Bergmann 2013

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Electrochemical Advanced Oxidation Processes,formation of halogenate and perhalogenate speciesacritical reviewM. E.H. Bergmann a , A. S. Koparal b & T. Iourtchouk aa Departments of Electroengineering and Applied Biosciences and Process Technology ,Anhalt University , Bernburger Str. 55, D-06366 Koethen/Anh, Germany Phone: +49(0)349667 2313 Fax: +49(0)3496 67 2313b Department of Environmental Engineering , Anadolu University , Eskiehir , Turkey Phone:+90(0)222 3350580 ext 6406 Fax: +90(0)222 3350580 ext 6406Accepted author version posted online: 20 Aug 2013.

To cite this article: Critical Reviews in Environmental Science and Technology (2013): Electrochemical Advanced OxidationProcesses, formation of halogenate and perhalogenate speciesa critical review, Critical Reviews in Environmental Scienceand Technology, DOI: 10.1080/10643389.2012.718948

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Electrochemical Advanced Oxidation Processes, formation of halogenate and perhalogenate

species a critical review.

M.E.H. Bergmann*, A.S. Koparal** and T. Iourtchouk*

* Departments of Electroengineering and Applied Biosciences and Process Technology, Anhalt

University, Bernburger Str. 55, D-06366 Koethen/Anh., Germany Tel. +49(0)3496 67 2313, Fax:

+49(0)3496 67 2642, E-mail: [email protected], [email protected]

** Department of Environmental Engineering, Anadolu University, Eskiehir, Turkey

Tel. +90(0)222 3350580 ext 6406, Fax: +90(0)222 3239501, E-mail: [email protected]

Abstract

Advanced Oxidation Processes (AOPs) are widely used and suggested for environmentally-oriented

applications. New combinations of single methods are described in literature. An overview about methods

is given focusing on innovative papers of the previous years. At the same time, there are an increasing

number of indications and evident demonstrations that the occurrence of harmful by-products is possible.

Chlorate, bromate and perchlorate belong to these by-products of inorganic nature. Corresponding cases

are considered and discussed. By studying Electrochemical Advanced Oxidation Processes (EAOPs) it

was found that radical generating electrodes show strong tendencies of chlorate and perchlorate

formation in aqueous systems containing chloride ions. Also, bromate and perbromate formation is

possible. Therefore, the authors propose these components as new inorganic criterions in

environmentally-oriented water treatment. A new project is described considering electrochemical

drinking water disinfection in close co-operation between researchers, health and water treatment officials

and cell producers.

Keywords

Electrochemical Advanced Oxidation Processes, disinfection by-products, chlorate, perchlorate, bromate,

perbromate

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1. INTRODUCTION

Advanced Oxidation Processes [1] are based on the application of highly-oxidative species such

as radicals. Mostly, they are aimed at treatment of organic compounds in aqueous waste systems.

There is no clear classification and terminology in AOPs and EAOPs. The principles for

generating highly-oxidative compounds may be divided into physical, chemical and combined

(Fig. 1) but there are many overlapping cases during application. To generate species with high

oxidation potential, activation by application of energy is necessary. Typical technologies of

activation are electrical discharge processes, heat activation and chemical reaction paths,

catalytic reactions, illumination/irradiation, electrochemical activation, cavitation (ultrasonic and

hydrodynamic) and others. One of the most popular among these is electrochemical oxidation,

which produces highly-reactive radicals such as hydroxyl radicals:

OH- OH + e- (1)

H2O OH + H+ + e- (2)

For instance, the extraordinary high reactivity of hydroxyl radicals is shown in Table 1.

The terminus Electrochemical Advanced Oxidation Processes is related with the

discovery, study and application of boron doped diamond (BDD) anodes after the

publication of Marselli et al. in 2003 [2-4]. After initiation, intensive studies on BDD

electrodes for electrochemistry started worldwide. First research on fundamentals and

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possible applications was located in groups of Pleskov, Natishan, Morris, Swain, Martin,

Vinokur, Anderson, Kang, Comninellis, Fujushima, Einaga, Rao, Brillas, Ferro, Battisti,

Haenni, Panizza and many others. Especially the ability of BDD anodes to produce

hydroxyl and other radicals leaded to promising approaches for destructing organics in

waste systems, synthesizing oxidants in industrial scale and producing ozone for

disinfection purposes as well as for electroanalysis and corrosion protection. A series of

books summarizes the state of manufacturing, property characterization and

applications [5-12]. The history is well described in [12]. BDD electrodes are currently

produced by six companies.

Nowadays, more and more papers are published dealing with new combinations such

as photoelectrocatalysis [13-14], Electro-Fenton [15], microwave-activated

electrochemistry [16], Photoelectro-Fenton [17-21], and many others.

EAOPs are able to mineralize organic material to relatively harmless substances: CO2,

N2 and other non-toxic compounds. Therefore, the overwhelming majority of studies

emphasize benefit of technology, i.e. detoxification of wastewaters, decolouring or

disinfection. However, a more careful consideration is necessary:

If the process is of environmental relevance, environment may be affected by

effluents in the case of incomplete conversion.

If the method is extended to drinking water, health of many people may be

directly affected.

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Moreover, mainly organic by-products are a subject of discussion [22-25]. Inorganic by-

products are usually not considered; bromate is an exceptional case.

Currently, new efforts are apparent to adapt principles of AOPs for drinking water

treatment (large variety of method combination, new materials etc.). As a logic

consequence, search for disinfection by-products has to be enforced.

This paper is aimed to give a brief overview about the developments in the field of

EAOPs focusing on papers with overview character published in recent years.

Based on literature analysis and our own experimental results on Electrochemical

Advanced Oxidation Processes the authors underline the enlarged necessity in more

accurate studies for inorganic treatment by-products in aqueous systems and suggest

new inorganic assessment criterions as firstly mentioned in [26].

