Electrochemical Ozone Production as an Environmentally Friendly Technology for Water Treatment

D�bora V. Franco1

Wilson F. Jardim1

Julien F. C. Boodts2

Leonardo M. Da Silva3

1Institute of Chemistry – UNICAMP,Brazil.

2Institute of Chemistry – UFU, Brazil.

3Department of Chemistry – UFVJM,Brazil.

Research Article

Electrochemical Ozone Production as anEnvironmentally Friendly Technology for WaterTreatment

Perspectives, advances and environmental aspects concerning electrochemical ozoneproduction applied to water purification are presented and discussed in relation tothe conventional corona process (silent electric discharge). Ozone generated using alaboratory-made electrochemical reactor was applied for the discoloration/degrada-tion of dyes used in the Brazilian textile industry and for degradation of endocrinedisruptors. A constant ozone load of 0.35 l 0.02 g/h was used throughout. The study,concerning color removal from dye solutions, revealed that total discoloration is rap-idly achieved. The degradation rate of the textile dyes evaluated by TOC is littleaffected by the dye composition and considerably influenced by the pH and ozona-tion time. Analysis of the COD/TOC-ratio indicates that ozonation increases oxidationfeasibility of the organic matter (dye by-products) when compared to the originalcompounds. Ozonation of mixed aqueous solutions containing different endocrinedisruptors revealed these compounds are totally degraded with a very high removalrate.

Keywords: Clean Technologies; Electrochemistry; Endocrine Disruptors; Ozone; Water Treatment;

Received: June 26, 2007; revised: August 14, 2007; accepted: September 9, 2007

DOI: 10.1002/clen.200700080

1 Introduction

Water pollution control and the development of clean technologiesfor water, air and soil treatments comprise one of the most impor-tant challenges for modern society [1]. The development of cleanprocesses for several industrial activities, together with environ-mental protection (sustainable development) is now imperative, inorder to ensure preservation of our water resources for the use inproximate future [1 – 15].

As previously discussed by Da Silva and Jardim [5], depending onthe desired water quality some processes are more suitable thanothers for its treatment. Physical separation of suspended solids andbiological treatments are economically viable in several cases (e. g.,municipal wastewater, food and farm processing water). However,there are situations in which the effectiveness of these treatments issignificantly reduced (e. g., in the presence of recalcitrant com-pounds and toxic substances). So, different chemical processes need

to be used as an auxiliary (coadjutant) process in order to obtain amore efficient effluent treatment [1 – 3, 5, 11].

Chemical oxidation (CO) is considered a complete technology forthe degradation of several organics in order to meet the regulationsfor toxicity as well as total organic carbon (TOC) reduction. There-fore, CO comprises one of the most important auxiliary process forthe on site degradation (removal) of hazardous and toxic recalci-trant organics [5, 11, 12].

The use of integrated processes based on the combination of COas a pre-treatment step followed by activated sludge treatment,where the former is used to provide the transformation of the recal-citrant pollutants to more easily biodegradable intermediates,deserves special attention since they can be economically viable [5,12].

An environmentally acceptable oxidant used in the CO processshould possess the following features [11, 13, 14]:(i) have a high reaction rate with the pollutant;

(ii) neither produce nor leave undesirable by-products during thecourse of the reaction;

(iii) be readily available, and

(iv) be reasonably inexpensive to purchase.

In this context, the commercially available oxidants for applica-tions in water pollution control are: ozone, hydrogen peroxide,chlorine, chlorine dioxide, sodium and calcium hypochlorite, andpotassium permanganate.

Ozone is a clean and powerful oxidant and, therefore, the ozona-tion process can be used in several cases [1 – 10]. O3 applications inintegrated treatment systems (ITS) for water treatment present at

Correspondence: Leonardo M. Da Silva, Department of Chemistry –UFVJM, Campus Presidente Juscelino Kubitschek, Rua da Gl�ria 187,39100-000, Diamantina, MG, Brazil.E-mail: [email protected]

Abbreviations: AOP, Advanced Oxidation Processes; CBR, Column Bub-ble Reactor; CO, Chemical Oxidation, DBP, Disinfection By-Products; ED,Endocrine Disruptors; EOP, Electrochemical Ozone Production; ITS, Inte-grated Treatment Systems; MEA, Membrane Electrode Assembly; OUE,Ozone Utilization Efficiency; TOC, Total Organic Carbon; TPCBP, Transi-ent Persistent Colored By-Products

i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com

34 Clean 2008, 36 (1), 34 –44

Clean 2008, 36 (1), 34– 44 Electrochemical Ozone Production 35

least two important consequences [5, 8]: (i) biodegradability of thedissolved organics increases and (ii) introduction of a considerableamount of oxygen into the water creates excellent conditions dur-ing the biologically active filtration process.

Several studies show that the CO processes known as AdvancedOxidation Processes (AOP) (e. g., O3, O3/UV, H2O2, H2O2/UV, H2O2/Fe2+

(Fenton's reagent), and TiO2/UV (heterogeneous photo-catalysis),etc.), which are based upon the attack by the hydroxyl radical (HO9 –E0 = 2.80 V) on the organic pollutants, play a key role in the develop-ment of ITS for pollution control of contaminated waters [1, 2, 5,16 – 23].

