Product and by-product formation in electrolysis of dilute chloride solutions

The results of an experimental study on electrochemical disinfection of water are presented. Attention was paid to the behaviour of chlorine compounds during electrolysis of water containing chlorides, with particular regard to the selectivity of the process towards the production of oxidising agents. Two reactor configurations were tested: a stirred tank cell and a filter press cell inserted in a hydraulic circuit. Both cells were equipped with boron doped diamond (BDD) anodes. Experiments were performed in batch and continuous mode. The effect of such operating parameters, current density, stirring rate or recirculating flow rate, on the behaviour of the process was investigated. The results at BDD anodes show that low current densities and perfect mixing of the system should be adopted in order to obtain high values of the concentration of oxidising agents avoiding the formation of such undesired by-products as chlorite, chlorate and perchlorate ions. Runs were also performed in which BDD was substituted by a commercial (Ti/RuO₂) DSA anode and the results obtained with the two materials are compared.     

Studies on electrochemical disinfectant production using anodes containing RuO2

The present work contains results for artificial water electrolysis in discontinuous operation using laboratory reactors without separators. Rotating anodes with mixed oxide coatings containing IrO₂/RuO₂ were used. The experimental parameters were the chloride concentration, current density, rotation rate, cathode material, pH and water composition. Active and total chlorine concentrations and current efficiencies were obtained. It was shown that even for very low chloride concentrations, chlorine formation occurs, but side effects and side reactions significantly lower the efficiency in this case. Nitrite and ammonia formation was found to reduce the efficiency of chlorine formation. Partial polarization curves were obtained in kinetic experiments using solutions containing chloride and sulphate ions. An erratum to this article can be found at     

Formation of chlorite and chlorate from chlorine dioxide with Han river water

This study was designed to elucidate the behavior of chlorine dioxide in drinking water systems. Furthermore, the factors that influence the formation of chlorite, chlorate in terms of reaction time, concentration of chlorine dioxide, pH, temperature and UV irradiation were experimentally reviewed. At 20 ‡C, pH 7, 70–80% of chlorine dioxide injected was converted to chlorite and 0–10% of that was transformed into chlorate within 120 min with 2.91 mg/L of DOC. The amount of chlorite formed also increased when pH and temperature increased. As DOC content increased, the residual chlorine dioxide decreased but the amount of chlorite and chlorate were increased. These experiments revealed that chlorate was a dominant by-product under UV irradiation. The models that were obtained by the regression analysis for the formation of chlorite and chlorate from chlorine dioxide with Han River water are as follows: Chlorite (mg/L)=10^-2.20[ClO₂]^0.45[pH]^0.90[temp]^0.27[TOC]^1.04[time]^0.20, Chlorate (mg/L)=10^-2.61[ClO₂]^1.27[pH]^-0.50[temp]^1.28 [TOC]^0.31[time]^0.12     

Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides

In this work the products of the oxidation at BDD anode of chloride ions in aqueous solutions were identified during galvanostatic electrolyses performed in a filter-press reactor operating both in batch and continuous mode. A set of experiments were preformed in order to study the effect of operating conditions (current density, residence time, hydrodynamics and chloride concentration) on distribution and concentration of electrolysis by-products. As a comparison experiments were also performed using a commercial DSA anode. A simple mathematical model was formulated, and the model predictions agree with the experimental data in a wide range of experimental conditions. The results of this work showed that at low chloride concentrations electrolysis with BDD anode produce a mixture of powerful oxidant: low current density, high mass transfer conditions and low residence time were found as optimal conditions to maximize the concentration of oxidants and minimize the concentration of chlorates. The proposed reaction mechanism may also justify the controversial effect of chloride ions in wastewater treatments: the electrolysis carried out with BDD anodes and electrolyte containing chloride concentration higher 1 g/L could meet the target of the process only if the active chlorine is effective in oxidation of the pollutant that must be removed.     

Mechanism and kinetics of chlorine dioxide reaction with hydrogen peroxide under acidic conditions

The reaction of chlorine dioxide with hydrogen peroxide was studied in a well stirred batch reactor in a pH range of 3.60 to 5.07, which is of interest for commercial chlorine dioxide bleaching of chemical pulp. The reaction rate was determined by following the consumption of chlorine dioxide and hydrogen peroxide and the formation of chlorite. The rate equation was established. It was found that the concentration dependencies of chlorine dioxide, hydrogen peroxide and hydroxide ion were all first-order. A reaction mechanism compatible with the rate equation was proposed. Since it was found in previous work that chlorite in chlorine dioxide solution by the addition of small amount of hydrogen peroxide potentially led to a decrease in the formation of organically bound chlorine during chlorine dioxide bleaching, two methods were suggested to implement this technique in a bleach plant.     

