Laboratory Scale Water Circuit Including a Photocatalytic
Reactorand a Portable In-Stream Sensor To Monitor Pollutant
DegradationPatrick Nickels,,Hang Zhou,,Sulaiman N.Basahel,Abdullah
Y.Obaid,Tarek T.Ali,Ahmed A. Al-Ghamdi,El-Sayed H.
El-Mossalamy,Abdulrahman O. Alyoubi,and Stephen A. Lynch*,London
Centre for Nanotechnology,University College,London,U.K.Bio Nano
Consulting,U.K.Chemistry Department,King Abdulaziz University,Saudi
ArabiaPhysics Department,King Abdulaziz University,Saudi
ArabiaABSTRACT: We describe a lab-scale closed-circulating test
systemfor photocatalytic wastewater treatment. The systemcomprises
a UV-LED photoreactor, a microcirculating fluid pump, and an
in-stream sensor unit. The reactor can hold volumesup to 250 mL and
is optimized to study the degradation of pollutant concentrations
in the microgram to milligram per liter rangeusing photocatalysts
fixed to a planar surface within the reactor vessel. The test
pollutant used was methyl orange. The in-streamsensor unit consists
of aliquidflowcell withtransparent windows,
allowingthetransmissionof light
fromanLEDtobemonitoredbyaphotodiode. Theconcentrationof
thepollutant isevaluatedinreal-time. Thesystemislightweight,
cheap,portable, andflexible, ideal forlaboratoryorfieldworkuse,
andcouldbeeasilyup-scaledandusedforin-linequalitycontrolmonitoring
in a wastewater treatment plant.1. INTRODUCTIONWater pollutionis a
global problem. Compounds includingnatural organic matter and
synthetic organic microcontami-nants, forexample, hydrocarbons,
pharmaceuticals, endocrine-disrupting compounds like
polychlorinated biphenyls, fertilizersandpesticides,
arereleasedconstantlyintotheenvironmentbyindustry, households,
andagriculture.1Regularwastewaterplantshelptoremovemost of
thepollutantsviaregularandcost-effective treatment steps like
sedimentation, filtration, andbiological processes, all of which
are deemed relatively effectivefor the treatment of wastewater.
However, biologically toxicandnondegradableorganicscanstill remain.
Advancedtreat-mentprocessessuchasactivatedcarbonandadvancedoxida-tion
processes are being adopted;2but these can be
expensivetorunandresultinincreasedwatercosts.3Theuseof
semi-conductor photocatalysts togeneratereactiveoxygenspeciesfor
advanced oxidation processes in water treatment
technologyhasbecomeoneofthemostpromisingtechniquestoprovidea
cheapandenergyefficient methodfor thedisinfectionofwater.46Other
advantages are that foulingcanpossiblybeinhibited by the
photocatalytic activity, and ideally the catalyticmaterial does not
need refueling or replacement and can, therefore,run continuously.