2. ELECTROCHEMICAL ADVANCED OXIDATION PROCESSES STATE OF THE

ART

2.1 Quick look to methods application emphasizing the target of treatment

To emphasize the benefit of EAOPs most of the papers are concerned with the

treatment targets, for example, decolouring and removal of odours, removal of

surfactants, general or special waste water and effluent treatment, detoxification and

disinfection. Sometimes treatment fields may overlap. The following citations describe

main subjects in more details.

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2.1.1 Decolouring

Colour is a sign of special constituent presence. If colour disappears the constituent

concentration decreases. It does not mean that the system is now harmless but

disappearance of colour is an indicator for oxidation reactions. Many papers can be

mentioned to deal with EAOPs decolouring [27-38]. Two papers give an overview over

electrochemical decolouring processes [39,40].

Decolouring is also an indicator method for EAOPs. Often, Acid Blue 22, Acid Orange 7,

Acid Orange 52, Methylene Blue, Prussian Blue, Reactive Red, Acid Yellow 36,

Reactive Orange 122, and other systems (textile industries wastewaters, olive oil mill

wastewaters) are used for this purpose.

2.1.2 Treatment of waste, process and natural waters

The large amount of publications in these fields cannot be overseen. The reader has to

distinguish between pure fundamental and application-orientated studies. Five larger

overviews were presented [41-45].

Although sometimes reported, treatment of wastes at concentration in the range of g L-1

level and at large volume is not sophisticated because of consuming unacceptable

amount of energy and time. Consequently, low-concentrated systems (contaminant

concentration at mg L-1 and g L-1 level) are more suitable for economic treatment.

(Most guidelines and recommendations use mass concentration data related to one

Litre like mg L-1 and g L-1. Thats why these units are preferred in this paper).

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One possible classification of considered systems is the removal of contaminants such

as pharmaresidues, surfactants, herbicides and insecticides, and pathogens

(disinfection).

2.1.3 Removal of pharmaresidues

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It is estimated that 10 % of medicals prescribed each year worldwide (hundred thousand tons)

are released to environment. Detected hormonal activities of pharmaresidues (endocrine

disruptors) from natural and drinking water [46-49] in animals and human beings reported in the

1990ies, which resulted in a high number of works dealing with EAOPs for pharmaceutical

residual removal [50-53]. The background was the insufficient treatment by conventional

methods [54]. Mostly, model substances in mg L-1 range of concentrations are studied.

Representatives are Bisphenol A [55,56] , Ibuprofen [57], Fluoxeine and Metoprolol [58,59], and

others as summarized by Esplugas [60]. The problems are the same as mentioned above:

unknown intermediates may be formed at large extent. When process water at mg L-1 range of

concentration is treated, long treatment times are necessary. At g L-1 range analytical control

of educts and by-products complicates application. A new paper for instance describes and

quantifies mineralization of beta-blockers such as atenolol, metoprolol tartrate and propranolol

hydrochloride by Electro-Fenton and Photoelectro-Fenton including BDD application and solar

light photolysis. Aromatic intermediates and carboxylic acids like oxalic and oxamic were

temporarily found. Finally, inorganic substances (nitrate, ammonium etc.) remained in the

treated systems [61].

Although halogenates and perhalogenates are in the focus of this paper it must be

underlined that ammonium and nitrate have to be taken into consideration of potential

risks for environment. Ammonium is toxic for fish, and nitrate supports eutrophication.

Recently, Sirs and Brillas published a new review paper [62], see also [63] and Table 2.

2.1.4 Removal of surfactants and personal care products

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Surfactants in effluents from home chemicals and personal care products are usually

responsible for foam formation. Biological waste water treatment plants are not able to

remove them totally. Some studies suggest EAOPs for surfactant removal [84].

Widespread use is not known. Surfactants are sometimes considered as a common

group of pharmaceutical and personal care products [60].

2.1.5 Removal of biocides/herbicides

Approximately 25 years ago, environmental problems brought agro-chemicals into the

focus of discussion. Nowadays, pesticides and herbicide degradation is widely studied

[85-86]. Problems lie in the complicated organic chemistry scheme during oxidation and

the formation of many by-products. Chloride addition accelerates degradation but

results in chlorinated by-product formation [85]. Therefore, new publications describe

techniques such as electrochemical oxidation using BDD anodes that are capable of the

total mineralization of organic contaminants [87]. In section 2.2.2.2. papers describing

degradation of several biocides/herbicides by the Electro-Fenton methods and its

modification are listed.

2.1.6 Disinfection

Besides process water from production or cleaning and rinsing processes, chiller waters

belong to this group of contaminated systems. Disinfection here is the main target of

treatment, just as the case in the disinfection of drinking water, swimming pool water

and other aqueous systems. Free available chlorine species (HOCl, OCl-) are formed

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from chlorine radicals and mainly responsible for the disinfection effect in waters

containing chlorine [88-90]:

Cl- Cl + e- (3)

Cl- + OH Cl + OH- (4)

In waters free of chloride oxidants like H2O2 and ozone (formation reaction in section

6.1) may react with microorganisms.

For drinking water, strong regulations and approaches regarding (mainly) organic

disinfection by-products exist [91]. Bergmann et al. critically discuss Inline electrolysis

mechanisms for disinfection [92,93].

2.2 Quick look to methods application emphasizing the principle of treatment

2.2.1 Non-combined electrochemical methods

Electrochemists had to use platinum and lead dioxide electrodes for adjustment of high

oxidation potential (permanganate, peroxide, persulphate, and ozone generation,

chromium oxidation etc.) for a long time. Later, it was found that mixed oxide (MIO)

electrodes are also able to develop radical chemistry mechanisms. A characterisation

was published elsewhere [94-95]. A new challenge was the presentation of doped thin

film diamond electrode characterized by high chemical stability, large electrochemical

window and the possibility to operate anodically at potentials higher than 2 V (SHE)

[96]. Boron doped diamond electrodes are most common electrodes. Nearly one

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thousand papers were published studying properties of doped diamond electrodes and

their applicability [97-100]. Velegraki et al. studied toxicity test on water treated by

doped diamond electrodes. Their results showed that degradation by-products are

consistently more toxic than the parent compound even after deep oxidation [68].

