RECENT ENVIRONMENTAL APPLICATIONS OF …ser.cienve.org.tw/download/18-3/jeeam18-3_155-172.pdfJ. Environ. Eng. Manage., 18(3), 155-172 (2008) RECENT ENVIRONMENTAL APPLICATIONS OF DIAMOND

J. Environ. Eng. Manage., 18(3), 155-172 (2008)

RECENT ENVIRONMENTAL APPLICATIONS OF DIAMOND ELECTRODE: CRITICAL REVIEW

Carlos A. Martínez-Huitle1,* and Marco A. Quiroz Alfaro2 1Laboratory of Electrochemistry University of Milan, DiSTAM,

Milan 20133, Italy 2Departamento de Química y Biología Universidad de las Américas-Puebla

Puebla 72820, México

Key Words: Diamond electrode, water disinfection, water treatment, electrochemical oxidation, inorganic pollutants, strong oxidants

ABSTRACT

Conductive diamond electrode has been studied for the application in wastewater treatment, electroanalysis, energy conversion, organic synthesis and sensor areas. Recently, there has been increasing interest in the use of diamond electrodes in some emerging environmental applications: electrochemical oxidation for removing synthetic dyes; water disinfection; elimination of organic and inorganic pollutants and production of strong oxidants. These new electrochemical applications of diamond electrodes open new perspectives for easy, effective, and versatile processes for pollution abatement. This article highlights and summarizes the current results dealing with recent environmental applications using conductive diamond electrodes.

*Corresponding author Email: [email protected]; [email protected]

INTRODUCTION Diamond is an extremely hard crystalline form

of carbon and is considered an excellent material for many applications due to unusual physical and chemi-cal properties. Interest in diamond has been further in-creased by the discovery about the possibility to pro-duce polycrystalline diamond films with good me-chanical and electronic properties.

Many efforts were spent during the 60s and 70s to investigate diamond synthesis until it was success-fully achieved with Chemical Vapour Deposition (CVD) technique (low pressures) with excellent dia-mond growth rates [1-4], showing the good prospect of diamond films for some industrial applications. Electrochemical studies of synthetic diamond elec-trodes were started several years ago with the first pa-per on diamond electrochemistry [5]. Diamond films have been the subject of applications and fundamental research in electrochemistry, opening up a new branch known as the electrochemistry of diamond electrodes. Recently, electrically conductive films of boron-doped diamond (BDD) have gained popularity in a variety of electrochemical applications. The most important properties of this electrode are a large potential win-

dow, lower adsorption, corrosion stability in very ag-gressive media, high efficiency in oxidation processes, very low double-layer capacitance and background current. Therefore, diamond electrodes are a suitable material for several purposes: synthesis of chemicals, electroanalysis [6], energy conversion and destruction of organic/inorganic pollutants [7]. Moreover, the ver-satility of these materials has also been extended to develop sensors and biosensors [6,8]. Over the last few years, the number of publications has increased considerably about the synthesis and/or applications of this new material [6,9-16], general electrochemical properties [9,16], water treatment [6,7,10,13,16], elec-trosynthesis [10,13,14,16], photoelectrochemistry [9,11,16], and electroanalytical applications [6,12,16]. Consequently the main focus of this review summa-rizes and highlights most important recent progresses in the emerging environmental applications of dia-mond electrodes.

ELECTROCHEMICAL WASTEWATER

TREATMENT Recently the applications of electrochemistry for

environmental pollution abatement have been thor-

156 J. Environ. Eng. Manage., 18(3), 155-172 (2008)

oughly investigated [7,17,18]. The feasibility of elec-trochemical conversion/destruction of organic sub-strates in wastewater, in particular, has attracted much attention since pioneering studies. During the last two decades [7], research work has focused on the effi-ciency in oxidizing various pollutants with different electrodes, on the improvement of the electrocatalytic activity and electrochemical stability of the electrode materials, investigation of factors affecting the process performance and kinetics of pollutant degradation. Experimental investigations focusing on the behaviour of different anodic materials allowed the exploration and description of the mechanisms of the electro-chemical oxidation processes to convert organic pol-lutants to CO2 and water at different electrodes. Con-sequently, the results have consented to consider the electrochemistry as an attractive alternative to tradi-tional methods for treating wastewaters containing or-ganic pollutants. The results obtained with other an-odic materials are interesting for their excellent elec-trocatalytic performances in the elimination of the or-ganic pollutants; currently, diamond electrode repre-sents the most investigated material for water treat-ment due to their significant properties in the electro-chemical elimination of organic pollutants from water (high overvoltage for oxygen evolution and the possi-bility to anodically produce hydroxyl radicals with high current efficiency (CE)), demonstrating that many biorefractory compounds can be mineralized with high CE.

ELECTROCHEMICAL OXIDATION OF

ORGANIC POLLUTANTS IN WATER Diamond electrodes represents an attractive an-

ode material for the degradation of refractory or prior-ity pollutants such as ammonia, cyanide, phenol, chlorophenols, aniline, surfactants, alcohols, herbi-cides (Fig. 1) and many other compounds [7,19-55]. Unlike PbO2, SnO2 and TiO2, the BDD thin films de-posited on Si, Ta, Nb and W by CVD have shown ex-cellent stability during electrochemical experiments. In the case of Ti/BDD, the preparation of these elec-trodes has been successfully achieved the degradation of different organic compounds. In fact, Ti/BDD has been used for the destruction of several pollutants [56,57], like dyes, carboxylic acids and phenol.

During the study of electrochemical behaviour surface, Gandini et al. [32,58] found that a BDD elec-trode which does not provide an active site for the ad-sorption of reactants has no electrocatalytic activity for the direct oxidation of some aliphatic alcohols or carboxylic acids. However, Cañizares et al. [51] sug-gested the possible existence of a direct electron trans-fer with indirect oxidation via electrogenerated hy-droxyl radicals, based on the overlapping oxygen evo-lution (at about 2.3 V vs. SCE) observed in the volt-ammograms of different carboxylic acids. On the ba-

Fig. 1. TOC abatement with specific charge for the

degradation of 100 mL of 230 mg L−1 2,4-D solutions of pH 3.0 at 300 mA and 35 °C. Method: (a, ○) anodic oxidation with a 10 cm2 Pt anode and a 3.1 cm2 O2-diffusion cathode, (b, □) electro-Fenton with a 10 cm2 Pt anode, a 3.1 cm2 O2-diffusion cathode and 1 mM Fe2+ in the solution, (c, Δ) anodic oxidation with a 3 cm2 BDD anode and a 3 cm2 graphite cathode, (d, ▲) electro-Fenton with a 3 cm2 BDD anode, a 3.1 cm2 O2-diffusion cathode and 1 mM Fe2+ in the solution [33].

sis of these experimental results, both authors have demonstrated that, at a fixed potential in the region of oxygen evolution, the current density increases with carboxylic acid concentration, indicating that the pathway for the oxidation of these compounds in-volves intermediates that are formed during oxygen evolution (indirect mechanism).