1.1 Ozone Technology for Water Treatment

Ozone is the most powerful disinfectant available [8, 14, 24, 25] andis capable of oxidizing several different organic pollutants [4, 5, 18,26 – 28]. Therefore, ozone application under optimized conditions(e. g., [O3(aq)], contact time, temperature, etc.), alone or combinedwith hydrogen peroxide and/or UV-light, comprise efficient proc-esses for water treatment (disinfection and/or organic pollutantremoval) [2].

The U.S. Environmental Protection Agency (EPA) presented a gen-eral guide for controlling the presence of disinfection by-products(DBP), such as haloacetic acids and trihalomethanes in water, whicharise from the disinfection of water using chlorine and other halo-gens [8]. According to the EPA, the presence of DBP in a water supplycreates a serious health risk for the population since many of thesecompounds are indeed carcinogenic [1]. Thus, the elimination ofDBP using alternative CO methods of disinfection such as ozonationis welcomed in several cases and, therefore, the use of ozone forwater treatment is gaining popularity around the world [1]. Recentstudies also revealed that ozonation is very efficient for the removalof several pollutants in water classified as endocrine disruptors (ED)[27].

Besides potable water treatment, other potential ozone applica-tions are: purification of water loops used in the pharmaceuticaland electronic industries; surface sterilization; wood pulp bleach-ing; treatment of textile wastewater, etc. [2, 4, 9, 16, 29 – 31].

Ozone is an unstable gas (s1/2 L 40 – 90 min) and is sparingly solu-ble in water (12 mg/dm3; 258C) [4, 5, 14, 32]. Depending on the dis-solved oxygen concentration and the solution pH, the ozone stand-ard redox potential can be very high (1.51 to 2.07 V interval) [33].

Ozonation of recalcitrant pollutants dissolved in water comprisesa chemical process involving the mass transfer of the gaseous phase(O2 + O3) into the liquid phase, where the irreversible reaction withthe pollutant takes place [34]. The ozonation process can be theoret-ically treated by means of the film model, in which the ozone masstransfer is assumed to proceed via stationary molecular diffusionthrough a stagnant film [34, 35].

Two processes can control the overall reaction rate in this case: (i)the mass transfer from the gas phase to the liquid phase and (ii) thechemical reaction taking place in the liquid phase [10, 14, 34, 35].Considering that ozone and oxygen are both not very soluble inwater, one can suppose that no mass transfer limitation is observedwithin the gas phase and only the liquid phase mass transfer resist-ance needs to be considered.

The ozonation process based on the film model can be describedby the dimensionless parameters known as [35]: the Hatta number(Ha), the instantaneous enhancement factor (Ei) and the reactionnumber (R). In this case, R incorporates the volumetric mass transfer

coefficient (kLa) and the reaction rate constant (k), while Ha includesthe reaction rate constant (k) and the mass transfer coefficient (kL).

The increase of small bubbles production, using an efficient dif-fuser system, leads to a large interfacial area of gas-liquid contact,thus increasing the kLa-value and, consequently, the gas hold-upinside the bubble column is increased [36]. According to Gao et al.[36], the volumetric mass transfer coefficient kLa has no connectionwith the saturated concentration of ozone and the pH. Also, depend-ing on the gas flow rate, kLa-values obtained during ozonation inbubble columns are in 0.2 to 0.8 min – 1 range [36].

The most effective way to increase the ozone mass transfer rate isto increase the interfacial area available for mass transfer bydecreasing the size of the gas bubbles dispersed in solution and byincreasing their residence time [5, 37]. Thus, the mass transfer ratewill be a maximum when a given mass of the gaseous mixture (O2/O3) is introduced in the liquid using an efficient diffuser systemwhich forms a great number of very small bubbles, thus consider-ably increasing the reaction zone area located in the gas/liquidinterface [10, 29].

Gas-liquid contactors (diffuser systems) currently used in waterand wastewater systems include submersible orifices, such as noz-zles, sintered-metal or sintered-glass plates, sprinklers, rubber mem-branes and electrostatic spraying microbubble generators [5]. Incombination with these bubble generation devices, the use ofmechanical agitation can be used to increase the contact betweenphases [29]. Such devices provide kinetic energy to the bulk fluid inorder to provide efficient bubble dispersion.

Da Silva and Jardim [5] reported a simplified scheme for repre-senting the ozonation of organic pollutants taking place at the gas/liquid interface. According to this reaction scheme, ozone absorp-tion in the liquid phase is followed by the irreversible degradationreaction, as follows:

O3(g) fi O3(l) (mass transfer step) (1)

[O3(l) Q R (Rad)(l)] + mX(l)fi Products(l) (degradation step) (2)

where O3(g) and O3(l) represent ozone present in the gaseous andliquid phases, respectively; [O3(l) Q R (Rad)(l)] describes the O3-decom-position that yields oxygenated free radicals (e. g., HO9); m is the stoi-chiometric coefficient, and X is a given target organic compound.

