1. Photocatalytic processes to remove pollutants from water
Workshop on advanced oxidation processes for industrial wastewater
Photocatalytic processes to remove pollutants from water
Santi Esplugas
Chemical Engineering Department.
University of Barcelona SPAIN
Madrid , October 4th 2007

2. Structure of the Presentation
Introduction
Advanced Oxidation Processes
Photochemical Oxidation Processes
- UV Photolysis
- H2O2/UV
- Ozone/UV
- TiO2 Photocatalysis
- PhotoFenton
Conclussions and Recommendations
Photocatalysis

3. Introduction: REMOVAL OF POLLUTANTS FROM WATER
*PHYSICAL METHODS (non destructive ):
Mechanical separation
Sedimentation
Filtration
Adsorption and ion exchange
Reverse osmosis
*CHEMICAL METHODS (destructive ):
Precipitation
Ion exchange
Chemical Oxidation (and reduction):
biochemical oxidation
chlorine
hydrogen peroxide
ozone
hydroxyl radical (OH •) AOPs

4. OH• Characteristics (redoxpotential)
* very powerful oxidation compound (F2 > OH• > O3 > )
* oxidation power related to chlorine (Eº = 1.36 V)
Rate constants of °OH radical with organics: x 107 mol-1 L s-1

5. OH• Characteristics (no especific oxidant)
Compounds that can be oxididez by OH radicals
Acids
Formic, gluconic, lactic, malic, propionic, tartaric
Alcohols
Benzyl, tert-butyl, ethanol, ethylene glycol, glycerol, isopropanol, methanol, propenediol
Aldehydes
Acetaldehyde, benzaldehyde, formaldehyde, glyoxal, isobutyraldehyde, trichloroacetaldehyde
Aromatics
Benzene, chlorobenzene, chlorophenol, creosote, dichlorophenol, hydroquinone, p-nitrophenol, phenol, toluene, trichlorophenol, xylene, trinitrotoluene
Amines
Aniline, cyclic amines, diethylamine, dimethylformamide, EDTA, propanediamine, n-propylamine
Dyes
Anthraquinone, diazo, monoazo
Ethers
tetrahydrofuran
Ketones
Dihydroxyacetone, methyl ethyl ketone

6. OH• Characteristics (no especific oxidant)
Scavenger effect (at high concentration)
Inorganic ions: Sulfate, chloride, carbonate,..
Alcohols: methanol, terbutanol,..
Hydrogen Peroxyde 6

7. Biodegradability enhancement by AOPs
dyes WO
pulp&paper
Fenton
textil O3
surfactants

8. (light conditions) atmospheric pressure and room temperature
AOP classics
homogeneous
heterogeneous
Photolysis
O3/UV
TiO2/UV
Fe+3/UV/H2O2
O3/H2O2 O3
sonolysis
electrolysis
Se divide principalmente en dos partes, una primera parte donde se estudiaran los procesos llamados homogéneos, y son aquellos procesos donde todos los reactivos son líquidos o en disolución. Y una segunda parte dedicada a los procesos heterogéneos, se trata de reacciones gas-liquido, donde el reactante gaseoso es el ozono. Los tratamientos estudiados en la primera parte son la fotolisis mediante radiación ultravioleta, y el proceso que combina radiación ultravioleta y un oxidante ( en este caso se utilizaron peróxido de hidrogeno y sales de hierro III). En cuanto a la segunda parte se estudiaron los procesos de ozonización. Ozonización combinada con radiación UV, y ozonización combinada con peróxido de hidrogeno, también se realizaron experimentos combinando ozono, radiación y peróxido a la vez.
Todos estos tratamientos, están basados principalmente en el poder oxidativo del radical ·OH, considerado uno de los oxidantes más potentes.
radiation UV
Radical ·OH
(light conditions) atmospheric pressure and room temperature

9. Suitability of water treatment according to COD10
100
1000
Classical AOP
Wet Oxidation
Incineration
COD (g.L-1) 20
200
autothermic 2
Andreozzi, R., Caprio, V., Marotta, R. (1999) Advanced Oxidation Processes (AOP)
for water purification and recovery”, Catalysis Today 53 (1) 51-59

10. COD removal - AOPs Applications
AOPs application regions with regards to economic consideration
From these examples of applications and considering all others we know today from different sources we can draw out a figure of the use of AOP for COD removal in waste water treatment. With regards to economic consideration it appears than ranging few 10 mg/L to few 100mg/L the UV based systems and O3/H2O2 combination are set out. The Fenton process and pressurised Fenton processes are much more dedicated to COD ranging 800mg/l to 30000mg/L while the WAO process since to get interest for COD of 30000mg/L up to mg/L. Above this value incineration should be the ultimate solution. It is clear than higher is the COD and lower is the quantity of flow which is really economically acceptable. To complete this purpose about waste water treatment we can mention the case of concentrate coming from gas treatment for hydrophilic COV with a chemical washing process. In such case in which we get dirty water which contains few g/l of COV (case of formaldehyde and phenols) and with a small volume (few m3/d) AOPs can be used to recover the washing water with the same figure.
Suty,H., Coste, M. “The AOPs tools in the treatment of waste water effluents”
3rd European Meeting on Solar Chemistry and Photocatalysis :Environmental
Applications Barcelona, 30 June and 1, 2 July 2004