Thus, the investigation into, and development of,efficient
photocatalysts and reactors has become a
worldwidechallenge.Titaniumdioxideisthemost
widelystudiedphotocatalyticmaterial to date. Crystalline TiO2 is a
compound semiconductorandhasabandgapthatliesintherange3.13.4eV,
depend-ing on the exact crystal structure (anatase, rutile, or
brookite).7Bandgap excitation is achieved using photonswith
wavelengthslying in the near-UV band (shorter than 380 nm). TiO2 is
widelyavailableand, duetoits ubiquitous useas awhitepigment,is
inexpensive. It is biologically compatible andvery
stable;suchproperties havebrought it
accreditationevenasafoodadditive.8There are two methods to treat
wastewater in a photo-catalytic process: either to suspend the
catalyst in a powder orgranuleforminthewater,
(aso-calledslurrysystem)orcoatthe catalyst on a surface over which
the water flows (commonlyreferred to as a fixed bed system).9A
possible advantage of theslurrysystemisthat thereisamuchhigher
surface-to-liquidinterface area and, therefore, a more efficient
generationofreactiveoxygenspeciesordirectinteractionwithpollutants.10However
inthis study, afixedbedreactor systemhas beeninvestigated to avoid
the possible need for a post-reaction separa-tion of catalyst from
the water.A key step in the development process is to understand
howwell thephotocatalyst behavesunderdifferent
environmentalconditions.11The efficacy of a photocatalyst is
usually evaluatedby monitoring the degradation rate ofa
specificcompound inaqueous solutionunder controlledconditions;
these includeconcentration of solutionand photocatalyst,
irradiance, pH,and volume.12One compound that is commonly used as a
testmodel pollutant is the relatively benign chemical
methylorange13duetoitsstrongcolorvisibletothenakedeyeandtheuseof
conventional spectrometers toassess
theconcen-trationbyabsorptionspectroscopy. Additionally, it
hasmanyproperties of common organic pollutants such as benzene
rings,sulfonate, andaminegroups. Thereareseveral
methodsthatcanbeusedtomeasuretheconcentrationof suchanagent
insolution. The most obvious of these is conventional
spectroscopyReceived: October 15,2011Revised: December
12,2011Accepted: January 5,2012Articlepubs.acs.org/IECR XXXX
American Chemical Society A dx.doi.org/10.1021/ie202366m | Ind.
Eng.Chem. Res. XXXX, XXX, XXXXXX(be it UVvis or FTIR); however,
other possibilities include highpressure (performance) liquid phase
chromatography (HPLC) orliquid chromatographymass spectroscopy
(LCMS).Commondisadvantages to these methods are the equipment
needed is oftennot easily portable and it is usually very
expensive, often requiringa trained user. Furthermore, in a real
world application, water willbe circulating through a
photocatalytic reactor with either
artificiallightornaturalsunlightastheradiationsource.
Insuchawatercircuit, it is important to monitor the concentration
of thechemical(s) requiring removal. Therefore, it is of great
interest tohave sensor systems in place that can record the
concentration inreal-time. Of the existing analysis methods
previously mentioned,some could be adapted to perform real-time
monitoring, but themodifications would be
expensive.Thesetupdescribedinthisstudyisacompact, robust,
andcheapsolutiondesignedtounderstandtheefficacyof
photo-catalyticreactionsinreal-time.
Itisaclosedwatercircuitthatintegrates a real-time in-stream sensor
and photocatalytic reactor.The reactor consists of a vessel with
inlets and outlets, and a sub-strate coated with photocatalytic
material covers the base. Water iscirculated through the reactor
using a centrifugal pump, providingconstant mixingandflow.
ToinitiateaphotocatalyticreactionUV-LEDs, mountedonthecoverof
thereactor, illuminatethephotocatalyst. UV-LEDs have recently
become a popular choice
asaUVlightsourceinreactorsbecauseoftheircheapprice, longlifetime,
high quantum yield, and small size,1418and importantlythey have
been shown to be effective for chemical degrada-tion.1922A liquid
cell that measures light transmittance is used toenable the
concentration of any selected chemical in the system tobe
monitored. This setup enables the study of a range of parametersand
optimal conditions for photocatalytic reactions accordingly.2.
EXPERIMENTAL SECTION2.1. Overview. Figure1shows aschematicof
theclosedwater circuit system containing a reactor where the
photocatalyticdegradationof
chemicalsororganicpollutantsiscarriedout.Thewaterflowisdriventhroughacentrifugal
micropumptoguaranteeconstant mixinginthereactorvessel.
Theheartofthe monitoring system can be seen on the left side in
Figure 1.Aliquidcell isplacedintheflowcircuit; herealight
(LEDsource)passes throughthewater/pollutant streamsothat
ameasurementofthelightabsorptioncanbemade.