Other studies were carried out by Comninellis and co-workers, they investigated the anodic oxidation of various pollutants with Si/BDD electrodes, as well as the mechanism by which the organic substrates are oxidized at this electrode surface through the forma-tion of a film of hydroxyl radicals at the Si/BDD sur-face, which may represent a “reaction cage” for the process [7,26,30-34,37,40,54,58-61]. On the basis of these experimental results, Comninellis [19,62] pro-posed a detailed mechanism for the oxidation of or-ganics with concomitant oxygen evolution at BDD electrode surface, based on the initial mechanistic scheme of anodic oxidation of organic compounds with simultaneous oxygen evolution on non-active anodes and on active anodes [63].

So far, many papers have reported that the elec-trochemical oxidation of organic pollutants at BDD electrodes is completely mineralized by the reaction with electrogenerated •OH radicals, confirming the mechanism [19,62]. Most relevant results have been summarized in Table 1 wherein the CE obtained is very high, ranging from 33% to more than 95%, depending on pollutant characteristics and oxidation

Specific charge (Ah L-1)

TO

C (

mg

L-1

)

Martínez-Huitle and Quiroz Alfaro: Review in Applications of Diamond Electrode 157

Table 1. Some examples of organic compounds oxidized on diamond electrodes.

Pollutant J, i or E1 Current

efficiency (%)2 Removal efficiency

(%)3 Conditions3 Ref.

Chloranilic acid J = 6.3-50 mA cm-2 GCE = 70-40 90 H2SO4 T = 25-60 °C [22]

Multicomponent mixtures

J = 100-300 A cm-2 near 100 COD removal from 5000 to 300 mg L-1

Na2CO3 0.1 M supporting electrolyte, pH = 7.0 using H2SO4 5% (v/v)

[25]

isopropanol J = 300 A cm-2 > 95 < 90 [isopropanol] = 0.17 M [26]

Phenolic compounds

J = 153-510 mA cm-2 90 [Phenolic compounds]=100-500 mg L-1, Re number = 3500-15000

[28]

Isopropanol J = 30 mA cm-2 85 90 1 M H2SO4, transport limitations [32]

Herbicides i = 100-450 mA ACE = 8-12 40 (1 h); 70-90 (3 h) pH = 3, 0.5 M H2SO4, [Herbicide] = 100 mg L-1

[33]

Phenol J = 300 A cm-2 CE = 33 90 1 M HClO4, T = 25 °C, [phenol] = 20 mM, pH = 2, charge loading 4.5 Ah L-1

[37]

Phenol J = 5 mA cm-2 ICE = 50 90 HClO4 1 M, T = 25 °C, [phenol] = 20 mM

[37]

Kodak 6 first developer

J = 1000 A cm-2 73, COD Initial COD = 32,500 mg L-1, V = 30 mL, I = 0.31 A, electrolysis time 6.25 h

[38]

Kodak E6 color developer

J = 1000 A cm-2 80, COD Initial COD = 19,050 mg L-1, V = 30 mL, I = 0.31 A, electrolysis time 4.75 h

[38]

Phenol solution J = 1000 A cm-2 94, COD Initial COD = 3,570 mg L-1, V = 60 mL, I = 0.31 A, electrolysis time 18 h

[38]

Hydroquinone solution

J = 500 A cm-2 97, COD Initial COD = 23,530 mg L-1, V = 60 mL, I = 0.15 A, electrolysis time 38 h

[38]

Pentachlorophenol E = 3.0, 2.0 and 0.9 V CE = 96 to 90 95-80 buffer pH = 5.5, [PCP] = 5.0×10-5 M [39]Benzoic acid i =1.5 A ICE = 100 90, COD 0.5 M HClO4, [BA] = different

concentrations [44]

Polyhydroxy benzenes

J = 15-60 mA cm-2 ICE = 100 from COD 4000-2000 mg L-1

90, COD T = 15-60 °C, initial concentration 1.1-36 mM, pH = 2 and 12, transport limitations

[47]

Maleic, formic and oxalic acids

J = 30 mA cm-2 ICE = 20-80 90 H2SO4/Na2SO4, pH = 2, T = 20 °C [51]

Phenol J = 15-60 mA cm-2 100-50 (5 Ah L-1

passed) and > 50 (after 5 Ah L-1

passed)

[phenol] from 350 to 1500 mg L-1, T = 25 °C

[53]

Polyacrylates J = 0.1-0.3 A dm-2 37-100 99 HClO4 1 M, T = 30-60 °C [54]

Dyes J = 100 A m-2 70-90 80-97, COD T = 30 °C, V = 25 mL, initial dyes concentration = 1000 mg L-1, initial pH = 4.7-6.7

[56]

Acetic and maleic acids

J = 200 A m-2 CE (acetic) = 64; CE (maleic) = 61

97, COD (acetic); 96, COD (maleic)

V = 30 mL, Na2SO4 = 2000 mg L-1, T = 30 °C

[57]

Phenol J = 100 A m-2 CE = 79 97, COD (after 4.85 Ah L-1)

V = 30 mL, Na2SO4 = 2000 mg L-1, T = 30 °C

[57]

Orange II J = 200 A m-2 CE = 55 91, COD (after 6.25 Ah L-1)

V = 30 mL, Na2SO4 = 2000 mg L-1, T = 30 °C

[57]

Reactive Red HE-3B

J = 200 A m-2 CE = 47 95, COD (after 6.25 Ah L-1)

V = 30 mL, Na2SO4 = 2000 mg L-1, T = 30 °C

[57]

Carboxylic acids (acetic, formic and oxalic)

J = 30 mA cm-2 CE (acetic and formic) = 90; CE (oxalic) = 70

90-98 1 M H2SO4, T = 30 °C, transport limitations in the case of oxalic acid

[58]

CN- J = 360 A cm-2 CE = 41 95 1 M KOH [59]2-naphthol J = 15-60 mA cm-2 ICE = 100

(8 Ah L-1) 90 [naphthol] = 2-9 mM [60]

1J = current density, i = current, E = potential. 2CE = current efficiency, ACE = apparent current efficiency, GCE = general current efficiency, ICE = instantaneous current

efficiency. 3COD = chemical oxygen demand, V = volume, T = temperature, [ ] = concentration.


conditions. Beck et al. [64] compared the Si/BDD with Ti/SnO2, Ta/PbO2 and Pt for the oxidation of phenol and found that at a charge loading of 20 Ah L-1, the total organic carbon (TOC) was reduced from an initial value of 1500 to about 50 mg L-1 at Si/BDD, and to about 300, 650 and 950 mg L-1 at Ti/SnO2, Ta/PbO2 and Pt, respectively.