Currently, many new projects are orientated to targeted ozone production on BDD

anodes. Some combined or hybrid processes including electrochemical principles are

mentioned as follows.

2.2.2 Combined electrochemical methods

2.2.2.1 Electrochemistry-UV and electrochemistry-visible light

Photoelectrochemical treatment of solutions may influence the composition and reaction

scheme near the electrode and in the bulk of solution. For example, Xiao et al.

developed a unique photoelectrochemical process for the treatment of ammonia in

wastewater containing chloride ions by combined use of electrogenerated active

chlorine and photogenerated active radicals (UPE). In this process, first of all Cl- ions

were electrochemically oxidized to Cl2. Second step was Cl2 hydrolysation to generate

active chlorine. The last step was active chlorine dissociation to OH and Cl radicals

under UV irradiation in the UPE process. Thus, these in situ electrogenerated active

chlorine and photogenerated chlorine radicals were responsible for the synergistic effect

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on ammonia degradation [101]. Combination of these methods has been considered for

wastewater treatment by some researchers [102-112]. The use of visible light and

electrochemistry [113] is a relatively new combination.

2.2.2.2 Electro-Fenton and Anodic Fenton Treatment (AFT)

Photolysis and ultrasound were not found effective alone, while photocatalysis or

assisted photochemistry need an additional reagent such as H2O2 or TiO2.

Electrochemistry was proposed to be a new Advanced Oxidation Process as an

alternative process to produce Fentons reagent in the bulk solution [114].

The so-called Fenton process is known from the 1890s. It includes generation of

hydroxyl radicals by reacting Fe2+ ions with hydrogen peroxide:

Fe2+ + H2O2 Fe3+ + OH + OH

(5)

In the Electro-Fenton process, hydrogen peroxide that is necessary for a Fenton

reaction effect is formed by cathodic reduction of dissolved oxygen:

O2 + 2H+ +2e H2O2 (6)

Oxidized Fe3+ may be again electrochemically reduced:

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Fe3+ + e Fe2+ (7).

More than a hundred papers exist in the field. Song studied Electro-Fenton systems to

demonstrate their efficiency in wastewater treatment as a representative of EAOPs

[115]. Brillas (Chapter 17 in [12]), and Jiang and Zhang [116] summarize the state of the

art in wastewater treatment.

Important papers of the Brillas and Oturan groups on pharmaceutical residue treatment

are cited here in section 2.1.3. Recently, a lot of studies have been conducted on

Electro-Fenton application of wastewater treatment [117-124].

Treatment of biocides/herbicides has gained very high interest in many papers

published recently [125-130].

It is a logic conclusion that an Electro-Fenton process may be combined with direct

anodic or mediated oxidation of pollutants. Brillas and co-workers studied herbicide

destruction by applying this combination of methods [87].

When the Electro-Fenton process is carried out under highly acidic conditions (pH=2-3)

and with sacrifice iron electrode, the process is called Anodic Fenton Treatment (AFT)

[131]. The technology was mainly studied for biocide/herbicide destruction [132-137].

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Combination of Electro-Fenton processes with biochemistry results into the Bio-Electro-Fenton

process [138].

2.2.2.3 Ultrasound-Electrochemical Oxidation

Ultrasound- electrochemical oxidation (US-EC process) is one of the novel combined

technologies. Researchers have shown the obvious enhancement on electrochemical

degradation of pollutants by ultrasound [139,140]. It is widely accepted that ultrasound

with low frequency cause physical effect which can clean electrode surface and improve

mass transport, while US with high frequency usually causes chemical effects, which

can produce active substances such as hydroxyl free radicals. In the article [140], Zhao

et al. mention an ultrasound enhanced electrochemical oxidation of phenol on boron-

doped diamond and Pt electrodes. The rate of electrochemical degradation of phenol

and current efficiency on both BDD and Pt electrodes can be enhanced with the help of

US. The enhancement effect on BDD is much more obvious than for Pt. Lower amount

of intermediates are produced with BDD than with Pt. In the presence of ultrasound, the

variety of intermediates does not change on both electrodes. However, production and

degradation rate of intermediates can be promoted by ultrasound, in particular on BDD

[139]. Ultrasonic assisted electrochemical oxidation was also used in many other

studies [141-146].

According to Compton et al., power ultrasound applied to boron-doped diamond

electrodes allows the electrochemical reduction of dioxygen to hydrogen peroxide under

conditions of extremely high rates of mass transport and in the presence of cavitation. A

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colorimetric test reaction with iodide13 was used to confirm the formation of hydrogen

peroxide after electrolysis. In contrast, under the same experimental conditions but

without electrolysis the sonolytic formation of H2O2 was found to be negligible [147].

2.2.2.4 Sonoelectro-Fenton

Combined electrochemical technologies have gained major attention in recent years for

environmentally friendly wastewater treatment. Oturan et al. worked on a novel hybrid

technique, namely Sonoelectro-Fenton, based on the simultaneous action of ultrasonic waves and

Electro-Fenton using a 3D cathode. The method was used for the removal of organic pollutants

in aqueous medium. In this study, result showed that high performance arises from the coupling

between ultrasound irradiation and the in situ electrogeneration of Fentons reagent [148].

