Laboratory Scale Water Circuit Including a Photocatalytic Reactor and

Article pubs.acs.org/IECR

Laboratory Scale Water Circuit Including a Photocatalytic Reactor and a Portable In-Stream Sensor To Monitor Pollutant Degradation Patrick 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 Arabia ⊥ Physics Department, King Abdulaziz University, Saudi Arabia ‡

ABSTRACT: We describe a lab-scale closed-circulating test system for photocatalytic wastewater treatment. The system comprises a UV-LED photoreactor, a microcirculating fluid pump, and an in-stream sensor unit. The reactor can hold volumes up to 250 mL and is optimized to study the degradation of pollutant concentrations in the microgram to milligram per liter range using photocatalysts fixed to a planar surface within the reactor vessel. The test pollutant used was methyl orange. The in-stream sensor unit consists of a liquid flow cell with transparent windows, allowing the transmission of light from an LED to be monitored by a photodiode. The concentration of the pollutant is evaluated in real-time. The system is lightweight, cheap, portable, and flexible, ideal for laboratory or fieldwork use, and could be easily up-scaled and used for in-line quality control monitoring in a wastewater treatment plant.

1. INTRODUCTION Water pollution is a global problem. Compounds including natural organic matter and synthetic organic microcontaminants, for example, hydrocarbons, pharmaceuticals, endocrinedisrupting compounds like polychlorinated biphenyls, fertilizers and pesticides, are released constantly into the environment by industry, households, and agriculture.1 Regular wastewater plants help to remove most of the pollutants via regular and cost-effective treatment steps like sedimentation, filtration, and biological processes, all of which are deemed relatively effective for the treatment of wastewater. However, biologically toxic and nondegradable organics can still remain. Advanced treatment processes such as activated carbon and advanced oxidation processes are being adopted;2 but these can be expensive to run and result in increased water costs.3 The use of semiconductor photocatalysts to generate reactive oxygen species for advanced oxidation processes in water treatment technology has become one of the most promising techniques to provide a cheap and energy efficient method for the disinfection of water.4−6 Other advantages are that fouling can possibly be inhibited by the photocatalytic activity, and ideally the catalytic material 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 worldwide challenge. Titanium dioxide is the most widely studied photocatalytic material to date. Crystalline TiO2 is a compound semiconductor and has a bandgap that lies in the range 3.1−3.4 eV, depending on the exact crystal structure (anatase, rutile, or brookite).7 Bandgap excitation is achieved using photons with wavelengths lying in the near-UV band (shorter than ∼380 nm). TiO2 is widely available and, due to its ubiquitous use as a white pigment, is inexpensive. It is biologically compatible and very stable; © 2012 American Chemical Society

such properties have brought it accreditation even as a food additive.8 There are two methods to treat wastewater in a photocatalytic process: either to suspend the catalyst in a powder or granule form in the water, (a so-called slurry system) or coat the catalyst on a surface over which the water flows (commonly referred to as a fixed bed system).9 A possible advantage of the slurry system is that there is a much higher surface-to-liquid interface area and, therefore, a more efficient generation of reactive oxygen species or direct interaction with pollutants.10 However in this study, a fixed bed reactor system has been investigated to avoid the possible need for a post-reaction separation of catalyst from the water. A key step in the development process is to understand how well the photocatalyst behaves under different environmental conditions.11 The efficacy of a photocatalyst is usually evaluated by monitoring the degradation rate of a specific compound in aqueous solution under controlled conditions; these include concentration of solution and photocatalyst, irradiance, pH, and volume.12 One compound that is commonly used as a test model pollutant is the relatively benign chemical methyl orange13 due to its strong color visible to the naked eye and the use of conventional spectrometers to assess the concentration by absorption spectroscopy. Additionally, it has many properties of common organic pollutants such as benzene rings, sulfonate, and amine groups. There are several methods that can be used to measure the concentration of such an agent in solution. The most obvious of these is conventional spectroscopy Received: Revised: Accepted: Published: 3301

October 15, 2011 December 12, 2011 January 5, 2012 January 5, 2012 dx.doi.org/10.1021/ie202366m | Ind. Eng.Chem. Res. 2012, 51, 3301−3308