Thesignalfromaphotodiodeis
thenprocessedbyanalogueelectroniccircuitry, andtheresultingsignal
correspondstotheconcen-trationof theabsorbingchemical, which,
inthis instance, ismethyl orange.2.2. Chemicals and Photocatalyst
Preparation. Methylorange (MO) sourcedfromSigma-Aldrichwas
dissolvedindeionized(DI)water intypical concentrations
rangingfrom100 to 10 ppm. Drops of the MO solution were added into
thereactor containing DI water.We monitored the real-time
photo-diode signal during this process and observed that a
homogeneousmixture was produced on a time scale of seconds. This
time scalewas negligible when compared to the rate-constant of any
of thereactions we studied.Thephotocatalyst usedfor
theseexperiments was EvonikTiO2 Aeroxide P25, which we will
subsequently refer to in thispaper as P25. P25 consists of a
mixture of 20% rutile and 80%anataseTiO2.
TheresponsiblephotocatalyticmechanismthatmakesP25oneofthemostphotocatalyticallyactivematerialsonthemarket23is
under constant debate; somebelievetherutile TiO2 acts as an
antenna, which due to a smaller
bandgapabsorbsalargerrangeofwavelengths,
whileothersclaimthatattheinterfacebetweenthetwomaterialschargeseparationand
prolongation of lifetimes enhance the
photocatalystsactivity.24,25Forcoating,
weadaptedaspin-castingmethodwhereTiO2nanoparticle suspensions were
formed by mixing TiO2 (400 mg)withethanol
(4mL)andTritonX-100surfactant
(250L).26Thinfilmswerefabricatedbyspin-castingtheTiO2suspensiononto
3 in.glass wafers.For several cycles,0.5 mL of suspensionwas drop
cast onto the substrate surface and then spun at 300 rpmfor 20 s.
The wafer was rapidly heated to 450 C for 10 min. Thefunction of
this processing step was to remove any traces of theorganic
surfactant used for spin coating. We have chosen atemperatureof450
Cbecausethisiswell belowtheannealingtemperature requiredfor
microstructural transformationof thefilm, as discussed by Zhang et
al.272.3. Characterization. Powder X-ray diffraction (XRD)
ex-periments on the TiO2 powder and coated TiO2 samples
wereperformed at room temperature using a Philips PW 3040
DY640diffractometerequippedwithagraphitemonochromatorusingCuKradiation(=0.1541nm).
Thesampleswerescannedover a 2 range of 1080oin steps of 0.02o. To
verify the surfacecoverage and morphology of the TiO2 on the glass
wafers bothbefore and after reaction, field-emission scanning
electronmicroscopy(SEM, Carl ZeissXB1540)at 5kVaccelerationvoltage
was employed. The thicknesses of the films were mea-sured using a
Dektak profilometer.2.4. ReactorAssembly.
Thewatercircuitwasassembledbyconnectingasmall batchreactor,
madefromaglasswithinletsandoutlets, inserieswithasmallcentrifugal
pumpandthe liquid cell. Figure 2 shows a schematic of the reactor
vesselon the left panel. The glass reactor vessel has a height of
70 mmand has an inner diameter of 84 mm. The reactor cover holds15
UV-LEDs and the coated wafer is fixed to the reactor
base.ThedistanceoftheUV-LEDstothephotocatalystis65mm.The inlets and
outletsare 4 mm diameter glass tubes situated10 mm from the base of
the reactor. The reactor was filled withthe test liquid in volumes
ranging from 100 to 250 mL. In mostexperiments 100 or 150 mL was
used, which give water depthsof approximately 20 or 30 mm,
respectively.For the UV light source, we have used Ultra
BrightDeepVioletLED370EUV-LEDssourcedfromThorlabs. TheFigure 1.