Martinez and Uribe [149] studied the enhanced degradation of azure B dye using a Sonoelectro-

Fenton process. Babuponnusami and Muthukumar [150] made a comparison between Fenton,

Electro-Fenton, Sonoelectro-Fenton and Photoelectro-Fenton by oxidizing phenol. The last two

methods showed higher reaction rates than the others.

2.2.2.5 Photoelectro-Fenton

By combining Electro-Fenton with photochemical technologies the so-called

Photoelectro-Fenton process can be defined as a relatively new method of EAOPs. Life

cycle assessment showed the advantage of Solar Photoelectro-Fenton over other

combined technologies if applicable [151]. Reactor design description and comparison

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with Electro-Fenton and Fenton technologies was presented by Ting and co-workers

using benzene sulfonic acid mineralization [152]. De Luna and co-workers studied the

acetaminophen degradation [19]. Recent advances in the field of synthetic dye

treatment were discussed by Peralta-Hernandez and co-workers [153].

There are still other combined methods existing such as peroxy-coagulation [154, 155],

which cannot be discussed here in details but it becomes clear that the short overview

reveals the great importance of EAOPs. EAOPs belong to the most dynamically

developing technologies in chemical engineering. The number of papers is increasing

each year. Despite the academic character of many research papers it can be expected

that application of the technologies will significantly grow. Therefore, possible problems

and risks have to be controlled more systematically.

3 PROBLEMS OF ELECTROCHEMICAL ADVANCED OXIDATION PROCESSES

In this part in order to examine thoroughly the formation of inorganic by products that is

the particular purpose of this study, all the AOPs in which this problem generally occurs

must be considered. Although benefit from Electrochemical Advanced Oxidation

Processes appears to predominate, the methods are not unproblematic. In this section,

main problems of EAOPs are addressed. Besides the economy, first of all, occurrence

of by-products must be mentioned. Focusing on inorganic by-products halogenates and

perhalogenates are considered in particular. This question is not totally new. In drinking

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water disinfection limiting concentration values were defined in special rules for selected

halogenates. Perhalogenates are under discussion. Chlorate, bromate, perchlorate and

perbromate are the most important representatives of these substances. Therefore,

halogenate and perhalogenate formation potential is shown here using an extended

literature research. Then, the formation of halogenates and perhalogenates is

demonstrated using electrochemical processes with BDD anodes (sections 4 and 5) as

an accepted method of EAOPs.

3.1 Economy

In the most published studies AOPs are proved to be able to oxidize components in

aqueous systems. Process efficiency is mostly not studied though simple calculation,

which often shows that the practical application is far from reality. So, it is necessary to

pay more attention to economical points. The biggest problem is formed by high

investment and energy costs in large scale application. Consequently, economical

considerations have to include

- the volume or flow rate of treated system

- the rates of reaction and mass transfer

- the ratio of contaminant concentration to oxidant concentration

- the chemical complexity of the treated system

- the specific investment and treatment costs per mass or volume unit of treated

system at defined final concentration level

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Descriptively, electrode and photocatalytic processes depend on mass transfer from

and towards the reaction surface. Electrodes, catalysts and catalyst carriers should be

available and cheap. Recent work has been focused on economical consideration for

special AOPs [156].

3.2 By-product formation

Many papers are structured according to the following the scheme: Application of a

selected treatment principle, analysis of the concentration of the main contaminant,

derivation of a pseudo-reaction order. (Due to big analysis problems only in rare cases

reaction mechanism based on measured products or intermediates are discussed.) This

approach is highly critical because nothing is mentioned about the environmentally-

related properties of the system treated. In principle, it is known that in treatment

processes organic by-products can be formed, which are more dangerous than the

initial species [157,158]. Toxicological studies are rare in the field [159,160]. Control of

TOC decay and different toxicity measurements are methods to control the problem.

Little is known about certain inorganic by-products such as chlorite, chlorate and

perchlorate when waters having high importance with respect to environment or health

are treated. Occurrence of bromate in drinking water after ozonation is an old-familiar

problem; there are also efforts to overcome the problem [161,162]. Logic consideration

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leads one to the conclusion that the formation probability of similar by-products by

EAOPs is very high. Factors supporting this are

extremely high reaction rates in the range of 106-1010 M-1 s-1 for second-order

reaction constants (or conversion times in micro- and millisecond range),

high treatment times as typical for technologies with recirculation solutions,

presence of halogen atoms or ions in compounds and solutions in significant

concentration.

Halogenide ions may be present prior to treatment or be released as a result of

oxidizing organics containing halogen atoms.

Papers mostly describe experiments for total conversion conducted over a longer period

of time. Table 3 contains some new selected publications with respect to anodic

oxidation processes using boron-doped diamond anodes, which are able to generate

OH radicals, H2O2, and ozone.

In drinking water disinfection, the residence time inside an electrochemical cell with

single pass (not the treatment time; that often continues many hours or even days after

passing through cell.) is usually in the range of one second. But in treatment systems

with recirculation, higher specific charge load may be reached. In addition, enlarged

perchlorate formation is observed if the anode generates radicals at a high extent [169].

Other typical systems of interest are sewage and industrial wastewaters, and bilge

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waters. Inorganic by-product formation is often neglected both by equipment producers

and users but publications of many authors demonstrate the high formation potential.

4. DEMONSTRATION OF HALOGENATE AND PERHALOGEATE FORMATION IN

LITERATURE

4.1 Chlorate and perchlorate

Siddiqui found chlorate when chlorinated water was treated by ozone and H2O2/O3. The

carbonate radical supported chlorate formation [170]. Enlarged chlorate formation in

swimming pool water disinfected electrochemically was found recently [171,172].

