The photo-electrochemical disinfection of water using a novel bubble column

reactor

P. A. Christensen

Acknowledgements

The people who really did the work PDRAs: Dr. J. C. Harper and Dr. Sze Tai Lau Students: Ms. S. Kosa, Mr. J. Tinlin, Ms. P. Meynet Staff: Dr. J. Gunlazuardi, (University of Indonesia); Dr. T. P. Curtis, (Department of Civil Engineering); Dr. T. A. Egerton, (Department of Chemistry); Professor K. Scott, Department of Chemical and Process Engineering Funding and support: EPSRC, Ineos Chlor, Huntsman PU

“You will have a blade upon your weapon; when you “ease yourself” outside, you will dig with the blade, and cover what comes from you”

Deuteronomy Chapter 12 Verse 12 and 13

Housesteads Roman Fort, Hadrain’s Wall, Northumberland, GOC

Netties

N

(Latrines)

The latrine at Housesteads Roman fort

Structure of Presentation

•Aim•Introduction-Photocatalysis by TiO2

•Reactor design•Reactor characterisation•Light sources•Electrodes•Disinfection data: E. coli•Disinfection data: Cryptosporidium•Disinfection data: E. coli -the effect of Fe•A warning for the academic•Conclusions•Further work

Aim

To develop a photoelectrochemical detoxification system to facilitate the

recycling of wastewater from industrial processes.

To explore the potential of such a system for disinfection.

(Does EFE take place?)

Introduction: Photocatalysis by TiO2

an example of an AOP

Advanced

Oxidation

Process

OxidantRedox

potential/V vs NHE

Cl2 1.36O3 2.07

.OH 2.80F2 3.03

Solar Detoxification

'Solar'bacteria

PlantsAdvanced Oxidation Processes

Photo-Fenton Reaction Semiconductor Photocatalysis

Slurry

Titanium Dioxide

Immobilised Film

'Promoted' Non-promoted

'Promoted' Non-promoted

Non-electrochemical Electrochemical

One sun Concentrated sunlight

Solar Furnace Technology

Combustion Hydrogenation

Scheme 1: Summary of the processes currently employed in Solar Detoxification

Homogeneous Heterogeneous

'Free' iron Coordinatediron

Gas phaseAqueous phase

PhotocatalyticOxidation

From P. A.Christensen andG. M. Walker,"Opportunites forthe UK in SolarDetoxification",ETSUS/P4/00249/REP,HMSO, London,1996.

The generation of hydroxyl radicals at irradiated TiO2 ( < 410 nm)

h

-OHads

-OHadsh+

e-.OHads

e-h+

e-

TiO2 particle

Semiconductor Photocatalysis

Slurry

• Minority carrier length 0.1 m

• Small particle size

- Long settlement times

- Membrane filtration

• Low throughput

Immobilised film

• Severe MT limitations

• AND - EFE • BUT - reactor design

V

e-O2

H2O

e- h+ h

e-

e-

‘Fatal Attraction’The Electric Field Enhancement Effect

(How?)TiO2 photoanode Counter electrode

Reactor Design

(Not designed specifically for disinfection)

Reactor Design Criteria

The reactor should exploit the EFE effect.

(The reactor should be capable of treatingwater containing strongly absorbing pollutantssince this is characteristic of a number oftypical effluent streams arising from industrialactivity).

The mass transport characteristics of thereactor should be as efficient as possible.

The supply of oxidant, (O2), to the counterelectrode should be sufficient to ensure thatthis process is not rate limiting as has beenobserved elsewhere.

The relative geometry of the lamp andphotoanode should maximise absorption oflight by the latter.

The reactor should be economically feasibleand require low maintenance.

These criteria led to the design of a concentric tube reactor which offered maximum exposure of the photoanode to the UV light whilst minimising the light path and achieving good mass transport conditions.