Schematic of the closed water circuit for evaluation
ofphotocatalytic reactions. The reactor is driven by
UV-LEDsilluminatinga substrate coatedwithphotocatalytic material
onthebase of the reactor vessel. A micropump provides constant
mixing andmassflowover thecatalyst andservesthein-streamsensor unit
tomonitor the concentration of pollutants in real time by detecting
lightabsorption.Industrial & Engineering Chemistry Research
Articledx.doi.org/10.1021/ie202366m | Ind. Eng.Chem. Res. XXXX,
XXX, XXXXXX Bemissionspectrumisindicatedontherightpanel
inFigure2witha mainemissionpeakat 375nmanda line
widthofapproximately10nm. Thus, theemittedlightlieswell
intheabsorption spectrum of P25. Each UV-LED has a half
viewingangle of 19 and a forward optical power of 2 mW at the
drivecurrent of 20 mA.The arrangement of the 15 UV-LEDS is shown in
Figure 3awithaslight prolongationalongoneaxis. Thereal
lightfieldwasphotographedandisshowninFigure3b. Theideal
lightfieldgenerated, at adistanceof 65mm, (thepositionof
thephotocatalystsurface)givesanalmostcircularillumination, asshown
in the simulation in Figure 3c. The real illuminated areadeviates
due tononideal soldering of the UV-LEDs onthelidplate andpossible
inhomogeneous molding of the lightemittingsemiconductor chips.
Theintensitydistributionwasmeasured with a Newport 918D-UV-OD3
detector and powermeter(resultsareshowninFigure3d)at
astepdistanceof1cm. Themaximumirradianceis 2.1W/m2,
withthepeakcenter shiftedslightlytotheright of theideal position.
Theintegrated power of the measured irradiated
fieldfromthemeasurement is 31.2 mW, which has to be corrected by a
factorof 4/duetothecircular apertureof thedetector
andthesquare-typemeasurementmatrixandamountsto24.5mWoftotal
irradiant power. The intensity of the light can be
changedbyapotentiometer set inseries totheUV-LEDs. For
mea-surements of the light intensity, we have plotted the
irradianceobservedinthecenterofthelightfieldandassumedalinearrelationship
with total power. The total area of the coated waferis 45.6cm2. All
15UV-LEDs irradiate approximately three-quarters of the coated
surface.The reactor vessel design was chosen to ensure both
efficientmixing of the MO solution and at a steady but controlled
massflowrateover thephotocatalyticsurface. InFigures
4a,bweshowtwo-dimensional computational fluiddynamics
(CFD)simulations and subsequent distribution of flowrates
indi-cated by velocities for two different designs, respectively.
Bothdesigns have a central circular chamber, the design in Figure
4ahas opposing inlets and outlets, whereas the design in Figure
4bhas a linear arrangement for the inlet and outlet. CFD
simula-tions were performed with EasyCFD in the steady state
regimewith turbulent flow, isothermal, and nonbuoyant settings. A
fastconverging steady state solution depending on the grid size
wasconfirmed. The in and out mass flow rate was set to 0.5
L/minsimilar tothe real pumprate. The first design(Figure 4a)shows
a slow flow in the middle of the reactor and faster flowat the edge
and also has turbulence due to the direction changeat theoutlet
fromthewater streamcomingfromtheinlet.It, therefore,
givesbettermixingpropertiesasopposedtotheFigure 2. (a) Schematic
showing the reactor, which consists of a glassvessel equipped with
inlet and outlet, with the photocatalyst fixed onits base. UV-LEDs
are fixed into the cover of the reactor. (b) Emissionspectrum of
the illuminating LEDs showing a peak emissionwavelength centered on
375 nm.Figure 3. (a) Photograph of the UV-LED pattern in the
reactor cover,(b) photograph of the resulting illuminated area on
the photocatalystcoatedwafer,
(c)simulatedirradiantpowerdistribution,
basedupongeometricarrangementassumingGaussiandistributionof
thepowerforeachLED onthephotocatalyticdisk, and
(d)measuredirradiantpower distribution reaching up to 2.1
mW/cm2.Figure 4.(a,b) Two investigatedchamber geometries in.The
designandresultsof two-dimensional CFDsimulationsarepresented.
Forillustration purposes we have included arrows indicating the
speed andflow direction in the vector diagrams. The first design in
panel a wasimplemented in the reactor. (c) Mixing of methyl orange
in the reactorvessel monitored by the in-stream sensor. Each step
represents addingone drop (ca. 0.05 mL) of a 100 ppm MO solution to
150 mL of clearwater.Industrial & Engineering Chemistry
Research Articledx.doi.org/10.1021/ie202366m | Ind. Eng.Chem. Res.
