Aluminium Electrochemistry in Electrocoagulation Reactors Martin Mechelhoff Imperial College London Department of Chemical Engineering London SW7 2AZ (UK) 213 th ECS Meeting, Phoenix, 18 th 23th May 2008
Conventional Coagulation raw water coagulant (Al III or Fe III ) + ph adjustment treated water outlet sludge outlet sludge outlet Advantage: well known process Disadvantages: large footprint large amounts of liquid chemicals performance exhausted 2
Electrocoagulation Reactors Water for purification passed through electrochemical reactor Anode usually Fe or Al; cathode often steel Inter-electrode gap: < 5 mm Dissolution, hydrolysis and precipitation of metal ions from electrode Simultaneous evolution of H 2 gas Destabilisation of suspension coagulation Power Supply H 2 O Main advantage: No addition of chemicals Anode Me + H 2 + OH - Cathode 3
Performance and Challenges removal [%] But: 100 90 80 70 60 50 40 30 20 10 0 Comparison Chemical dosing - Electrocoagulation at inlet ph 7.8 DOC chemical dosing colour (UV254) electrocoagulation Jiang, J.Q.; Graham, N. et al. (2002); Water Research, 36, 4064-4078 High cell voltages, thus high specific energy consumption Electrode passivation slow reaction rates 4
Potential-pH diagram for Al-H 2 O system for 10-6 M Al(III) activity at 298 K 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0-0.2-0.4-0.6-0.8-1.0-1.2-1.4-1.6-1.8-2.0-2.2-2.4-2.6 electrode potential E Electode Potential (SHE) / V H + H 2 Al 3+ Al(OH)3 (am) Al 2 O 3 (c) Al 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ph overpotential η Al(OH) 4 - O 2 H 2 O 5
Electrode Reactions Power supply 3 Al Al + + 3e 2 2 3 Al Anode + 2Al + 3H O Al O + 6H + 6e ( Al+ 3H O Al( OH) + 3H + + 3e ) 2 3 + H 2 bubbles _ Inert Cathode Precipitation: 2HO+ 2e H + 2OH 2 2 ( O2 2H2O 4e + + 4OH ) 3 Al + + 3 H2O Al( OH) 3 + 3H + Water flow 6
Voltammetric Behaviour of Al Passivation impedes dissolution log (j/a m -2 ) 1 0.5 0-0.5-1 -1.5-2 -2.5-3 -3.5-4 2HO+ 2e H + 2OH 2 2-2.3-2 -1.7-1.4-1.1-0.8-0.5-0.2 0.1 0.4 electrode potential E(SCE) / V 2Al + 3H O Al O + 6H + 6e 2 2 3 ( Al+ 3H O Al( OH) + 3H + + 3e ) 2 3 + High purity Al RDE mirror finish 2000 rpm 5x10-4 kmol m -3 Na 2 SO 4 1 mv s -1 7
Impedance Spectrum Three overlapping, slightly depressed semi-circles -Z (max) of second semi-circle increases with growing potential 2.5 2.0 0.5 Hz -Z'' / ohm m2 1.5 1.0 0.5 0.0 1 Hz 1 Hz 0 1 2 3 4 Z' [ohm m2] -1.0 V -0.6 V -0.2 V 10 4 ca. 10-2 Hz Stationary Al1050 (99.5%) 5x10-4 M Na 2 SO 4 + 10 mgl -1 humic acid + 1.5x10-4 mm NaCl 8
Effect of Potential on Maximum Z -Z (max) linearly dependent on applied potential Lower values when adding humic acid and NaCl interference with surface properties, ε=? Na 2 SO 4 R 2 = 0.994 1.6 1.4 1.2 -Z'' / ohm m 2 R 2 = 0.9593 R 2 1.0 = 0.9559 0.8 Na 2 SO 4 + humic acid 0.6-1.2-1 -0.8-0.6-0.4-0.2 0 electrode potential (SCE) / V Na 2 SO 4 + humic acid + NaCl 0.4 0.2 0.0 9
Passive Layer Thickness Equivalent electrical circuit used for data fitting: R1 R2 R3 Q2 Q1 layer thickness d / m 3.0E-09 2.5E-09 2.0E-09 1.5E-09 1.0E-09 5.0E-10 R 2 = 0.9991 1 d = 1.9 nmv η 0.0E+00-1.2-1 -0.8-0.6-0.4-0.2 0 Electrode potential (SCE) / V 5x10-4 kmol m -3 Na 2 SO 4 With ε = 9 10
Cell Voltage at Constant Current Density At Yorkshire Water s electrocoagulation pilot plant: Constant current density applied: ca. 67 A m -2 Rapid cell voltage increase due to passive layer growth / fouling 400 350 300 cell voltage / V 250 200 150 100 50 0 0 50 100 150 200 250 300 time / min 11