The occurrence of bromate and perbromate on BDD anodes during electrolysis of aqueous systems containing bromide: first systematic experimental studies

Bromide electrolysis was carried out on laboratory-scale cells in the range of 1–1,005 mg [Br⁻] dm⁻³ using boron-doped diamond (BDD) anodes. These studies were part of fundamental research activities on drinking water electrolysis for disinfection. Synthetic water systems were mostly used in the experiments, which varied the temperature between 5 and 30 °C, the current density between 50 and 700 A m⁻², and the rotation rate of the rotating anode between 100 and 500 rpm (laminar regime). Hypobromite and bromate were found as by-products, as expected. Bromite was not detected. Higher bromate levels were formed at higher current density, but no clear relationship was observed between bromate concentration and the rotation rate or temperatures between 5 and 30 °C. Bromate yields higher than 90% were found at higher charge passed. Perbromate was found as a new potential synthesis or disinfection by-product (DBP), but no perbromate was detected at the lowest bromide concentrations and under drinking water conditions. The perbromate yield was about 1%, and somewhat lower when bromate was used as a starting material instead of bromide. At a temperature of 5 °C more perbromate was detected compared with experiments at 20°. Approximately 20 times more perchlorate was formed compared with perbromate formation in the presence of chloride ions of equimolar concentration. State of mechanistic considerations is presented and a mechanism for perbromate formation is proposed. The reaction from bromate to perbromate was found to be limited that is in contrast to the earlier studied chlorate-to-perchlorate conversion. In the measured concentration range, reduction processes at the mixed oxide cathode showed a much higher impact on the resulting concentration for perbromate than for bromate.     

Electrochemical waste water treatment using high overvoltage anodes Part II: Anode performance and applications

The performance of highly doped SnO₂ anodes for the oxidative treatment of biologically refractory waste water was compared with PbO₂ and Pt. The oxidation of a wide range of organic compounds proceeds with an efficiency which is about 5 times higher than with platinum anodes. The oxidation efficiency was found to be independent of the pH of the water. In chloride containing media, SnO₂ anodes produce less chlorine gas than platinum anodes and hence show less potential to form hazardous chlorinated organic by-products. The design of a simple plate-and-frame reactor with undivided cells for waste water treatment using SnO₂ anodes was based on two experimental findings: (a) no interference of the cathode with the oxidation has been found: (b) the rate of oxidation is not limited by mass transfer, indicating the participation of homogeneous reactions in the overall oxidation. The new anode material reduces the specific energy requirement of electrochemical oxidation of organics in waste water to 30 to 50 kWh kg^–1 of COD removed. This makes the process an interesting alternative to chemical oxidation using oxidants such as ozone and hydrogen peroxide, or wet oxidation using oxygen at elevated temperature and pressure.     

Mechanism of the ClO2 generation from the H2O2‐HClO3 reaction

The development of chlorine containing species during the hydrogen peroxide‐based chlorine dioxide generation process has been determined. Accordingly, two distinct phases, namely the induction period and the steady‐state phase, were identified. In the induction period, it was observed that chloride and chlorous acid are generated, while chlorine, a byproduct from some methanol‐based processes, is not detectable. The absence of chlorine is explained by the fast reaction kinetics between hydrogen peroxide and chlorine, which results in the formation of chloride. In the steady‐state phase, due to the accumulation of chloride and chlorous acid during the induction period, the reaction between chloric acid and chlorous acid, which is responsible for the generation of chlorine dioxide in the hydrogen peroxide‐based ClO₂ process, becomes possible. Chloride is a catalyst in such a reaction.     

The Evaluation Of Ozonation and Chlorination On Disinfection By-Product Formation for a High-Bromide Water

High-bromide raw water was ozonated or chlorinated with and without hydrogen peroxide to study the effect of the disinfectants on the disinfection by-product (DBP) formation. Less bromate was formed when ozonation was made at the ambient pH of 5.8 as compared to ozonation at pH 7, showing the effectiveness of pH reduction in controlling the bromate formation. When chlorine dose was 1 mg/L instead of 2.3 mg/L, the trihalomethane formation was 50 μg/L instead of >100 μg/L, and the proportional distribution of the trihalomethanes was similar. The use of ozone for this water could provide good results in respect of the DBP formation.     