According to the film model [34, 35], the mass transfer step (seeEq.(1)) in the gas/liquid interface is described by the instantaneousenhancement factor Ei, which represents the mass transfer accelera-tion caused by the degradation step (see Eq. (2)).

The irreversible chemical reaction can be characterized as “slow”or “fast”. If the “degradation step” is considered slow in comparisonwith the “mass transfer step”, one has the limit case where Ha a 0.3and Ei = 1. In this case, the ozonation process remains in the moreexternal region of the film and the overall reaction rate is governedby the chemical reaction (the rate constant k can be calculated fromthe “standard kinetics” taking place within the bulk). In contrast, ifthe degradation step is considered fast, Ha A 3 and EiA 1. In this case,the ozone concentration is considerable depleted in the film regionand the overall reaction rate is governed by mass transfer step.

Intermediate cases where 0.3 a Ha a 3 are representative of manyozonation reactions where both chemical kinetics and the masstransfer rate are important for the overall reaction rate. A modelconcerning these complex cases was presented by Benbelkacem etal. [34, 35].


36 D. V. Franco et al. Clean 2008, 36 (1), 34 –44

The O3-decomposition into oxygenated radicals considered in the“degradation step” (see Eq. (2)) depends on pH and/or the applica-tion of UV-light and/or H2O2 [2, 5, 19]. The O3-decomposition influ-enced by the pH originates from the fact that the hydroxide ion(HO – ) initiates a chain of reactions when ozone enters into water[14]. The chain reaction is sustained by HO29 formation, which canthen initiate further reactions. The hydroxyl radical (HO9) is themost important species formed during ozone decomposition [32].

Therefore, ozone can react in aqueous solutions directly with sub-strates, such as molecular-ozone, or indirectly, via radical-inter-mediates formed during ozone decomposition. Depending on theextent of the ozone decomposition in water, the selectivity of theozonation process can be high (direct reaction) or low (indirect reac-tion) [5, 14].

From the above considerations, in most cases degradation oforganic pollutants via ozonation takes place as a result of the com-bined “direct” (O3) and “indirect” (HO9) oxidation pathways [14, 24,26, 32], which can be represented by the scheme presented below[38]:

Considering that ozone and the hydroxyl radical react very rap-idly with several organic pollutants (e. g., textile dyes, endocrine dis-ruptors, etc. [5, 9, 24 – 27, 32]), the mass transfer step can be the ratedetermining one in many ozone applications.

1.2 Advances in Electrochemical Technology forOzone Generation

The increasing popularity in recent years concerning ozone applica-tions are mainly due to two factors [1 – 5, 8]: (i) costs associated withozone production considerably decreased in the last decade and (ii)ozone presents environmental advantages over chlorine.

Ozone is an unstable gas and, therefore, its generation should becarried out in situ [5]. The main commercially available technologyfor ozone generation is based on the corona process [5, 29, 32],which comprises the application of a silent electric discharge in acooled/dried gaseous phase containing oxygen (O2 or air) [32, 39, 40].The fundamental processes leading to ozone generation during thesilent electric discharge are depicted in the scheme presented inFig. 1.

The efficiency for the oxygen fi ozone conversion process (O2 + O9fi O3) taking place during silent electric discharge (corona process)depends on several factors [32, 39, 40]: (i) oxygen source (O2 or air); (ii)gas temperature and (iii) presence of impurities in the gaseousphase. The efficiency presented by most of the commercially avail-able corona devices, using air as the O2 source, is l0.4 – 2.0 wt% andrequires an “apparent” energy demand (where costs concerning gasrefrigeration and heat exchange are not considered) of l14 – 28Wh/g [5, 4, 32].

Considering that nitrogen is not an inert gas inside the coronadevice, this experimental approach using air presents the environ-mental inconvenience of releasing strong pollutants such as NOxcompounds and HNO3 into the atmosphere [32, 39, 40].

As a consequence, the service life of the corona device can bereduced due to wear caused by acidity [32]. When dry oxygen isused, this inconvenience is circumvented and the efficiency reachesvalues of l1 – 6 wt%, requiring an apparent energy demand of l6 –12 Wh/g.

In contrast to ozone, the NOx compounds released during thecorona process cannot be used as a cleaning reagent for pollutantremoval in contaminated waters and, therefore, its production isundesirable from an environmental point of view.

The main problem with the corona technology is that the energyefficiency decreases rapidly as one tries to increase the ozone gener-ation rate [32]. This inconvenience comes from the fact that theozone decomposition rate by the electric spark increases on increas-ing the O3 concentration [4, 32]. Therefore, most commercial ozona-tion systems, based on the corona technology, are commonlydesigned to operate in the 2 – 3 wt% or 1 – 2 wt% range using oxygenor air, respectively.

The theoretical heat of formation of ozone is 0.85 Wh/g [15]. Eval-uating performance of different commercial corona devices, usingoxygen as the reagent, one can observe that about 60 to 80% of thecorona power is released as heat. Therefore, considering that ozonegeneration is very sensitive to temperature changes, and that airand pure oxygen are poor heat conductors, one concludes that anefficient temperature control is absolutely required in any coronadevice, thus increasing the overall costs for ozone production (realenergy demand).