On the other hand, during oxidation of organic compounds, such as phenol, diuron, 3,4-dichloroaniline and triazines, the crucial point to ob-tain high yields is the rate of mass transfer of the reac-tant towards the electrode surface [27-29]. Thus, an in-ject cell was developed to obtain high mass transfer coefficients. With this cell, at a current density of 15 mA cm-2, a faradic yield of 100% was achieved, up to the almost complete disappearance of the organic pol-lutant.

Some papers have also compared the behaviour of BDD with other electrodes, such as SnO2, PbO2, IrO2, for the oxidation of organic pollutants. Chen et al. [57] reported that the CE obtained with Ti/BDD in oxidizing acetic acid, maleic acid, phenol, and dyes was 1.6-4.3 times higher than that obtained with the typical Ti/Sb2O5-SnO2 electrode. Others have demon-strated that Si/BDD electrodes are able to achieve faster oxidation and better incineration efficiency than PbO2 in the treatment of naphthol [41,42], 4-chlorophenol [61] and chloranilic acid [22]. In con-trast, the oxidation of oxalic acid [20], which requires a strong adsorption of the organic on the electrode sur-face, is lower than on PbO2, but is higher on Pt, Au and IrO2 anodes.

Diamond electrodes have been also used as cathode materials for the electrochemical reduction of high nitrate concentrations [65-68]. Levy-Clement et al. [66] found that at applied potentials between -1.5 and -1.7 V the amount of NO3

- is reduced 10% and that it is mainly transformed into gaseous products, then it increases to 29% at -2 V with almost equal parts of nitrite and nitrogenous gas formed, without the production of ammonium.

Another aspect to be taken into consideration is the production of powerful oxidants, like the per-oxodisulphate [69,70]; these species can participate in the oxidation of the organic substrates, allowing higher efficiencies.

In particular cases, for high organic concentra-tions and low current densities, the chemical oxygen demand (COD) decreased linearly and the instantane-ous CE (ICE) remained about 100%, indicating a ki-netically controlled process. While for low organic concentrations or high current densities, the COD de-creased exponentially and the ICE began to fall due to the mass-transport limitation and the side reactions of oxygen evolution. For example, the trend of the COD and ICE was obtained by Panizza et al. during the electrochemical oxidation of 2-naphthol [60]. In order to describe these results these authors developed a

comprehensive kinetic model that allowed them to predict the trend of the COD and CE for the electro-chemical combustion of the organic with BDD elec-trodes and to estimate the energy consumption during the process [44,60,71]. Additionally, Carey et al. [38] patented the use of diamond films as anodes for or-ganic pollutants oxidation.

Other applications of BDD for the destruction and/or determination of organic pollutants were ob-tained by Avaca et al. [39,72-80] where the compari-son between the BDD and other anodic materials has permitted the use of alternative techniques during the oxidation of the organic pollutants. The BDD elec-trode is the best choice for several electroanalysis studies, as demonstrated by the electrochemical oxida-tion of 4-nitrophenol in 0.1 M Britton-Robinson buffer electrolyte [72].

It is important to remark that at diamond elec-trodes several intermediates are generally produced during the oxidation of the original organic substrates. Starting from an aromatic compound, hydroxylated derivatives are found as initial intermediates, but in the final stages of the oxidation process several car-boxylic acids are produced, the last being usually ox-alic acid. The formation of these acids increases the process time and highlights possible mass transport limitations; interestingly, some anode materials are more efficient than others for their elimination [20]. As seen from data in Table 1, many studies have been carried out on electrochemical treatment of organic compounds at BDD electrodes, obtaining their com-plete mineralization of organics to CO2 at high poten-tial electrolysis; these electrodes produce hydroxyl radicals from the water discharge on its surface. In this frame, BDD anodes are promising materials for industrial-scale wastewater treatment thank to their properties, achieving high CE values during com-pletely mineralized of many biorefractory compounds. There has been increasing interest in the use of dia-mond electrodes in some emerging environmental ap-plications: electrochemical oxidation for removing synthetic dyes, water disinfection, elimination of inor-ganic pollutants and production of strong oxidants.

RECENT ENVIRONMENTAL APPLICATIONS

1. Electrochemical Oxidation of Dyes

The interest of wastewater treatment containing

synthetic dyes has increased in the last years. Syn-thetic dyes are extensively used in many fields of up to-date technology, e.g., in various branches of the textile industry, leather tanning industry, paper pro-duction, food technology, agricultural research, light-harvesting arrays, photo-electrochemical cells, and in hair colourings [81]. The discharged dyes cause con-siderable environmental pollution and are serious health-risk factors. Many researchers have investi-


gated the application of electrochemical oxidation al-ternatives for removing dyes from water. Based on the good performances obtained by electrochemical oxi-dation of different organic pollutants with diamond electrodes [7,6,10,13,16], these materials have been tested for removing dyes from aqueous solutions. A collection of data using diamond anodes for the deg-radation of some important synthetic dyes can be found in Table 2. Different parameters have been in-vestigated including the current density, CE and con-sumption energy, among others.

Ti/BDD electrodes for anodic oxidation of vari-ous dyes were investigated by Chen et al. [56]. They found that these electrodes were much more active than Ti/Sb2O5-SnO2 anodes. All dyes tested (Table 2), including Orange II and 16 reactive dyes, were effec-tively mineralized with CE being 51-90%. COD was also reduced from initial 400-1120 mg L-1 to 8-93 mg L-1 at a current density of 100 A m-2 and after treat-ment, most solutions turned colourless. The energy consumption process ranges from 8.9 to 17.9 kWh m-3. Operating variables could affect the process efficiency significantly; low current density, high pH, and high temperature are beneficial for dye degradation (Table 2). Chen et al. [57] also studied the electrochemical oxidation of various dyes at Ti/BDD and Ti/Sb2O5-SnO2. CE obtained on the Ti/BDD electrode is 47-79%, 1.6-4.3 times higher than that obtained on the Ti/Sb2O5-SnO2 electrode. The high CE of the Ti/BDD electrode for pollutant oxidation is attributed to the difficulty of O2 evolution on Ti/BDD [57].