To reservoir

AirSparge

From reservoir

SIDE VIEW

UV

LAMP

NiCounterelectrode

TiO2 Working electrode

PLAN VIEW

UV lamp

Glass walls

Gauze 125 mmx 115 mm

The Bubble Column Reactor

Electrode cassettes

PUMP

N2 INLET

LAMP HOUSINGALUMINIUM CAGE

ANNULUS

RESERVOIR

The Bubble Column Reactor

(Single electrode cassette)

Reactor Characterisation

2.00x10-5 2.20x10-5 2.40x10-5 2.60x10-5 2.80x10-5

5.8x10-6

6.0x10-6

6.2x10-6

6.4x10-6

6.6x10-6

6.8x10-6

Qv / m3 s-1

kL / m s-1

22

24

26

28

30

32

Re

Calculated mass transfer coefficient (kL, ) and Reynolds ()number through electrode annulus as a function of volumetricflow rate. The reactor was characterised via the reduction of

1mM Fe(CN)63- in aqueous 0.5M K2CO3. Ni mesh WE, Pt/Steel

mesh CE.

Mesh electrodes arenot promoting

(sufficient)turbulence bythemselves.

0.00 0.05 0.10 0.15 0.20 0.25 0.300

2

4

6

8

10

k L / 1

0-5 m

s-1

Gas-Liquid Fraction ( )

Mass transfer coefficient as a function of gas-liquidfraction (). Volumetric flow rate = 2 x 10-5 m3 s-1.

As soon as bubble flow initiated, marked increase in kL.

0.0 0.1 0.2 0.3 0.4 0.5 0.60

500

1000

1500

2000

2500R

e

uG / m s-1

Dependence of Reynolds number on bubble rise velocity.Volumetric flow rate = 2 x 10-5 m3 s-1

Reynolds numbersufficiently highto maintainturbuluent flow

Applying the correlations obtained by Akita andYoshida [K. Akita and F. Yoshida, Ind. Eng. Chem.Proc. Des. Dev., 13 (1974) 84], the estimated thebubble size distribution (Sauter mean diameter) was inthe range of approximately 3-8 mm, consistent withexperimental observations.

The gas-liquid mass transfer coefficients, kg, were thencalculated as a function of bubble rise velocity (ug) andwere found to be c. three times greater than thecorresponding solution-to-electrode mass transfercoefficients, kL.

Hence, for the photoelectrochemical oxidation ofdilute solutions:

-the dissolution of oxygen into the liquid phase isunlikely to be the rate limiting process-

(provided the gas velocities are not so high as toinduce slug flow)

The light source(s)

300 350 400 450

0.0

0.2

0.4

0.6

0.8

1.0

2 x 8W

2 x 32W

400W /100

Inte

nsity

/a.u

.

Wavelength /nm

Emission profiles from the 8W, 32W and 400W lamps used in our experiments. Actinometry shows that the emission of the

400W lamp is 100 x that of the 2 x 32W lamps at 435 nm

Anatase Rutile

Photonic Efficiencies

Defining the ‘charge carrier’ photonicefficiency as:

PE(%) = 100 x (no of electrons in the external circuit)no of incident photons

PEThermal = 1.7% and PESol-gel = 0.6%;for Io 8 x 10-10 Ein cm-2 s-1.

The Electrodes (Photoanodes)

(As coatings on a mixture of substrates: 1 cm2 and 25 cm2 Ti plates, and

11.5 cm x 12.5 cm Ti mesh rolled into a cylinder)

Thermal TiO2 films were prepared by first cleaning the Ti metal substrates (ie. mesh) with ethanol and then placing them in a furnacepreheated at the required temperature for 10 min.in air, after which time they were allowed to cool slowly.

The TiO2 sol-gel electrodes were prepared from the product of the acidcatalysed reduction of titanium di-isopropyl acetoacetonate with water[L. Kavan and M. Gratzel, Electrochim. Acta, 40 (1995) 643 - 652].Dip-coated films were applied by immersing the titanium substrate intothe gel product and withdrawing slowly to ensure good coverage of thecatalyst. The resulting amorphous layer was heated at the required temperature for 10 mins. The procedure was repeated five times.

