92.5 exp (−10.5/RT) [2CP] 0.95 [O 2] 0.56 [H 2O] 0.45. The undesired by-products in the SCWO of 2CP are also extensively reduced in the channels of zeolite catalysts. By EXAFS spectroscopy, CuO and CuCu are identified as the main oxidation active species in the zeolite channels. Cl-bonded CuO species in zeolite Y are not observed.

Rate Enhancement by Cations in Supercritical Water Oxidation of 2-Chlorophenol

Interactions between cations and Cl species in the supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) has been investigated in a high-pressure quartz-lined reactor at 673−773 K. Experimentally, we found that the S/D ratio (defined as the amount of 2CP converted to CO2 and H2O/disappearance of 2CP) for SCWO of 2CP was enhanced by cations (such as Li+, Na+, K+, Ca2+, Fe2+, or Fe3+). The S/D ratio increased as the charge density of cations increased (Fe3+> Fe2+ > Li+ > Ca2+ > Na+ > K+). Due to the extremely low solubility of metal salts in the supercritical water, abstraction of Cl in 2CP via an intermediate ((OH)PhClδ-- - - - -Mδ+) was postulated. Formation of these metal chloride (such as KCl, CaCl2, and FeCl3) precipitates in the SCWO of 2CP were identified by X-ray diffraction (XRD) spectroscopy. Since the formation of toxic highly chlorinated phenols and heavy polycyclic aromatic hydrocarbons (PAHs) was notably reduced, abstraction of Cl of 2CP by cations may occur in the early stage of the SCWO process.

Shape selectivity of trace by-products for supercritical water oxidation of 2-chlorophenol effected by CuO/ZSM-48

Shape selectivity of trace by-products in supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) catalyzed by CuO/ZSM-48 has been investigated. Experimentally, destruction efficiency of 2CP in the SCWO process is effectively enhanced by CuO/zeolite catalysts. In the two-dimensional (2D) channels of ZSM-48, formation of undesired by-products such as polycyclic aromatic hydrocarbons (PAHs) and higher chlorinated phenols (via the Cl-reinsertion) is extremely suppressed in the SCWO of 2CP compared to those observed for zeolites ZSM-5 and Y with three-dimensional (3D) channel structure and larger pore sizes, respectively. The main oxidation active species on CuO/ZSM-48 surfaces were CuO and Cu 2O determined by X-ray photoelectron spectroscopy (XPS). Ratios of the surface species CuO/Cu 2O are between 13.4 and 14.1 in the SCWO process. However, the existence of the Cu–O and Cu–Cu species with a Cu–O/Cu–Cu ratio of 3.5 in the copper catalyst is also observed by extended X-ray absorption fine structure (EXAFS) spectroscopy. Furthermore, since the absence of Cu–Cl species in the XPS and EXAFS spectra, one would suggest that copper in the channels of ZSM-48 is unlikely involved in the abstraction of Cl species from 2CP in the SCWO process.

Photocatalytic Oxidation of Chlorophenols in Single-Component and Multicomponent Systems

The photocatalytic oxidation of single-component and multicomponent systems consisting of chlorophenols in aqueous TiO2 suspension was investigated using long, medium, and short wavelength UV radiation. In the single-component experiments, 2-chlorophenol (2-CP), 2,4-dichlorophenol, 2,4,6-trichlorophenol, pentachlorophenol, and 4-chloro-3-methylphenol (4-Cl-3-MP) were found to degrade at the same rate. The rate of mineralization of 4-Cl-3-MP was higher than that of the others. Power law kinetic models for the temporal prediction of the degradation and mineralization profiles fitted the experimental results well for the single-component systems. The application of these models to represent the simultaneous oxidation of a binary mixture of 2-CP and 4-Cl-3-MP was examined for several equimolar and nonequimolar mixtures. Competitive inhibition kinetics were observed. Under equimolar feed conditions the rates of degradation of the total mixture were comparable to those for single-component systems and could be ...

Photocatalytic oxidation of chlorophenols in the presence of manganese ions

The decomposition of chlorophenols in aqueous solution with UV-illuminated TiO2 suspensions in the presence of manganese ions was studied. It was found that the removal rate of chlorophenols would be the highest at pH 3 in the presence of 1.18×10−4 M manganese ion. The effect of ionic strength on the 2-CP decomposition can be ignored in the range from 0.1 to 0.005 M for NaClO4. This study is also to explore the relationship between the adsorption rate and reaction rate. Results showed that the more the adsorption rate the more the decomposition rate for the three chlorophenols. Manganese ions can increase the photocatalytic oxidation of 2-chlorophenol in terms of DOC. The relationship between temperature and reaction rate for 2-CP is k = 0.0043T - 1.2146.

Photocatalytic oxidation of chlorophenols in the presence of manganese ions

The decomposition of chlorophenols in aqueous solution with UV-illuminated TiO2 suspensions in the presence of manganese ions was studied. It was found that the removal rate of chlorophenols would be the highest at pH 3 in the presence of 1.18×10−4 M manganese ion. The effect of ionic strength on the 2-CP decomposition can be ignored in the range from 0.1 to 0.005 M for NaClO4 This study is also to explore the relationship between the adsorption rate and reaction rate. Results showed that the more the adsorption rate the more the decomposition rate for the three chlorophenols. Manganese ions can increase the photocatalytic oxidation of 2-chlorophenol in terms of DOC. The relationship between temperature and reaction rate for 2-CP is k = 0.0043T– 1.2146.

On the performance of Ti/SnO2 and Ti/PbO2 anodesin electrochemical degradation of 2-chlorophenolfor wastewater treatment

Electrochemical oxidation of 2-chlorophenol was studied at Ti/PbO2 and Ti/SnO2 anodes. The performance of the electrodes was evaluated in terms of faradaic yield and fraction of toxic intermediates removed during the electrolysis. Results showed that, although similar average faradaic yields were obtained using Ti/PbO2 or Ti/SnO2 anodes, the latter material is preferred because of its better ability to oxidise toxic compounds. An effective electrochemical treatment (ηF≅50%) may be accomplished, in which electrolysis at Ti/SnO2 can be stopped when, in spite of a relatively high COD, only a small amount of easily biodegradable oxalic acid is present in the effluent.

Oxidation of 2-Chlorophenol Effected by Na+ and CuO/Zeolites in Supercritical Water

Abstract Supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) is very effected in the presence of 0.02 M Na+ that reduces the formation of by-products (higher chlorinated phenols). The undesired by-products in the SCWO of 2CP catalyzed by CuO in zeolites Y, ZSM-5, and ZSM-48 is also reduced extensively. Chlorine-bonded CuO species in the channels of ZSM-5 are not observed by EXAFS (extended X-ray adsorption fine structure) spectroscopy.