11. Photochemical Oxidation Processes
PHOTOCHEMICAL PROCESSES
WAVELENGTH
UV (photolysis)
< 190 nm
H2O2/UV
< 300 nm
O3/UV
< 320 nm
O3/H2O2/UV
TiO2/UV (photocatalysis)
< 400 nm
Fe+2/H2O2/UV (photoFenton)
< 550 nm
Fe+2/e-/UV (photoelectroFenton)

12. PHOTOCHEMISTRY -9 12

13. Energy of photon with wavelength  E = h.c/
PHOTOCHEMISTRY
Energy of photon with wavelength 
E = h.c/
h (Planck) = J.s
c (light) = m/s
= m
Einstein = 1 mol of photons
Quantum yield  (stoichiometric coefficient)
 = mol reacted/mol of photons absorbed 13

14. PHOTOCHEMISTRY UV radiation ~ 100 - 400 nm visible ~ 400 - 700 nm
near infrared ~ nm
far infrared nm
UVA – 400 nm
UVB – 320 nm
UBC – 280 nm
UV-VIS ENERGY ENERGY
200 nm kcal.mol kJ.mol-1
700 nm kcal.mol kJ.mol-1 UV
VISIBLE
wavelength, nm 14

15. PHOTOCHEMISTRY photon balance x
q =photon flow density vector, Einstein/(m2.s)
= absorbance, cm-1
x = position, cm x
0,01
0,9900
0,0100
0,1
0,9048
0,0952 1
0,3679
0,6321 3
0,0498
0,9502 9
0,0001
0,9999 15

16. SOLAR PHOTOCHEMISTRY
4 - 7% UV radiation ( <400 nm) 16

17. WATER PHOTOLYSIS O-H bond strength in H2O: ca. 497 kJ mol -1
(corresponding wavelength = 240 nm)
H2O + hν → °OH + H°
Φ = 0.42 at 172 nm
H° + O2 → HO2°
Termination reactions:
2 °OH → H2O2
2 H° → H2
HO2° + °OH → H2O + O2
2 HO2° → H2O2 + O2
240 nm 17

18. WATER PHOTOLYSIS VCV lamps ( = 180 nm) Excimer lamps
Advantage: no gaseous, liquid or solid additive
Disadvantage: very limited penetration depth of the photons
high cost of 180 nm radiation (Xe lamp)
ozone production
Potential application: microelectronics, gas treatment 18

19. H2O2 PHOTOLYSIS O-O bond strength in H2O: ca. 213 kJ mol -1
(corresponding wavelength = 560 nm)
H2O2 + hν → 2 °OH
pKa = 11.6
HO2- + hν → °OH + O°-
O°- + H2O → °OH + OH-
ε254
H2O L mol-1 cm-1
HO L mol-1 cm-1
extinction coefficient
absorbance (cm-1)
concentration 19

20. H2O2 PHOTOLYSIS Drawbacks Advantages handling/safety precautions
no residues
H2O2 is miscible with H2O
H2O2 is readily available, transportable, storable
handling/safety precautions
very weak absorption except at high pH (several tens of kW-lamps) ; inner filter effect by organics
needs UVC (254 nm). Gemicidal
°OH scavenging by H2O2 (°OH + H2O2 → HO2° + H2O)
Drawbacks
Advantages 20

21. OZONE/UV Ozone does not need radiation (dark) to produce OH radicals
O3 stripped
Direct-O3 reaction
O3 (l)
Mox +M
OH P R
OH- or R
+Si
O3 (g)
Chain reaction
Radical-type reaction
R = free radicals, which catalyze
the ozone decomposition
M = solute
Si = free radical scavenger
Mox = oxidized solute
P = products, which do not
catalyze the ozone decomposition 21

22. OZONE/UV Absorbs < 320 nm (UVB-UVC) ε254: 18.6 for O3 L mol-1 cm-1
Relative absorbance
wavelength, nm
320 nm
ε254: 18.6 for O3 L mol-1 cm-1
concentration
(L mol-1)
absorbance
(cm-1)
extinction coefficient 22

23. OZONE/UV mechanism
hydroxyl radical
hydroperoxyl radical
scavenger 23

24. OZONE/UV Advantages Disadvantages no residues
easy O3 dosification
UV absorption higher than that of H2O2 (0.1 kW-lamps)
Disadvantages
need to produce O3 (but bactericidal use of O3)
necessity of destroying residual gaseous O3
efficiency limited by O3 hydrosolublity, gas-liquid transfer and scavenging of °OH (°OH + O3 → HO2° + O2)
Problems
Unwanted stripping of VOCs
Br- + 3 O3 → BrO O2 (overall reaction) 24

