Experimental design methodology applied to electro-Fenton treatment for degradation of herbicide chlortoluron

The degradation of herbicide chlortoluron in aqueous medium by electro-Fenton process using a carbon felt cathode and a platinum anode was studied. The great oxidation ability of this process is due to the large production of hydroxyl radical (OH) by electrochemically induced Fenton's reagent. Hydroxyl radicals are very powerful oxidizing agents which react on organics up to complete mineralization. The influence of some experimental parameters such as initial concentration, current intensity and processing time on the degradation and mineralization rate of chlortoluron by hydroxyl radicals has been investigated. The evolution of chlortoluron concentration with processing time shows a pseudo first order kinetics (k_abs = (4.8 ± 0.2) × 10⁹ mol⁻¹ L s⁻¹). A Doehlert matrix was applied for determination of the optimal working conditions. Optimal parameters for maximum mineralization efficiency (TOC removal ratio of 98%) was achieved after 8 h of treatment using a chlortoluron initial concentration of 0.125 mM and an applied current of 300 mA. The mineralization of aqueous chlortoluron solutions was confirmed by identification of the end-products such as carboxylic acids and inorganic ions. Their evolution during electro-Fenton treatment was studied.     

Treatment of a mixture of three pesticides by photo- and electro-Fenton processes

In the present work, a comparative study of a mixture of three pesticides (chlortoluron, carbofuran and bentazon) has been investigated by advanced oxidation processes such as photo-Fenton and electro-Fenton. These processes are based on the in situ production of hydroxyl radical, a highly strong oxidant, which allows the degradation of organic pollutants until their mineralization into CO₂ and H₂O. For the photo-Fenton process, the effect of key parameters such as initial catalyst (Fe³⁺) concentration and hydrogen peroxide (H₂O₂) dosage were studied. Under optimal operating conditions, the evolution of total organic carbon (TOC) has been investigated for the two processes. Obtained results showed that more than 90% of TOC removal was obtained after only 2 h of photo-Fenton treatment whereas the electro-Fenton process needed 8 h of treatment. Nevertheless, the comparison of cost treatment shows that the photo-Fenton process is more expensive than electro-Fenton. The evolution of pesticide's concentration during treatment was determined by high performance liquid chromatography (HPLC). Inorganic ions released such as chloride, nitrate, sulphate and ammonium ions are identified and their kinetic evolution was measured by ion chromatographic analyses.     

A comparative study on the efficiency of electro-Fenton process in the removal of propham from water

Electro-Fenton process has been widely used in the treatment of organic pollutants lately. Its oxidation efficiency mainly depends on the electrode materials. In this study, boron doped diamond (BDD), carbon sponge (CS) and platinum (Pt) electrodes were used at four different configurations as anode and cathode. The oxidation efficiencies of BDD anode and CS cathode were investigated together for the first time in the electro-Fenton process. Propham was used as the model pollutant. The obtained results indicate that the decay rate of propham and the mineralization rate of propham aqueous solutions were highest in the case of BDD and CS electrodes as expected. The obtained mineralization current efficiency (MCE) value was 81% at 100 mA in the presence of 0.2 mM Fe³⁺ for 30 min electrolysis. The oxidative degradation intermediates of propham showed different accumulation characteristics in all configurations. The oxamic acid resisted to mineralization but it rapidly degraded in the presence of BDD anode.     