In an earlier paper, Chen and Chen [82] found that oxidizing Orange II at Ti/BDD electrodes, there was no effect of electrolyte concentration on oxidation efficiency in the investigated range of 1500–3000 mg L-1 Na2SO4. Although alkaline media was favourable for Orange II oxidation, the pH effect was not signifi-cant. Even at a pH value of 1.15, nearly 100% of CE was obtained at a charge loading < 1.67 Ah L-1. In ad-dition, it was observed that polymeric intermediate products were formed during anodic oxidation. How-ever, the amount of the polymeric products depended on operational conditions; at high pH, high tempera-ture, low current density and low initial Orange II concentration tended to suppress the formation of polymeric products. Moreover, at high temperature, the residual COD after treatment decreased signifi-cantly (22 mg L-1) than that obtained at low tempera-tures (186 mg L-1).

Sakalis et al. [83] studied an electrochemical method for wastewater treatment in the textile indus-try based on an innovative electrochemical cell, using boron-doped diamond electrode supported on a nio-bium substrate (Nb/BDD) and platinised titanium (Pt/Ti) as anodes. Several parameters affecting the procedure such as the nature and the quantity of the electrolyte, pH and the applied potential were studied. In addition, biological oxygen demand (BOD), COD,

TOC, energy consumption and efficiency of the an-odes, as well as the Cl− and hypochlorite concentra-tions were determined using both Pt/Ti and Nb/BDD as anodes. They have determined that chloride and sulfate ions are mainly responsible for the indirect de-colouration of the wastewater, while direct decolour-ation in the tested potential values (12-18 V) was al-most absent. According to data in Table 2, colour re-moval up to 90% was achieved resulting in practically colourless final wastewater, while BOD, COD and TOC are decreased up to about 50, 93 and 52%, re-spectively, using Nb/BDD. Moreover, it is very im-portant to remark that the COD/BOD ratio was sig-nificantly reduced; indicating less toxic and more bio-degradable wastewater, which can easily be mineral-ized by subsequent biological treatment.

Alternatively, the electrochemical oxidation of Blue Reactive 19 dye, using a filter-press reactor with a Nb/BDD anode, was investigated. In this paper, Andrade et al. [84] also made a comparative study be-tween some anodic materials, where the electrooxida-tion of the dye was carried out at 50 mA cm-2, 2.4 L h-1 volume flow rate, temperature of 25 °C and elec-trode area of 5 cm2. The performances of the elec-trodes in the dye decolourization were quite similar, achieving 100% decolourization, and in some cases 90% decolourization was achieved by applying only ca. 0.3 Ah L-1 (8 min of electrolysis). In the case of Nb/BDD, after 2 h of electrolysis the obtained TOC reduction was 82% [84]. On the other hand, the elec-trooxidation of the Blue Reactive 19 dye was also in-vestigated by Fryda et al. [59] using a Nb/BDD anode, obtaining 90% decolourization of the dye solution af-ter 10 min and 82% TOC reduction after 2 h of elec-trolysis.

From an examination of data in Table 2, Si/BDD is the most widely investigated anode material for electrooxidation of several dyes. Fernandes et al. [85] showed the influence of electrolyte concentration, ini-tial dye concentration and current density on the deg-radation rates that were the fundamental parameters for effective oxidation of C.I. Acid Orange 7 (AO7) on Si/BDD. They compared the rates of colour and COD removal in each case. Results showed an almost complete colour and COD removal, higher than 90% (Fig. 2). An important application was realized by Fernandes et al., when they treated an industrial efflu-ent, obtaining values of 98 and 77% of colour and COD removals, respectively.

Recently, Carvalho et al. [86] tested the electro-chemical oxidation of the biotic degradation products of the textile dye AO7, using model solutions and also with industrial effluents. For these effluents, oxidation tests were carried out for different electrolytes and at different current densities in order to investigate the efficiency. The results showed a high elimination of the persistent pollutants and a COD removal higher than 70% for model solutions. For the real effluents


Table 2. Relevant examples of dyes oxidized on diamond electrodes

Pollutant J, i or E1 Charge loading (Ah L-1)

Energy consumption

Current efficiency

(%)

Removalefficiency

(%)2 Effects studied Ref.

Anode = Ti/BDD

Cibacron yellow J = 100 3.52 14.4 kWh m-3 68 > 95, colour

pH = 4.7-6.73, [dye]= 1000 ppm, 25 mL, 30 °C

[82]

Cyfaiw yellow J = 100 3.02 9.7 kWh m-3 64 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Cycafix navy blue

J = 100 2.77 8.9 kWh m-3 72 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Monozol black J = 100 2.52 10.3 kWh m-3 84 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Monozol blue J = 100 2.52 10.3 kWh m-3 75 > 95, colour

[dye]= 1000 ppm, 25 mL, 30 °C [82]

Monozol red J = 100 2.39 10.2 kWh m-3 82 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Monozol T-blue J = 100 4.03 17.9 kWh m-3 74 > 95, colour

[dye]= 1000 ppm, 25 mL, 30 °C [82]

Monozol yellow J = 100 2.52 10.9 kWh m-3 70 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Procion blue J = 100 3.02 13.9 kWh m-3 90 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Reactive blue J = 100 2.9 9.3 kWh m-3 86 > 95, colour

[dye]= 1000 ppm, 25 mL, 30 °C [82]

Reactive red J = 100 2.52 11.5 kWh m-3 51 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Samafix red J = 100 2.52 11.1 kWh m-3 76 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Samafix yellow J = 100 2.52 10.6 kWh m-3 52 > 95, colour

[dye]= 1000 ppm, 25 mL, 30 °C [82]

Unicion green J = 100 4.03 16.7 kWh m-3 58 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Unicion red J = 100 2.27 9.7 kWh m-3 77 > 95, colour

[dye] = 1000 ppm, 25 mL, 30 °C [82]

Orange II J = 100-400 6.52 50-59 90, colour pH = 8.5-12.3, 30 °C [82]

Orange II J = 100-400 1.67 100 95, colour Electrolyte concentration, pH = 1.15, current density, temperature

[84]

Orange II J = 200 6.25 55 100, colour; 95, COD

pH = 1.0, comparison with Ti/Sb2O5-SnO2

[83]

Reactive red HE-3B

J = 200 6.25 47 100, colour; 98, COD

pH = 1.0, comparison with Ti/Sb2O5-SnO2

[83]

Basic red 29 J = 2.5-10 0.2-0.5 kWh g-1

97.2, colour; 91, COD

pH = 3, bipolar trickle tower reactor, 0.03 M Na2SO4, 5-10 min

[94]

Anode = Nb/BDD

Blue reactive 19 J = 500 0.3 1.86 kWh m-3 90-100, colour; 82, TOC

Electrolyte concentration, pH, current density, temperature

[86]

