Degradation Pathway of Ozone Oxidation of Phenols and Chlorophenols as Followed by LC-MS-TOF

ABSTRACT

This contribution summarizes the temporal intermediates produced during the ozonation of phenol, 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol, as followed by liquid chromatography coupled to a time of flight mass spectrometer (LC-MS-TOF). A 2 × 10^?3 M solution of the phenolic compounds was ozonated in a sintered glass reactor at an ozone dose of 0.14 mg/min (O₂/O₃ flow 10 mL/min). Identified intermediates showed that catechol pathway was the predominant route for phenol oxidation with acrylic acid being the end product. Hydroxylative dechlorination of 2-chlorophenol also gave catechol, and the reaction products were similar to that of phenol. Hydroxylation and ring opening via the 4-chlorocatechol pathways were the predominant route for the ozonation of 4-chlorophenol, while 2,4-dichlorophenol followed both hydroxylation and hydroxylative dechlorination mechanism. Several novel intermediates and coupling products were identified and reaction schemes leading to breakdown products are provided for each phenolic compound.     

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.     

Photooxidation of Some Mono-, Di-, and Tri-chlorophenols in Aqueous Solution by Hydrogen Peroxide/UV Combinations

A number of chlorophenols, namely 2-, 3-, and 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol, have been decomposed in aqueous solution by using hydrogen peroxide as the oxidizing agent under UV radiation emitted by a 400 W high-pressure mercury lamp in a thermostatted Pyrex-glass column photoreactor, and the organic-bound chlorine has been converted into the environmentally harmless inorganic chloride. For H₂O₂/chlorophenol mole ratios between 1: 1 and 16: 1, the photooxidation reaction approached pseudo-first order kinetics, and the rate constants increased with increasing ratio of the oxidant. A theoretical model for the degradation pathway is proposed. The fact that the reaction order was not simply unity could be attributed to the complex reaction sequence processing via a radical mechanism through colored intermediates possibly comprising hydroquinone, catechol and resorcinol.     

Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects

The liquid phase hydrodechlorination (HDC) of 2-chlorophenol (2-CP) and 2,4-dichlorophenol (2,4-DCP) has been studied over 1% (w/w) Pd/C and Pd/Al₂O₃ under conditions of minimal mass transport constraints. The HDC of 2,4-DCP generated HCl and 2-CP as the only intermediate partially dechlorinated product which reacts further to yield phenol; cyclohexanone was formed over Pd/Al₂O₃, but not over Pd/C, prior to complete dechlorination. Pd/Al₂O₃ is characterized (on the basis of TEM analysis) by a narrow distribution of smaller Pd particles to give a surface area weighted mean particle DIAMETER = 2.4 nm that is appreciably lower than the value of 13.2 nm established for Pd/C, where the latter is characterized by a broader distribution of larger (spherical) particles. The addition of NaOH served to increase fractional dechlorination by suppressing HDC inhibition due to the HCl that is generated. Reuse of the catalysts revealed an appreciable deactivation of Pd/C and a limited loss of activity in the case of Pd/Al₂O₃. Deactivation of Pd/C can be linked to a decrease (up to ca. 60%) in the initial BET surface area allied to appreciable leaching (up to ca. 40%) of the starting Pd content through the corrosive action of HCl and, while the average Pd diameter is essentially unaffected, there is evidence of a preferred leaching of larger Pd particles. The stronger metal/support interactions prevalent in Pd/Al₂O₃ results in limited Pd leaching and comparable initial HDC activities during catalyst reuse with/without NaOH addition. Inclusion of HCl in the reaction mixture (pH 5–1.5) resulted in a marked decline in the initial HDC rate associated with Pd/Al₂O₃ and a lesser drop in HDC activity for Pd/C. The difference in response to bulk solution pH variations are discussed in terms of the nature of the reactive species in solution and the amphoteric behavior of the Pd supports.     