Anodic,cathodic and combined treatments for the electrochemical oxidation of an effluent from the flame retardant industry

The electrochemical oxidation of an effluent from the manufacture of phosphorus based flame retardants was studied. To reach a residual concentration of reduced phosphorus lower than 10 mg L⁻¹, in compliance with Italian law for industrial wastewater disposal, anodic oxidation using a boron-doped diamond (BDD) anode and electro-Fenton (EF) treatment were tested. The effects of some factors are optimised and a comparison of the reaction pathways is also presented. A combined treatment using EF with BDD conducted in an undivided cell is shown not to enhance the data obtained with BDD while a novel combined treatment using EF and BDD in a divided cell shows promising results when an anionic membrane is used as separation. In this last case the cell operates as two different batch reactors working with the same current. The anodic compartment, fed with raw effluent, provides partial oxidation, while the cathodic compartment, fed with the partially anodically oxidised solution, completes the treatment. When the effluent is transferred in the cathodic compartment, the anodic one is fed with fresh untreated solution. The advantage of this kind of coupling consists in the simultaneity of the two treatments which allows total oxidation with notable saving of charge and time.     

Reaction sequence for the mineralization of the short-chain carboxylic acids usually formed upon cleavage of aromatics during electrochemical Fenton treatment

Electrochemical Fenton treatment of aromatic pollutants in aqueous medium always leads to the formation of short-chain carboxylic acids, which account for the slower degradation rate at the final stages of the process. In order to gain further insight into the fate of such compounds, bulk electrolyses of 200 ml aqueous solutions of eleven C₁-C₄ carboxylics, namely formic, glyoxylic, oxalic, acetic, glycolic, pyruvic, malonic, maleic, fumaric, succinic and malic acid have been carried out by electro-Fenton process with 0.1 mM Fe²⁺ as catalyst, at room temperature and pH 3.0, applying a constant current and using an open and undivided cell equipped with a carbon-felt cathode and a Pt anode. In situ cathodic electrogeneration of Fenton's reagent leads to the formation of the very oxidizing species hydroxyl radical (OH) in the medium, allowing the degradation and total mineralization of all carboxylics studied. Various goals have been accomplished: (a) identification of the degradation intermediates for each carboxylic acid and study of their time course, (b) discussion and proposal of the reaction mechanisms under the action of OH/O₂, (c) analysis of the decay kinetics and determination of the absolute rate constants, which agree well with those available in literature for processes involving OH, (d) verification of the great oxidation ability of the process to degrade mixtures containing all the carboxylics, upon variation of some experimental parameters such as current, concentration and cathode dimensions and, finally, (e) elucidation of a detailed reaction sequence for their mineralization, indicating the plausible pre-eminent pathways.     

Organic pollutants oxidation by needle/plate plasma discharge: On the influence of the gas nature

A gas–liquid plasma reactor is used as an Advanced Oxidation Process for water treatment. The plasma discharge is carried out at atmospheric pressure using AC high voltage and different gases (N₂, O₂, Ar) or gas mixtures. The originality of this work lies in the measurement of the number, frequency and mean energy of micro-discharges under the different gaseous atmospheres using iCCD camera visualization. Treatments in argon present the best results. This may be due either to the positive role of metastable argon in the degradation of the pollutant and/or to the way the oxidant species (HO radicals) are brought to the solution at the liquid-plasma interface. In fact, unlike discharge in oxygen, discharge in argon is characterized by low energy micro-discharges evenly distributed in time.     

Oxidation pathways of malachite green by Fe-catalyzed electro-Fenton process

A very detailed scheme for the Fe³⁺-catalyzed electro-Fenton mineralization of malachite green as a model triarylmethane dye is presented. Bulk electrolyses of 250-mL aqueous solutions of 0.5 mM malachite green with 0.2 mM Fe³⁺ as catalyst have been carried out at room temperature and pH 3.0 under constant current in an undivided cell equipped with a graphite-felt cathode and a Pt anode to assess the performance of the electro-Fenton system. In situ electrogeneration of Fe²⁺ and H₂O₂ from quick cathodic reduction of Fe³⁺ and dissolved O₂ (from bubbled compressed air), respectively, allows the formation of the very oxidizing species hydroxyl radical (OH) in the medium from Fenton's reaction. A pseudo-first-order decay kinetics with an apparent rate constant of k_1,MG = 0.244 min⁻¹ was obtained for total destruction of malachite green by action of OH at 200 mA, requiring 22 min for total decoloration of the solution. In the same experimental conditions, overall mineralization was reached at 540 min. Up to 15 aromatic and short-chain carboxylic acid intermediates were identified along the treatment. The evolution of current efficiency was calculated from the chemical oxygen demand (COD) removal. Based on the time course of most of the by-products and the identification of inorganic ions released, some plausible mineralization pathways are proposed and thoroughly discussed. It has been found that the electro-Fenton degradation of malachite green proceeds via parallel pathways, all of them involving primary splitting of the triaryl structure initiated by attack of OH on the central carbon, thus yielding two different N-dimethylated benzophenones. Successive cleavage of the aromatic intermediates generates oxalic acid as the ultimate short-chain carboxylic acid, whereas N-demethylation of some of them produces formic acid as well. Oxalic acid and its Fe²⁺ complexes, as well as formic acid, can be slowly but totally mineralized by OH.     

