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Electrode Material for the Electrochemical Oxidation of Organic Pollutants for Wastewater Treatment Christos Comninellis Swiss Federal Institute of Technology GGEC-ISIC-SB-EPFL 1015- Lausanne, Switzerland Slide 2 Laboratory scale equipment used for electrochemical oxidation of organics for wastewater treatment One compartment cell in batch operation Slide 3 Bench scale equipment used for electrochemical oxidation of organics for wastewater treatment One compartment cell in continuous or batch operation Slide 4 Pilot-plant equipment used for electrochemical oxidation of organics for wastewater treatment One compartment cell in continuous or batch operation Slide 5 Involved reactions in the electrochemical treatment Oxidation: R + H 2 O RO + 2H + + 2e - (main reaction) H 2 O 1/2O 2 + 2H+ + 2e- (side reaction) Reduction: 2H + + 2e - H 2 Global reaction: R + H 2 O RO + H 2 (main reaction) H 2 O 1/2O 2 + H 2 (side reaction) Anode Cathode (wastewater) Slide 6 Definition of global parameters Operation at constant current I (A) t (s) V(volt) t (s) V av ICE (-) t (s) ACE Average rate of COD elimination (mol O 2 /s) : Electrochemical oxygen demand (mol O 2 /l) : Electrochemical COD conversion (%) : Instantaneous rate of COD elimination (mol O 2 /s) : Slide 7 Proposed model for the anodic oxidation of organics in acid media i)Water discharge to hydroxyl radicals ii) Oxidation of organics R with electro-generated OH radicals (main reaction) iii) Oxygen evolution (side reaction) : represents an active site on the anode surface Slide 8 (main reaction) (side reaction) Instantaneous current efficiency Definition of instantaneous current efficiency (ICE) and average current efficiency (ACE) in the electrochemical treatment process Average current efficiency : electrolysis time Slide 9 (side reaction) (main reaction) Measurement of the instantaneous current efficiency (ICE) Two main techniques have been used for ICE measurements: F: 96485 C mol -1 V: electrolyte volume (m 3 ) I : applied current (A) COD: Chemical oxygen demand (mol O 2 m -3 ) t: time interval : Calculated O 2 flow rate from Faradays low (m 3 O 2 /s) : measured O 2 flow rate during electrolysis (m 3 O 2 /s) I) COD method II) O 2 flow rate method Slide 10 Influence of anode material on ICE (side reaction) (main reaction) Relation between M-OH adsorption enthalpy and a)Chemical reactivity of OH radical (main reaction) b)Electrochemical reactivity of OH radical (side reaction) Low M-OH adsorption enthalpy (physisorption of OH radical on M) results in an increase of the chemical reactivity of OH radicals high Oxidation power anodes (favor main reaction) High M-OH adsorption enthalpy (chemisorption of OH radical on M) results in an increase of the electrochemical reactivity of OH radicals low Oxidation power anodes (favor side reaction) Slide 11 Active component ElectorodeOxidation potential (V) Adsorption Enthalpy of M-OH Oxidation power of the anode RuO 2 RuO 2 -TiO 2 (DSA-Cl 2 ) 1.4-1.7Chemisorption of OH radical IrO 2 IrO 2 -Ta 2 O 5 (DSA-O 2 ) 1.5-1.8 PtTi/Pt1.7-1.9 PbO 2 Ti/PbO 2 1.8-2.0 SnO 2 Ti/SnO 2 -Sb 2 O 5 1.9-2.2 BDDp-Si/BDD 2.2-2.6Physisorption of OH radical Oxidation Power of anode material Slide 12 Investigation of the oxygen evolution reaction (side reaction) eH)OH(IrOOH 222 eH )OH(IrO 32 223 O 2 1 eH)OH(BDDOH 2 1 )OH(BDD 2 O 2 eH IrO 2 : low Oxidation power anode (electrocatalytic) BDD : high Oxidation power anode (non-electrocatalytic) Slide 13 Investigation of the oxidation reaction (main reaction) Oxalic acide oxidation on BDD and IrO 2 EE EE IrO 2 low Oxidation power anode BDD high Oxidation power anode 0 1000 2000 3000 4000 5000 04812 specific charge [Ah L ] IrO 2 BDD oxalic acid conc. [mol L -1 ] Slide 14 log ACE x NH 2 x OH x COOH x SO 3 H x NO 2 Influence of anode material on