XXXX, XXX, XXXXXX Cdesign suggestedin Figure4b,wheremost
oftheliquid flowsdirectly ina linear streamfromthe inlet to the
outlet. Inaddition, the design in Figure 4a guarantees a constant
flow rateand mass exchange in the center of the photocatalytic
wafer, theareathat receivesthehighest photonfluxandisexpectedtohave
the highest photocatalytic activity, while at the same
timeproviding a fast mixing and an instant sensor reading of the
realconcentration. Figure 4c shows the concentration of
MO,measuredbythesensor upondropwiseadditionof approxi-mately
0.05mLof 100ppmMOsolutionintothe reactorcontaining 150 mL ofwater.
The concentrationincreases in astepwise manner and demonstrates
fast and homogeneousmixing in three to five seconds.2.5. In-Stream
Sensor Unit. The sensor system consists ofan aluminum milled liquid
flow cell with front and back quartzobservationwindows(seeFigure5).
Quartzhasbeenchosenfor this application because it is transparent
in the range 2002500nm. Other windowmaterials couldbe
usedtoaccessalternativespectral bands. At oneof thewindows thereis
alight-tight tubecontaininganilluminatingLEDwithspectralproperties
matchingthevisibleabsorptionof methyl
orange,alongwithcollimationoptics. Theemissionspectrumof
theLED(Hyper blue LEDLB3333fromOSRAMOptoSemi-conductor GmbH) is
taken from the datasheet and presented inFigure 6. When compared to
the measured absorption of MOin Figure 6a, both spectra have the
same maximum wavelengthat 465 nm. At the other window of the liquid
cell, is a light-tighttube containing a photodiode (visible light
photodiode BPW21from OSRAM Opto Semiconductor GmbH). Here the
spectralrange is chosen to match the illuminating LED.Thefractionof
illuminatinglightnotabsorbedbytheMOsolution registers on the
photodiode in the form of an electricalsignal I(Figure7).
Acalibratedreferencesignal representingthe intensity I0 of a total
absence of MO is produced either bythe photodiode on a similar
reference cell containing purewater or alternatively by an
adjustable constant voltage source.Bothsignalsarethenpassedfirst
throughatrans-impedanceamplifier and second a logarithmic
amplifier. In the last stage, adifferential amplifier compares the
amplified logarithmic signals.Inthis way, theoutputsignal
producedisproportionalto thequotient of the sample I and reference
signal I0:= III I log log log00(1)which, inturn, isproportional
totheconcentrationCof themonitored chemical,according to the
BeerLambert law:=I I 10alC0(2)where I and I0are the intensities of
the transmitted andincident light, is the absorption coefficient, l
the path length,and C the concentration. The resultant logarithmic
quotient istherefore directly proportional to the concentration of
themonitored chemical.3. RESULTS AND DISCUSSIONBefore considering
the properties of the catalytic reactor/sensorsystem,
somebasicmaterial characterizationof theTiO2filmwas performed. The
aimof this exercise was to establishwhether the coating process
itself affected the catalyticproperties of the TiO2. Factors suchas
the microcrystallinestructure and the film uniformity has been
studied in previousinvestigations.47,25XRD results before
andafterconfirmthatnomajorphasetransitionshaveoccurredaftercoating.
Somebroadening of the peaks was observed (Figure 8) but this
wasattributedtothedecreasedsamplevolumeinthefilm, com-pared to the
powdered state. Analysis of the SEMimages(Figure 9) shows that the
coating method resulted in a uniformcoverage with an average
thickness of about 40 nm (measuredby Dektak). This measurement was
repeated after severalcatalyticreactionshadbeenperformed.
Whilethelater SEMFigure5. (a)Theabsorptionmeasurement is
performedinspecialflow-through cells, which have fittings for the
tubing in one directionand two windows on each side on one of the
orthogonal axes. On thewindows attachedare holders for the light
source (LED) andthephotodetector tomeasurethelight
absorptionintheliquid. (b)Aphotograph of the cell.Figure 6.