Catalytic behavior of the Fe/Fe system in the electro-Fenton degradation of the antimicrobial chlorophene

The catalytic behavior of the Fe³⁺/Fe²⁺ system in the electro-Fenton degradation of the antimicrobial drug chlorophene has been studied considering four undivided electrolytic cells, where a Pt or boron-doped diamond (BDD) anode and a carbon felt or O₂-diffusion cathode have been used. Chlorophene electrolyses have been carried out at pH 3.0 under current control, with 0.05 M Na₂SO₄ as supporting electrolyte and Fe³⁺ as catalyst. In these processes the drug is oxidized with hydroxyl radical (OH) formed both at the anode from water oxidation and in the medium from electrochemically generated Fenton's reagent (Fe²⁺ + H₂O₂, both of them generated at the cathode). The catalytic behavior of the Fe³⁺/Fe²⁺ system mainly depends on the cathode tested. In the cells with an O₂-diffusion cathode, H₂O₂ is largely accumulated and the Fe³⁺ content remains practically unchanged. Under these conditions, the chlorophene decay is enhanced by increasing the initial Fe³⁺ concentration, because this leads to a higher quantity of Fe²⁺ regenerated at the cathode and, subsequently, to a greater OH production from Fenton's reaction. In contrast, when the carbon felt cathode is used, H₂O₂ is electrogenerated in small extent, whereas Fe²⁺ is largely accumulated because the regeneration of this ion from Fe³⁺ reduction at the cathode is much faster than its oxidation to Fe³⁺ at the anode. In this case, an Fe³⁺ concentration as low as 0.2 mM is required to obtain the maximum OH generation rate, yielding the quickest chlorophene removal. Chlorophene is poorly mineralized in the Pt/O₂ diffusion cell because the final Fe³⁺–oxalate complexes are difficult to oxidize with OH. These complexes are completely destroyed using a BDD anode at high current thanks to the great amount of OH generated on its surface. Total mineralization is also achieved in the Pt/carbon felt and BDD/carbon felt cells with 0.2 mM Fe³⁺, because oxalic acid and its Fe²⁺ complexes are directly oxidized with OH in the medium. Comparing the four cells, the highest oxidizing power regarding total mineralization is attained for the BDD/carbon felt cell at high current due to the simultaneous destruction of oxalic acid at the BDD surface and in the bulk 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.     

过碳酸盐高级氧化技术在水处理中的研究进展

近年来,活化过碳酸盐高级氧化技术由于低成本、安全、稳定等特点,越来越多的学者将其应用于处理水中的各类有机污染物,有望成为一种新兴的环境友好型水处理技术。通过查阅近十年大量国内外文献,在总结了活化过碳酸盐降解有机污染物的机理的基础上,系统综述了均相活化、非均相活化、物理活化、联合活化等不同过碳酸盐的活化方法与活化反应机制,并针对活化过碳酸盐高级氧化技术应用于降解废水中的各类有机污染物、膜污染控制、污泥脱水及污泥厌氧消化等方面的研究现状展开阐述。最后指出目前水处理研究领域活化过碳酸盐高级氧化技术存在的问题,并对未来的重点研究方向进行了展望,以期为该技术在水污染治理中的进一步推广和应用提供参考。     