Although the discovery, by Werner von Siemens [4, 32], thatozone can be generated by means of the corona process goes backmore than 140 years, it is only in the last 20 years that significanttechnical progress has been made in this field [32, 39, 41]. The AT-ozone generators based on the corona process developed by the Ozo-nia Ltd. Company (Switzerland) in early 1991 (see www.ozonia.com),resulted in great advances in large scale ozone production. AT-ozonegenerators operate at high frequencies (l800 Hz) and they weredeveloped based on the use of non-glass dielectrics. The ozone con-centrations in the gaseous phase furnished by the AT-ozone genera-


Figure 1. Scheme describing the fundamental processes taking placeduring silent electric discharge (corona process).


tors are l5 wt% (using air) and l16 wt% (using dry oxygen). As aconsequence, the specific (apparent) energy consumption for ozonegeneration was reduced up to 50% in comparison with the classicalcorona devices.

Another commercially available technology for ozone generationis based on electrochemistry (water electrolysis: 3H2O fi O3 + 6H+ +6e – ) [7, 42]. Although ozone was discovered by Sch�nbein in 1840during a water electrolysis experiment [5], investigations of Electro-chemical Ozone Production (EOP) were abandoned for many years[43].

The EOP process was “resuscitated” in the last two decades as aconsequence of great advances in the research fields of new materi-als science and electrochemical engineering [4, 7, 28, 30, 31, 42,44 – 52]. Nowadays, the EOP technology is an attractive alternativefrom the economical point of view for many important applica-tions, and ozone production can be carried out using different elec-trochemical reactors (f.i., see refs. [7, 30, 38, 52]).

From the environmental point of view, EOP can be considered anenvironmentally friendly technology in comparison with thecorona process using air, since NOx emission is not possible withEOP. Indeed, the only products released during water electrolysisare oxygen/ozone and hydrogen [4, 31]. In some cases, where theelectrochemical reactor uses an air cathode, H2 generation isreplaced by oxygen reduction obtained from air [44, 47, 51, 53, 54].

Ozone is classified as an air pollutant [1]. Therefore, in order to beconsidered a clean oxidant, any ozonation system aimed at pollu-tion control (e. g., treatment of contaminated waters) should bedesigned as a “closed system”. This experimental approach ensuresthat the generated ozone is quantitatively absorbed by the pollu-tant, thus avoiding its undesirable release in the atmosphere.

After the ozone application in water treatment, the only productreleased into the atmosphere is oxygen [1, 5]. Therefore, taking intoaccount the adequate conditions required for its secure applicationin many processes involving pollution control, ozonation carriedout electrochemically can be indeed classified as an environmen-tally friendly technology.

Contrary to the corona technology, where a gas phase reactiontakes place forming ozone, ozone generation from water electroly-sis is rather complex, comprising a sequence of surface reactions [6,55] that take place on certain inert electrode materials (e. g., glassycarbon, platinum, etc.). The fundamentals concerning the EOP elec-trode process were extensively discussed and revised by Da Silva etal. [4 – 6, 38, 55 – 57]. The electrode mechanism proposed by Da Silvaet al. [6, 55] for the EOP process has been used and discussed by sev-eral authors [47, 52, 58, 59].

Considering the ionic conductor medium used in the electro-chemical reactor, the electrochemical ozonation system can be arbi-trarily classified in two categories: (i) Type-(I) [44, 45, 47, 60], wherean inert electrode material is immersed in specially designed liquidelectrolytes (the use of specific electrolytes can improve ozone pro-duction efficiency by affecting the O9(ads) residual life at the anodesurface [55]), and (ii) Type-(II) [30, 31, 47, 51 – 54, 61, 62], where aMembrane Electrode Assembly (MEA) [46, 49], using a solid polymerelectrolyte (e. g., Nafionm 117 membrane) and a porous inert elec-trode material, is responsible for the ionic transport from anode tocathode by means of the “zero-gap” approach. Obviously, the use ofconventional (liquid) electrolytes in reactors of Type-(II) is not neces-sary.

EOP technology presents a higher efficiency for O3 generationwhen compared to corona technology [4, 30, 38, 44, 45, 52]. Depend-

ing on the electrochemical reactor used, the efficiency varies in thel3 to 47 wt% range and the EOP specific power consumption is inthe l20 to 85 Wh/g range [4, 30, 52].

The choice of the type of electrochemical reactor depends on theparticular ozone application. For instance, reactors of Type-(I) maybe used in substitution of the corona devices in many applicationswhere a very high ozone concentration in the gaseous phase (oxy-gen/ozone) is imperative in order to permit a rapid removal of recal-citrant organics and/or immobilization of pathogenic agents. As inthe case of the corona device, reactors of Type-(I) require the use of acooling system and a gas diffuser.

Reactors classified as Type-(II) can be mainly used for purificationand ultra-purification of water (e. g., potable water, water used inthe pharmaceutical industry, etc.). These reactors present the bene-fit of permitting that the ozone micro-bubbles formed inside theMEA be directly released into the streaming (treated) water, thusresulting in a very high OUE-value (L100%). In these ozone applica-tions using reactors of Type-(II), the total operational costs are con-siderably minimized due two factors: (i) a very high OUE-value canbe achieved (L100%) and (ii) the use of a cooling system is not neces-sary, since these reactors operate at their maximum performance atambient temperatures (25 to 358C). Besides, the heat producedinside the reactor is promptly removed by the circulating water.Contrary to the corona process (see previous discussion), circulatingwater is a good heat absorber, thus permitting easy heat dissipationto the environment.