Electrode Preparation

Geometric surface area

High surface area.

450 500 550 600 650 7000.100.150.200.250.300.350.400.450.500.550.600.650.700.750.800.850.90

I h m

A/c

m2 @

1.2V

vs A

g/A

gCl

Heating temperature °C

The effect of heating temperature on the photocurrent densityof the sol-gel TiO2 films formed on 5 cm x 5 cm Ti plates.

2 x 32W ‘sunbed’ lamps.

Tapwater

+ Methanol

We generally find that the photocurrents observed usingthermal TiO2 films do not show any significant change onaddition of, eg. methanol, in contrast to those observed atthe ‘sol-gel’ films, where significant increases areobserved, as well as lower onset potentials.

This suggests that methanol cannot compete effectivelyfor holes at the surface of thermal films.

‘Kill’ mechanism?

450 500 550 600 650 700 750 800 10000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40I hv

@ 1

.2V

/mA

cm

-2

Preparation temperature /°C

No change onaddition ofmethanol

Note - max c. 750 C

The effect of heating temperature on the photocurrent density of the resultant thermal TiO2 film. 5 cm x 5 cm Ti plates, 2 x 32W

‘sunbed’ lamps.

Cyclic voltammogramms, (forward sweeps only, for clarity), recorded using the 500 cm3 BCR, single sol-gel electrode cassette, Ni counter electrode mesh, 1.4 mM

Na2SO4, 2 x 8W UVA lamps.

Disinfection data

(Data for batch operation, reactor volume 500 cm3, unlessotherwise stated)

The effect of film fabrication temperature on the activity of thermal TiO2 mesh electrodes towards the killing of E. coli.

107 cm-3 (500 cm3), single electrode cassette, Ni counter electrode mesh, 1.4 mM Na2SO4, 2 x 8W UVA lamps, V = 1.3V.

Batch mode.

‘100% kill’ = 5 log inactivation; 107 – c. 102 cm-3

Dependence of kill on fabrication temperatureconsistent with dependence of Ihv, and hence with kill

via .OH.

3.0 3.5 4.0 4.5 5.01.5

2.0

2.5

3.0

3.5

Sol-gel Film

Thermal Film

Log(

r i /ce

lls m

l-1 m

in-1)

Log(Ci /cells ml-1)Kinetic studies on disinfection in 250 cm3 batch reactor Log(Initial Rate) vs Log(Cell conc). 2 x 8W UVA lamps

(Philips Lighting UK Ltd. Model TL/02/8, = 300 nm - 475 nm, max = 360 nm) positioned 6 cm above the reactor vessel.

The light intensity, (Optronic Laboratories Inc. 730A radiometer/chemical actinometry), was c. 8 x 10-10 Ein cm-2

s-1. 1V vs AgCl.

Electrodecharacteristics

critical

5 x 5 cm2 plate photo-anode in 250 cm3 cell

Comparison of the BCR data obtained with the thermal and sol-gel TiO2 films with those obtained using P25 slurry. 107 cm-3, 2 x 8W UVA lamps/sparge/ 1.4 mM Na2SO4. Photo-electrochemical experiments: single

electrode cassette, Ni counter electrode mesh.

Electrodecharacteristics

critical

0

10

20

30

40

50

60 Initial stock viability

ThermalBlank

ThermalRun

Sol-gelRun

Sol-gelBlank

% V

iabl

e oo

cyst

s

The inactivation of Cryptosporidium oocsysts in tap water in a 135 cm3 batch reactor, 2 x 8W UVA lamps, 25 cm2 sol-gel

TiO2 electrode, 1.2V vs Ag/AgCl applied potential. The error bars represent multiple comparisons at the 95% level

Electrodecharacteristics

critical

Flow system, sol-gel electrode mesh, 107 E. coli cm-3 (500 cm3 BCR + 2 dm3 reservoir, flow rate = 100 cm3 min-1), single

electrode cassette, Ni counter electrode mesh, 1.4 mM Na2SO4, 2 x 8W UVA lamps, V = 3.0V.