Absorptionspectrumshowing the absorptionvalues formethyl orange
andthe matching emissionof theblue LED, whichisused in the
concentration sensor. The UV-LED spectrum is plotted asa
reference.Figure 7. Flow diagramof the read out and signal
processingelectronics. The signal from the photodiode (I) is
amplified and passedthrough comparator electronics to produce an
output directlyproportional to the concentration.The other input to
the differentialamplifier is the calibrated reference intensity
(I0).Industrial & Engineering Chemistry Research
Articledx.doi.org/10.1021/ie202366m | Ind. Eng.Chem. Res. XXXX,
XXX, XXXXXX Dimages did show some minor changes to the surface
morphology,showing some additional agglomeration of the particles,
the aver-age thickness of the film remained more or less constant
at 40 nm.Fromthis,
weconcludedthatthefilmshadremainedrelativelystable during the
catalytic reactions, and consequently leaching ofthe TiO2
nanoparticles into the water was negligible.The batch reactor,
micropump and liquid cell
wereconnectedbyflexible3mmdiametertubingandloadedwithDI water. The
sensor systemwas calibratedtozerooutputbefore mixing the MO
solution into the reactor. To initiate thephotocatalyticreaction,
theTiO2coatedglasswaferwasfixedon the bottom and was illuminated by
the UV-LEDs.Figure 10 depicts typical results from an experiment
measur-ing thedegradationofMO, wheretheinitial concentrationofMO in
the solution is 0.6 ppm,and after approximately six toeight hours
the orange solutionbecomes colorless.
ControlexperimentsusingawaferpreparedwithoutTiO2andexperi-ments
with no UV illumination confirmed that the decoloriza-tion is due
to the photocatalytic reaction. Figure 10a shows themeasured data
from the output of the sensor unitan almostperfectexponential
decay. Hence, weareassumingfirstorderkinetics,where the
concentration C at time t is described by= C C kt exp(
)0(3)withinitial concentrationC0, andobserveddecayratek.
Theratekisdeterminedbytheslopeofalinearfitto ln(C/C0)over t
(seeFigure10b). Fromthedata, weobserveadecayratekof 0.5h1.
Takingintoaccount theamount of water(150 MLinthis instance) and the
initial concentration of0.52 ppm, we can estimate the cleaning
capacity to be in therange of 0.0036 mol L1h1. This rate depends on
the geo-metry of the reactor, which includes the ratio of
photo-catalyticsurfaceareaandwatervolume. Toachieveanim-proved
cleaning rate,this ratio has to be optimized
throughthereactordesign.For continuous operation it is essential to
demonstrate that thereactor is stable and can be operated for many
cycles. Figure 11ashows four consecutive runs, where in each run
the concentrationwas set to0.35ppminthereactor vessel at
afillinglevel of100 mL. As can be seen in the plot, the rate gives
similar results foreachrun. Thisdemonstrates, therefore,
thatthesystemisstableand the efficacy of the photocatalyst is
conserved.In Figure 11b the UVvis spectra of the contaminated
modelwater (MO solution) and the clean water after the
photocatalyticreaction is shown. The water containing MO exhibits
the typicalpeakaround465nm. After thereaction,
thepeakdisappears,demonstratingcompleteremoval. Baiocchi
etal.28haveshownthatinthephotocatalyticprocesstheMOmoleculeisdecom-posed
into smaller molecules. The reaction proceeds through anumber of
steps including demethylation, hydroxyl attack on thephenyl ring,
and eventually cleavage of the azo bond.The finalend products are
sulfate, water, and carbon dioxide.29Figure8. Powder
X-raydiffractionpatternsof (a)thepristineP25TiO2 and (b) P25 TiO2
coated onto the glass wafer.Figure 9. Representative SEM images of
the coated wafer surface before (a) and after (b) photocatalytic
reaction.Figure 10. (a) Degradation measurement of methyl orange
solution inthereactor, showinganexponentialdecayofconcentration(C)
withdecay rate (k). (b) Negative logarithm of the concentration
divided bythe initial concentration (C0) and the observed decay
rate (k).Industrial & Engineering Chemistry Research
Articledx.doi.org/10.1021/ie202366m | Ind. Eng.Chem. Res. XXXX,