Minimization Of Disinfection By-Products Formation In Water Purification Process Using Chlorine Dioxide — Case Studies

The formation of the by-product chlorite after using chlorine dioxide for the disinfection of drinking water depends on the quantity of the organic matter dissolved in the water. A further decisive factor for the chlorite formation is the level of residual free chlorine dioxide. The chlorine dioxide demand decreases by application of activated carbon filtration, especially after the use of a combination of ozone and activated carbon treatment of the water. Nevertheless, higher chlorine dioxide residuals are a source of chlorite and chlorate formation. The concept of the “Minimum Chlorine Dioxide Dosage (MCDD)” is developed in order to give a clue to the water companies for an optimized chlorine dioxide dose without compromising the disinfection efficiency. By application of the MCDD, the residual level of chlorine dioxide is focused to 0.05 mg/L after 0.5 h contact time. In the range of the MCDD the ratio of the chlorite formation and the chlorine dioxide demand is nearly independent of the level of DOC.     

Inorganic Reactions in Chlorine Dioxide Bleaching of Softwood Kraft Pulp

Abstract

A softwood kraft pulp (27 kappa) was bleached with chlorine dioxide to various end pH values. The formation of chlorite, chlorate, and chloride was measured to quantify the amount of chlorine dioxide wasted as a function of pH during a D_o (pre‐bleaching) stage. Chlorate formation increased with a decrease in final pH. Conversely, residual chlorite increased with an increase in the final pH. After 120 min of bleaching the total residual chlorite and chlorate showed that no substantial increase in residual oxidant occurs when bleaching to an end pH below 3.4. As a result, the brightness and permanganate numbers for low pH chlorine dioxide bleached pulps did not differ when bleaching to an end pH between 3.4 and 1.8. However, decreasing the pH below 3.4 did result in increased organic chloride formation. The results are contrary to previous studies where the maximum bleaching efficiency for a D₁ stage was reported to occur when the end pH was between 3 and 4.     

Effect of manganese (II) added to the electrolyte on the properties of lead anodes used in oxidation of ferrous sulphate to ferric sulphate

The present paper deals with the possibilities of increasing the corrosion resistance of lead anodes during anodic oxidation of ferrous sulphate to ferric sulphate. This was achieved by preliminary passivation of lead anodes in a sulphuric acid and ferrous sulphate solution to which manganous sulphate was introduced. Owing to the passivation of lead anodes by the 20 mA cm^–2 current a potential of 2.2 V vs SEH is established and on their surface, apart from PbO₂, MnO₂ is also formed. On the basis of voltametric measurements of lead anode oxidation, analysis of potential variations during their selfdepassivation, X-ray microanalysis and scanning analysis of the passivated electrode surface the role of MnO₂ in increasing the corrosion resistance in sulphuric acid solutions containing ferrous sulphate was determined. Also a mechanism explaining the effect of MnO₂ formed on the lead anode surface on limitation of the destruction process of the passive oxide layer during oxidation of the ferrous to ferric ions was suggested.     

Fe-exchanged Y zeolite as catalyst for wet peroxide oxidation of reactive azo dye Procion Marine H-EXL

This paper evaluates the degradation of a reactive azo dye, Procion Marine H-EXL, by catalytic wet hydrogen peroxide oxidation (CWHPO). The catalyst was prepared by ion-exchange, starting from a commercially available ultrastable Y zeolite. All experiments were performed on a laboratory scale set-up. The effects of different reaction parameters such as initial pH, catalyst and hydrogen peroxide concentrations on the oxidation of the dye aqueous solution were assessed. Apart from the conventional parameters, the toxic potential of the dye’s degradation products was investigated using the bioluminescence test. HPICE analysis was also performed to obtain detailed information on the resulting oxidation products (organic and inorganic anions). The results indicate that after only 10 min at 50 °C, 20 mmol/l H₂O₂ and 1g/l FeY_11.5 the color removal was as high as of 97% at pH=3 and 53% at pH=5. More than 96% removal of the dye could be attained in 30 min at pH=5, t=50 °C, 20 mmol/l H₂O₂ and 1 g/l FeY_11.5 which corresponds to about 76% reduction of the initial COD and 37% removal of the initial TOC. A preliminary study of catalytic oxidation with hydrogen peroxide of the synthetic textile wastewater containing the specific dye is also presented. Leaching tests indicate that the activity of the catalyst is not due to leached iron ions, although an amount of 0.1–4.0 ppm of iron ions was found in aqueous solution. The catalyst allows almost total elimination of the dye and a significant removal of COD and TOC without the significant leaching of Fe ions. It was also observed that by using this catalyst, it is possible to extend the range of pH values for which Fenton-type oxidation can occur and no iron hydroxide sludge is formed.     