Anodic oxidation of mecoprop herbicide at lead dioxide

The electrochemical oxidation of an aqueous solution containing mecoprop (2-(2-methyl-4-chlorophenoxy)propionic acid) has been studied at PbO₂ anodes by cyclic voltammetry and bulk electrolysis. The influence of current density, hydrodynamic conditions, temperature and pH on the degradation rate and current efficiency is reported. The results obtained show that the use of PbO₂ leads to total mineralization of mecoprop due to the production of oxidant hydroxyl radical electrogenerated from water discharge. The current efficiency for the electro-oxidation of mecoprop is enhanced by low current density, high recycle flow-rates and high temperature. In contrast, the pH effect was not significant. It has also been observed that mecoprop decay kinetics follows a pseudo-first-order reaction and the rate constant increases with rising current density.     

Mineralization of herbicide 3,6-dichloro-2-methoxybenzoic acid in aqueous medium by anodic oxidation, electro-Fenton and photoelectro-Fenton

The mineralization of acidic aqueous solutions with 230 and 115 ppm of herbicide 3,6-dichloro-2-methoxybenzoic acid (dicamba) in 0.05 M Na₂SO₄ of pH 3.0 has been studied by electro-Fenton and photoelectro-Fenton using a Pt anode and an O₂-diffusion cathode, where oxidizing hydroxyl radicals are produced from Fenton's reaction between added Fe²⁺ and H₂O₂ generated by the cathode. While electro-Fenton only yields 60-70% mineralization, photoelectro-Fenton allows a fast and complete depollution of herbicide solutions, even at low currents, by the action of UV irradiation. In both treatments, the initial chlorine is rapidly released to the medium as chloride ion. Comparative electrolyses by anodic oxidation in the absence and presence of electrogenerated H₂O₂ give very poor degradation. The dicamba decay follows a pseudo-first-order reaction, as determined by reverse-phase chromatography. Formic, maleic and oxalic acids have been detected in the electrolyzed solutions by ion-exclusion chromatography. In electro-Fenton, all formic acid is transformed into CO₂, and maleic acid is completely converted into oxalic acid, remaining stable Fe³⁺-oxalato complexes in the solution. The fast mineralization of such complexes by UV light explains the highest oxidative ability of photoelectro-Fenton.     

臭氧氧化处理炼油废水的生化处理出水

采用臭氧氧化法处理经三级生化处理后生化性很差的炼油废水,研究了臭氧氧化法处理废水较佳的反应条件和反应规律。试验结果表明,在最佳的反应条件下,在进水COD的质量浓度为160mg/L时,经臭氧氧化处理后,废水的COD降低40%以上,BOD5与COD的质量比从0.13提高到0.3以上,废水满足进入生化系统进行进一步处理的水质要求。     

Sodium diclofenac removal from a pharmaceutical wastewater by electro-Fenton process

ABSTRACT

In this study, the experiments were carried out to evaluate the effect of five independent variables on the Chemical Oxygen Demand (COD) removal in a pharmaceutical wastewater contaminated with sodium diclofenac. The parameters were statistically optimized under response surface mythology. The optimum conditions for 97.21% of COD removal experimentally were at reaction time of 59.68 min, current density of 58.47 mA/cm², pH of 2.89, volume ratio of 0.32 ml/l and H₂O₂/Fe²⁺ molar ratio of 3.60.     