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(be it UV−vis or FTIR); however, other possibilities include high pressure (performance) liquid phase chromatography (HPLC) or liquid chromatography−mass spectroscopy (LC−MS). Common disadvantages to these methods are the equipment needed is often not easily portable and it is usually very expensive, often requiring a trained user. Furthermore, in a real world application, water will be circulating through a photocatalytic reactor with either artificial light or natural sunlight as the radiation source. In such a water circuit, it is important to monitor the concentration of the chemical(s) requiring removal. Therefore, it is of great interest to have sensor systems in place that can record the concentration in real-time. Of the existing analysis methods previously mentioned, some could be adapted to perform real-time monitoring, but the modifications would be expensive. The setup described in this study is a compact, robust, and cheap solution designed to understand the efficacy of photocatalytic reactions in real-time. It is a closed water circuit that integrates a real-time in-stream sensor and photocatalytic reactor. The reactor consists of a vessel with inlets and outlets, and a substrate coated with photocatalytic material covers the base. Water is circulated through the reactor using a centrifugal pump, providing constant mixing and flow. To initiate a photocatalytic reaction UV-LEDs, mounted on the cover of the reactor, illuminate the photocatalyst. UV-LEDs have recently become a popular choice as a UV light source in reactors because of their cheap price, long lifetime, high quantum yield, and small size,14−18 and importantly they have been shown to be effective for chemical degradation.19−22 A liquid cell that measures light transmittance is used to enable the concentration of any selected chemical in the system to be monitored. This setup enables the study of a range of parameters and optimal conditions for photocatalytic reactions accordingly.

measurement of the light absorption can be made. The signal from a photodiode is then processed by analogue electronic circuitry, and the resulting signal corresponds to the concentration of the absorbing chemical, which, in this instance, is methyl orange. 2.2. Chemicals and Photocatalyst Preparation. Methyl orange (MO) sourced from Sigma-Aldrich was dissolved in deionized (DI) water in typical concentrations ranging from 100 to 10 ppm. Drops of the MO solution were added into the reactor containing DI water. We monitored the real-time photodiode signal during this process and observed that a homogeneous mixture was produced on a time scale of seconds. This time scale was negligible when compared to the rate-constant of any of the reactions we studied. The photocatalyst used for these experiments was Evonik TiO2 Aeroxide P25, which we will subsequently refer to in this paper as P25. P25 consists of a mixture of 20% rutile and 80% anatase TiO2. The responsible photocatalytic mechanism that makes P25 one of the most photocatalytically active materials on the market23 is under constant debate; some believe the rutile TiO2 acts as an antenna, which due to a smaller bandgap absorbs a larger range of wavelengths, while others claim that at the interface between the two materials charge separation and prolongation of lifetimes enhance the photocatalyst’s activity.24,25 For coating, we adapted a spin-casting method where TiO2 nanoparticle suspensions were formed by mixing TiO2 (400 mg) with ethanol (4 mL) and Triton X-100 surfactant (250 μL).26 Thin films were fabricated by spin-casting the TiO2 suspension onto 3 in. glass wafers. For several cycles, 0.5 mL of suspension was drop cast onto the substrate surface and then spun at 300 rpm for 20 s. The wafer was rapidly heated to 450 °C for 10 min. The function of this processing step was to remove any traces of the organic surfactant used for spin coating. We have chosen a temperature of 450 °C because this is well below the annealing temperature required for microstructural transformation of the film, as discussed by Zhang et al.27 2.3. Characterization. Powder X-ray diffraction (XRD) experiments on the TiO2 powder and coated TiO2 samples were performed at room temperature using a Philips PW 3040 DY640 diffractometer equipped with a graphite monochromator using Cu Kα radiation (λ = 0.1541 nm). The samples were scanned over a 2θ range of 10−80o in steps of 0.02o. To verify the surface coverage and morphology of the TiO2 on the glass wafers both before and after reaction, field-emission scanning electron microscopy (SEM, Carl Zeiss XB 1540) at 5 kV acceleration voltage was employed. The thicknesses of the films were measured using a Dektak profilometer. 2.4. Reactor Assembly. The water circuit was assembled by connecting a small batch reactor, made from a glass with inlets and outlets, in series with a small centrifugal pump and the liquid cell. Figure 2 shows a schematic of the reactor vessel on the left panel. The glass reactor vessel has a height of 70 mm and has an inner diameter of 84 mm. The reactor cover holds 15 UV-LEDs and the coated wafer is fixed to the reactor base. The distance of the UV-LEDs to the photocatalyst is 65 mm. The inlets and outlets are 4 mm diameter glass tubes situated 10 mm from the base of the reactor. The reactor was filled with the test liquid in volumes ranging from 100 to 250 mL. In most experiments 100 or 150 mL was used, which give water depths of approximately 20 or 30 mm, respectively. For the UV light source, we have used Ultra Bright Deep Violet LED370E UV-LEDs sourced from Thorlabs. The