XXX, XXXXXX ETounderstandtheinfluenceofessential
parametersandtodemonstrate the capability of our setup, a series of
experimentswere performed with variations in the initial MO
concentration,the irradiance,and the liquid volume.Figure 12
presents observed decay rates from measurementsat varied initial
concentrations. All measurements were performedwith a filling
volume of 150 mL. It can be seen that there is asteep increase in
decay rate with increasing initial concen-tration,which plateaus at
higher initial concentrations.A reac-tionmodel
oftenusedtoexplainthiskineticbehavior istheLangmuirHinshelwood (LH)
model30where the adsorptionconstant Kadsdescribestherateof ad-
anddesorptionof thechemical
underinvestigationonthesurfaceandtheconstantkLH describes other
influences such as the light intensity. In themodel it followsthat
thedegradationrateridependsontheinitial concentration C0 in the
form of=+r kK CK C[ ]1 [ ]i LHads 0ads 0(4)This modelwas fitted to
our data(seeFigure 12dottedline)resulting invalues of kLH=0.32mol
L1h1andKads=0.45mol1L. Thesevaluesgiveanindicationof
thephoto-catalytic efficiency of our coated surface.The LHmodel has
beencriticizedas anoversimplifica-tion31owing to the very complex
nature of photocatalytic pro-cesses involving a series of steps
from light absorption, transferof excited states to the surface,
and production of active oxygenspecies before a reduction/oxidation
of a given
moleculecantakeplace.32Animportantparameteristheincidentlightintensity,
whichwill influencethechargecarrier dynamicsinthe semiconductor and
can affect both constants.33To see thedependenceof theirradiant
powerof
theUVinoursystem,thelightintensitywasvariedatconstantinitial
concentration(0.35 ppm) and constant filling levels of 100 mL. In
Figure 13,measurementsandderivedrateconstantsfor light
intensitiesfrom50to200Wpercm2areplotted. Theratesfollowanalmost
linear increase as indicated by the dotted line inFigure13b.
Thesmall deviationof thepoint measuredat100W/cm2can be attributed
to measurement errors.Totestthelinearincreaseof
theratewithlightintensitiesfurther, measurements at higher power
intensities up to amaximumof2000W/cm2wereperformed.
InFigures14a,bmeasurementsontwodifferent wafersareshown.
Wefoundthat our coatingprocess resultedininhomogeneous thick-ness
and together with the nonuniform light field (as seen
inFigure3)thesystemissensitivetotheexact positionof thewafer
resulting in large fluctuations in the decay rates.
Depend-ingonthepositionof thewafer
relativetotheilluminatingUV-LEDstheratescandoubleascanbeseeninFigure14a.An
overall trend in all the measurements is the linear
increaseintheirradiancerangebelow1000W/cm2andasaturationeffect that
appears at higher UV power. We have also tested areducedset of
fiveUV-LEDsandfoundthat asimilar effectoccurs. There appears to be
a transition where the rate and itsdependence on the light
intensity saturate at a similar positionaround1000W/cm2.
Itwasidentifiedthatthereactionratefollows a linear
relationshipwhenthe process is dominatedbythechemical reaction. If,
however, thelightfluxreachesathreshold the internal processes in
the semiconductor canbecomedominantandrecombinationof
chargescontrolsthereaction. Similar behaviour with respect to light
intensities hasbeen reported by Stefanov et al.34and Wang et
al.35In a third series of experiments we tested the dependence
ofdecay rate on water volume in the reactor (see Figure 15).
Asexpected, there was a clear reduction in degradation rate whenthe
volume and, therefore, the total number of molecules
whichhavetobedegradedincreases. Themeasuredratesfollowanexponential
curve, ascanbeseeninFigure15b. Thiscanbereasonably explained by
changes in the mass flow rate over thephotocatalytic surface,which
is kept constant.Figure 11. (a) Several cleaning cycles of freshly
added polluted water(methyl orange) demonstrates the possibility of
continuous operation.(b) UVvis of the prepared solution before and
after cleaning showsthecompleteremoval. Inthecontaminatedwater
methyl orangeisexpressing the typical peak around 465 nm.Figure 12.
