ELECTROCOAGULATION / ELECTROOXIDATION.

1. ESSEE 44th European Summer School on Electrochemical EngineeringPalić, Serbia and Montenegro17 – 22 September, 2006 ELECTROCOAGULATION / ELECTROOXIDATION. Dr. Manuel A. Rodrigo Department of Chemical Engineering. Facultad de Ciencias Químicas. Universidad de Castilla La Mancha. CampusUniversitario s/n. 13071 Ciudad Real. Spain. Department of Chemical Engineering. Universidad de Castilla La Mancha. Spain

2. CONTENTS • ELECTROCHEMICAL WASTEWATER TREATMENT TECHNOLOGIES • 1.1 What happens inside an electrochemical cell during the electrolysis of a wastewater? • 1.2 Types of electrochemical wastewater treatment technologies • 1.3 Advantages of electrochemical technologies in environmental remediation • ELECTROCOAGULATION • 2.1 What is coagulation? • 2.2 The electrochemically-assisted coagulation: fundamentals • 2.2.1 ANODE MATERIALS • 2.2.2 ELECTRODISSOLUTION • 2.2.3 ELECTROLYTIC GENERATION OF OXYGEN AND HYDROGEN • 2.2.4 MAIN PROCESSES INVOLVED IN THE ELECTROCHEMICALLY ASSISTED TECHNOLOGIES FOR COLLOID-POLLUTED WASTES • 2.3 Electrochemical cells • 2.3.1 TANK CELLS • 2.3.2 FLOW CELLS • 2.3.3. PROMOTION OF THE ELECTROFLOTATION PROCESS • 2.3.4 OTHER PROCESSES • 2.4 Electrocoagulation of soluble organics and break-up of emulsions. Removal of phosphates • 2.5 Advantages and disadvantages of electrocoagulation • ELECTRO-OXIDATION • 3.1 Fundamentals • 3.2 Electrode materials • 3.3 Electrochemical cell • 3.3.1 IS IT RECOMMENDED THE USE OF DIVIDED CELLS? • 3.3.2 STIRRED-TANK CELLS • 3.3.3 SINGLE-FLOW CELLS • 3.3.4 FILTER-PRESS CELLS • 3.3.5 OTHER CELLS • 3.4 Indirect electrochemical oxidation processes • 3.5 Advantages of the electrooxidation technologies • 3.6 Combined processes

3. 1.1 What happens inside an electrochemical cell during the electrolysis of a wastewater? Power supply e e e e - - - - Red Ox 5. Migration of anions influent 1. Electrooxidation 2. Electroreduction Anode Cathode Ox Red 5. Migration of cations effluent M M 3. Electrodissolution 4. Electrodeposition Mn+ Mn+

4. Concentrated solution Anionic membrane Cathionic membrane Anionic membrane Cathionic membrane Diluted solution anode cathode Anions Cations Feed solution • 1.2 Types of electrochemical wastewater- treatment technologies electrodialysis Electro-oxidation electrocoagulation Electrodeposition metal Rotational cathode anode Electrolyte flux

5. 1.3 Advantages of electrochemical technologies in environmental remediation • Environmental compatibility: “the main reagent used is the electron” No residues are formed. • Versatility: • Many processes occur simultaneously in any electrochemical cell. Plethora of reactors, electrode materials, shapes, configuration can be utilized and allow to promote different kinds of treatment technologies. • Point-of-use production of chemicals is facilitated by electrochemical technology • Volumes of fluid from microliters to thousand of cubic meters can be treated • Processes work at room temperature and atmospheric pressure • Selectivity: the applied potentials can be controlled to selectively attack specific compounds. • Easy operation. Amenability to automation. • Cost effectiveness

6. 2. ELECTROCOAGULATION 2.1 What is coagulation? Pollutants size 100 micras 10 micras 1 micra 100 nm 0.1 nm 10 nm 1 mm 1 nm 1 cm Dissolved comp. Colloids Suspended solids influent effluent Typical hydraulic residence time of a settler for wastewater treatment Sludge

7. Bulk solution + Negatively charged particle - - + + + + + + + + + + + + + - + - + + + + + + + - + + + + + + + + - Surface potential Diffuse layer -(electrostatic potential) Zeta potential Distance from the surface Interaction energy Ea Ea+Eb Distance between particles Eb Electrostatic repulsion energy: Ea Van der Waals attraction energy: Eb Resulting energy : Ea+Eb Coagulation is a chemical treatment which consists of the addition of chemical reagents to reduce the electrical repulsion forces that inhibit the aggregation of particles. Hydrolysing metal salts (iron, aluminium)

8. + + + + + + + + + + + + + + + + + + + + + + + + + + + Particles stabilized by electrostatic repulsion forces Compression of the diffuse layer by an increase of the ionic strength Neutralization of superficial charges by adsorption of ions Precipitation Charge Neutralization Interparticle bridging Enmeshment in a precipitate

9. Outlet Sludge Conventional Chemical Coagulation consists of the direct dosing of a coagulant solution to the wastewater. Chemical reagent Inlet sedimentation coagulation flocculation Flocculation is a physical treatment in which the collision of coagulated colloids is promoted in order to make possible the formation of larger particles. The result of both processes is a wastewater in which the size of the particles is enough to be separated by a settler or a flotation unit.