Addition of chlorine from hypochlorite feedstock produced electrochemically seems to

be a general problem. For example, Stanford et al. mention the presence of chlorate

and perchlorate in hypochlorite solutions [173]. In addition, Garcia-Villanova et al.

reported the presence of chlorite and chlorate [174]. Van Hege [175] describes

experiments of BDD-based oxidation of reverse osmosis membrane is concentrated by

chlorate formation in mM concentration range. Dordjevic and co-authors [176]

measured OCl, ClO2, ClO2- and ClO3- electrolysing hypochlorite solution. Photolysis of

free chlorine species may form chlorate [177]. Chlorate was found in

photoelectrochemistry [178] and in photoelectrocatalytic treatment [179] etc. A series of

papers [180-182] deals with by-product formation effects when solutions containing

chlorine are illuminated.

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Dasgupta measured perchlorate after ozonation of hypochlorite solution [182]. Studies

and discussions on perchlorate in drinking water were published by Bull and co-workers

[183] and Urbansky [184]. Review books on perchlorate were published [185,186].

Perchlorate was found in drinking water simulating cathodic protection systems using

iron electrodes [187]. Dasgupta and co-workers [182] describe perchlorate formation

during electric discharge in aerosols containing chloride and during ultrasonic treatment

of sea water. Kang et al. [188] studied photolytic perchlorate formation from oxychlorine

anions. It was even reported that perchlorate can be formed on illuminated wet beach

sand surfaces. More discussion is presented in [189].

Jung et al. [190] found chlorate and perchlorate in chloride electrolysis using Pt as the

anode material. Our previous studies mainly dealt with drinking water disinfection

[191,192]. Results were confirmed by Polcaro and co-workers [193,194] and in studies

by the Rodrigo group [195]. Presence of chloride is necessary for the formation of

chlorate and perchlorate; however, perchlorate is also formed starting, from

hypochlorous, chlorite and chlorate solutions [169] in electrochemical oxidation for

instance. Most of mechanisms have not been studied yet but probably chemical and

electrochemical steps form known and unknown intermediates and products. It is

noteworthy that the perchlorate problem is discussed in the U.S. for many years.

Several states set perchlorate limits for drinking water at g L-1 level. The compromise

USEPA advisory value is 15 g L-1. A review of electrochemical perchlorate formation

in drinking water was given by Bergmann et al. [196].

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4.2 Bromate and perbromate

By electrolysing bromide, bromine and bromate solutions on MIO and BDD anodes it

could be found that bromate is a typical product [197,198]. Even, perbromate was found

for the first time [199]. Echardt and Kornmller measured bromate in mg L-1 range of

concentration treating ballast water by electrolysis using BDD anodes [200]. Oh et al. do

not recommend application of Ti/Pt electrodes in seawater electrolysis due to the

bromate formation potential [201]. They also found in their studies perchlorate at lower

concentration. Selcuk et al. studied bromate formation in photocatalysis [202,203].

The origin of bromate and perbromate is bromide being a constituent of many water

matrixes at g L-1 level. Because of high environmental importance [204] bromate

formation was even studied in natural waters [205].

It is well-understood that ozonation of water containing bromide ions is accompanied by

bromate formation [206]. Bromine may be implemented into several THMs during

drinking water treatment [207]. Xin et al. detected bromate at levels lower than 10g L-1

when Yangtze water was disinfected under routine conditions by applying dark

chlorination and UV/chlorination processes [208]. UV irradiation enhanced the decay of

free chlorine and the formation of bromate. This effect is known from disinfection

chemistry [209]:

2HOCl + HOBr BrO3- + 3H+ + 2 Cl- (8).

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The formation of bromate in electrolytic chlorine production for disinfection purposes

was confirmed by Hutchinson and Weinberg et al., but the anodes used in the studies

were not of BDD type [210,211]. Bromate formation in hypochlorite solutions and water

treated with hypochlorite solutions was also reported by other authors [171,172].

There is no information on inorganic by-product formation for oxidation technologies

using chloride addition to accelerate destruction of organics [165,212,213]. Acceleration

is realized by oxidizing chloride to chlorine, a highly-efficient oxidant [214]. In our

experiments, it could be shown that metoprolol can be quickly oxidized on BDD anodes

only in the presence of chloride. Chlorate and perchlorate were found as by-products.

More results are given in section 5.

Randazzo et al. found chlorate by destructing chlorinated aliphatic hydrocarbons on

BDD anodes [215].

5. TOXICITY OF CHLORATE AND PERCHLORATE, BROMATE AND

PERBROMATE - RULES AND REGULATIONS

5.1 Chlorate

Summaries of documents in terms of chlorate are available at the WHO websites.

Chlorate is suspected to damage red blood cells. In rats, carcinogenic effects were

observed. Mutagenic action was found in some cases. No long-term studies exist but

are in progress. Chlorate dose of 36 g/kg of body weight per day for 12 weeks did not

result in any adverse effects in human volunteers. A long-term study is in progress,

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which should provide more information on chronic exposure to chlorate. So, many

chlorate poisonings have been reported [216]. Symptoms are methaemoglobinaemia,

anuria, abdominal pain and renal failure. For an adult human, the oral lethal dose is

estimated to be 230 mg of chlorate per kg of body weight [217]. Six separate doses of

sodium chlorate were given to ten male volunteers following a rising-dose protocol,

single doses of 0.01, 0.1, 0.5, 1.0, 1.8 and 2.4 mg of chlorate ion per litre in 1 litre of

drinking-water was ingested by each man. Due to very slight changes in group mean

serum bilirubin, iron and methaemoglobin, the authors concluded that they didnt have

adverse physiological effects. The tested highest dose found out as a single-dose

NOAEL is 2.4 mg L-1 (34 g/kg of body weight per day) [218].