Treatment of chlorophenols-bearing wastewaters through hydrodechlorination using Pd/activated carbon catalysts

The catalytic hydrodechlorination of 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol in aqueous solution over Pd/activated carbon catalysts (0.5% w/w Pd) was studied in a fixed bed reactor. The reactor was fed with a 100 mg/l solution of chlorophenol and a H₂/N₂ (1:1) gas stream. The ranges studied for temperature, pressure and space-time were 25-100 °C, 1.8-6.0 bar and 14-55 kg h/mol, respectively. A commercial and some home-made catalysts were tested. The carbon supports were subjected to oxidation with nitric acid and sodium persulfate. Chlorophenols conversion was found to peak for pressure values ≈2.4 bar. In these conditions, an increase of reaction temperature increases conversion. In the runs carried out at high space-time both the reactants conversion and the selectivity towards end chain reaction products was enhanced. The oxidation of the carbon support with nitric acid prior to impregnation improves conversion. At the optimum conditions (2.4 bar, 75 °C and nitric acid oxidation of carbon support) conversion values over 95% were reached for all chlorophenols. As a result of this treatment the toxicity of the initial solutions was reduced by more than 90%.     

Pd–Ni双金属复合物修饰泡沫镍电极对水中4-氯酚的电化学脱氯

以Pd–Ni双金属复合物修饰泡沫镍电极（Pd–Ni/Ni）为阴极,对水中4–氯酚（4–CP）进行了电催化脱氯。Pd–Ni双金属颗粒直径大约为50–100nm且均匀地分散在泡沫镍基体上。高效液相色谱（HPLC）分析结果表明Pd–Ni/Ni电极比Pd/Ni或Ni/Ni电极具有更高的催化效率,而苯酚是主要的脱氯产物。增大4–CP初始浓度和升高溶液温度均能使电流效率增大,而还原电流的增大则使电流效率减小。在表观电流密度2.500 mA•cm-2、溶液温度15 °C、4–CP初始浓度1 mmol·L^-1时,4–CP脱氯效率可达82%。另外,根据Pd–Ni/Ni,Pd/Ni和Ni/Ni三种电极的电化学阻抗谱的差异以及它们在不同电流条件下的脱氯效率,推断得到不同极化条件下的速率控制步骤。     

Nanostructured palladium-silver coated nickel foam cathode for magnesium-hydrogen peroxide fuel cells

A novel multiscale Pd-Ag catalyzed porous cathode for the magnesium-hydrogen peroxide fuel cell was prepared by electrodeposition of Pd onto Ag coated nickel foam surface from an aqueous solution of palladium chloride. The structure, morphology and composition of the electrodeposited catalyst layer were characterized using SEM, EDS and XPS analysis. Magnesium-hydrogen peroxide fuel cell tests with the Pd-Ag deposited cathode were carried out and compared with the Ag-deposited electrode. The effects of temperature, H₂O₂ flow rate and H₂O₂ concentration on cell performance were investigated, and the electrode stability test was carried out. The Pd-Ag deposited electrode showed higher catalytic activity for the reduction of hydrogen peroxide than that of the Ag-deposited Ni foam cathode, and gave much improved fuel cell performance. The magnesium-hydrogen peroxide fuel cell with nanostructured Pd-Ag coated nickel foam cathode presented a maximum power density of 140 mW cm⁻², but the Mg-H₂O₂ fuel cell with Ag coated Ni foam cathode gave only 110 mW cm⁻² under the same operation condition.     

Electrochemical characteristics of LSCF-SDC composite cathode for intermediate temperature SOFC

A kind of composite cathode, La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ-Ce_0.8Sm_0.2O_2−δ (LSCF-SDC), was presented in this paper. The electrochemical performance of the cathode on the electrolyte of SDC and YSZ coated with a thin SDC (YSZ/SDC) layer was studied by electrochemical impedance spectroscopy (EIS) and cathodic polarization techniques for their potential utilization in the intermediate temperature solid oxide fuel cell (IT-SOFC). Also studied was the relationship between the electro-catalytic characteristics and the electrode microstructure. Results showed that the LSCF-SDC composite electrode performed better on the SDC electrolyte than on the electrolyte of YSZ/SDC. The polarization resistance, R_p, of the cathode on the SDC electrolyte was 0.23 Ω cm² at 700 °C and 0.067 Ω cm² at 750 °C, much lower than the corresponding R_p of the same cathode on the YSZ/SDC electrolyte. At 750 °C, the cathodic overpotential of the composite cathode on the SDC electrolyte was 99.7 mV at the current density of 1.0 A cm⁻².     

