Electrode Material for the Electrochemical Oxidation of Organic Pollutants for Wastewater Treatment Christos Comninellis Swiss Federal Institute of Technology GGEC-ISIC-SB-EPFL Lausanne, Switzerland

Laboratory scale equipment used for electrochemical oxidation of organics for wastewater treatment One compartment cell in batch operation

Bench scale equipment used for electrochemical oxidation of organics for wastewater treatment One compartment cell in continuous or batch operation

Pilot-plant equipment used for electrochemical oxidation of organics for wastewater treatment One compartment cell in continuous or batch operation

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)

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) :

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

(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

(side reaction) (main reaction) Measurement of the instantaneous current efficiency (ICE) Two main techniques have been used for ICE measurements: F: 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

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)

Active component ElectorodeOxidation potential (V) Adsorption Enthalpy of M-OH « Oxidation power » of the anode RuO 2 RuO 2 -TiO 2 (DSA-Cl 2 ) Chemisorption of OH radical IrO 2 IrO 2 -Ta 2 O 5 (DSA-O 2 ) PtTi/Pt PbO 2 Ti/PbO SnO 2 Ti/SnO 2 -Sb 2 O BDDp-Si/BDD Physisorption of OH radical « Oxidation Power » of anode material

Investigation of the oxygen evolution reaction (side reaction)   eH)OH(IrOOH 222   eH )OH(IrO 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)

Investigation of the oxidation reaction (main reaction) Oxalic acide oxidation on BDD and IrO 2 EE EE IrO 2 « low Oxidation power anode » BDD « high Oxidation power anode » specific charge [Ah L ] IrO 2 BDD oxalic acid conc. [mol L -1 ]

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  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, (1992) X X

DMPO g.l -1, i = 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

Detection of HO. radicals formed by water discharge on BDD anode H 2 O HO. + H + + e -

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=25°C. Anodic production of H 2 O 2 on BDD

Competition Reaction of Hydroxyl Radicals with Carboxylic Acids Òxidation of Oxalic and Formic Acids k OH = L mol -1 s -1 k ’ OH = 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

(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

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%

COD(t) ICE (%) COD t or Q dm

Comparison of model and experimental data values (  naphthol)

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

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

Oxidation of organics on BDD electrodes Investigated organic compounds

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, (1992)

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

Combined Electrochemical – Biological treatment Partial Electrochemical oxidation Biological treatment non-bio toxic bio non-toxic Ah/l End of the Electrochemical treatment Ah/l EC min (Microtox) non-biobio toxicnon-toxic ( Zan-Wellens)

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 Å  RC OH R

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.

 (<4)  60 (>28) Partial oxidation of phenol Incineration of phenol Phenol conversion <20% ICE = 100% ICE < 100%

Mediated electrochemical treatment A red/ox couple (O/R ), present in the electrolyte, are firstly electro-generated by oxidation at the anode then oxidize the organic pollutant in solution in a homogenous chemical reaction. S R O ze - P R (sol) O (sol) + z e - (At the anode) O (sol) + S (sol) R (sol) + P (sol) (In solution ) Redox couple Cl 2 /Cl - H 2 O 2 /H 2 OS 2 O 8 2- /SO 4 2- O 3 /H 2 O E o (V)/NHE Advantage: Avoid problems related with mass transfer limitations

Use both anodic and cathodic reactions* 2 H 2 O 2 OH * + 2H + 2e - H2O2H2O2 2H + + O Anodic reaction Formation of hydroxyl radicals by water oxidation Cathodic reaction Formation of hydrogen peroxide by oxygen reduction Global reaction : 2H 2 O + O 2 H 2 O OH. 2F * Ch.Comninellis et al Electrochimica Acta, vol. 49, num. 25 (2004), p H 2 O 2OH. + 2H + +2e - O 2 + 2H + + 2e - H 2 O 2

Treatment of industrial wastewater Cases studied OriginCOD (mg/l) BOD 5 (mg/l) Anode usedRemarks Ciba-Geigy 12’0001’800 Ti/Pt NaCl : 0.5 g/l BASF* 1’250- P-Si/BDD E sp =22 kWh/kg COD Roche 49’2504’000 Ti/IrO 2 Presence of VOC Laboratory scale (Batch) Lonza 25’8003’000 Ti/IrO 2 NaCl : 50 g/l Pilot plant (1.6 m2) (continuous) *New Diamond and Front. Carbon. Technology. Ch.Comninellis et al. 14(4), (2004)