An X-ray photoelectron spectroscopic study of electrocatalytic activity for chlorine evolution on amorphous palladium-phosphorus alloys containing rhodium,iridium or platinum

The relationship between the electrocatalytic activity for chlorine evolution and the nature of the surface film formed on amorphous palladium-base alloy anodes during electrolysis of a hot concentrated sodium chloride solution was investigated. Chlorine evolution took place on the surface film in which cations were mainly those of additive group metals such as rhdoium, iridium or platinum ions. Amorphous PdIrP alloys showed a high activity for chlorine evolution, while a high rate of chlorine evolution on amorphous PdPtP alloys to produce chlorine gas at a further high rate even at very high overpotentials. This was interpreted in term of formation of a surface film of a low electronic conductivity due to an increase in concentration of a higher valency platinum ion Pt⁴⁺ at high overpotentials. A large amount of neutral chlorine was found on the surface of the amorphous PdRhP andPdPtP alloys polarized at high overpotentials. This was assigned mainly to adsorbed molecular chlorine; that is, the final products of the chlorine evolution reaction, and the difficulty of their release seemed responsible for the difficulty in increasing the rate of chlorine evolution on the amorphous alloys containing rhodium and platinum. The amorphous PdIrP alloys were significantly active for chlorine evolution because they were able to release chlorine molecules without difficulty in addition to the fact that the relative amounts of the higher valency cations in the surface film were not appreciably changed with potential.     

Ozonation/Post-Chlorination of Humic Acid: A Model for Predicting Drinking Water Disinfection By-Products

Experiments were performed to evaluate disinfection by-products in model humic acid solutions which were ozonated at three different ozone to carbon levels and then chlorinated. These experiments were conducted in order to help understand whether the ozone/post-chlorination process alters the amount and type of mutagenic by-products formed, from those produced by chlorination of humic acid alone. Disinfection by-products were identified by gas chromatography/mass spectrometry (GC/MS). Samples of clarified and sand-filtered Mississippi River water at a pilotscale drinking water treatment plant in Jefferson Parish, Louisiana, that were ozonated and post-disinfected with chlorine, also were analyzed by GC/MS. A comparison of the by-products in the pilot plant study versus those in our laboratory study showed that similar compounds were produced. The effect of bromide ion in the pilot plant water on by-product formation also is discussed.     

Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: A general review

Effluents of a large variety of industries usually contain important quantities of synthetic organic dyes. The discharge of these colored compounds in the environment causes considerable non-aesthetic pollution and serious health-risk factors. Since conventional wastewater treatment plants cannot degrade the majority of these pollutants, powerful methods for the decontamination of dyes wastewaters have received increasing attention over the past decade. This paper presents a general review of efficient electrochemical technologies developed to decolorize and/or degrade dyeing effluents for environmental protection. Fundamentals and main applications of typical methods such as electrocoagulation, electrochemical reduction, electrochemical oxidation and indirect electro-oxidation with active chlorine species are reported. The influence of iron or aluminium anode on decolorization efficiency of synthetic dyes in electrocoagulation is explained. The advantages of electrocatalysis with metal oxides anodes and the great ability of boron-doped diamond electrodes to generate heterogeneous hydroxyl radical as mediated oxidant of these compounds in electrochemical oxidation are extensively discussed. The effect of electrode material, chloride concentration, pH and temperature on the destruction of dyestuffs mediated with electrogenerated active chlorine is analyzed. The degradation power of these pollutants with an emerging electrochemical advanced oxidation process such as electro-Fenton, based on the mediated oxidation by homogeneous hydroxyl radical formed from Fenton's reaction between cathodically produced hydrogen peroxide and catalytic Fe²⁺, is examined. Recent progress of emerging photoassisted electrochemical treatments with UV irradiation such as photoelectro-Fenton and photoelectrocatalysis is also described.     