Ni-Fe双金属对氯代苯酚催化还原脱氯的试验

分别采用Ni-Fe双金属体系和单一零价铁对氯代有机物2,4-二氯苯酚、2-氯苯酚和4-氯苯酚进行了催化还原脱氯的研究。结果表明:单一零价铁能够对氯代苯酚还原脱氯,但效率不高,通常在10%～25%。在镍的催化作用下,零价铁对氯代苯酚的还原脱氯效率大大提高。当零价铁加入量为60 g/L,硫酸镍为0.6 g/L,初始氯代苯酚的质量浓度在25 mg/L左右,反应初始pH值控制在偏酸性的条件下,还原脱氯效率可达到70%以上。氯代苯酚降解的准一级速率常数和降解率满足以下规律:4-氯苯酚大于2-氯苯酚大于2,4-二氯苯酚。     

The “In-cell” and “Ex-cell” Fenton treatment of phenol, 4-chlorophenol and aniline

A comparative study of phenol, 4-chlorophenol and aniline degradation with the electro-generation of H₂O₂ at gas-diffusion electrodes was carried out under three different conditions: electro-Fenton^® treatment in an undivided cell; electro-Fenton treatment in the catholyte of a membrane cell divided by a proton-exchange membrane (in-cell electro-Fenton membrane process); and a treatment of polluted solution in the cathode space of a membrane cell with the generation of H₂O₂, followed by the addition of Fe(II) salt in the other reactor (ex-cell electro-Fenton process).An optimized cell design with no gap between the membrane and the anode, along with the appropriate choice of supporting electrolytes, ensured a voltage reduction with a membrane cell in comparison with that of an undivided cell. The accumulation of hydrogen peroxide in concentrations sufficient for the almost complete destruction (90–98%) of aromatic organic pollutants was achieved in all cases but the ex-cell process with the preparative electrolysis in the pilot scale membrane reactor separated by the proton-exchange membrane MK-40 showed higher treatment efficiency and lower specific energy consumption in comparison with known technologies. Damage of the gas-diffusion layer was observed in some tests which could be caused by alkaline conditions in the pores of the gas-diffusion cathode (GDE). The pH indicator paper showed a color specific for alkaline media in contact with the GDE treated in the solution with pH 3 in the bulk. A possible explanation could be that even in acid media, hydrogen peroxide generation in pores of the gas diffusion layer proceeds with formation of HO ₂ ^? which is common for alkaline media and consecutive protonation occurs at the interface with the acid solution.     

Electrochemical destruction of chlorophenoxy herbicides by anodic oxidation and electro-Fenton using a boron-doped diamond electrode

The degradation of herbicides 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous medium of pH 3.0 has been comparatively studied by anodic oxidation and electro-Fenton using a boron-doped diamond (BDD) anode. All solutions are totally mineralized by electro-Fenton, even at low current, being the process more efficient with 1 mM Fe²⁺ as catalyst. This is due to the production of large amounts of oxidant hydroxyl radical (OH) on the BDD surface by water oxidation and from Fenton’s reaction between added Fe²⁺ and H₂O₂ electrogenerated at the O₂-diffusion cathode. The herbicide solutions are also completely depolluted by anodic oxidation. Although a quicker degradation is found at the first stages of electro-Fenton, similar times are required for achieving overall mineralization in both methods. The decay kinetics of all herbicides always follows a pseudo first-order reaction. Reversed-phase chromatography allows detecting 4-chlorophenol, 4-chloro-o-cresol, 2,4-dichlorophenol and 2,4,5-trichlorophenol as primary aromatic intermediates of 4-CPA, MCPA, 2,4-D and 2,4,5-T, respectively. Dechlorination of these products gives Cl⁻, which is slowly oxidized on BDD. Ion-exclusion chromatography reveals the presence of persistent oxalic acid in electro-Fenton by formation of Fe³⁺-oxalato complexes, which are slowly destroyed by OH adsorbed on BDD. In anodic oxidation, oxalic acid is mineralized practically at the same rate as generated.     