2. EXPERIMENTAL SECTION 2.1. Overview. Figure 1 shows a schematic of the closed water circuit system containing a reactor where the photocatalytic

Figure 1. Schematic of the closed water circuit for evaluation of photocatalytic reactions. The reactor is driven by UV-LEDs illuminating a substrate coated with photocatalytic material on the base of the reactor vessel. A micropump provides constant mixing and mass flow over the catalyst and serves the in-stream sensor unit to monitor the concentration of pollutants in real time by detecting light absorption.

degradation of chemicals or organic pollutants is carried out. The water flow is driven through a centrifugal micropump to guarantee constant mixing in the reactor vessel. The heart of the monitoring system can be seen on the left side in Figure 1. A liquid cell is placed in the flow circuit; here a light (LED source) passes through the water/pollutant stream so that a 3302

dx.doi.org/10.1021/ie202366m | Ind. Eng.Chem. Res. 2012, 51, 3301−3308


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emitting semiconductor chips. The intensity distribution was measured with a Newport 918D-UV-OD3 detector and power meter (results are shown in Figure 3d) at a step distance of 1 cm. The maximum irradiance is 2.1 W/m2, with the peak center shifted slightly to the right of the ideal position. The integrated power of the measured irradiated field from the measurement is 31.2 mW, which has to be corrected by a factor of 4/π due to the circular aperture of the detector and the square-type measurement matrix and amounts to 24.5 mW of total irradiant power. The intensity of the light can be changed by a potentiometer set in series to the UV-LEDs. For measurements of the light intensity, we have plotted the irradiance observed in the center of the light field and assumed a linear relationship with total power. The total area of the coated wafer is 45.6 cm2. All 15 UV-LEDs irradiate approximately threequarters of the coated surface. The reactor vessel design was chosen to ensure both efficient mixing of the MO solution and at a steady but controlled mass flow rate over the photocatalytic surface. In Figures 4a,b we

Figure 2. (a) Schematic showing the reactor, which consists of a glass vessel equipped with inlet and outlet, with the photocatalyst fixed on its base. UV-LEDs are fixed into the cover of the reactor. (b) Emission spectrum of the illuminating LEDs showing a peak emission wavelength centered on 375 nm.

emission spectrum is indicated on the right panel in Figure 2 with a main emission peak at 375 nm and a line width of approximately 10 nm. Thus, the emitted light lies well in the absorption spectrum of P25. Each UV-LED has a half viewing angle of 19° and a forward optical power of 2 mW at the drive current of 20 mA. The arrangement of the 15 UV-LEDS is shown in Figure 3a with a slight prolongation along one axis. The real light field

Figure 4. (a,b) Two investigated chamber geometries in. The design and results of two-dimensional CFD simulations are presented. For illustration purposes we have included arrows indicating the speed and flow direction in the vector diagrams. The first design in panel a was implemented in the reactor. (c) Mixing of methyl orange in the reactor vessel monitored by the in-stream sensor. Each step represents adding one drop (ca. 0.05 mL) of a 100 ppm MO solution to 150 mL of clear water.

show two-dimensional computational fluid dynamics (CFD) simulations and subsequent distribution of flow rates indicated by velocities for two different designs, respectively. Both designs have a central circular chamber, the design in Figure 4a has opposing inlets and outlets, whereas the design in Figure 4b has a linear arrangement for the inlet and outlet. CFD simulations were performed with EasyCFD in the steady state regime with turbulent flow, isothermal, and nonbuoyant settings. A fast converging steady state solution depending on the grid size was confirmed. The in and out mass flow rate was set to 0.5 L/min similar to the real pump rate. The first design (Figure 4a) shows a slow flow in the middle of the reactor and faster flow at the edge and also has turbulence due to the direction change at the outlet from the water stream coming from the inlet. It, therefore, gives better mixing properties as opposed to the

Figure 3. (a) Photograph of the UV-LED pattern in the reactor cover, (b) photograph of the resulting illuminated area on the photocatalyst coated wafer, (c) simulated irradiant power distribution, based upon geometric arrangement assuming Gaussian distribution of the power for each LED on the photocatalytic disk, and (d) measured irradiant power distribution reaching up to 2.1 mW/cm2.