Remaining problems in the electrochemical treatment Bench scale and pilot plant measurements are needed for each wastewater treatment. For each wastewater the service life of electrode material should be estimated High investment cost of the electrochemical unit Problems inherent to hydrogen production at the cathode Formation of organochlorinated compounds and Cl 2 (g) if Cl - are present in the wastewater Stripping of VOC with the evolved gas Deposition of Ca and Mg carbonates at the cathode when working in neutral or basic media The composition of the wastewater should be constant (only in plant treatment) Industrial electrochemistry is not well know in industry

Conclusions Electrogenerated OH. are the active oxidants involved in the electrochemical oxidation of organic pollutants. The main side reaction in the electrochemical oxidation of organic pollutants is oxygen evolution. Two techniques have been presented for the determination of the instantaneous current efficiency (ICE) during the electrochemical oxidation of organic pollutants. Electrogenerated OH. are chemisorbed on «low oxidation power » anodes (partial oxidation, low ICE) and physisorbed on « high oxidation power » anodes (complete oxidation. high ICE) A model is presented for the oxidation of organics on BDD anode « high oxidation power » anodes Working at modulated current density using « high oxidation power » anodes ( BDD) can allow the suppression of the side reaction of oxygen evolution ( ACE=1) Many technological problems have to solved before the practical application of the technology

2 m 3 of a non-biodegradable organic industrial wastewater with a COD of 10 kg/m 3 and TOC of 5 kg/m 3 is to be pre-treated (before the biological treatment) by anodic oxidation, under galvanostatic conditions (i = 1kA/m2), using a filter press electrochemical reactor with 20 m 2 anode surface area. Considering that : After elimination of 60 % of the COD the waste water becomes biodegradable The average current efficiency, for elimination 60% of the initial COD, is 30% The average cell potential during the electrochemical treatment is 4.5 volts Calculate the treatment time and the specific energy consumption (kWh/kg COD eliminated). Problem 1

Problem 2 A continuous electrochemical plant is to be designed for the treatment of 2 m 3 /h of an organic industrial wastewater with a COD of 2.5 kg/m3. Considering that : The treatment has been carried out at constant current density (i=1 kA/m 2 ) The average current efficiency for the elimination of 80% of the COD is 35%. The average cell potential during the electrochemical treatment was 5 volts Calculate the required anode surface area and the specific energy consumption (kWh/kg COD eliminated).

Problem 3 3 m 3 of a non-biodegradable industrial wastewater with a COD of 12 kg/m 3 and TOC of 5 kg/m 3 has been pre-treated by anodic oxidation, to transform the non- biodegradable organic compounds to biodegradable. This has been achieved after 5 h of electrolysis under galvanostatic conditions (I=15 kA ) resulting in the elimination of 40% of the initial COD. Calculate the average current efficiency and the amount of O 2 formed during the treatment.

Problem 4 10 m 3 of a non-biodegradable industrial wastewater with a COD of 6 kg/m 3 has been pre-treated by anodic oxidation. This treatment has been achieved after 5 h of electrolysis under galvanostatic conditions (I=300 kA) resulting in a TOC of 1kg/m 3. Analysis of the wastewater after the electrochemical treatment shows that the final oxidation product was oxalic acid. Calculate : a) The concentration of oxalic acid and the COD after the electrochemical treatment. b) The average current efficiency c) The anode surface area used if the applied current density was 1kA/m 2.

A chemical plant produce 5 m 3 /h of a wastewater with the following composition: Phenol : 3kg/m 3 Acetone : 4kg/m 3 Na 2 SO 4 : 25 kg/m 3 H 2 SO 4 : 5kg/m 3 Calculate the anode surface area necessary to eliminate 60% of the COD by anodic oxidation, using a constant current density of 2 kA/m 2 and considering an average current efficiency of 50%. Problem 5

A chemical plant produce a wastewater with the following composition: COD : 2.5 kg/m 3 Na 2 SO 4 : 25 kg/m 3 H 2 SO 4 : 5kg/m 3 The treatment has been carried out in an electrochemical reactor characterized by an average mass transfer coefficient k m = 2x10 -5 m/s. Calculate: a)The initial limiting current. b)The initial current efficiency if the electrolysis has been carried out at 238 A/m 2 c)The instantaneous current efficiency (ICE) after elimination 90% of COD using a current density of 238 A/m 2 d)Propose an operation mode for an optimal operation of the treatment. Problem 6