Effectiveness and Mechanisms of Fenton-like Si-FeOOH/H2O2 in 2,4-Dichlorophenol Wastewater Treatment

This research measured the effectiveness of Si-FeOOH as a type of heterogeneous Fenton-like catalyst made from alkaline precipitation methods, in catalyzing the degradation of 2,4-dichlorophenol (2,4-DCP) wastewater. Factors including catalyst dosage, pH, and hydrogen peroxide dosage were investigated, showing: with an initial concentration of 2,4-dichlorophenol at 100 mg/L, a dosage of catalyst at 0.1 g/L, a dosage of H₂O₂ at 1.5 ml/L, pH at 5 and indoors temperature at 25 ± 1 °C, the degradation rate of 2,4-dichlorophenol, COD_Cr and TOC has reached 94.21%, 83.67%, and 76.11%, separately. Comparisons were made with FeOOH used as catalyst, showing that Fenton-like Si-FeOOH is a better catalyst than FeOOH.     

Electropolymerization of chlorinated phenols on a Pt electrode in alkaline solution Part I: A cyclic voltammetry study

Cyclic voltammetry studies were carried out on a platinum electrode in alkaline 1 M NaOH solutions containing 0.1 M of phenol, monochlorophenols, dichlorophenols, trichlorophenols and pentachlorophenol in order to compare their electropolymerization ability and the degree of deactivation of the electrode. These show that fouling of the Pt electrode occurs during the electrooxidation of all the phenols studied. Almost full deactivation of the electrode occurs in the case of phenol and 3-chlorophenol after five cycles, while 4-chlorophenol, 3,4-dichlorophenol, 2,4-dichlorophenol and 2,4,5-trichlorophenol deactivate the electrode completely after the first cycle. Partial deactivation of the electrode occurs in the case of 2-chlorophenol, 2,3-dichlorophenol, 2,6-dichlorophenol, 2,5-dichlorophenol and 2,4,6-trichlorophenol. A weak fouling of the electrode occurs in the case of 3,5-dichlorophenol, 2,3,6-trichlorophenol and pentachlorophenol. Such differences in the degree of electrode deactivation may be explained by the different structure (permeability) of the polymeric tars formed. The structure of the tars depends on the monomer structure. Presumably, more regular and dense polymer structures deactivate the electrode more rapidly. More branched, irregular high molecular weight substances deactivate the electrode more slowly.     

Effect of silver catalyst on the activity and mechanism of a gas diffusion type oxygen cathode for chlor-alkali electrolysis

Application of a gas-diffusion type oxygen cathode will contribute to energy saving in chlor-alkali electrolysis. For this purpose the development of gas-diffusion electrodes with high performance and durability is essential. We have investigated the performance for oxygen reduction and the mechanism of its on gas-diffusion electrodes with and without Ag catalyst in order to develop such oxygen cathodes with high performance and durability. It has been found that an electrode with no catalyst, that is, carbon support only in the reaction layer, shows electrochemical activity for oxygen reduction in 32 wt % NaOH at 80 °C and 1 atm O₂, but loading of 2 mg cm^–2 Ag of particle size 300 nm, not only improves the activity by about 100 mV but promotes the four-electron reduction to produce OH^–, while H₂O₂ is the predominant reaction intermediate in the absence of the Ag catalyst. The production of H₂O₂ has been demonstrated by conducting CV measurements to detect H₂O₂ in the anodic scan after a cathodic sweep up to 0.3 V vs RHE. It has been shown that the gas-diffusion type oxygen cathode with Ag catalyst has the high performance and durability necessary for chlor-alkali electrolysis.     