3 x longer for 5 x more!!

But- catalytic activity too low

The catalyst activities need improving…….a little thing(!?), but...

“…well, that certainly screws up our chance of conquering the universe!”

However-

- Iron

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

I h /m

Acm

-2 @

1.2

V v

s Ag/

AgC

l

Atm.% Fe

1 cm2 Sol gel TiO2/Ti photoanode, fabricated at 500 C. Photocurrents measured in tap water with and without added methanol as a function of the atom % of added Fe. 2 x 32W

sunbed lamps.

Steady decline in photocurrent as Fe content increases-reflected in ‘activity’?

Added methanol

No methanol

2 3 4 5 6 7 8 90

102030405060708090

100 Thermal ElectrodeFe Doped Electrode

Sol-gel ElectrodeICI Electrode

% k

ill o

f E.C

oli.

Photocurrent /mA

% Kill of E. Coli (107 cm-3) in the BCR at 1.3V after 15 minutes,using various TiO2 photoanodes

Kill mechanism?

But….

The potential limitations of Advanced Oxidation Processes involving the use of O3 and/or H2O2 were highlighted by the Cryptosporidiosis outbreak in Kitchener-Waterloo, Ontario where the post-outbreak review of operations found no relevant treatment breaks, indicating intrinsic problems associated with H2O2 and O3 -based treatment.

The final conclusion of the report* on theCryptosporidiosis outbreak in Kitchener-Waterloo,Ontario: “We are entering into a new era when we canmeet or exceed regulatory requirements and are stillconfronted by episodes of this nature” highlights theseconcerns.

*B. Pett et. al., Proc. AWWA WQTC, 1993, Miami,Florida, 1739

Effect of bicarbonate ion concentration on the kill of 107 E. coli cm-3 (500 cm3) in the BCR; 1.4 mM Na2SO4, 2 x 8W UVA lamps, single electrode cassette, Ni counter electrode mesh. Thermal film mesh, V =

1.3V; sol-gel film mesh V = 3.0V.

Effect of phosphate ion concentration on the kill of 107 E. coli cm-3 (500 cm3) in the BCR; 1.4 mM Na2SO4, 2 x 8W UVA lamps,

single electrode cassette, Ni counter electrode mesh. Thermal TiO2 mesh, V = 1.3V; sol-gel mesh V = 3.0V.

Conclusions•Although not designed for disinfection, the BCR showed good mass transport and light utilisation characteristics, with supply of oxidant to the counter electrode, and reduction thereat, not being rate limiting.

•The photoelectrochemical disinfection of water inoculated with a range of pathogens including Cp, E. coli and Clostridium Prefringens spores has been observed using simple TiO2 photoanodes and relatively low intensity UVA lamps and low applied voltages (< 3V). The kill rate is increased substantially over that observed at zero applied potential.

•The mechanism of disinfection remains unclear. However, it is clear that disinfection activity depends very much upon the nature of the TiO2 photoanode employed, and may not simply involve OH radicals as the active agent.

•In our hands, the photoelectrochemical disinfection of water proved more effective than photochemical disinfection using a TiO2 slurry.

•The catalytic activity of the photoanodes remains too low.

The most recent reactor design: the Tower reactor

Ti Rod

Ti-meshNi-mesh

PTFE wheel

60 mm

40 mm

40 mm

40 mm

40 mm

60 mm

Stainless Steel supporting rod

UV lamp housing

Sintered glass gas distributor

Ni Rod

Preliminary disinfection data from the tower reactor. 4.3 x 106 CFU ml-1, 4.6 dm3 aqueous 1.4mM Na2SO4, 5 x thermal TiO2/Ti mesh + Ni mesh cassettes. 2 x 25W UVA lamps, 50% intensity.

Reactor volume 7 dm3.

Electrochem(1.6V)

Photocatalytic

Photoelectrocat(1.6V)