Reactive orange 91

E = 5 1-3.8×102 kWh kg-1 COD

90, 80 and 40 for pH = 2, 7 and 10

85-90, colour

pH = 2.0-10, chlorine effect, electrolysis media (NaCl and Na2SO4), 20-30 min

[85]



Energy consumption

Current efficiency

(%)

Removalefficiency


Reactive red 184 E = 5 100, 90 and 30 for pH = 2, 7 and 10

85, colour pH = 2.0-10, 20-30 min [85]

Reactive blue 182

E = 5 100, 100 and 70 for pH = 2, 7 and 10

89, colour pH = 2.0-10, 20-30 min [85]

Reactive black 5 E = 5 100, 100 and 95 for pH = 2, 7 and 10

90, colour pH = 2.0-10, 20-30 min [85]

Anode = Si/BDD

Alizarin red S J = 300, 600, 900

30 90, colour; 98, TOC; 100, COD

pH = 1.0, 5 mM, 25 °C [98]

Acid orange 7 J = 100-200 1.2 0.31-0.6 Wh L-1

65-95, colour; 45-90 COD; 19-41, COD

pH = 7-8, anaerobic boitreatment residue, supporting electrolytes NaCl and Na2SO4, 4-24 h

[88]

Orange 7 J = 12.5-50 0.3-0.5 100, colour; 80-100, COD

pH = 3.5-5, rlectrolyte (Na2SO4 0.01, 0.02, 0.035 M and KCl 0.07, 0.1 M), current density, 10 h

[84]

Reactive black 5 E = -1.0 and -2.5

22 kWh kg-1 95 , colour; 65, COD

pH = 5-7.0, flow effect, potential, 40 min

[89]

Dispersed indigo J = 3.6-800 0.047-14.7 kWh m-3

10-43, colour

pH = 6-9, current density, 120 min

[90]

Natural yellow 28, vat blue 41, basic green 4, basic violet 16, acid orange 7

E = 2.2, 2.5 and -2.5

20-70 50-70, colour

pH = 7, 2 h [102]

Eriochrome black T, Methyl orange, Congo red

J = 300-600 35-50 kWh kg-1 COD

95, COD; 80, TOC

Natural pH, concentration effect, 25 °C, Na2SO4 5000 mg L-1, initial COD 100 or 1813 ppm, 250-1250 min

[100]

Indigo carmine i = 100, 300, 450 mA

100 at different times depending on J

100, colour pH = 3 and 10, pH 3 (120 min) and pH 10 (280 min), 25 °C, Na2SO4, evolution of NH4

+ and NO3

-, 240-600 min

[92]

Methyl red i = 500 mA 40 100, colour Comparison between Ti/RuSnO2, Pt, PbO2 and BDD, 6 h

[97]

Orange II J = 10, 270, 530, 800

100 and 4.4 100, colour pH = 1-10, batch reactor, divided and undivided cells, chlorine free solutions and containing Cl, 150 min

[101]

Alizarin red J = 600 50-80 100, colour; 100, COD

pH = 2.0, 25 °C, [Na2SO4] = 5000 mg L-1, different concentrations of dyes

[91]

Eriochrome black T

J = 300 50-80 100, colour; 90, TOC

pH = 2.0, 25 °C, Na2SO4 5000 mg L-1, different concentrations of dyes

[91]



Energy consumption

Current efficiency

(%)

Removalefficiency


Eriochrome black T

J = 300 200-800 kWh m-3

100, colour; 100, TOC; 98, COD

natural pH, 25 °C [95]

Methyl orange J = 300 200-800 kWh m-3



Congo red J = 300 200-800 kWh m-3



Rhodamine B J = 300 10 100, colour; 100, COD; 40, TOC

natural pH, 0.1 M NaCl supporting electrolyte, 25 °C, initial COD = 100 ppm

[93]

Methylene blue 100, COD; 60, TOC

natural pH, 0.1 M NaCl supporting electrolyte, 25 °C, initial COD = 100 ppm

[93]

Direct red 80 J = 15-25 6.65 kWh m-3 99-100, colour; 76-87, COD

pH = 4-6.7, electrooidation, 24-42 h

[103]

Indigo carmine J = 33, 100, 150

15-55, MCE depending on J

91, TOC pH = 2.0-4.0, electro-Fenton, 1.0 mM Fe+2, determination of inorganic ions, 9-13 h

[96]

1J = current density (A m-2), i = current (mA), E = potential (V). 2COD = chemical oxygen demand, TOC = total organic carbon, [ ] = concentration.

Fig. 2. Comparison between percentage of colour and

COD removal for the degradation essays performed with different initial dye concentration on BDD anode, at 10 mA cm-2 in a 0.035 M Na2SO4 electrolyte solution [85].

COD removals between 45 and 90% were obtained, and TOC removals varying from 19 to 41%. They demonstrated that the tests using NaCl as supporting electrolyte given higher rates of degradation than those with Na2SO4. On the other hand, Basic Yellow 28 and Reactive Black 5, which are respectively me-

thine and sulfoazo textile dyes were individually ex-posed to electrochemical treatment using diamond electrode. The results obtained were 90% colour re-moval and COD removal of up to 65% [87].

Bechtold et al. [88] investigated the anodic dec-olourisation of solutions containing dispersed indigo using Si/BDD. The formation and role play by per-oxodisulfate and small amounts of chlorine, formed as by product of the electrolysis, were determined during the dyestuff destruction. They observed that decolour-isation of the dispersed indigo cannot be attributed to hypochlorite, but it depends on operational conditions; the energy consumptions ranged from 0.047 to 14.7 kWh m-3. High indigo concentrations require longer duration of electrolysis and an estimation to decolour-ise 1 L in 45 h, applying a cell current of 1 A needs an energy consumption of 622 kWh m-3.

Saez and co-workers [89] investigated the elec-trochemical oxidation of a synthetic wastewater con-taining the model dyes Alizarin Red and Eriochrome Black T on a Si/BDD electrode by both cyclic volt-ammetry and bulk electrolysis. The results showed that complete COD and colour removal were obtained for both wastes. However, they established that the nature of the pollutant and specially the presence of functional groups seem to strongly influence the per-

Charge (mAh L-1)


formance and efficiency of the electrochemical proc-ess. The electro-oxidation of Alizarin Red behaves as a mass-transfer-controlled process, where an increase in the current density leads to a decrease in the CE. This behaviour was explained by direct or hydroxyl radical mediated oxidation. The electro-oxidation of Eriochrome Black T, however, achieved higher effi-ciencies at high current densities. They, thus, indi-cated that the mediated oxidation by electrogenerated reagent (such as peroxodisulphate) may be the main oxidation mechanism involved in Eriochrome Black T treatment. This study showed the suitability of the electrochemical process for completely removing the COD, TOC and effectively decolourising of wastewa-ters containing synthetic dyes, achieving values of 97 to 100% in all cases.