The only gaseous products released during water electrolysis byboth type I and II electrochemical reactors are oxygen/ozone andhydrogen. It is possible that the latter product can be injected intothe anode compartment of a hydrogen fuel cell (a clean energysource [63, 64]) in order to reduce the total costs related to EOP. Forinstance, considering an applied current of 130 A, ozone produc-tion is accompanied by the release of 5 g/h of pure hydrogen (L60dm3/h at 1 bar and 298 K), which can, in principle, be converted intoelectrical energy for the EOP-reactor using a fuel cell.

Figure 2 presents a scheme of an environmentally friendly hybridelectrochemical system proposed for ozone generation using hydro-gen as an auxiliary energy source.


Figure 2. Scheme representing the hybrid environmentally friendly elec-trochemical system proposed for ozone production using the electro-chemically generated hydrogen as an auxiliary energy source.


It is worthwhile to emphasize that when all operational costs con-cerning the ozone application and the environmentally friendlyaspects are considered, the unique features presented by the EOPtechnology can indeed compensate its higher apparent specificpower consumption when compared to the corona process.

The objective of this paper is to present in a clear view the envi-ronmental benefits and the technological aspects inherent to theelectrochemical technology devoted to ozone generation for appli-cation in water treatment. The article also describes the applicationof the electrochemically generated ozone for degradation of textiledyes and endocrine disruptors.

2 Experimental

2.1 Electrochemical Ozone Production

The degradation kinetics of textile dyes and endocrine disruptorsusing electrochemically generated ozone was investigated usingthe experimental set-up presented in Fig. 3.

Ozone production was carried out using the electrochemical reac-tor OZONA-ZY60 developed by Da Silva and Jardim (patent in prepa-ration) for potable water treatment. OZONA-ZY60 presents an EOPefficiency of l15 wt% and an ozone production rate of 6 g/h (at 130A and 308C). The EOP specific power consumption is 70 Wh/g andthe hydrogen production rate is l5 g/h.

In contrast to the case of the corona technology, where the esti-mated calculations for the true energy demand are very compli-

cated and, therefore, its performance is frequently presented basedupon the apparent energy consumption, the calculations for trueenergy demand for EOP are easy. For instance, disregarding the useof hydrogen as an auxiliary energy source, the total energy dissipa-tion during the ozone production is 92 Wh/g.

The gas mixture (O2/O3) leaving the anode compartment was sepa-rated from pure water using a gas separator flask and introducedinto the column bubble reactor (CBR) (V = 1 dm3) as indicated inFig. 3.

The electric current applied to the OZONE-ZY60 reactor wasadjusted in order to provide a constant ozone production rate of0.35 l 0.02 g/h. This value was chosen considering the geometricproperties of the CBR (liquid height, volume capacity and diameterof the porous coarse glass plate) in order to maximize OUE duringthe treatment.

The electrochemical reactor was powered using a 350A/12V d.c.current source model PU-21 from AMZ (Brazil). A model 77601-10MASTERFLEX peristaltic pump (Cole-Parmer) was used to circulatedistilled water inside the anode compartment at a volumetric flowrate of 60 dm3/h. Temperature control of the circulating water (30 l28C) was carried out by means of a laboratory-made heat exchangerusing tap water. The linear velocity of the fluid (water) was 1.32 cm/sand the space velocity 5.19 min – 1. The flow regime was turbulent(Re A 3500).

2.2 Ozonation of Textile Dyes under Controlled pH

Samples were prepared by dissolving the commercial dyes Yellow143 (RY 143) and Reactive Blue 264 (RB 264) (purity degree: 80 – 85%),furnished by CERMATEX Textile Industry Ltd. (Americana, Brazil), indistilled water, resulting in an initial dye concentration (IDC) of 100mg/dm3. This study was carried out as a function of the solution pH(2 – 12) using the CBR(A) presented in Fig. 3.

The discoloration kinetics study was carried out by measuringthe absorbance at a fixed wavelength (421 nm for RY 143 and 619nm for RB 264) during the first 60 min of the reaction, using amodel DR 4000 spectrophotometer from Hach.

Total organic carbon (TOC) was measured using a model 5000ATOC analyzer from Shimadzu. Chemical oxygen demand (COD)measurements were carried out following the standard procedure[65].

The gas mixture (O2 + O3) was bubbled (gas flow rate = 2.35 dm3/h)using a porous plate diffuser (Schott #2 coarse glass frit; A = 3.5 cm2)placed at the bottom of the CBR(A) (height = 40 cm and diameter =10 cm). In all cases, the solution was stirred in order to increase themicrobubbles residence time. These conditions were previouslyinvestigated in order to obtain a reproducible profile for the micro-bubble generation. A volumetric mass transfer coefficient kLa of 0.9l 0.1 min – 1 was obtained using the classical unsteady state method.A preliminary study using pure water revealed that the steady stateozone concentration in aqueous phase is reached after an ozonationtime of 1.0 l 0.1 min.

