Electrochemistry for waste treatment• Positives:

S potentially “green” – “only uses electrons”; does not“add a chemical to chase away another chemical”

S low cost of electricity (¢ per mole)S most often applicable to oxidationS treatment of wastes that are toxic or recalcitrant in

biological processesS well suited to aqueous wastesNegatives:S must have a supporting electrolyteS must avoid electrode foulingS need for long-lived and cheap electrodesS choice of reaction at counter electrodeS current efficiency declines as concentration fallsS must be engineered for high throughput

• Metal ion removal: variant of electrowinning only (a)from aqueous solution (b) at low concentrations. Applications:S primary leaching of low grade ores, or of tailings pile

run-offS spent solutions from electroplating, catalysts, etching

solutions, photographic fixing, batteriesS cathode is generally the same metal as is being

recovered; periodically removedS high current density –> H2 evolution

Electrogenerated oxidants• hydrogen peroxide, ozone, hypochlorite

S peroxide: difficulties of obtaining more than mMconcentrations in a single pass. Recent developmentsinclude trickle bed reactors

Anode: 2OH! —> ½O2 + H2O + 2eCathode: O2 + H2O + 2e —> HO2

! + OH!

Problems: reverse of cathode reaction at the anode; furtherreduction of HO2

! to OH!; catalytic decomposition ofperoxide by transition metal ions, especially Fe2+; requirementfor strongly alkaline solutions

S ozone: oxidation of water directly to O3 (but problemof oxidation to O2 instead)

3H2O —> O3 + 6H+ + 6e E/ = 1.51 V2H2O —> O2 + 4H+ + 4e E/ = 1.23 V

Essential to use anodes with high overvoltage for O2

Anodes include PbO2 and solid polymer electrolytes

Issue of explosive nature of O3(g), whose solubility inwater is not very high

S hypochlorite: Oxidation of brine in a non-separatedcell. Solution has disinfectant properties

Cathode: 2H2O + 2e —> H2 + 2OH!

Anode: 2Cl! —> Cl2 + 2eCl2 + 2OH! —> H2O + ClO! + Cl!

Net: Cl! + H2O —> ClO! + H2

Main applications are water purification for drinking andswimming e.g. on ships, for food processing; also forsterilization of treated sewage (superchlorination) at remotesites; disadvantage of organochlorine byproducts whenorganics are oxidized

Electrolytic methods for degradation of organics• most methods are oxidative• “electrochemical combustion” means complete

conversion to CO2 and H2O• electrochemical combustion usually undesirable (energy

intensive); more often the objective is partial oxidation,and the treated product is then further degradedbiologically

• probably the most serious problem is anode fouling bypolymerized/oligomerized byproducts.

Example: phenols: serious drinking water problem becausechlorophenols have very strong taste and odour

Substituted phenol Odour threshold, ppbNone > 10002– 24– 2502,4– 22,6– 32,4,6– > 1000

Simplest rationalization for oligomerization is:C6H5OH —> C6H5O0 + H+ + eC6H5O0 + C6H5OH —> dimer formation (still a

radical) via C or O attack (aromatic substitution)

More likely, the radical chemistry is initiated by attack of ahydroxyl radical (see below) on phenol

Steps in the development of an electrochemicalremediation technology

Basic science• voltammetry: at what potential does the substance oxidize

(or reduce)? Direct oxidation possible, or electrocatalyticor mediated methods?

• does the waste stream contain ionic components, or musta supporting electrolyte be added?

• initial choice of electrode• small scale electrolysis: what are the products and

chemical yields? What is the mass balance?• what is the current efficiency, and how does it vary with

contaminant concentration?

Engineering aspects• should batch, flow, recirculating batch cells ... be used?• divided or (much preferred) undivided cell?• can the process meet regulatory effluent standards?• potentiostatic or (much preferred) amperostatic

operation?• cell design and electrode design

Oxidation: Anodes

• Metal anodes favour direct electron transfer at theelectrode

P —> P0+ + e

Disadvantage: many organic pollutants have very highoxidation potentials, and so water is oxidized in preference tothe contaminant of interest

• Work of Comninellis on different mechanisms ofoxidation for metal oxide type anodes. These electrodesare essentially electrocatalytic, in that they do not requirethe loss of an electron from the substrate

Type 1: form OH radicals: PbO2 and Ti/SnO2 type DSAsMOx + H2O —> MOx(OHads) + H+ + eMOx(OHads) + P —> MOx + “P-OH”

P is oxidized by adsorbed or free (dissociated) OH,which is a very powerful oxidant

Type 2: form higher metal oxides: Ti/IrO2 type DSAsMOx + H2O —> MOx(OHads) + H+ + eMOx(OHads) —> MOx+1 MOx+1 + P —> MOx + “PO”

the higher oxide is the oxidizing agent

The “Zappi cell” (Zappi, Electrochemistry Forum, 2002)

Applications foreseen:

• food industry• textiles and leather• landfill leachates• chemical industry wastes

Boron-doped diamond (BDD) electrodes (Comninellis,Electrochemistry Forum, 2002)

Prepared by vapour deposition, e.g., 1% CH4 + 3 ppm Me3Bin H2 onto a substrate such as Ti. Allow very high oxidationpotentials to be used, but to date the films are (a) expensive toprepare (b) unacceptably fragile

Applications foreseen for BDD are waste treatment and theelectrosynthesis of compounds that require high oxidationpotentials

Comninellis, 2002

BDD electrode for oxidation of 4-chlorophenolRodrigo et al., J. Electrochem. Soc, 148, D60-D64 (2001)

• objective was electrochemical combustion

• electrode fouling (deposition) avoided by excursions tohighest anodic polarization possible

• oxidation involves both direct oxidation of substrate tophenoxy radicals and benzoquinone and also attack byOH radicals

Mediated electrolysis: Example, Ag(II)

The “mediator” is the substance oxidized (or reduced), andthis is the only electrochemistry in the system; the oxidized(reduced) form of the mediator then reacts chemically with thecontaminant

Ag+ —> Ag2+ + eAg2+ + H2O —> Ag+ + H+ + OH (radical)OH + P —> oxidation of P

Recall that characteristic OH chemistry is abstraction ofhydrogen or addition to multiple bonds

Example of destruction of aromatics by mediated Ag+

oxidation. Solution should not contain Cl!, else AgCl willprecipitate. Ag+ is needed only in catalytic amounts.

Other oxidations systems: Ce(III)/Ce(IV)

Reduction systems: Co(III)/Co(II); Cr(III)/Cr(II)

• the objective is to produce a higher (lower) oxidationstate that is close to the most reactive that can be formedin aqueous solution

Electrocoagulation technologies

• Use of sacrificial metal anodes (usually iron)

• Combination of electrochemistry and precipitation. Usually this involves co-precipitation of Fe(OH)3 or therelated phase hydrated FeOOH along with thecontaminant

• In combination with H2O2, OH radicals are formed:electro-Fenton reactionFenton reaction: Fe2+ + H2O2 + H+ —> Fe3+ + H2O + OH

Application of electrocoagulation to the removal of arsenic (J.Wang, unpublished)

• As is present as As(III) and As(V). As(V) coprecipitatesreadily with Fe(OH)3 at pH 5–8

• oxidation at Fe anode has several functions:• oxidation of As(III) to As(V)• oxidation of Fe to Fe2+

• oxidation of water to (e.g.) H2O2 and OH, whichoxidize As(III) and also Fe2+ to Fe3+

• at the pH of drinking water Fe(OH)3 precipitatesalong with As(V) / H2AsO4

!

• WHO limit of 10 ppb for As is achievable