The remediation of wastewaters containing in-digo carmine was also studied by Brillas and co-workers [90]. They demonstrated that degradation rate increases with increasing current and dye concentra-tion. Indigo carmine was more rapidly removed in al-kaline than in acid medium with 100% of TOC and colour removal. Isatin 5-sulfonic acid was identified as the main aromatic product formed, along with ox-alic and oxamic acids as ultimate carboxylic acids. In addition, the nitrogen of the dye converted into NH4

+ and NO3

−. Cañizares et al. [91] recently confirmed the ad-

vantage of using Si/BDD, as anode for the destruction of organics, achieving the almost complete COD re-moval (98%), TOC removal (40-60%) and high min-eralization percentages of synthetic aqueous wastes polluted with dyes (thiazine dye methylene blue, xan-thene dye rhodamine B), solvents and surfactants. In-dependently of the nature of the pollutant, the percent-ages of removal COD obtained were higher than those of mineralization, indicating the accumulation in the system of intermediate compounds with very low COD and with low oxidizability. Moreover, they sug-gested that the CEs of the processes seem to depend on the nature of the pollutant studied. They also stud-ied the effect of the chlorine during the oxidation of these organic pollutants, concluding that chlorine me-dia favours the oxidation process of dyes and aromatic surfactant, whereas its effect was less significant in the treatment of aliphatic compounds (glycolic com-pounds).

In the case of the electrochemical oxidation of Basic Red 29 (BR29), a bipolar trickle tower reactor with Si/BDD electrodes was used by Koparal and co-workers [92]. Several experimental conditions were studied such as effects of initial dye concentration, Na2SO4 concentration as supporting electrolyte, cur-rent density, flow rate and initial pH on the removal efficiency, and practically, complete BR29 removal (over 99%) was obtained in all the studies. The textile wastewater was also studied with 97% of colour and 91% of COD removal at the current density of 1 mA

cm-2. In addition, the authors evaluated the toxicity in both treated BR29 solution and textile wastewater with good toxicity reductions.

Faouzi et al. [93] compared electrochemical oxi-dation process with Fenton oxidation and ozonation for the treatment of wastes polluted with azoic dyes (Eriochrome Black T, Methyl Orange and Congo Red). Although the three technologies all involves hydroxyl radicals, the results showed important differences. They demonstrated that the efficiencies and the extent of mineralization depend strongly on the oxidation technique and on the concentration of pollutant. How-ever, good performances were achieved at Si/BDD electrode (Table 2). For diluted wastes, the electro-chemical oxidation was least efficient because the primary oxidation mechanisms that occur in Si/BDD were greatly affected by mass transport limitations. For highly-loaded wastes, in the Fenton process a fraction of refractory carbon remained at the final stages of the treatment and, consequently, the process was less efficient. The efficiency of ozonation and electrochemical oxidation were very similar, although the energy consumption required by the electrochemi-cal process to remove the same amount of COD or TOC was significantly smaller than that of ozonation.

Other studies [94-101] using Si/BDD to oxidize synthetic dyes from water have been performed, wherein several operational conditions and chemical effects have been studied (see Table 2), obtaining the complete elimination of dyes. Although other relevant information should be determined for its real applica-tion (e. g., economical considerations and service life time of the electrode), the literature does point out the validity of the electrochemical approach for the elimi-nation of different dyes.

2. Disinfection/Purification Water

The contamination of any type of water with mi-

cro-organisms constitutes a major sanitary concern. Current technologies for water disinfection are chemi-cal treatment with disinfectants or physical treatments, like radiation, UV-treatment, treatment with oxidants and membrane filtration. Any of them have at least one of the following characteristics: high cost, expen-sive maintenance, instantaneous treatment and han-dling of chemical products. Drinking water disinfec-tion has provided a major contribution to the reduction of world mortality during the last century. In general, chlorine is the most used chemical method of disinfec-tion, providing both primary and residual disinfection. Unfortunately, there are several disadvantages like un-favourable taste and odour, its ineffectiveness when used alone against some resistant microorganisms, and the generation of products potentially toxic such as organochlorinated compounds (e.g., chloroform) [102]. Therefore, the new electrochemical methods are de-veloped to establish chlorine-free systems, oxygen-


based disinfection, avoiding the drawbacks of chlorine and the generation of harmful by-products.

The electrochemical production of oxidants at the diamond surface can be exploited for the disinfec-tion of drinking water and removal of colour and odour to prevent waterborne diseases. These disinfect-ing methods are still under research, but the efficient direct in situ production of common chlorine-based disinfection agents [16,103-106], along with the high generation of reactive oxygen species (ROS) [16,19], via Eqs. 1-11 may achieve more accurate dosage and simplify the handling of chemicals.

2Cl- → Cl2 + 2e- (1)

Cl2 + 2OH- → H2O + OCl- + Cl- (2)

Cl2 + 4H2O → 2ClO2 + 8H+ + 8e- (3)

H2O → •OH + H+ + e- (4)

•OH → •O + H+ + e- (5)

2•O → O2 (6)

2•OH → H2O2 (7)

O2 + •O → O3 (8)

−42HSO → −2

82OS + 2H+ + 2e- (9)

−32HSO → −2

62OC + 2H+ + 2e- (10)

−342PO → −4

82OP + 2e- (11)

A DiaCell® reactor with a Si/BDD anode was applied for disinfection of Legionella pneumophila at 104-106 CFU mL-1 [104,105]. The cell operated in continuous by circulating either tap water without or with addition of 75 mg L-1 Cl- or deionized water with 330, 476 or 440 mg L-1 of NaCl, NaHCO3 or Na2SO4, respectively, at 160 L h-1. Total inactivation of Le-gionella cells (> 90%) was reached when the tap water was electrolyzed at 150 mA cm-2 and the contact time was longer than 1 h. The bacteria abatement in tap wa-ter (with 3.5 mg L-1 Cl-) was at least three times faster with the electrochemical disinfection from the dia-mond cell than with conventional chlorine dosing. A low level of electrogenerated oxidant (< 1 mg L-1) was sufficient for a rapid disinfection. The inactivation ef-ficacy increased gradually as the electrolyzed water contained more chloride, even at low current densities. Using 80 mg L-1 Cl-, for example, Legionella cells were completely inactivated by applying a current density as small as 50 mA cm-2 with contact time of 1 min. Bicarbonate solutions electrolyzed in the dia-mond cell also inactivated the bacteria due to the for-mation of a low content of oxidant peroxodicarbonate from Eq. 10. The generation of this oxidant can then explain the rapid Legionella abatement attained with electrolyzed tap water, which contains a high HCO3