Chlorate is nitrate reductase-deficient mutants for Aspergillus nidulans according to

Cove [219]. It has been also found out that there is a mutagenic effect of chlorate in

Chlamydomonas reinhardtii and Rhodobacter capsulatus. Chlorate failed to induce

mutations in the BA-13 strain of Salmonella typhimurium. According to Prieto and

Fernandez, significant increases in mutants were observed at concentrations of 45

mmol L-1 and above for C. reinhardtii [220]. Chromosomal abnormalities were not seen

neither in the micronucleus test nor in a cytogenetic assay in mouse bone marrow cells

following gavage dosing with chlorate [221]. Many other partially controversial

investigations on several organisms exist in the field.

WHO guidelines give recommendation of 0.7 mg L-1 for drinking water. In Switzerland,

not more than 200 g L-1 chlorate is allowed. German regulations limit chlorate

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concentration in order not to be higher than 5.4 % of added chlorine (in solid form).

Chlorate is a normal by-product in chlorine and chlorine/chlorine dioxide disinfection

chemistry and must be tolerated at certain extent [222,223]:

2HOCl + OCl- ClO3- + 2H+ + 2Cl- (9)

HOCl + ClO2- + OH- ClO3- + Cl- + H2O (10)

In electrochemical chloride oxidation, chlorate may be formed both by electrochemical

and chemical mechanisms. In complex electrochemical systems, other mechanisms

exist, for example, if chlorine dioxide and ozone are intermediates for a brutto-reaction

[224]:

2ClO2 + O3 + H2O 2ClO3- + O2 + 2H+ (11).

5.2 Perchlorate

Except for the efforts in U.S. states and by EPA, no regulations and recommendation

exist for perchlorate. Perchlorate is a natural component of Chile fertilizers and potash

ore in the United States and Canada. (Recently, it was found even on Mars.) As

mentioned above perchlorate seems to be formed under photolytic and photocatalytic

conditions in nature [183]. Perchlorate is produced and used in explosives, rocket fuel,

fireworks and road flares. Incorrect waste management lead to environmental pollution.

Perchlorate was detected in a 2005 AWWA occurrence study in 26 states of the USA,

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mostly below 12 g L-1. The problem of contamination by fireworks [225] is probably

underestimated. The tetraedric structure of perchlorate is responsible for its relatively

high stability. Perchlorate is persistent in surface waters and underground aquifers for at

least several months. Based on the early studies of U.S. researchers perchlorate is now

a subject of ecotoxicology [226] and searched in many areas, such as surface waters

[227], food and ingredients [228,229], fish and bird mortality [230,231].

Many experiments with animals have shown negative perchlorate uptake effects on

thyroid function [232,233]. On human exposure perchlorate inhibits iodine uptake thus

influence thyroid metabolism processes, inhibiting hormone production and supporting

cause thyroid cancer. Against this background, many publications deal with perchlorate

destruction using physical, physical-chemical, chemical and biological methods. They

are not considered in this paper. Some medical studies discussed the opposite.

Perchlorate was found in breast milk of mothers. Lower values of 100 g L-1 iodine

(WHO definition) were found in urine of 36% of woman under enlarged perchlorate

exposure. The discussion on setting perchlorate standards in the U.S. is in progress. 1

g L-1 levels for drinking water were requested by EPA scientists in 2002. According to

new information, in January, 2009, the Environmental Protection Agency (EPA)

released an interim drinking water health advisory of 15 parts of perchlorate for every

billion parts of water - based on the recommendation of the National Research Council

(NRC). This reference dose is an estimate of a daily oral exposure by the human

population (including sensitive subgroups) that is likely to be without a significant risk of

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deleterious effects during a lifetime. Perchlorate is known to affect the production of

thyroid hormones, which are considered critical for brain development.

There is not a single mechanism of perchlorate formation in technical electrochemical

processes and detailed information is not available. But it is a fact that those

electrochemical electrodes with OH radical generation show preferred perchlorate

formation potential, higher by the factor of 1000 compared to BDD anodes with mixed

oxide anodes [234]. Radical formation is also possible on Pt, PbO2 anodes, and others.

Surprisingly, perchlorate was detected in water electrolysis using iron anodes with Mn

deposits, simulating processes of corrosion protection [235]. The reasons that

perchlorate was not discussed more seriously in the past were a low interest of AOPs

users and difficulties in analysis at g L-1 level. These difficulties are now overcome

[191,236,237]. By combining IC-MS/MS concentrations of 0.005 g L-1 are still

detectable.

5.3 Bromate and others

Bromate was able to cause kidney cancer in experiments with animals. WHO recommends 0.010

mg [bromate] L-1 as a limiting concentration. Mechanisms of bromate formation were presented

by Caizares and v. Gunten [238] for DOC-free waters. The mechanisms in electrochemical

formation of bromate and perbromate are not yet known.

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Finally, it must be mentioned that the by-product discussion is not only limited by

halogenates and perhalogenates. Electron transfer and radicals reactions are able to

produce peroxodisulphate from hydrogensulphate ions [239,240]:

2HSO4- S2O82- + 2H+ + 2- (12)

HSO4- + OH SO4- + H2O (13)

2SO4- S2O82- (14)

It was reported that peroxomonosulphate and peroxomonosulphate-based disinfectants

can cause dermatitis [241] and tubercolocidal activity [242].

6 EXPERIMENTAL CONDITIONS OF SELECTED ELECTROCHEMICAL STUDIES

Formation of inorganic by-products from EAOPs is relatively new and there is no

systematic experimental study in literature. Some researchers deal with the formation of

inorganic by-products as a marginal part of their EAOP application to water or

wastewater treatment. Therefore, experimental conditions of selected electrochemical

studies [234,243,244] for formation of chlorate, perchlorate, bromate and others have

been summarized in this section.