was photographed and is shown in Figure 3b. The ideal light field generated, at a distance of 65 mm, (the position of the photocatalyst surface) gives an almost circular illumination, as shown in the simulation in Figure 3c. The real illuminated area deviates due to nonideal soldering of the UV-LEDs on the lid plate and possible inhomogeneous molding of the light 3303



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from OSRAM Opto Semiconductor GmbH). Here the spectral range is chosen to match the illuminating LED. The fraction of illuminating light not absorbed by the MO solution registers on the photodiode in the form of an electrical signal I (Figure 7). A calibrated reference signal representing

design suggested in Figure 4b, where most of the liquid flows directly in a linear stream from the inlet to the outlet. In addition, the design in Figure 4a guarantees a constant flow rate and mass exchange in the center of the photocatalytic wafer, the area that receives the highest photon flux and is expected to have the highest photocatalytic activity, while at the same time providing a fast mixing and an instant sensor reading of the real concentration. Figure 4c shows the concentration of MO, measured by the sensor upon dropwise addition of approximately 0.05 mL of 100 ppm MO solution into the reactor containing 150 mL of water. The concentration increases in a stepwise manner and demonstrates fast and homogeneous mixing in three to five seconds. 2.5. In-Stream Sensor Unit. The sensor system consists of an aluminum milled liquid flow cell with front and back quartz observation windows (see Figure 5). Quartz has been chosen

Figure 7. Flow diagram of the read out and signal processing electronics. The signal from the photodiode (I) is amplified and passed through comparator electronics to produce an output directly proportional to the concentration. The other input to the differential amplifier is the calibrated reference intensity (I0).

the intensity I0 of a total absence of MO is produced either by the photodiode on a similar reference cell containing pure water or alternatively by an adjustable constant voltage source. Both signals are then passed first through a trans-impedance amplifier and second a logarithmic amplifier. In the last stage, a differential amplifier compares the amplified logarithmic signals. In this way, the output signal produced is proportional to the quotient of the sample I and reference signal I0:

Figure 5. (a) The absorption measurement is performed in special flow-through cells, which have fittings for the tubing in one direction and two windows on each side on one of the orthogonal axes. On the windows attached are holders for the light source (LED) and the photodetector to measure the light absorption in the liquid. (b) A photograph of the cell.

for this application because it is transparent in the range 200− 2500 nm. Other window materials could be used to access alternative spectral bands. At one of the windows there is a light-tight tube containing an illuminating LED with spectral properties matching the visible absorption of methyl orange, along with collimation optics. The emission spectrum of the LED (Hyper blue LED LB3333 from OSRAM Opto Semiconductor GmbH) is taken from the datasheet and presented in Figure 6. When compared to the measured absorption of MO

log

I = log I − log I0 I0

(1)

which, in turn, is proportional to the concentration C of the monitored chemical, according to the Beer−Lambert law:

I = I010−alC

(2)

where I and I0 are the intensities of the transmitted and incident light, α is the absorption coefficient, l the path length, and C the concentration. The resultant logarithmic quotient is therefore directly proportional to the concentration of the monitored chemical.

3. RESULTS AND DISCUSSION Before considering the properties of the catalytic reactor/sensor system, some basic material characterization of the TiO2 film was performed. The aim of this exercise was to establish whether the coating process itself affected the catalytic properties of the TiO2. Factors such as the microcrystalline structure and the film uniformity has been studied in previous investigations.4−7,25 XRD results before and after confirm that no major phase transitions have occurred after coating. Some broadening of the peaks was observed (Figure 8) but this was attributed to the decreased sample volume in the film, compared to the powdered state. Analysis of the SEM images (Figure 9) shows that the coating method resulted in a uniform coverage with an average thickness of about 40 nm (measured by Dektak). This measurement was repeated after several catalytic reactions had been performed. While the later SEM

Figure 6. Absorption spectrum showing the absorption values for methyl orange and the matching emission of the blue LED, which is used in the concentration sensor. The UV-LED spectrum is plotted as a reference.

in Figure 6a, both spectra have the same maximum wavelength at 465 nm. At the other window of the liquid cell, is a light-tight tube containing a photodiode (visible light photodiode BPW21 3304



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Figure 8. Powder X-ray diffraction patterns of (a) the pristine P25 TiO2 and (b) P25 TiO2 coated onto the glass wafer.