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.     

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.     

Liquid phase catalytic hydrodechlorination of 2,4-dichlorophenol over carbon supported palladium: an evaluation of transport limitations

The catalytic hydrodechlorination (HDC) of aqueous 2,4-dichlorophenol (2,4-DCP) solutions over Pd/C catalysts (1-10% w/w Pd) has been investigated at 303 K in a stirred slurry reactor. The experimental results have shown that 2,4-DCP is converted to phenol quantitatively and 2-chlorophenol (2-CP) is the only intermediate product within detect limitations . The system is 100% selective in terms of dechlorination and phenol hydrogenation only proceeds once complete dechlorination has been attained. The reaction pathway is illustrated and HDC progress is related to pH changes in solution. The mass-transfer limitations have been evaluated experimentally using the diagnostic criteria associated with varying hydrogen flow rate, stirring speed, catalyst concentration and particle size. Experimental results combined with parameter estimation have revealed the influence of mass transfer at the liquid/solid interface and intraparticle diffusion in limiting HDC rate. These effects can be minimized for the less active 1% w/w Pd/C catalysts where the stirring speed , hydrogen flow , catalyst concentration and particle sizes . The selectivity trends associated with 1% w/w Pd/C were the same whether the system operated under physical transport or chemical control. The selectivity with respect to 2-CP was however limited by mass-transfer processes in the HDC reaction using higher Pd loadings.     

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.     

Photocatalyzed Degradation of Phenol, 2,4-Dichlorophenol,Phenoxyacetic Acid and 2,4-Dichlorophenoxyacetic Acid over SupportedTiO2 in a Flow System

The photodegradation of phenol, 2,4-dichlorophenol, phenoxyacetic acid and 2,4-dichlorophenoxyacetic acid using TiO₂ as photocatalyst is investigated. The photodegradations of these compounds have been conducted in a continuous mode by means of a flow system, in which TiO₂ remains fixed onto glass pearls. The use of this system gives high yields of degradation for the chemicals tested, except for 2,4-dichlorophenol for which a slow dechlorination process is observed. The rate of photodegradation depends on the pH of the solution, the point of zero charge of TiO₂ and the pK_a of the chemicals being the key parameters. The main aromatic intermediates detected have been hydroquinone,paraquinone and hydrohydroquinone during phenol degradation; phenol and hydroquinone during phenoxyacetic acid degradation; chlorohydroquinone and chlorophenol during 2,4-dichlorophenol degradation; and dichlorophenol during 2,4-dichlorophenoxyacetic acid degradation. Finally, some long term irradiations with phenol as model compound have been performed, showing high degrees of photodegradation. It has been observed that only a periodic evacuation of the effluent out of the reactor is needed to sustain high percentages of photodegradation.     

Preparation and characterization of PbO2 electrodes doped with different rare earth oxides

PbO₂ electrodes doped with rare earth oxides (Re-PbO₂), including Er₂O₃, Gd₂O₃, La₂O₃ and CeO₂, were prepared by anodic codeposition in order to investigate the effect of rare earth oxide dopants on the properties of PbO₂ electrodes. The physicochemical properties of the Re-PbO₂ electrodes were analyzed by spectral methods and electrochemical measurements. The surface morphology of the Re-PbO₂ electrodes held the characteristics of the dopants and the crystal grain of PbO₂. The crystal structure of the PbO₂ electrodes was also influenced by doping with different rare earth oxides. The presence of Er₂O₃ and La₂O₃ in the PbO₂ films could enhance the direct anodic oxidation, which was helpful to mineralize 4-chlorophenol. The 4-chlorophenol decay on the Re-PbO₂ electrodes was analyzed and good fitting was found using the relation for the pseudo-first order reaction. Of the electrodes examined, the Er-PbO₂ electrode exhibited the best performance for the degradation of 4-chlorophenol. The removal rates of COD and 4-chlorophenol during the 9 h electrolysis at a current density of 20 mA cm⁻² were 80.7 and 100%, respectively, with the current efficiency being 16.0-10.1%.