10. 9 z1 z2 z3 z4 8 7 6 pH Nitrate media 5 4 Sulphate media 3 2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 OH-/Al Coagulation by hydrolysing aluminium salts Al3+ 0 Concentration of monomeric hydrolysis products of Al(III) in equilibrium with the amorphous hydroxides at zero ionic strength at 25ºC Al(OH)2+ -2 Al(OH)4- AlT -4 Al(OH)2+ -6 Log [Alx(OH)y3x-y ] / mol dm-3 Al(OH)3 -8 -10 Precipitate Polymeric species Monomeric species -12 0 2 4 6 8 10 12 14 pH Typical titration curve for neutralization of aluminium salt solutions

11. 100 monomers [Al13O4(OH)24]7+ 80 Ali/AlT 60 40 [Al(OH)3]* 20 [Al2(OH)2]4+ [Al2(OH)x](6-x)+ pH 0 3 3,5 4 4,5 h= OH/ AlT 0,25 1 2 2,2 2,25

12. Coagulation by hydrolysing iron salts 0 Concentration of monomeric hydrolysis products of Fe(III) in equilibrium with the amorphous hydroxides at zero ionic strength at 25ºC -2 Fe(OH)3 -4 -6 Log [Fe(OH)y3x-y ] / mol dm-3 -8 Fe3+ Fe(OH)4- -10 Fe(OH)2+ Fe(OH)2+ -12 0 2 4 6 8 10 12 14 pH

13. 2.2 The electrochemically assisted coagulation: fundamentals Electrocoagulation An alternative to the direct use of a solution containing the coagulant salts, is the in situ generation of coagulants by electrolytic oxidation of an appropriate anode material (e.g. iron or aluminium). This process is called electrocoagulation or electrochemically assisted coagulation. • Electrochemical processes involved: • Electrodissolution • Electrolytic generation of oxygen and hydrogen M Electro-dissolution e- Unstabilized small particles Aggregated particles Mn+ flocculation coagulation + colloids macromolecules emulsions

14. 2.2.1 ANODE MATERIAL Aluminium Iron M Electro-dissolution e- Mn+ coagulation +

15. Faraday’s value Chemical dissolution Experimental Influence of current density Influence of pH 2.2.2 ELECTRODISSOLUTION Electrochemical process Faradaic Efficiencies can be over 100% Chemical process

16. pH profile in the electrochemical cell Cathode Anode pH profile Direction of electrolyte flux

17. 2.2.3 ELECTROLYTIC GENERATION OF OXYGEN AND HYDROGEN Anodic processes Cathodic processes H20 H2O e- e- O2 H2 + -

18. Air-dissolved flotation Bubbles diminish the overall density of the system and the particle floats

19. Oxygen and hydrogen bubbles turbulence adhesion Gaseous microbubbles link to pollutant particles. Consequently, the density of the new species decreases and this promotes the flotation of the particle Promotes soft mixing conditions and improves flocculation processes Electrochemically assisted flocculation (electroflocculation) Electrochemically assisted flotation (electroflotation)

20. Anodic processes Cathodic processes pollutants e e e - - - Al(III) species Electrodissolution Electrocoagulation flocs Electroflotation Electroflocculation H2O H2O e- e e e - - - H+ + O2 H2 + OH- 2.2.4. MAIN PROCESSES INVOLVED IN THE ELECTROCHEMICALLY ASSISTED TECHNOLOGIES FOR COLLOID-POLLUTED WASTES

21. 2.3 Electrochemical cells purpose • Only electrodissolution • Electrocoagulation/electroflocculation • Electrocoagulation/electroflocculation electroflotation Type of cells

22. Power supply - e - e Sludge Floated sludge Inlet Flotation H H O O Outlet n + M 2 2 Anode Cathode (Pollutant) n + M H 2 hydrated OH - Precipitated Settling Settled sludge Sludge 2.3.1 TANK CELLS Mixing can be accomplished either by mechanical stirrers or by the evolved gases Coagulation/flocculation Sedimentation/flotation The process combines Contrarily to electrooxidation processes, mass transport does not control the overall rate of the process