the ACE Oxidation of benzene derivatives under conditions were there is no mass transport limitation. i) low oxidation power anodes (Pt) ACE decrease with increasing the Hamet constant of X substituent. log ACE = -2 - 1.3 This indicates that the reaction is electrophilic in nature* ii) high oxidation power anodes (BDD) ACE is practically independent of the Hamet constant of X substituent. log ACE x NH 2 OH x COOH SO 3 H x ACE = 1 *GWA 11,792-797 (1992) X X Slide 15 DMPO 10 -3 g.l -1, i = 10 -4 A.cm -2. Detection of HO. radicals formed by water discharge on BDD anode H 2 O HO. + H + + e - Fenton BDD : 1h BDD : 2h Electron spin resonance (ESR) spectra in the presence of 5,5- dimethyl 1-pyrroline-1-oxide (DMPO) spin-trap Slide 16 Detection of HO. radicals formed by water discharge on BDD anode H 2 O HO. + H + + e - Slide 17 Anodic production of H 2 O 2 on BDD at different current densities: ( )230 A cm -2, ( )470 A cm -2, ( )950 A cm -2 and (x)1600 A cm -2 during electrolysis of 1M HClO 4 on BDD electrode T=25C. Anodic production of H 2 O 2 on BDD Slide 18 Competition Reaction of Hydroxyl Radicals with Carboxylic Acids xidation of Oxalic and Formic Acids k OH = 1.3 10 8 L mol -1 s -1 k OH = 1.4 10 6 L mol -1 s -1 HCOOH(COOH) 2 HClO 4 C formic = C oxalic = 0.5 M Electrolyte HClO 4 1M j = 238 A m -2 Formic acid Oxalic acid Slide 19 (a) water discharge to hydroxyl radicals, (b) oxygen evolution by electrochemical oxidation of hydroxyl radicals, (c) formation of the higher metal oxide at low oxydation power anodes, (d) oxygen evolution by chemical decomposition of the higher metal oxide (e) oxidation of the organic compound, R, via hydroxyl radicals at high oxydation power anodes, ; (f) oxidation of the organic compound via the higher metal oxide at low oxydation power anodes,. Proposed model for the anodic oxidation of organics in acid media Slide 20 Modeling of organics oxidation on high oxidation power anodes (BDD) Batch reactor Operation at constant current Estimation of the instantaneous current efficiency ICE a) i appl. < i lim ICE = 100% b) i appl. > i lim ICE < 100% Slide 21 COD(t) ICE (%) COD 0 100 t or Q dm -3 0.00 Slide 22 Comparison of model and experimental data values ( naphthol) Slide 23 Oxidation of 4-chlorophenol (4-CP) on BDD anode Influence of 4-CP concentration Conc. A B A B B : complete combustion A : Partial oxidation Slide 24 Oxidation of 4-chlorophenol (4-CP) on BDD anode Influence of current density 60 mA/cm2 i 15 mA/cm2 A A B B A : Partial oxidation B : complete combustion Slide 25 Oxidation of organics on BDD electrodes Investigated organic compounds Slide 26 Economical considerations Operation at constant current I (A) t (s) V(volt) t (s) V av. ICE (-) t (s) ACE Anodic surface area needed for the elimination of a given amount of COD (kg COD/h) * Specific electrical energy consumption (kWh/kg COD):* i = 1 kA/m 2 P= 1 kg COD/h ACE= 1 *GWA 11,792-797 (1992) Slide 27 i appl. < i lim i appl. > i lim i appl. = i lim (I) (II) I (A) t (h) (I) (II) i appl. = i lim Modulated current operation i appl. > i lim Slide 28 Combined Electrochemical Biological treatment Partial Electrochemical oxidation Biological treatment non-bio toxic bio non-toxic Ah/l 0.5 1.0 End of the Electrochemical treatment Ah/l EC 50-10 min (Microtox) non-biobio toxicnon-toxic ( Zan-Wellens) Slide 29 Partial oxidation or incineration of organics on BDD anode in acid medium Rate of OH production in the RC Rate of R transport in to the RC r R = k m [R] (mol m -2 s -1 ) Anode Solution Reaction cage (O 2,OH,H 2 O 2,Org...) RC : few RC OH R Slide 30 Partial oxidation or incineration of phenol We can define: the parameter the stoichiometry factor for a given reaction as the number of moles of OH (per mol of R) involved in the reaction. Slide 31 1.3-1.6 (28) Partial oxidation of phenol Incineration of phenol Phenol conversion