Fenton氧化法深度处理垃圾渗滤液

为了去除垃圾渗滤液中难于生物降解的有机物,采用Fenton氧化法深度处理垃圾渗滤液。得出试验最佳反应条件为:H2O2和Fe2+不混合分3次投加,H2O2和Fe2+的质量比为2∶1,Fe2+的浓度为0.04mol/L。在最佳条件下,进水CODCr的质量浓度为1521mg/L时,反应3h,出水CODCr的质量浓度为120mg/L,可以达标排放。药剂费用估算为6元/t。     

高级氧化技术处理染料废水的研究进展

由于染料废水中含有高浓度难降解有机污染物,对其有效处理一直是个难题.综述了近几年国内外采用湿式氧化法、Fenton法、光化学与光催化氧化法、电化学法、臭氧氧化法、微波辅助氧化法和超声氧化法等高级氧化技术处理染料废水的进展情况,并指出了高级氧化技术在染料废水处理中的发展趋势.     

过硫酸盐高级氧化降解水体中有机污染物研究进展

基于硫酸根自由基（sulfate radical,SO₄ ^-·）氧化原理的活化过硫酸盐（persulfate,PS）氧化法是近年来高级氧化工艺（advanced oxidation process,AOP）的研究热点,以经济、高效、环境友好、安全稳定的优势在水处理、环境保护等领域开辟了新的思路。此前,学者们发现过硫酸盐高级氧化根据活化反应条件（如温度、光照、pH、过渡金属及催化剂等）的不同,会产生不同的自由基参与氧化反应,对降解结果也会产生不同程度的影响。本文根据相关自由基氧化机理,从产生硫酸根自由基的单一氧化、复杂活化体系硫酸根自由基与其他自由基复合氧化以及强化降解等方面,分析了近几年国内外学者对过硫酸盐降解典型有机污染物的研究及在催化剂开发方面所做的工作,指出了许多新颖的过硫酸盐活化手段及其降解效果与不足,并就未来的发展进行了展望,以期为过硫酸盐氧化法未来更好地发展和应用探索出路。     

Kinetics of phenol mineralization by Fenton-like oxidation

The kinetics of hydrogen peroxide decomposition and the mineralization rate of phenol in homogeneous aqueous solution (pH < 3) via Fenton-like reaction were studied. Results were correlated with the generation of hydroxyl radicals as well as with iron speciation. Batch experiments were carried out in de-ionized water in a completely mixed batch reactor under a wide range of experimental conditions (3500 ≤ [H₂O₂] ≤ 8250 mg/L; 100 mg/L ≤ [Fe] ≤ 2350 mg/L; 2.5 ≤ [H₂O₂]/[Fe] ≤ 83; 0 mg/L ≤ [TOC] ≤ 1000 mg/L). Results demonstrated that the rate of hydrogen peroxide decomposition, phenol mineralization and ferrous ions formation depended on both the initial concentration of the phenol and on the weight ratio between hydrogen peroxide and iron. A linear correlation was found between the mineralization rate of phenol and the decomposition rate of hydrogen peroxide indicated that 10 g of hydrogen peroxide was required to mineralize 1 g of phenol.     

电Fenton法处理难降解有机物的研究现状及趋势

电Fenton法作为一种新兴的废水处理技术,在处理难降解有机废水方面国内外已开展了广泛的研究,并且发现此法对于处理高浓度难降解有机废水效果良好.简要介绍了Fenton法与其他高级氧化技术相比在处理难降解有机废水方面的优缺点,指出电Fenton法克服了普通Fenton法、光Fenton法等的缺点并且成为近来人们研究的重点.综述了电Fenton法的分类、较其他方法的优点,以及利用电Fenton法处理不同类型有机废水的国内外研究现状.指出了电Fenton法还存在的问题,并对其今后的发展趋势进行了说明.     

高级氧化法处理硒冶炼废水的研究

从化学键能出发,着重探讨了臭氧氧化法、Fenton氧化法处理硒冶炼废水中有机污染物的机理,并对高级氧化法进行优化组合。实验结果表明,单独使用臭氧处理废水,在酸性条件下受酯化反应的影响,加入双氧水能抑制酯化反应的进行。臭氧/H2O2+Fenton氧化法是最好的联合技术,CODCr去除率可达到88%。