images did show some minor changes to the surface morphology, showing some additional agglomeration of the particles, the average thickness of the film remained more or less constant at 40 nm. From this, we concluded that the films had remained relatively stable during the catalytic reactions, and consequently leaching of the TiO2 nanoparticles into the water was negligible. The batch reactor, micropump and liquid cell were connected by flexible 3 mm diameter tubing and loaded with DI water. The sensor system was calibrated to zero output before mixing the MO solution into the reactor. To initiate the photocatalytic reaction, the TiO2 coated glass wafer was fixed on the bottom and was illuminated by the UV-LEDs. Figure 10 depicts typical results from an experiment measuring the degradation of MO, where the initial concentration of MO in the solution is 0.6 ppm, and after approximately six to eight hours the orange solution becomes colorless. Control experiments using a wafer prepared without TiO2 and experiments with no UV illumination confirmed that the decolorization is due to the photocatalytic reaction. Figure 10a shows the measured data from the output of the sensor unitan almost perfect exponential decay. Hence, we are assuming first order kinetics, where the concentration C at time t is described by

C = C0 exp( − kt )

Figure 10. (a) Degradation measurement of methyl orange solution in the reactor, showing an exponential decay of concentration (C) with decay rate (k). (b) Negative logarithm of the concentration divided by the initial concentration (C0) and the observed decay rate (k).

0.52 ppm, we can estimate the cleaning capacity to be in the range of 0.0036 μmol L−1 h−1. This rate depends on the geometry of the reactor, which includes the ratio of photocatalytic surface area and water volume. To achieve an improved cleaning rate, this ratio has to be optimized through the reactor design. For continuous operation it is essential to demonstrate that the reactor is stable and can be operated for many cycles. Figure 11a shows four consecutive runs, where in each run the concentration was set to 0.35 ppm in the reactor vessel at a filling level of 100 mL. As can be seen in the plot, the rate gives similar results for each run. This demonstrates, therefore, that the system is stable and the efficacy of the photocatalyst is conserved. In Figure 11b the UV−vis spectra of the contaminated model water (MO solution) and the clean water after the photocatalytic reaction is shown. The water containing MO exhibits the typical peak around 465 nm. After the reaction, the peak disappears, demonstrating complete removal. Baiocchi et al.28 have shown that in the photocatalytic process the MO molecule is decomposed into smaller molecules. The reaction proceeds through a number of steps including demethylation, hydroxyl attack on the phenyl ring, and eventually cleavage of the azo bond. The final end products are sulfate, water, and carbon dioxide.29

(3)

with initial concentration C0, and observed decay rate k. The rate k is determined by the slope of a linear fit to −ln(C/C0) over t (see Figure 10b). From the data, we observe a decay rate k of 0.5 h−1. Taking into account the amount of water (150 ML in this instance) and the initial concentration of

Figure 9. Representative SEM images of the coated wafer surface before (a) and after (b) photocatalytic reaction. 3305



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can take place.32 An important parameter is the incident light intensity, which will influence the charge carrier dynamics in the semiconductor and can affect both constants.33 To see the dependence of the irradiant power of the UV in our system, the light intensity was varied at constant initial concentration (0.35 ppm) and constant filling levels of 100 mL. In Figure 13,

Figure 11. (a) Several cleaning cycles of freshly added polluted water (methyl orange) demonstrates the possibility of continuous operation. (b) UV−vis of the prepared solution before and after cleaning shows the complete removal. In the contaminated water methyl orange is expressing the typical peak around 465 nm.

To understand the influence of essential parameters and to demonstrate the capability of our setup, a series of experiments were performed with variations in the initial MO concentration, the irradiance, and the liquid volume. Figure 12 presents observed decay rates from measurements at varied initial concentrations. All measurements were performed

Figure 13. (a) Measurements at constant initial concentrations (0.35 ppm) varying the UV irradiance in the center of the light field illuminating the fixed photocatalyst. (b) A plot showing the obtained decay rates against irradiance. The line is a guide, demonstrating the linear increase of the rate with an increase of the irradiance.