Characterisation of Electrodes Objective: Investigate factors controlling passivation behaviour Define means of diminishing passivation Procedure: Use of ca. 30 mm 2 plate electrodes with various degrees of surface roughness Measurement of electrode potential as function of time in response to applied current density 12
Effect of Surface Roughness electrode potential / V(SCE) 12 10 8 6 4 2 0 0 50 100 150 200 time / s 1.4 Limit of potentiostat Rough Al electrode Smooth Al electrode Solution without Cl - Fresh surface electrode potential E / V(SCE) 1.2 1 0.8 0.6 0.4 0.2 0 0 100 200 300 400 500 600 time / s 13
Effect of Storage Time in Solution 1.4 electrode potential E / V(SCE) 1.2 1 0.8 0.6 0.4 0.2 0 0 100 200 300 400 500 600 5 time / s 4.5 After 72 hours in solution Fresh surface Rough electrode Solution without Cl - electrode potential / V(SCE) 4 3.5 3 2.5 2 1.5 1 0.5 0 0 100 200 300 400 500 600 time / s 14
Effect of Chloride Ions 12 electrode potential / V(SCE) 10 8 6 4 2 0 0 50 100 150 200 1.4 time / s 1.2 Without Cl - electrode potential / V(SCE) 1 0.8 0.6 0.4 0.2 0 With Cl - Smooth electrode Fresh surface -0.2-0.4 0 100 200 300 400 500 600 time / s 15
Impedance Spectrum after Corrosion Initially, high -Z (max)-value thick passive layer Significantly reduced after constant current layer thinned 20 16 before -Z'' / ohm m 2 12 8 0.2 Hz 4 after 0.05 Hz 0 0 4 8 12 16 20 Z' / ohm m 2 10 4 0.05 Hz Stationary High purity Al Fresh electrode 5x10-4 kmol l -1 Na 2 SO 4 + 10 g m -3 humic acid + 1.5x10-4 kmol l -1 NaCl 16
Effect of Real Surface Area of Al Electrode AFM image of mirror finish surface: Micro-roughness may create cracks in passive layer 17
Finite Element Model of Electrochemical Reactor Reactor design and reactions included: Out-flow Dissolution of Al: 0.01 m Production of H 2 : 3 Al Al + + 3 e Anode: Al 0.14 m Cathode: inert 2 H 2 O + 2 e H 2 + 2 OH Homogeneous precipitation of Al 3+ in two steps: 0.19 m 0.002 m 1 3 2 Al + + H O Al OH + + H + 2 ( ) 2 Al OH H O Al OH H 2 ( ) + + 2 2 ( ) 3+ 2 + In-flow: c(al 3+ )=0, ph 7 18
Model Prediction of ph Profile Anode Potential +0.05 V(SCE) Resultant current density ca. 6.8 A m -2 Al Anode Cathode (inert) fluid flow inter-electrode gap 19
Effect of ph Profile on Dissolution Low local ph at Al anode dissolves oxide layer Electode Potential (SHE) / V 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0-0.2-0.4-0.6-0.8-1.0-1.2-1.4-1.6-1.8-2.0-2.2-2.4-2.6 H + H 2 Al 3+ Al(OH)3 (am) Al 2 O 3 (c) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ph at anode Al ph Al(OH) 4 - O 2 H 2 O 20
Conclusions Dissolution of Al anode enabled by low local ph due to Al 3+ hydrolysis generating H + and precipitating Al(OH) 3 This leads to local dissolution and thinning of passive layer spontaneous depassivation Rough and freshly prepared surfaces more likely to exhibit spontaneous depassivation Corrosion promoters, e.g. Cl -, enhance depassivation 21
Thanks Yorkshire Water, Bradford (UK), for financial support Prof. Geoff Kelsall (Imperial College, Dep. of Chemical Engineering) Prof. Nigel Graham (Imperial College, Dep. of Civil and Environmental Engineering) 22
Questions? 23
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High Frequencies Frequency range 10 4 50 Hz 0.08 0.06 -Z'' / ohm m2 0.04 0.02 IR drop 0.00 0.00 0.03 0.06 0.09 0.12 0.15 0.18 Z' / ohm/m2-1.0 V -0.6 V -0.2 V 25
Frequency range 10 4 10-3 Hz Full Spectrum 16 14 12 -Z'' / ohm m2 10 8 6 4 2 0 0 5 10 15 20 25 30 Z' / ohm/m2-1.0 V -0.6 V -0.2 V -0.2 V fitted 26