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 thesupport of this project (T/80/429).REFERENCES(1)Corcoran, E.; Nellemann, C.; Baker, E.; Bos, R.; Osborn, D.;Savelli, H. SickWater? The Central Role of Wastewater Managementin Sustainable Development; United Nations Environment Pro-gramme, UN-HABITAT, GRID-Arendal: Arendal, Norway, 2010;www.grida.no.(2) Cheremisinoff, N. P. 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