Electrochemical treatment of diluted cyanide aqueous wastes

The electrochemical oxidation of diluted cyanide aqueous wastes has been studied in a single compartment electrochemical flow cell. It has been determined that the anode material influences greatly the process's performance. Boron doped diamond and PbO₂ anodes can oxidize these wastes in the presence of both sulfate or chloride anions. On the contrary, dimensional stable anodes cannot oxidize cyanide in sulfate‐containing wastewaters, and require the presence of chloride ions. The oxidation of cyanides leads to the formation of cyanate in a first step, and later to the formation of carbon dioxide and nitrogen. There is a net consumption of hydroxyl ions during the process. Energy consumptions in the range 20–70 kWh m^?3 are required to decrease the initial pollutant load by 70–80%. Global current efficiencies in the range 3–8% are obtained. These low current efficiencies are justified by the low cyanide concentrations that the wastes used in this work contain. Copyright © 2005 Society of Chemical Industry     

Manganese based materials for diesel exhaust SO2 traps

The storage of SO₂ in manganese based materials was investigated in flow reactor experiments. Manganese oxide precipitated with ammonia and hydrogen peroxide stored about 76 wt.% of SO₂ at a high diffusion rate into the bulk. Doping with potassium increases the SO₂ storage rate substantially at 200 °C, but has an only minor effect at 400 °C. Kinetic studies showed that the storage of SO₂ in pure and potassium doped manganese oxide is controlled by the kinetics of the sulfate formation reaction on the catalyst surface up to complete sulfation, whereas the storage on manganese cerium mixed oxide is limited by internal diffusion of the formed sulfate. The sulfate formation reaction was found to be first order with respect to both SO₂ and manganese oxide. For the potassium doped catalyst sulfur was found to be bound on manganese sites being transferred to potassium afterwards.     

Practical Studies of the Electrolysis and Volatilization of the Bromide from Drinking Water to Minimize Bromate Production by Ozonation

In four recently published articles, a process for the oxidation of bromide to bromine and the volatilization of bromine from drinking water sources was presented. This process was shown to be able to remove up to 35% percent of the bromide found naturally in the California State Water Project. Although bromide itself is quite harmless, it has been shown to react with commonly used disinfectants to produce compounds or disinfection by-products (DBPs) of suspected carcinogens. Bromide reacts with ozone to form bromate. This article presents two studies of pilot scale, flow-through electrolytic reactors that oxidize bromide to bromine and volatilize bromine at <pH 3.5, which occurs at the anode as a result of the oxidation of water. One reactor had 14 anodes that were 91 cm deep and the other had 13 anodes 1.2 cm deep. The bromide removal rates were studied at several different water flows and power settings for different bromide concentrations for both reactors. The results show removal of bromide is impacted by water flows and power settings for different bromide concentrations. Effluent from the deep reactor did show some reduction in bromate concentration as compared to control samples but the results were inconsistent. This appeared to be caused by significant differences in the ozone demand produced by different experimental conditions, difficulty determining the concentration of chlorine, and the use of hydrogen peroxide as a dechlorinating agent. Using the shallow reactor, these difficulties were overcome by developing a more consistent determining chlorine concentration, using much larger ozone doses to overwhelm the ozone demand, and by using ascorbic acid instead of hydrogen peroxide. With these changes, it could be shown that the electrolytic reactor not only lowered the concentration of bromide in the water but when ozonated, the amount of bromate formed was reduced in direct proportion to the amount of bromide removed for an equal dose of ozone.     

The occurrence of perchlorate during drinking water electrolysis using BDD anodes

Electrochemical studies were carried out to estimate the risks of perchlorate formation in drinking water disinfected by direct electrolysis. Boron Doped Diamond (BDD) anodes were used in laboratory and commercially available cells at 20 °C. The current density was changed between 50 and 500 A m⁻². For comparison, other anode materials such as platinum and mixed oxide were also tested. It was found that BDD anodes have a thousandfold higher perchlorate formation potential compared with the other electrode materials that were tested. In long-term discontinuous experiments all the chloride finally reacted to form perchlorate. The same result was obtained when probable oxychlorine intermediates (OCl⁻, ClO₂⁻, ClO₃⁻) were electrolysed in synthetic waters in the ppm range of concentrations. The tendency to form perchlorate was confirmed when the flow rate of drinking water was varied between 100 and 300 L h⁻¹ and the temperature increased to 30 °C. In a continuous flow mode of operation a higher chloride concentration in the water resulted in a lower perchlorate formation. This can be explained by reaction competition of species near and on the anode surface for experiments both with synthetic and local drinking waters. It is concluded that the use of electrodes producing highly reactive species must be more carefully controlled in hygienically and environmentally oriented applications.