The pH was periodically monitored using a combined glass elec-trode and adjusted during ozonation to a specified value (pH l 0.05)by adding NaOH (1.0 mol/dm3) using a micro-syringe (Hamilton).Discoloration was investigated using 500 mL samples. Ozonatedsamples (3 cm3) were periodically withdrawn using a syringe forabsorbance measurements. These aliquots were re-injected into thereactor flask after each measurement (l45 s).


Figure 3. Flow diagram representing the experimental set-up used forozone generation and its application for degradation of textile dyes andendocrine disruptors using different column bubble reactors.


2.3 Ozonation of Endocrine Disruptors

The degradation of different endocrine disruptors (ED) via ozona-tion was carried out using the CBR(B) (height = 30 cm and diameter= 10 cm, see Fig. 3). The sample solutions were prepared based onthe solubility limit presented in water by the different compounds(diclofenac, ibuprofen, bisphenol-A, estrone and 17a-ethinylestra-diol), resulting in a total ED concentration of 600 lg/dm3. This studywas carried out using 1000 cm3 samples as functions of the ozona-tion time. A total of ten samples were collected covering an intervalof 1 h. The ozonation by-products from the ozonated samples wereseparated by liquid-liquid extraction (water-dichloromethane 1:1(v/v)) and stored at 78C in appropriate vessels. Analysis of the ozo-nated samples was carried out using specific ED standards via highperformance liquid chromatography (UV-detection) with a Shi-madzu equipment.

3 Results and Discussion

3.1 Influence of the pH on Total Discoloration

Figure 4 shows the UV-VIS spectra for textile dyes RY 143 and RB264, used as model recalcitrant compounds, before and after ozona-tion (t = 1 h). Both dyes show a maximum absorbance in the visibleregion, corresponding to 421 and 619 nm for RY 143 and RB 264,respectively.

The final discoloration was reported as an absorbance reductionhigher than 99% obtained for a given ozonation time which wasdefined as td.

Analysis of the spectra shown in Fig. 4 reveals that ozonation,under the specific experimental conditions, leads to a considerabledecrease in absorbance in the visible region, while in the UV regionabsorbance did not show drastic changes. The absorption bandslocated in the 200 to 300 nm interval are associated with the elec-tronic transitions in the aromatic ring present in the daughter aro-matic compounds generated from partial oxidation of the parentdye molecule [66]. Analysis of the absorbance in the UV-regionrevealed the total amount of aromatics decreased by about 20 to40% for both RY 143 and RB 264.

Table 1 shows td values as functions of the pH for RY 143 and RB264. The comparison of td values obtained at different pH valuesreveals that total discoloration (F99%) is attained faster in alkalinesolution, indicating that the contribution of the indirect oxidativepathway is important for discoloration. It was also found that dis-coloration depends on the nature of the dye (td(RB264)A td(RY143)), reveal-ing that the destruction of the chromophore centers in RB 264 ismore pronounced (unfortunately, information concerning theirmolecular structures are not available for more detailed discus-sions).

These results reveal that alkaline conditions are more suitable fordiscoloration of dye effluents. This is an interesting result sincemost of the textile effluents present a high pH (A10) [66].

3.2 Discoloration Kinetics

Ozonation kinetics of textile dye solutions depend markedly on pHand ozone load [2, 16, 18]. The influence of the pH can be under-stood considering that when ozone is absorbed in water, hydroxideions (HO – ) initiate a chain reaction leading to ozone degradation,generating several oxygenated radicals (e. g., HO9) [10, 11, 14].

It is worth emphasizing that there is a distinction between totaldiscoloration of the dye solution and the total degradation (miner-alization) of the dye molecules [66]. For most textile dyes, discolora-tion via chemical oxidation (e. g., ozonation) occurs when the chro-mophore bond(s) is(are) removed, while many colored by-productsof the parent dye molecule may remain stable in solution [66].Therefore, discoloration may be the initial step in the degradationroute of a dye molecule, which is not necessarily accompanied byquantitative carbon removal (considerable degree of mineraliza-tion).

Discoloration requires a lower oxidant load than mineralization,thus becoming an interesting pre-treatment step for the traditionalbiochemical process, which frequently presents a low efficiency forcolor removal [2].

In all cases, an exponential decay for the absorbance reduction asfunction of the ozonation time was verified and a rather good linear


Figure 4. UV-VIS spectra for textile dyes RY 143 (A) and RB 264 (B)before and after ozonation at pH 12. IDC = 100 mg/dm3.

Table 1. Dependence of td values on pH for RY 143 and RB 264. IDC =100 mg/dm3.

pH RY143 RB 264td/min td/min

2 l 0.05 25 147 l 0.05 23 12

12 l 0.05 20 11


behavior (r A 0.998) was verified using the pseudo-first order kineticmodel. These findings reveal the reaction leading to discolorationcan be considered slow and, therefore, the chemical reactionbetween O3 and/or HO9 and the chromophore centers governs theoverall reaction rate.