In laboratory-scale discontinuous experiments a cell working with a rotating anode

(mixed oxides, Pt or Condias BDD material on niobium) 4 mm above a cathode in a

thermostated dark glass cell (200 mL), mostly at 20C was used (Fig. 2a). The current

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density was varied between 50 A m-2 and 500 A m-2 treating artificial and real drinking

waters. Discontinuous mode of operation was applied in drinking water treatment

experiments. For discontinuous and continuous operation and for varying the chloride

concentration in experiments with a flow-through cell shown in Fig. 2b, a plastic

container (hard-plastic material) was filled by 300 L of drinking water and the water was

pumped by a centrifugal pump continuously through a rotameter and then through the

CSEM electrolyser equipped with 2 pairs of 90mm diameter BDD electrodes

(conducting diamond on silicon). The electrodes had diamond layers in the m range

and boron content between 2000 and 4000 ppm. In discontinuous mode, water was

pumped by the same pump from the 300 L container through a rotameter and then

through the electrochemical reactor back to the container. Drinking water from the

regional Koethen waterworks without and with chloride addition was used: pH=7.4-7.9,

42-46 mg[Cl-] L-1, 145-165 mg[SO42-] L-1 , 10-13.9 mg [NO3-] L-1. Chemicals of analytical

purity and higher grade with relatively low impurity influence on UV spectra were used.

Analyses were performed by HPLC. Perchlorate was analysed using a Metrohm

Metrosep Dual 4 column. Mostly, nitrite, nitrate, chloride, hypochlorite, chlorite, chlorate

and sulphate have been analysed by IC (Knauer/Alltech system with Novasep A-2 anion

column and electrochemical detector). Samples were collected at pre-determined time

intervals, immediately diluted if necessary and analysed by ion chromatography (IC)

and partially by spectrophotometry. Samples for IC, if not analysed in the course of or

directly after the experiment, were frozen and analysed later. Experiments were

repeated at least for once. For studies of polarization curves, an EG&G potentiostat

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model 283 together with a rotating disc electrode stand (model 616) was used in IR

compensation mode. The anode was a 1cm2 BDD disc (Condias) on niobium. A

mercury oxide reference electrode in an outer beaker connected with a salt bridge

containing 0.25 M NaOH was used to measure the potential. The cathode was a 15 mm

x 30 mm platinum sheet.

6.1 Chlorate and perchlorate formation

In all experiments using the BDD anodes, chlorate and perchlorate could be detected,

at very high flow rates or low charge in g L-1 range of concentration. Fig. 3

demonstrates high sensitivity of the HPLC analysis showing a perchlorate result at mg

L-1 level. For comparison, a peak response for perbromate at trace concentration level

is given. Both components are clearly distinguishable.

Fig. 4 shows results of 1.2 M sodium chloride/0.52 sodium sulphate electrolysis.

Chlorate is steadily formed and degraded finally to perchlorate. If the electrolysis time is

extended all chloride is reacted to perchlorate. This simple example shows that

treatment systems with recirculation have to be considered with more care because

DBP accumulation may occur. Balancing the Cl amount (not shown here) one obtains

that in principle all Cl is distributed between the species chloride, active chlorine,

chlorate and perchlorate at least for experiments mostly conducted at 200 A m-2. So,

chlorite and chlorine dioxide, if formed, are very short-lived. Stripping effects and

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aerosol formation are obviously marginal under these conditions. Small quantities of

chlorate and perchlorate may be reduced at the cathode (not shown here).

In continuous operation mode, even at residence times in the scale of few seconds

perchlorate was found in experiments using the technical cell and real drinking water.

Compared with mixed oxide electrodes, chlorate formation in the experiments using

BDD anodes is more intensive when radical mechanisms are relevant. Because in

drinking water chlorate is limited by regulations the process must be controlled also with

respect to chlorate formation. More perchlorate was found at higher current densities

(Fig. 5) and lower flow rates in the experiments [234].

The influence of chloride ions is often not well-understood. Higher chloride

concentration (usually higher than 50 mg L-1) results in lower perchlorate formation in

continuous and discontinuous experiments if majority of chloride ions has not yet

reacted (at relatively small specific charge passed). This behaviour can be explained by

species competition (Cl-, OCl-, ClO3- and others) for the relatively small number of active

sites on the BDD surface or reaction partners in the electrolyte layer and has been

recently discussed [245]. Another reason is the different reactivity of educts and

intermediates with OH radicals. For example, the rate constant for chlorate oxidation by

OH is thousandfold lower than that of chloride oxidation. As mentioned above, when all

chloride is finally converted to perchlorate (at relatively high specific charge passed),

perchlorate concentration again follows the chloride concentration).

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Chlorate concentration usually follows the chlorine concentration (Fig. 6). Extending the

experiment toward longer times the typical maximum (Fig. 4) reveals.

Finally, chlorate and perchlorate were found in discontinuous experiments using Pt and

mixed oxide anodes. This is consistent with other research results describing the

occurrence of radicals and O3 at MIO, PbO2, Pt, and other anode materials [95,246].

Results are not presented here because these electrode material applications do not

classically belong to Electrochemical Advanced Oxidation Processes.

It was discussed above that there is no mechanism to explain perchlorate formation

starting from chloride. At least, following fundamental mechanism steps have to be

taken into account:

- Electrochemical electron transfer reactions

- Chemical reactions with radicals

- Disproportion reactions

- Chemical reactions with ozone

Their probability of occurrence depends on the electrode material, electrode potential,

pH, water matrix and concentration levels.

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Fig. 7 represents an anodic polarization curve for BDD. Regions for OH and beginning

dioxygen and ozone formation are indicated. The role of OH and O radicals depends

on many factors; the subject is still under discussion.

The reaction scheme (Eqs. 15-18) for BDD anodes is accepted [3,8] but this does not

mean that all reaction will proceed if the electrode potential is chosen sufficiently high.

When for example enough reactive species are in the vicinity of OH generating active

sites they would react with OH radicals and O, and no ozone would be formed.

Moreover, other radicals may be formed as shown by radiolysis experiments.