Degradationrates of methyl orange depending ontheinitial
concentration of methyl orange used. The graph shows derivedvalues
(black squares) from measurements and a fit (dotted line) usingthe
LangmuirHinshelwood kinetic rate model.Figure 13. (a) Measurements
at constant initial concentrations(0.35ppm) varying theUV
irradiancein the center of the lightfieldilluminating the fixed
photocatalyst.(b) A plot showing the
obtaineddecayratesagainstirradiance. Thelineisaguide,
demonstratingthelinear increase of the rate with an increase of the
irradiance.Industrial & Engineering Chemistry Research
Articledx.doi.org/10.1021/ie202366m | Ind. Eng.Chem. Res. XXXX,
XXX, XXXXXX FAlthoughenergy efficient, due to UV-LEDs, the reactor
designwouldneedtobeenhancedinordertoreachperformancesofasuspension
based system.21To enhance the reactors cleaningcapacity the
geometric arrangement of light source, liquid andcatalyst or
periodicilluminationhas tobeoptimized.14Anotherpossibility is to
increase the ratio of coated surface to water volume,introducing
coated light guides.224. CONCLUSIONSWe have assembled a
cheap,robust,and small closed circulat-ingwater
systemandhaveintegratedasensor unit that canmeasure the
concentration of chemicals in a water stream.
Wehavealsodevelopedaphotocatalytictestreactoranddemon-stratedits
functionbymeasuringthedegradationof methylorange.
Thesensorsystemallowsustomonitorthedegrada-tion of the
concentration in real-time and also records degradationcurves.
Fromthedata, wecancalculatethefirst-orderratecon-stant,
whichisameasureof theefficiencyof thereaction. Oursystem provides
the possibility to investigate a range of importantparameters that
can affect the reaction rate. We have demonstratedits ability by
showing its stability in operation and by investigatingthe
dependence of the reaction rate on initial concentration,
lightintensity, and liquid volume to catalyst surface.While our
setup is designed specifically to study photo-catalytic degradation
of methyl orange, in principle, it could beused to monitor any
liquid-phase chemical or biochemicalreaction in real time. Some
alternative reactions that our systemmay be able to be adapted and
optimized to study include moni-toring fermentation reactions to
detect changes in turbidity,
detec-tingchangesinmetabolicproductconcentrations, andassessingthe
effect of antibiotics on bio-organisms.Recently, efforts have been
made to introduce standards(e.g., BSI:ISO10678:2010) to enable
comparison of
theefficiencyofnewphotocatalystsdevelopedindifferentlabora-tories
or companies. Our cheap and simple setup could
poten-tiallybeincorporatedintostandardprocedures whichwouldallow
different laboratories and companies to benchmark theirnew
photocatalysts against a competitor.Because our system is very
cheap it would be easy to scale-upby purchasing additional units.
For example, several tens of ourinventioncouldbeoperatedinparallel
for thepriceof oneUVvis spectrometer.In a real water purification
plant, the water is assessedthroughout the treatment process as
part of quality control. Oursensor could be easily adapted as an
in-line quality-testing tool thatcould raise an alarm should the
water quality fall outside samplelimits.AUTHOR
INFORMATIONCorresponding Author*Tel.: +44 (0)29 208 75315. Fax: +44
(0)29 208 74056.Address: School of Physics and Astronomy, Cardiff
University,Queens Buildings, The Parade, Cardiff CF24 3AA,
UnitedKingdomACKNOWLEDGMENTSThe authors acknowledge Hanbin Ma and
Jun Yu for valuableinput on the circuit design, Deena Modeshia and
MauriceMouradforassistingwiththecharacterisationstudies,
FelicitySartain for project management of this work, and the
Deanshipof Scientific Research at King Abdulaziz University for
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