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Characterisation of a laboratory electrochemical ozonation system and its application in advanced oxidation processes

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An electrochemical reactor for oxygen/ozone production was developed using perforated planar electrodes. An electroformed $eta$ -PbO2 coating, deposited on a platinised titanium substrate, was employed as anode while the cathode was a platinised
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  Characterisation of a laboratory electrochemical ozonation system and its applicationin advanced oxidation processes LEONARDO M. DA SILVA 1 , DE ´ BORA V. FRANCO 2 , JULIANE C. FORTI 3 , WILSON F. JARDIM 1 and JULIEN F.C. BOODTS 2, * 1 Instituto de Quı´ mica, Universidade Estadual de Campinas, Cidade Universita´ ria Zeferino Vaz, 13083-970, Campinas,SP, Brazil  2 Instituto de Quı´ mica, Universidade Federal de Uberla ˆ ndia, Campus Santa Mo ˆ nica, Av. Joa ˜ o Naves de A´ vila 2160,38400-902, Uberla ˆ ndia, MG, Brazil  3 Departamento de Quı´ mica, Faculdade de Filosofia Cie ˆ ncias e Letras de Ribeira ˜ o Preto, Universidade de Sa ˜ o Paulo,Av. Bandeirantes 3900, 14040-901, Ribeira ˜ o Preto, SP, Brazil (*author for correspondence, tel.:+55-34-32394143, fax:+55-34-32394208, E-mail: jfcboodts@ufu.br) Received 4 April 2005; accepted in revised form 2 September 2005 Key words:  advanced oxidation processes, degradation, Electrochemistry, ozone, reactive dyes Abstract An electrochemical reactor for oxygen/ozone production was developed using perforated planar electrodes. Anelectroformed  b -PbO 2  coating, deposited on a platinised titanium substrate, was employed as anode while thecathode was a platinised titanium substrate. The electrodes were pressed against a solid polymer electrolyte tominimise ohmic drop and avoid mixing of the gaseous products (H 2  and O 2 /O 3 ). Electrochemical ozone production(EOP) was investigated as function of current density, temperature and electrolyte composition. Electrochemicalcharacterisation demonstrated ozone current efficiency, F EOP , ozone production rate (g h ) 1 ),  m EOP , and grams of O 3 per total energy demand (g h ) 1 W ) 1 ),  m EOP  increase on decreasing electrolyte temperature and increasing currentdensity. The best reactor performance for EOP was obtained with the base electrolyte (H 2 SO 4  3.0 mol dm ) 3 )containing 0.03 mol dm ) 3 KPF 6 . Degradation of reactive dyes used in the textile industry (Reactive Yellow 143 andReactive Blue 264) with electrochemically-generated ozone was investigated in alkaline medium as function of ozone load (mg h ) 1 ) and ozonation time. This investigation revealed ozonation presents very good efficiency forboth solution decolouration and total organic carbon (TOC) removal. 1. Introduction Oxidation of organic pollutants (e.g. dyes of the textileindustry, pesticides, etc.) can be carried out using appro-priate anodically formed oxidants, as in the case of ozone[1–5]. The so-called Advanced Oxidation Processes(AOP) allows the optimisation of ozone application byincreasing the concentration of hydroxyl radicals(HO Æ ) E  0 =2.80 V) resulting from O 3 -decomposition inaqueous solutions, permitting a significant increase indecomposition rate for recalcitrant pollutants [1, 3, 5–7].Different ozonation systems based on electrochemicaltechnology have been described [6, 8–11]. Electrochem-ical reactors for ozone production can be constructedmaking use of a solid polymer electrolyte and high-porosity 3D-electrodes [6, 8–10]. In this configurationozone is released directly in the electrolyte-free waterstream. Such technology gives a high O 3 -concentrationin the gaseous phase (  14 wt. %), and presents high gasdispersion into the aqueous phase. Ozone can also begenerated in another reactor configuration using hollowcylindrical fluorocarbon-impregnated carbon anodesand solid polymer electrolyte technology [1]. Inherentadvantages of this ozonation system are its