OH O + H+ + e (15)

O + O2 O3 (16) 2O

O2 (17)

2OH H2O2 (18)

The extremely low concentration of species such as chloride, hypochlorite and

hypochlorous acid, and chlorate (thousandfold lower than water molecules) let us

conclude that water molecules preferably react at the anode with OH and O radical

formation, whereas the participation of O radicals in the stepwise perchlorate formation

(Eq. 19) is still speculative. Other chemical reactions are mentioned in [196] but, for

example, chemical chlorate formation is by many orders of magnitude lower than a

radical-based chemistry, comparing corresponding rate constants.

OH, e- OH, e- OH, e- e-, OH

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Cl- OCl- ClO2- ClO3- ClO4- (19)

The intermediates Cl, OCl, ClO3 are not shown but their potential existence was

described in other publications. In [196] it was concluded that obviously OH does not

react with the chlorate radical due to the relatively low rate constant. New results seem

to confirm this [247].

6.2 Bromate and perbromate formation

Generally, it is difficult to separate bromate from drinking water matrices. To overcome

the problem, minimization of bromate formation is the method of choice [248]. In

contrast to ozonation, electrochemical bromate and perbromate formation is not well-

studied. Several authors investigated electrode processes against the background of

bromate production. Ferro and co-workers studied the kinetics of Br- and Br3- oxidation

on Pt [249]. In dealing with BDD anodes, Ferro concluded that bromide oxidation is

mass transfer-controlled [250]. Cettou and co-workers [251] discuss bromate formation

by addition of the O radical to the hypobromite ion at Ti/RuO2 anode:

BrO- + 2O BrO3- (20).

The authors recently studied [244] bromide and bromate electrolysis in drinking water

and synthetic aqueous solutions. The chosen range of current density was 50-500 A m-

2, the range of bromide concentration was 1-1000 mg L-1. A boron doped diamond

(O, Cl) (O) (O) (O)

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electrode was used in the discontinuous experiments. Even at small bromide

concentration of 1 mg L-1 bromate could be detected by ion chromatography, whereas

perbromate has not been found in systems with bromide concentrations as typical for

drinking water. Usually current efficiency in electrolysis of artificial solutions containing

about 100 mg L-1 bromide or more is lower than 1%. Fig. 8 describes bromate formation

in regional drinking water at 5 mg[Br-] L-1 and for relatively low specific charge passed.

Additionally, curves for chlorine, chlorate and perchlorate formation are given. Probably,

kinetic effects, which are not yet known in detail, prevent bromate oxidation to

perbromate [244]. Nevertheless, the potential of perbromate formation on BDD anodes

has been clearly demonstrated and confirmed by Sez and co- workers [252].

Fig. 9 depicts concentration profiles for bromide, bromine, bromate and perbromate for

another experiment electrolysing bromide at 288.8 mg L-1. Only small charge amount is

separated for perbromate formation. Bromate is the main product.

In a first approach, possible mechanisms as shown in Eq. 21 form bromate and

perbromate

Br- OBr- BrO2- BrO3- BrO3 BrO4- (21)

But more studies are necessary for clarifications of mechanisms.

OH, O e-, Br

O O Br OH e-, OH

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7 AVOIDING HEALTH RISKS, FIRST STEPS

Apart from the U.S. with the perchlorate discussion there, no great awareness with

respect to perhalogenates as by-products in AOP exists worldwide. Because drinking

water treatment is of exceptionally high importance, systematic studies on direct

electrochemical disinfection have been carried out by our research group in a current

co-operation research project [26]. Main subjects have been

- the study of electrode materials focusing on mixed oxide and BDD anodes,

- definitions of disinfection by-product classes,

- microbiological studies,

- kinetics studies (chemical and electrochemical reaction),

- mathematical modelling

- construction details.

Concluding the results a new project conception for deeper understanding of water

electrolysis effects has been worked out. The main idea is to establish normative

controlling methods and administrative regulations for a better control. At present, Inline

electrolysis of drinking water is not accepted in all European countries. Project structure

is given in Fig. 10. Detailed results will be published soon. Both laboratory-scale and

technical cell were studied under largely varied water conditions. Disinfection ability was

generally confirmed using Escherichia coli, Enterococcus faecium and the

bacteriophages MS1 and PRD1.

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Inorganic and organic by-products were analyzed. Cytotoxicity (necrosis, ROS

formation) and genotoxicity measurements (Ames test, micro nucleus) showed mainly

negative results for selected water types and treatment conditions.

Bromate formation can be expected if the bromide concentration is not exactly limited.

Thus, inorganic treatment by-products should be more in the focus of studies of AOPs

and should serve as criterions for the efficiency and environmental importance of the

treatment method chosen. This approach is in agreement with suggestions of v. Gunten

et al. [253] discussing THM and bromate formation in drinking water ozonation.

8 CONCLUSIONS

EAOPs are a powerful instrument in environmental protection but they are not a priori

environmentally friendly methods. As a result of literature research and application of

special methods of AO the suspicion of by-product formation was confirmed.

Consequently, environmentally-oriented processes must be controlled with respect to

by-products with toxicity potential [198].

When chloride ions are present, formation of chlorate and perchlorate is possible. Thus,

chlorate and perchlorate may be used as new assessment parameters for an

environmentally friendly process. In presence of bromide ions bromate formation is the

most probable reaction generating unwanted by-products.

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The chlorate and perchlorate formation is high if the OH generation rate is high, higher

electrode potentials in electrochemical systems and recirculation are used. At higher

chloride concentration, lower perchlorate formation is possible at short-term treatment.

Additional ions may influence the mechanisms by radical consumption.

In drinking water treatment processes [263] radical generating technologies should be

avoided.

Technical cells, which are a subject of a new scientific-technical co-operation project

that was finished recently, should be controlled by standard test methods.

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