- concentration of 324 mg L-1. In contrast, water elec-

trolyzed with sulphate had not impact on Legionella cells due to the low oxidizing power of peroxodisul-fate formed via Eq. 9. Tröster et al. [107] also reported the better performance of a diamond anode in com-parison to common electrode materials like Pt and IrO2 for the treatment of a solution containing 1.4×108 CFU of Escherichia coli and glucose with COD of 9 g L-1. The use of diamond anode not only yields a con-siderable reduction in bacteria population, but also a simultaneous removal of COD. Haenni et al. [106] showed that the DiaCell® reactor oxidants, such as ROS formed from Eqs. 4-8, can be efficiently utilized for the disinfection of chloride-containing swimming pool water. The Si/BDD anode exhibits continuous chlorine productivity and higher disinfection perform-ance against bacteria in comparison to directly added NaOCl. Other interesting electrochemical applications with diamond films involve the disinfection of water circuits and process water in industries and energy supply, humidifiers in air-conditioning systems, cool-ing towers (inactivation of algae, Legionella and germs), warm water systems in hotels and hospitals (Legionella removal), biologically cleaned wastewater (sewage), free-chlorine systems ballast water and medical instruments [107-109].

New evidences on the oxidant action of ROS (•OH, •O, H2O2, and O3) in the electrochemical disin-fection with diamond films have been obtained by electrolyzing chloride-free waters. Polcaro et al. [110] have recently reported the treatment of bacterial sus-pensions of E. coli, Enterococcus faecalis and coli-forms in 1 mM Na2SO4 using a stirred tank reactor containing a Si/BDD anode. The results showed that the concentration of oxidants accumulated in electro-lyzed solution (H2O2, O3 and peroxodisulfate formed from Eqs. 7-9) under continuous treatment depends on flow rate. An increase in oxidant species with increas-ing the effluent flow can be observed. However, higher concentrations of oxidants are produced at high current density and low stirring velocity (character-ized by its Reynolds number). The reduction of mi-croorganism populations from 1×103 CFU mL-1 to the detection limit was achieved in 60, 100 and 300 s for E. coli, coliforms and enterococcci cells, respectively. The goodness of these results is evident when they are compared with similar electrochemical processes with other anode materials. For example, Kerwick et al. [111] treated in batch 10 L of 0.030 M Na2SO4 or 0.036 M NaH2PO4 at 6 L min-1 through the ZappiTM cell, showing a 4 log inactivation of E. coli and bacte-riophage MS2 cells after long electrolysis times of 60-75 min at 24-27 mA cm-2 due to the production of ROS. Patermarakis and Fountoukidis [112] exposed total coliforms (200-26,800 cell mL-1) in tap water to alternating current of 2.5 mA cm-2 using Ti electrodes, but the culturable counts were reduced only by an or-der of magnitude in 15.7 min. Matsunaga et al. [113] reduced E. coli in tap water to less than 2% of the


Fig. 3. Morphological change of Escherichia coli cells

(TEM) resulting from the electrolysis at 100 mA cm-2 for 5 min using a Nb/BDD anode. [E. coli]0 = 108 CFU mL-1, [KH2PO4]0 = 0.2 M, pH = 7.1, 25 °C, (A) before electrolysis, (B) after electrolysis [114].

initial number (102 cell mL-1) after 10 min of elec-trolysis with a carbon-cloth electrode at 0.7 V. The good efficiency of the direct electrochemical disinfec-tion with a Si/BDD anode in diluted Na2SO4 solutions corroborates the important role of ROS. A detailed study by Jeong et al. [114] was performed about the inactivation of E. coli cells with an electrochemical chloride-free system containing 0.2 M phosphate buffer and using Nb/BDD as anode. The morphologi-cal changes of cells after 5 min of electrolysis at 100 mA cm-2 were examined by transmission electron mi-croscopy (TEM) and atomic force microscopy (AFM). Evidence of drastic changes in the nature of the con-tents of cells is clearly shown in Fig. 3, as well as in the structure of their walls, after electrolysis [114]. The cells become mostly empty and their membranes appear to be no uniform. AFM images of the same cells before and after electrolysis showed that, while the surface of the untreated cells appears to be smooth and flat, the treated cells have a rough and sunken sur-face, as if they had shrunk when the inner contents es-caped from them. These morphological changes can be interpreted by the attack of ROS disrupting the in-tegrity of the cell membrane and leading to the lyses of the cells. A greater inactivation was also found with decreasing temperature from 35 to 4 ºC or pH from 7.1 to 5.6, which was mainly related to the formation of more amount of O3 or •OH, respectively. This study clearly shows that strong oxidants as ROS formed by electrolyzing water at diamond films can cause a sig-nificant inactivation of microorganisms, as much as chlorine in electro-chlorination. The potential role of these strong ROS, which possess higher oxidizing power than chlorine, should be further investigated in the treatment of spore forming microorganisms that are difficultly inactivated by only chlorine. Therefore, the development of new approaches to disinfecting waters by using diamond films may lead to an entirely new class of electrochemical chlorine-free systems.

3. Destruction of Inorganic Pollutants in Water Electrochemical processes with diamond elec-

trodes can also be useful for the treatment of water contaminated with inorganic pollutants such as nitrate [24,66,115,116], nitrite [24,116] or cyanide [26,66, 117]. Depending on the pH of the solution different products (nitrite, ammonia and N2) is formed during the reduction of nitrate [66,115,116]. Hydrogen evolu-tion is a competing side reaction. CEs between 10 and 30% [66] are not as high as for the anodic destruction of organic compounds. For the treatment of cyanide contaminated water again diamond anodes are used to oxidize cyanide ions [26,38,117]. The oxidation of cyanides leads to the formation of cyanate in a first step, and later to the formation of CO2 and N2 [117]. Diamond electrodes can also be used for direct con-version of sulfide to sulphate with CE of 90% [118].