25. Photocatalysis Heterogeneous photocatalysis
Zeolite with photosensitizers
Semiconductor based catalyst: TiO2, ZnO, CdS
Homogeneous photocatalysis
Photosensitizers
(ruthenium(II) poly(pyridyl),...)
PhotoFenton
Fe+3/UV/H2O2 25

26. Zeolite with photosensitizers
Fe2+ N
[Fe(bpy)3] (1 nm)
2,2‘ - bipiridin (bpy)
Zeolite 26

27. Zeolite with photosensitizers
Absorbance 27

28. TiO2/UV
good UV absorption
low quantum yield (  < 0.03) 28

29. TiO2/UV Advantages Disadvantages
No consumable additive
Use of Solar UV
Simplicity  (easy maintenance). Robustness. Flexibility
Much lower sensitivity to pH than UV-AOPs based on H2O2 , O3
Reduction of some pollutants: Mn+, CCl4
Disadvantages
Separation of the catalyst from the solution
UV irradiation is poorly utilized by TiO2 despite high absorption
Poisoning of catalyst by organic matter.
Low quantum yield 29

30. mechanism through de Fe(IV) ??
PhotoFenton T mV pH H 2 O OH Fe 3+ 2+
Fenton = Fe+2-H2O2
mechanism through de Fe(IV) ?? 30

31. Fenton mechanism 31

32. Fenton Advantages Disadvantages Operation pH 2-4 (optimal =3)
Very cheap
Simplicity  (easy maintenance). Robustness. Flexibility
Disadvantages
Operation pH 2-4 (optimal =3)
Generation of sludge through the removal of iron ions.
Exploration of Fenton at different pH (neutral and basic) under research (iron complexes, Cu,..)
Possibility of supporting Fe ions on a solid under research 32

33. PhotoFenton Fe+3/UV/H2O2
Fe2(SO4)3
UV lamps
(360 nm)
Fe(III) in the presence of UV :
Fe3+(aq) + H2O + hn OH· + Fe2+ +H+
quantum yield for the formation of Fe(II)
at 313 nm
0.017 at 360 nm
Faust, B. C.; Hoigne, J., Atmos. Environ. 24A (1990) pp
Magnetic Stirrer 33

34. PhotoFenton Fenton Fe(III) in the presence of UV :
Fe3+(aq) + H2O + hn OH· + Fe2+ +H+
Fenton 34

35. °OH radicals are not the only active species
PhotoFenton
Additional reactions in Photo-Fenton
Fe3+ + H2O2 → H+ + Fe(HO2)2+
Fe(HO2)2+ + hν → Fe(HO2)2+*
Fe(HO2)2+* → FeIII-O° ↔ FeIV=O + °OH
Fe(HO2)2+* → FeII + HO2°
°OH radicals are not the only active species
UV-visible photodegradable compounds 35

36. Solar PhotoFenton
parabolic CPC
PSA – Almeria 36

37. PhotoFenton Advantages Disadvantages Operation pH 2-4 (optimal =3)
Very cheap
Simplicity  (easy maintenance). Robustness. Flexibility
Increase mineralization (TOC reduction)
Use of visible radiation
Disadvantages
Operation pH 2-4 (optimal =3)
Generation of sludge through the removal of iron ions.
Cost of UV-visible lamps
Waters with suitable UV light transmission
Fouling of the surface of UV tubes 37

38. SUMMARY Photochemical Oxidation Processes
Limitations of using Photoxidation
Low ε (H2O2)
Gas-liquid transfer (O3)
Charge recombination (TiO2)
Limited water volume (H2O-VUV)
Rate decreases due to °OH scavenging by the
sensitizer (H2O2, O3)
products (HCO3-/CO32-, SO42-, HxPO43-x)
the reactant (Fe2+)
Advantadges
High oxidizing power
No residues (except homogeneous Photo-Fenton)
Disadvantages
Cost of UV-Visible radiation
Waters with suitable UV light transmission
Good chance for coupling AOP with biological treatment 38

39. sequencing photoreactor + bioreactor
Photoreactor 1.5 L
Black blue lamps (3 x 8W, 360 nm) = 6 Einstein/s 39

40. sequencing photoreactor + bioreactor
Thermostatic Bath
Air
27ºC
Volcanic stones
Neutralizing stage
H2O2
FeSO4 UV
UV lamps
Magnetic Stirrer 40

41. Photochemical Oxidation Processes
Application fields
Because of the use of photons: low water amounts or flow rates
Because of low oxidizing species production: low pollutant concentrations 41

42. members of the group PhD Santiago Esplugas Jaime Giménez
Esther Chamarro
Carme Sans
Bsc-Engineer
Bernardí Bayarri
Alessandra Diniz Coelho Bruno Domenjoud
Marc Esplugas
Renato Falcao
Óscar González
Maria Homs
Fabiola Mendez
Mar Mico
Maria Navarro

43. Thank you