During the curse of the ozonation reaction it is possible that thedaughter products (colored by-products) may compete with theparental dye molecules for the oxidant (ozone and/or hydroxyl radi-cal). In this case, the apparent reaction order for discoloration proc-ess can be determined using the least square regression fit of theintegrated form of the material balance expression [67]:

d½cro�dt

¼ �kobs½cro�n ð3Þ

Considering the discoloration process follows a pseudo first ordermodel (n = 1), integration of Eq. (3) furnishes the next relation:

ln½cro�½cro�o

� �¼ �kobs t ð4Þ

where [cro] and [cro]o are the instantaneous and the initial chromo-phore concentrations, respectively [38].

In principle, the overall pseudo-first order rate constant, kobs, con-tains the effect of the intrinsic kinetics and may reflect more thanone mass transfer-chemical regime. The expression for kobs can bepresented as follows [67]:

kobs ¼ aðG=VÞXð½HO��Y ½cro�0½HO��ÞZ ð5Þ

where: G/V = ozone application rate; [OH9] = hydroxyl radical concen-tration and [OH – ] = hydroxyl anion concentration. a, X, Y and Z arethe empirical constants for the particular reaction.

Applying Lambert-Beer's law in Eq. (4), discoloration kinetics canbe investigated using the next relation [38]:

lnAAo

� �¼ �kobs t ð6Þ

where the ratio A/A0 represents the normalized absorbance meas-ured at a fixed wavelength.

Figure 5 shows kinetic profiles for discoloration, while Tab. 2gathers kobs as a function of the pH for RY 143 and RB 264.

Two linear segments in the pseudo-first order profile wereobtained in all cases. The existence of two linear segment in thekinetic profile was also verified by Hsu et al. [29], who proposed thatslope changes in the kinetic profile, after a given threshold t-valuewere due to changes in the chemical nature of primary substancesin solution, forming transient persistent colored by-products(TPCBP). The presence of two kobs in the kinetic profile supports theexistence of two corresponding half-life time constants, s1/2, for dis-coloration. As a consequence, td is considerable influenced by forma-tion of TPCBP [29].

Analysis of Tab. 2 clearly reveals that the discoloration kineticsare rather complex. It was verified that the pH does not lead to a sig-nificant change in kobs. Thus, we can argue that the “direct oxidativeprocess” governs the discoloration process.

The linear behavior in the pseudo-first order profiles supportsthat ozonation of parental dye molecule and TPCBP are both slowchemical processes. Considering that G/V, [cro]o, [OH – ] and [OH9] (seeEq. (5)) are both constants in the present case, one can propose thatchanges in kobs as functions of the ozonation time and the dyenature, can be attributed to modifications suffered in the intrinsickinetics between the oxidant (O3 and/or HO9) and the different chro-

mophore centers present in the parental and non-parental dye mol-ecules (TPCBP).

From a theoretical point of view, the discoloration kinetics can bedivided into three stages. In the initial stage, the overall discolora-tion process is governed by chemical reactions involving the oxi-dant (O3 and/or HO9) and the more reactive chromophore centers ofthe parental molecule.

In the second (transition) stage, the discoloration process is influ-enced by changes suffered in the intrinsic nature of the oxidationprocess as a consequence of the competition between the new chro-mophore centers (TPCBP) formed during in the initial ozonationstage and the remaining chromophore centers present in the paren-tal molecule. Finally, the last stage of discoloration takes place viaoxidation of the chromophore centers of TPCBP.


Table 2. Dependence of kobs on pH for RY 143 and RB 264. IDC = 100mg/dm3.

pH 2 l 0.05 pH 7 l 0.05 pH 12 l 0.05

DYES kobs-1/min – 1

kobs-2/min – 1

kobs-1/min – 1

kobs-2/min – 1

kobs-1/min – 1

kobs-2/min – 1

RB 264 0.260 0.478 0.210 0.380 0.284 0.581RY 143 0.371 0.076 0.360 0.070 0.202 0.099

Figure 5. Kinetic profiles representative of discoloration via ozonation.(A) RY 143, pH 2; (B) RB 264, pH 2. IDC = 100 mg/dm3. T = 248C.


From the above discussion, changes in kobs as functions of the ozo-nation time can be attributed to modifications in the [dye]/[TPCBP]ratio, which results in a considerable modification in the intrinsickinetics between the oxidant (O3 and/or HO9) and the chromophorecenters.

A comparison of kobs-1 and kobs-2 presented in Tab. 2 reveals that forboth acidic and neutral conditions, that the primary oxidation ofRB 264 comprises a slower reaction between the oxidant and theparent dye molecule, since discoloration of TPCBP comprises thefaster process. These findings indicate that the chemical reactionbetween oxidant and TPCBP is the easy one, thus revealing adecrease in the recalcitrant nature of the particular oxidation reac-tion leading to discoloration during ozonation. This unusual behav-ior reflects the complex influence of intrinsic properties of the chro-mophore on the discoloration kinetics. Unfortunately, a deeper the-oretical analysis concerning these findings is not possible becausethe molecular dye structures are not available in the present case.