high currentefficiencies (  35%), lower cell voltage and eliminationof hydrogen management through the use of aircathodes [6, 12]. The disadvantage of this configurationis that decomposition of organics is limited by the ozonemass transfer rate from the gas to the liquid phase,where it is needed for reaction with organics.Ozonation systems based on electrochemical technol-ogy are a promising alternative to the conventionalozonation systems (corona process) especially in the casewhere high O 3 -concentrations are required (e.g. decom-position of resistant organic pollutants such as dyes andpesticides). Journal of Applied Electrochemistry (2006) 36:523–530   Springer 2006DOI 10.1007/s10800-005-9067-x  This paper reports the characterisation of an electro-chemical ozonation system based on the use of perfo-rated planar electrodes and its application to thedegradation of some reactive dyes used in the textileindustry with electrochemically generated ozone. 2. Experimental details 2.1.  Electrochemical reactor The reactor consisted of symmetric perforated planarelectrodes(10  8  0.15 cm)pressedagainstasolidpolymerelectrolyte,SPE,(Nafion  117).Thegeometricareaoftheperforatedelectrodeswas137 cm 2 ,whiletheelectrodeareain intimate contact with the solution was 80 cm 2 . Elec-trodes were perforated to avoid blockage of the electricfield and permit proton transport between cathode andanode. The  b -PbO 2  electrode was prepared by electrode-position at constant current from acid Pb(NO 3 ) 2  solutiononto both faces of a steel micro-sphere blasted perforatedTi-support, previously etched for 10 min in boiling oxalicacid (10% w/w) and then platinised. Pt electrodepositionwas carried out at constant current density (30 mA cm ) 2 )for 20 min from a solution containing 10 g dm ) 3 H 2 PtCl 6 +10 mg dm ) 3 Pb(CH 3 CO 2 ) 2  Æ 3H 2 O, at 24   C. b -PbO 2  was electrodeposited at constant current densityof 30 mA cm ) 2 for 1 h onto both sides of the platinisedTi-support from a solution containing 0.01 mol dm ) 3 HNO 3 +0.2 mol dm ) 3 Pb(NO 3 ) 2 , at 60   C. The averagethickness of the  b -PbO 2  layer, estimated by weighing, wasapproximately 40  l m. Fluka ‘‘purum’’ products wereused throughout.The cathode consisted of a perforated platinisedtitanium substrate prepared under identical conditionsas described above using an electrodeposition time of 1 h. Nafion   117 SPE was pre-treated in boiling nitricacid (50%) for 30 min and hydrated in boiling deionisedwater for 2 h [13].Figure 1 shows a scheme of the reactor while Figure 2presents the experimental set-up used in the investiga-tion. The reactor compartments were manufacturedfrom 1-inch thick acryl plates, while Viton   tubes wereemployed to circulate the electrolyte between the anodicreactor compartment and the all-glass electrolyte reser-voir/gas separator flask (5 dm 3 ). The anodically formedgases (O 2 +O 3 ) were separated from the electrolyte inthe gas separator flask and transported to the spectro-photometer using N 2  as carrier gas. In the dye degra-dation investigation the O 2 /O 3  mixture was passeddirectly into the Reactor Flask containing the reactivedye.2.2.  Equipment and techniques Ozone concentration in the gaseous phase was analysedby UV absorption measurements at 254 nm, using ahomemade gas flow cell. Absorbance was read after15 min of cell polarisation when steady state conditionswere observed. EOP partial current,  j  EOP , and EOPcurrent efficiency,  F EOP , were calculated using theequations [2, 14]:  j  EOP  ¼ ð AV  o zF  Þ = ð e l  Þ ð 1 Þ Fig. 1.  Scheme of the electrochemical reactor for ozone production. (a) cathodic compartment; (b) rubber gasket (Viton  ); (c) cathode,Ti 0 /Pt 0 ; (d) SPE, Nafion   117; (e) anode, Ti/Pt/  b -PbO 2 ; (f) anodic compartment. 