23. Power supply - e - e Sludge Floated sludge Inlet Flotation H H O O Outlet n + M 2 2 Anode Cathode (Pollutant) n + M H 2 hydrated OH - Precipitated Settling Settled sludge Sludge The activity of the anode can decrease with time due to the formation of insoluble hydroxides or sludge layer. These can be avoid by using motion electrodes or by using turbulence promoters Hydrogen evolution can disturb the sedimentation process. For this reason, if possible, it is better to separate the cathodic process from the sedimentation

24. HydroShock™ ElectroCoagulation

25. + - + - + - + - + - + - + - + - Multiple channels Single channel 2.3.2 FLOW CELLS The activity of the electrodes can be decreased by passivation. To solve this problem reverse of polarity (the anode acts as a cathode during a small period) are advised. This can be easily done in a cell designed with the only purpose of aluminium dosing… Normally, these cells do not promote the electroflocculation and the electroflotation processes except for especial designs. Hence its main goal is the electrodissolution and the electrocoagulation Electrode configuration in cells for aluminium dose

26. Cathodes (-) Anodes (+) … and both, monopolar and bipolar connections, allow this change of polarity! Bipolar electrodes + - cathode anode - - - - + + + + However, it is more complex for cells that combine electrocoagulation and electroflotation in different compartments

27. The turbulence generated by the evolved gases can be used in both types of flow. However, vertical flow allows to improve the separation by electroflotation as compared with horizontal flow. Horizontal flow Vertical flow

29. 2.3.3. PROMOTION OF THE ELECTROFLOTATION PROCESS If electroflotation processes have to be promoted it has to be taken into account that: e- e- - - Current density (j) influences on both: number of bubbles and the average size of bubbles Flow rate can also be used to control the average bubble size

30. And also that the electroflotation can be carried out in the same or in a different cell Power supply Efluent EF Divided electrocoagulation/ electroflotation Separator EC Power supply Efluent EF Combined electrocoagulation/ electroflotation Separator EC

31. influent air 2.3.4 OTHER PROCESSES

32. + + + + + + + + + + + + + + + + + + + + + + + + + + + 2.4 Electrocoagulation of soluble organics and break-up of emulsions. Removal of phosphates Emulsion stabilized by electrostatic repulsion forces Compression of the diffuse layer by an increase of the ionic strength Neutralization of superficial charges by adsorption of ions Coalescence of phases Inter-droplet bridging

33. HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO HO OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - SO3 - N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N M3+ N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 NO2 M3+ Dissolved organic matter Binding of monomeric cationic species to anionic sites of the organic molecules, neutralising their charge and resulting in reduced solubility compounds Binding of polymeric cationic species to anionic sites of the organic molecules, neutralising their charge and resulting in reduced solubility compounds + + + NO2 + + SO3 - + N HO + N + + OH Enmeshment in a precipitate Adsorption on a superficially charged precipitate

34. Precipitation of phosphates Log dissolved P -2 FePO4 AlPO4 -4 -6 2 8 10 4 6 pH Electrodissolution cell Treated wastewater clarifier wastewater

35. 2.5 Advantages and disadvantages of electrocoagulation In literature some advantages are reported for electrocoagulation processes including: 1) A promotion in the flocculation process due to the movement of the smallest charged colloids inside the electric field generated in the electrochemical cell and also to the turbulence created by the bubbles (electroflocculation process) 2) A promotion in the separation process due to the hydrogen bubbles produced in the cathode during the electrolysis, which can carry the solids to the top of the solution, where they can be easily collected and removed (electroflotation process) 3) A more compact residue, as it is reported that the electrocoagulation process produces a smaller amount of sludge that the chemical coagulation, and that the solids produced are more hydrophobic 4) A more easy operation mode as no mixing of chemicals is required, the dosing of coagulants can be easily controlled by manipulating the cell voltage (or the current density), and thus the operating costs are much lower compared with most of the conventional technologies 5) Very simple. Suitable for small WWTP 6) Lower operating cost. However, higher investment

36. 3.ELECTRO-OXIDATION 3.1 Fundamentals When can be applied? Wastewater polluted with soluble organic pollutants Is it possible the recovery of the pollutant as a valuable product? High calorific power? no Biodegradable? no Non AOP oxidation AOP oxidation Electrochemical oxidation no

37. Electro-oxidation technologies: use of an electrolytic cell to oxidize the pollutants contained in a wastewater pollutant 1. Direct electrolysis Oxidation of the pollutant on the electrode surface H2O pollutant 2. Advanced oxidation processes With some anode materials it is possible the generation of OH· OH· e- PO43- 3. Chemical oxidation + On the electrode surface several oxidants can be formed from the salts contained in the salt P2O84- pollutant