On the contrary, analysis of the kobs as function of pH for RY 143clearly reveals kobs-2 a kobs-1 in all cases, thus permitting the assump-tion that oxidation of the related TPCBP comprises the slower oxida-tive process leading to discoloration. These results reveal the com-mon trend where the color removal of TPCBP comprises the moredifficult process (the chromophore centers present in TPCBP aremore recalcitrant when compared to the ones present in the paren-tal dye molecule).

3.3 Influence of the Solution pH on Degradation of theTextile Dyes

Degradation kinetics of RY 143 and RB 264 using electrochemicallygenerated ozone was carried out measuring the reduction of totalorganic carbon (TOC). Measurements of the chemical oxygendemand (COD) were carried out in order to provide informationabout the susceptibility of the organic matter by chemical oxidation[12].

Figures 6 and 7 show the influence of the solution pH on TOC andCOD removal as functions of the ozonation time for RY 143 and RB264, respectively.

Analysis of the results reveals COD removal is more pronouncedthan is TOC reduction. An important feature is that reductions ofboth COD and TOC are more pronounced in the basic medium,where the indirect oxidative processes via hydroxyl radical takesplace. Also, in the case of the acid medium, one can observe that,after a given threshold t-value, changes in COD and TOC are negli-gible.

In both the acidic and alkaline medium, the TOC and COD valueslevels off for t A td (see Tab. 1). This behavior reveals that the indirectoxidative pathway, operative in alkaline media, governs the organicmatter removal (degree of mineralization) for longer ozonationtimes.

A TOC reduction of 72 and 64%, for pH 12, and of 15 and 38%, forpH 2, respectively, were obtained for RY 143 and RB 264 after 1 h ofozonation. Analysis of Fig. 7 reveals a COD reduction of 81 and 78%,for pH 12, and of 35 and 50%, for pH 2, respectively, for the RY 143and RB 264 after 1 h of ozonation.

Rationalization of these findings permits correlation the superiorperformance for dye degradation in the alkaline medium with thehigher oxidizing potential presented by the hydroxyl radical (indi-rect oxidative pathway), which is formed by ozone decompositionin the alkaline medium.

The feasibility of oxidation of organic compounds can be eval-uated by means of the c COD/TOC ratio [12]. Table 3 shows the c-val-ues the RY 143 and RB 264, shown as functions of the solution pHbefore and after treatment (1 h of ozonation).

Analysis of Tab. 3 reveals that c decreases with increasing ozona-tion time, especially in alkaline conditions, thus revealing that ozo-nation leads to an increase in the oxidation feasibility of the organicmatter (ozonation by-products). These findings using the simulatedtextile effluent as a recalcitrant effluent model support ozone appli-cations as an important alternative pre-treatment step for inte-grated processes for wastewater treatment combining chemical oxi-dation and biological processes.

3.4 Degradation of Endocrine Disruptors

Figure 8 shows the dependence of total ED concentration on ozona-tion time. The quantitative HPLC analysis of the ozonated samples(V = 1000 cm3), comprising the summation of the several different


Figures 6. Dependence of TOC removal on ozonation time and pH forRY 143 (TOC0 = 98 mg/dm3) and RB 264 (TOC0 = 92 mg/dm3).

Table 3. Dependence of c on pH for RY 143 and RB 264. t = 1 h. IDC =100 mg/dm3.

DYESa) pH 2 l 0.05 pH 7 l 0.05 pH 12 l 0.05

RB 264 2.4 1.8 1.3RY 143 1.8 1.5 1.2

a) c0 (RB 264) = 2.8 and c0 (RB 143) = 2.6


peak areas, reveals that ozone application leads to a total degrada-tion of the ED mixture after l3 min of ozonation. These results arein agreement with literature reports [27], and confirm that ozona-tion is an efficient process for treatment of waters containing ED.

Analysis of Fig. 8 reveals ozonation leads to a very fast degrada-tion of the ED mixture and, therefore, the experimental findings donot permit the performance of a complete kinetic treatment. Thetreatment of the experimental findings obtained at short ozonation

times (t f 3 min) furnished an average degradation rate of 30 lgdm – 3min – 1.

4 Concluding Remarks

The inherent environmentally friendly aspects presented by theelectrochemical ozone production process were discussed. Also thepossibility was suggested of using the hydrogen generated electro-chemically during ozone generation in a hybrid electrochemicalsystem comprising a fuel cell coupled to the ozone reactor. Theimplementation of this hybrid electrochemical system will result ina significant development in the field since the reduction in theenergy demand for the ozone production is obtained using a cleanenergy source.

Electrochemically generated ozone was applied for discoloration/degradation of textile dyes and degradation of a mixture of endo-crine disruptors. These studies revealed that ozonation promotes atotal discoloration of dye solutions and an increase in the oxidationfeasibility of the organic matter (dye by-products). The ozonation ofsolutions containing a mixture of different endocrine disruptorsshowed these compounds are totally removed after only 3 min ofozonation.

Acknowledgements

The authors wish to thank the Brazilian Foundations FAPEMIG(process No EDT-410/07), FAPESP (process No 04/01796-4R) and CAPES.We also thank Prof. Carol Collins for her technical assistance withthe English.

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