524  U EOP ð % Þ ¼ ½ð AV  o zF  Þ = ð e lI  T  Þ 100  ð 2 Þ where:  A =absorbance at 254 nm;  V   o =volumetric flowrate of (N 2 +O 2 +O 3 ) (dm 3 s ) 1 );  z =number of electrons( z =6);  e =ozone absorptivity at 254 nm (3024cm ) 1 mol ) 1 dm 3 [15  l  =optical path length (0.63 cm); I  T =total current (OER+EOP) (ampe `re);  F  =Faraday’sconstant (96485 C mol ) 1 ). The EOP specific powerconsumption,  P  0EOP , was calculated using the equation[16]: P 0EOP ð Wh g  1 Þ ¼ ð UzF  Þ = ð 1 : 73    10 5 U EOP Þ ð 3 Þ where  U   is the cell voltage.The reactor performance for the EOP was investi-gated as functions of the operating parameters analysingthe ‘‘ozone production rate’’,  m EOP , and the parameter‘‘gain of ozone mass per total power consumption’’, # EOP , which were calculated according to Equations 4and 5, respectively [16]: m EOP ð g h  1 Þ ¼  3600 ð  j  EOP M  Þ = ð zF  Þ ð 4 Þ # EOP ð g W  1 h  1 Þ ¼  m EOP = I  T U   ð 5 Þ where  M   is the ozone molecular weight (48 g mol ) 1 ).The electrochemical reactor was powered by a 80 A/12 V d.c. current source. The electrolytes used consistedof sulphuric acid solutions, in the presence or absence of fluor-compounds. In all cases the electrolyte was circu-lated in the anodic compartment using a model 7018-21MASTERFLEX peristaltic circulation pump (Cole-Parmer). The linear velocity of the electrolyte was1.30 cm s ) 1 and the space velocity 5.19 min ) 1 . The flowregime was turbulent (Re>3000). Temperature controlwas achieved by means of a model FC55A01 FTScooling system connected to the all-glass electrolytereservoir/gas separator flask. The electrolyte tempera-ture at the anode surface was monitored using a model61 FLUKE digital thermocouple. Fig. 2.  Experimental set up employed for ozone generation and investigation of decolouration/degradation of textile dyes. 525  2.3.  Ozonation of aqueous textile dye solutions The decolouration/degradation efficiency of the ozona-tion process was evaluated using as model recalcitrantcompounds reactive dyes employed in the textile indus-try. The decolouration/degradation of C.I. ReactiveYellow 143 (RY 143) and C.I. Reactive Blue 264 (RB264) dyes were carried out in alkaline solutions(pH=10) containing 150 ppm of these compounds.The samples were prepared by dissolving the commer-cial dyes, furnished by CERMATEX Textile IndustryLtd. (Americana, Brazil), in distilled water.Decolouration of 40 ml samples of reactive dyes wasfollowed spectrophotometrically measuring the absor-bance at their maximum wavelength of absorbance (421and 619 nm for RY 143 and RB 264, respectively) asfunctions of the ozonation time. A model D4000 HACHSpectrophotometer was used throughout. Removal of the total organic carbon (TOC) was investigated bymeasuring the TOC-decay as a function of ozonationtime. TOC measurements were carried out using amodel 2000 Shimadzu TOC Analyser.Gas dispersion in the reactor flask was done passingthe gas mixture (O 2 +O 3 ) through a gas diffuser (Shott#2 coarse glass frit) placed at the bottom of the ozonereactor flask. Non-reacted ozone at the flask outlet wasanalysed spectrophotometrically at 254 nm as afunction of ozonation time. Figure 2 shows the set upused for ozone generation and degradation of thetextile dyes. 3. Results and discussion 3.1.  Characterisation of the electrochemical ozonationsystem Previous studies [2, 14] showed galvanostatic polarisa-tion of   b -PbO 2  electrodes produces a transient behav-iour of the electrode potential and EOP-currentefficiency. Therefore, the galvanostatic polarizationexperiments were carried out by recording the cellvoltage,  U  , and measuring ozone absorbance after15 min of polarisation when a steady response wasobserved. Figure 3 shows the dependence of   U   and F EOP  on  I  T , for different temperatures.Figure 3a shows that  U  -values are little affected bytemperature for  I  T  £  30 A; however, for higher  I  T -valuesa reduction in temperature is accompanied by anincrease in the cell overvoltage. Figure 3b and c showthe dependence of   F EOP  on  I  T  and temperature for the1.0 and 3.0 mol dm ) 3 H 2 SO 4  electrolytes, respectively. F EOP  increases on increasing  I  T  and decreasing temper-ature, giving values in the   0.5–4% current