4. Production of Strong Oxidants

The exceptional properties of BDD electrodes

can allow the production of powerful oxidants, with high redox potential.

a) The oxidation of Ag+ to Ag2+ in concentrated HNO3. This redox couple can be used as mediator in the partial oxidation of organic compounds (applica-tions in synthesis), or for the electrochemical combus-tion of organic compounds (applications in wastewa-ter treatment). The anodic oxidation of Ag+ to Ag2+ can be performed on platinum, gold and antimony-doped SnO2 electrodes. However, these electrodes suffer from limited anodic stability in concentrated HNO3, and low CE for Ag2+ formation. Ag2+ can be produced with high CE by oxidation of Ag+ at a BDD anode under potentiostatic. In fact, preparative elec-trolysis in a solution of 10 M HNO3 + 100 mM AgNO3, applying a constant potential of 2.2 V, results in 11% conversion of Ag+ to Ag2+ after 2-h electroly-sis, with CE of 81% [119].

b) The oxidation of sulfate to peroxodisulfate in concentrated H2SO4. The peroxodisulfate is an impor-tant oxidant in many applications, e.g., in etching printed circuits and in acrylonitrile polymerization as well as wastewater treatment, dye oxidation and fiber whitening. The efficiency of the electrochemical pro-duction of peroxodisulfate depends on the electrode material. High oxygen overpotential anodes must be used to minimize the side reaction of oxygen evolu-tion. The conventional electrochemical process for peroxodisulfate synthesis uses smooth platinum an-odes. However, some investigations have been per-formed at BDD electrodes [69], in order to find the optimal conditions for peroxodisulfate formation. Therefore, the influence of the operating conditions (temperature, H2SO4 concentration) on the CE of per-oxodisulfate formation has been investigated [69]. At low H2SO4 concentration (< 0.5 M) the main side re-


action is the discharge of water to O2. The chemical decomposition of peroxodisulfate also takes place at this low H2SO4 concentration. While, at high H2SO4 concentration (> 2.0 M) the main anodic reaction is the electrochemical oxidation of sulfate to peroxodi-sulfate. Small amounts of monopersulfate and H2O2 are also formed by the chemical decomposition of peroxodisulfate.

c) Oxidation of Mn2+ to MnO4-. The electro-

chemical oxidation of Mn2+ has been previously stud-ied to produce manganese oxyhydroxide (MnOOH), MnO2 and MnO4

- at PbO2 electrode, but the PbO2 electrode can be leached into the solution, depending on the experimental conditions. Both Mn3+ and Mn7+ are important as strong oxidants, which have been used for both analytical and synthetic purposes as well as for the destruction of organic pollutants. The elec-trochemical oxidation of Mn2+ at BDD electrodes was achieved, wherein all three high valence states, i.e., Mn3+, Mn4+, and Mn7+, are produced, depending on experimental conditions [120]. The oxidation of Mn2+ to Mn7+ takes place at a potential significantly more positive than its thermodynamic potential of 1.5 V vs. SHE or 1.7 V vs. Ag/AgCl (in saturated KCl). This is attributed to the lack of capabilities of the BDD elec-trode for efficient oxygen transfer. The results of spec-troelectrochemical measurements confirmed that Mn7+ is a major product at a concentration lower than about 20 mM, whereas Mn3+ is a primary product at higher concentrations. CEs about 37% were obtained.

d) Oxidation of Fe3+ to Fe6+. The electrochemical generation of Fe6+ has been shown to be obtained by a direct oxidation of metallic iron rods in strongly alka-line media, where the ferrate salt is stable. According to cyclic voltammograms recorded at various scan rates for the oxidation of Fe2+ to Fe6+ via Fe3+ by Lee et al. [121]; the first anodic peak at about +1.0 V is due to the oxidation of Fe2+ to Fe3+ whose cathodic counter part was observed below about 0.60 V (Fig. 4). The sluggish electron transfer rate of this reaction makes the peak separation vary to a large extent de-pending on the voltage scan rate. The second anodic peak observed above about 2.3 V, which is 8-10 times of the first anodic peak, confirmed the oxidation of Fe3+ to ferrate.

e) Ozone production. Ozone is a strong oxidant, which is widely used to supplement or replace chlo-rine in a variety of processes associated with water treatment [122,123]. Oxidants kill microorganisms and precipitate various chemicals. However, there is still a need for a safe, inexpensive process for the pro-duction of ozone for water treatment in swimming pools and small drinking water plants. Ozone is greatly preferable to chlorine as a disinfectant, but the low ozone concentration available using an electric discharge in the gaseous phase (corona process) or UV light absorption (photochemical process) has pre-vented ozone from being applied in several green

Fig. 4. Cyclic voltammograms at a BDD in (a) 0.1 M

HClO4, and with 6 mM FeSO4 at scan rates of (b) 10, (c) 50, (d) 100, (e) 250, (f) 500 and (g) 1000 mV s-1. The electrochemical cell was a single compartment cell, with the surface of the BDD electrode exposed at the bottom of the cell through an O-ring supported opening with a Pt mesh counter electrode, and Ag/AgCl reference electrode (in saturated KCl) [121].

chemical processes, for example, the decomposition of persistent organic pollutants, where a higher ozone concentration is necessary. Electrochemical ozone production is a promising technology due to the pos-sibility of producing ozone in higher concentrations than in conventional methods. Today, however, most ozone is produced by corona discharge. Only a minor amount of ozone is produced by electrolysis with lead dioxide anodes in solid polymer electrolyte (SPE) re-actor with CE around 15%. In conventional electro-chemical reactors ozone is produced on diamond an-odes with a CE about 5% [124,125]. But combining diamond anodes and the SPE technology [126] leads to electrochemical ozone production efficiencies of more than 20% [127,128].

CONCLUDING REMARKS AND

PERSPECTIVES This review provides an overview of the recent

environmental applications of diamond electrodes in the fields of water treatment, drinking water disinfec-tion, elimination of organic and inorganic compounds, and generation of strong oxidants. The literature dem-onstrates that conducting diamonds offer significant advantages over other electrode materials in term of CE and stability for a variety of electrochemical proc-esses. The recent advances with diamond films elec-trodes suggest that their new applications should be rapidly developed because of their better performance with respect to other anode materials. In the cases of elimination of dyes and drinking water disinfection, the fast abatement and total oxidation of organic sub-

I (A

cm

-2)


stances can be achieved due to the great amounts of ROS produced during water electrolysis with dia-mond-coated electrodes. Imagine, for example, the use of such practical commercial technology for disinfec-tion, washing, swimming pools, sterilization of medi-cal articles, drinking water disinfection and treatment of purulent and septic diseases of humans and animals, as well as application to waste dyestuff treatment plants, poultry factories, and livestock farms. This opens new perspectives for an easy, effective and free-chemical water treatment by means of the electro-chemical technology with diamond films. However, more research is needed to assess possible problems involved with the formation of by-products, costs and preparation of industrial scale anodes.

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Discussions of this paper may appear in the discus-sion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication.

Manuscript Received: November 19, 2007 Revision Received: January 12, 2008

and Accepted: January 16, 2008

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