38. Direct electrolysis consists of the direct oxidation of a pollutant on the surface of the anode. To be oxidized the organic must arrive to the anodic surface and interact with this surface. This means that electrocatalytic properties of the surface towards the oxidation of organics can play an important role in the process. Likewise, it means that in certain conditions mass transfer can control the rate and the efficiency of the electrochemical process Organic pollutant e- intermediates (aromatics, carboxylic acids) ... e- CO2 + H2O O2 Cl- Cl2

39. Organic pollutant e- intermediates (aromatics, carboxylic acids) ... e- CO2 + The potentials required for the oxidation of organics are usually high. This implies that water can be oxidized and the generation of oxygen is the main side reaction. This is a non desired reaction and it influences dramatically on the efficiencies H2O O2 Cl- Cl2

40. Organic pollutant e- intermediates (aromatics, carboxylic acids) ... e- CO2 + H2O O2 Cl- Frequently the potential is high enough to promote the formation of stable oxidants, through the oxidation of other species contained in the wastewater. This can have a beneficial effect on the efficiency as these oxidants can oxidize the pollutant in all the volume of wastewater Cl2

41. Organic pollutant e- Organic pollutant 2. Mass transport, which can be promoted by a proper cell design ... e- CO2 1. Electrode material, which influences on the nature of the products and on the importance of the side reactions + H2O 3. The presence of compounds in the wastewater that can be transformed into oxidants, promoting mediated electrochemical oxidation processes O2 Cl- Cl2

42. 3.2 Electrode material DESIRABLE PROPERTIES MECHANICAL STABILITY. CHEMICAL STABILITY MORPHOLOGY. ELECTRICAL CONDUCTIVITY CATALYTIC PROPERTIES RATIO PRICE/ LIFETIME.

43. Typical materials include Platinum Stainless stell Metals Carbon oxides Grafite Doped diamond material DSA Ti/SnO2 Ti/PbO2 low efficiency electrodes High efficiency electrodes

44. Low efficiency electrodes SOFT OXIDATION CONDITIONS phenol Quinones, polymers, carboxylic acids Many intermediates Small conversion to carbon dioxide Slow oxidation rates Small current efficiencies Formation of polymers from aromatic pollutants is favoured e- Fouling by polymers + Pt IrO2 Mediated oxidation by a higher oxidation state of the species that conforms the electrode surface?

45. High efficiencies electrodes HARD OXIDATION CONDITIONS phenol Carbon dioxide few intermediates Large conversion to carbon dioxide Large current efficiencies only limited by mass transfer e- + Confirmed for conductive-diamond Suggested for PbO2/SnO2 OH· generation? BDD Ti/PbO2

46. Active electrodes Pt Stainless steel DSA Non-active electrodes Ti/SnO2 Ti/ PbO2 Doped diamond Drawbacks of non-active electrodes: Conductive diamond: large price >6000 euros/sqm PbO2/SnO2: Dissolution of toxic species

47. H O 2 R R · Cred OH R Cox RO Cox RO R R RO RO RO ROLE OF THE HYDROXYL RADICALS Direct oxidation process Mediated oxidation process Interfase Interfase Interfase Electrolyte Electrolyte e - e - Electrolyte e - Electrochemical oxidation Anode (+) Anode (+) Anode (+) Electrochemical Reaction Mass Transport Electrochemical Reaction Mass Transport Electrochemical Reaction Mass Transport Kinetic or mass transport controlled Kinetic controlled

48. Cathodic material The organic-oxidation processes that occur in an electrochemical cell are usually irreversible. Hydrogen evolution is the main cathodic reaction. e- e- e- e- H2O 0.5H2+ OH- cathode cathode anode anode Deposit of carbonates OH- + HCO3- Increase in the cell potential e- e- Increase in the energy consumption H2O 0.5 O2+ 2H+ Polarity reversal cathode anode

49. 3.3 Electrochemical cell DESIRED CHARACTERISTICS FOR A ELECTROCHEMICAL CELL SIMPLE MECHANICAL DESIGN. SMALL PRICE. EASY TO USE. LOW MAINTENANCE COST. ENHANCED MASS TRANSFER. HOMOGENEOUS CURRENT DISTRIBUTION ON THE ELECTRODES. LARGE DURABILITY SAFETY

50. V hW ea + h Cell potential hdiff hW ea + h + hreaction hW Direction of charge flux Electrolyte ANODE CATHODE 3.3.1 IS IT RECOMMENDED THE USE OF DIVIDED CELLS? 1. The membrane increases the cell potential and consequently the operating cost. 2. Most organic-oxidation processes are irreversible Power supply - + Turbulence promoters e- e- Membrane Catholite Anolite Anode Cathode