efficiencyinterval. In agreement with the fundamental studiespresented by Foller and Tobias [17],  F EOP -valuesincrease with increasing sulphuric acid concentration.This behaviour differs from the set-up using high-porosity 3D-electrodes (lead dioxide supported onporous titanium substrate), where a maximum in  F EOP is reached at moderate temperatures (  30   C) [8, 9].Due to the rather different surface current distributionand bubble adherence at the electrode surface, compar-ison of the behaviour of perforated planar electrodesand high-porosity 3D-electrodes is very difficult [8, 9,18, 19]. While with planar electrodes in contact withconventional electrolytes all regions of the electrodesurface are, in principle active, in the case of high-porosity 3D-electrodes making use of electrolyte-freewater the active surface area is restricted to the electroderegions in intimate contact with the solid polymerelectrolyte [18, 19]. Figure 4 shows the dependency of the EOP specificpower consumption,  P  0EOP , ozone production rate,  m EOP , Fig. 3.  Dependence of cell voltage,  U  , and EOP current efficiency, F EOP , on total current,  I  T , and temperature. Electrolyte: (a) and (b)1.0 mol dm ) 3 H 2 SO 4 ; (c) 3.0 mol dm ) 3 H 2 SO 4 . 526  and the gain of ozone mass per total power consump-tion,  # EOP , on  I  T  and temperature for the 3.0 mol dm ) 3 H 2 SO 4  electrolyte.  P  0EOP -values decrease with increasing  I  T  and decreas-ing temperature, giving values in the 1.5–0.4 kW h g ) 1 interval (Figure 4a). Comparison with the literature[8–10] shows these values are higher for perforatedplanar electrodes. Figure 4b and c show that both  m EOP and  # EOP  are affected by temperature and currentdensity. Maximum ozone production occurs at highcurrent densities, where minimum power consumption isobserved. Figure 4c also shows that the best EOPperformance (maximum  # EOP -value) is reached at high I  T -values and low temperatures.Several studies [2, 5, 12, 17] have shown that theintroduction of additives (e.g. NaF, HBF 4 , KPF 6 ) to thebase electrolyte (e.g. H 2 SO 4 ) significantly increases EOPcurrent efficiency. Foller and Kelsall [12] reported EOPcurrent efficiencies of up to 45% with an electrochemicalreactor using tubular glassy carbon as anode andconcentrated fluorboric acid (62 wt. %) as electrolyte.Fundamental aspects of the influence of the electrolyteand electrode material on the EOP process werediscussed previously [2, 14, 16].Figure 5 shows the influence, for several tempera-tures, of the introduction of KPF 6  to the electrolyte(3.0 mol dm ) 3 H 2 SO 4 ) on the current efficiency andspecific power consumption for the EOP process.Comparing Figures 3 and 5 it is observed thatintroduction of KPF 6  considerably increases  F EOP . Asdiscussed by Foller and Kelsall [12], EOP currentefficiencies higher than 20%, obtained at high currentdensities, imply a specific generation rate per unitelectrode area of up to three times that of coronadischarge technology. Figure 5b shows that the EOPenergy demand decreases at high  I  T -values and lowtemperature, reaching a minimum of 60 Wh g ) 1 at 0   Cfor  I  T  >40 A. This result is identical to that reported forhigh-porosity 3D-electrodes [8–10].Ozone production rate,  m EOP , and gain of ozone massper total power consumption,  # EOP , calculated for theelectrolyte containing 0.03 mol dm ) 3 KPF 6  as functionsof   I  T  and temperature are presented in Figure 6. Fig. 5.  Dependence of EOP current efficiency,  F EOP , and EOP spe-cific power consumption,  P 0EOP , on total applied current,  I  T , andelectrolyte temperature. Electrolyte: 3.0 mol dm ) 3 H 2 SO 4 +0.03 moldm ) 3 KPF 6 . Fig. 4.  Dependence of EOP specific power consumption,  P 0EOP , ozoneproduction rate,  m EOP , and gain of ozone mass per total power con-sumption,  # EOP , on total applied current,  I  T , and electrolyte tempera-ture. Electrolyte: 3.0 mol dm ) 3 H 2 SO 4 . 527
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