measurements and derived rate constants for light intensities from 50 to 200 μW per cm2 are plotted. The rates follow an almost linear increase as indicated by the dotted line in Figure 13b. The small deviation of the point measured at 100 μW/cm2 can be attributed to measurement errors. To test the linear increase of the rate with light intensities further, measurements at higher power intensities up to a maximum of 2000 μW/cm2 were performed. In Figures 14a,b measurements on two different wafers are shown. We found that our coating process resulted in inhomogeneous thickness and together with the nonuniform light field (as seen in Figure 3) the system is sensitive to the exact position of the wafer resulting in large fluctuations in the decay rates. Depending on the position of the wafer relative to the illuminating UV-LEDs the rates can double as can be seen in Figure 14a. An overall trend in all the measurements is the linear increase in the irradiance range below 1000 μW/cm2 and a saturation effect that appears at higher UV power. We have also tested a reduced set of five UV-LEDs and found that a similar effect occurs. There appears to be a transition where the rate and its dependence on the light intensity saturate at a similar position around 1000 μW/cm2. It was identified that the reaction rate follows a linear relationship when the process is dominated by the chemical reaction. If, however, the light flux reaches a threshold the internal processes in the semiconductor can become dominant and recombination of charges controls the reaction. Similar behaviour with respect to light intensities has been reported by Stefanov et al.34 and Wang et al.35 In a third series of experiments we tested the dependence of decay rate on water volume in the reactor (see Figure 15). As expected, there was a clear reduction in degradation rate when the volume and, therefore, the total number of molecules which have to be degraded increases. The measured rates follow an exponential curve, as can be seen in Figure 15b. This can be reasonably explained by changes in the mass flow rate over the photocatalytic surface, which is kept constant.

Figure 12. Degradation rates of methyl orange depending on the initial concentration of methyl orange used. The graph shows derived values (black squares) from measurements and a fit (dotted line) using the Langmuir−Hinshelwood kinetic rate model.

with a filling volume of 150 mL. It can be seen that there is a steep increase in decay rate with increasing initial concentration, which plateaus at higher initial concentrations. A reaction model often used to explain this kinetic behavior is the Langmuir−Hinshelwood (L−H) model30 where the adsorption constant Kads describes the rate of ad- and desorption of the chemical under investigation on the surface and the constant kLH describes other influences such as the light intensity. In the model it follows that the degradation rate ri depends on the initial concentration C0 in the form of

ri = kLH

K ads[C0] 1 + K ads[C0]

(4)

This model was fitted to our data (see Figure 12 dotted line) resulting in values of kLH = 0.32 μmol L−1 h−1 and Kads = 0.45 μmol−1 L. These values give an indication of the photocatalytic efficiency of our coated surface. The L−H model has been criticized as an oversimplification31 owing to the very complex nature of photocatalytic processes involving a series of steps from light absorption, transfer of excited states to the surface, and production of active oxygen species before a reduction/oxidation of a given molecule 3306



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have also developed a photocatalytic test reactor and demonstrated its function by measuring the degradation of methyl orange. The sensor system allows us to monitor the degradation of the concentration in real-time and also records degradation curves. From the data, we can calculate the first-order rate constant, which is a measure of the efficiency of the reaction. Our system provides the possibility to investigate a range of important parameters that can affect the reaction rate. We have demonstrated its ability by showing its stability in operation and by investigating the dependence of the reaction rate on initial concentration, light intensity, and liquid volume to catalyst surface. While our setup is designed specifically to study photocatalytic degradation of methyl orange, in principle, it could be used to monitor any liquid-phase chemical or biochemical reaction in real time. Some alternative reactions that our system may be able to be adapted and optimized to study include monitoring fermentation reactions to detect changes in turbidity, detecting changes in metabolic product concentrations, and assessing the effect of antibiotics on bio-organisms. Recently, efforts have been made to introduce standards (e.g., BSI:ISO 10678:2010) to enable comparison of the efficiency of new photocatalysts developed in different laboratories or companies. Our cheap and simple setup could potentially be incorporated into standard procedures which would allow different laboratories and companies to benchmark their new photocatalysts against a competitor. Because our system is very cheap it would be easy to scale-up by purchasing additional units. For example, several tens of our invention could be operated in parallel for the price of one UV−vis spectrometer. In a real water purification plant, the water is assessed throughout the treatment process as part of quality control. Our sensor could be easily adapted as an in-line quality-testing tool that could raise an alarm should the water quality fall outside sample limits.

Figure 14. Rate constants were derived from measurements made at constant initial concentrations, varying the UV irradiance in the center of the light field illuminating the fixed photocatalyst on two different wafers (a,b). The first wafer in panel a shows two series where the position of the wafer is changed leading to large fluctuations in the rate. The second wafer in panel b shows two series at lower and higher irradiant power. The lines in both graphs are guides to the eye showing trends in the change of rates with increasing UV power. Both wafers show a region of linear increase at lower power but the rate reaches saturation at higher power, the threshold being around 1000 μW/cm2.

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AUTHOR INFORMATION

Corresponding 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, United Kingdom

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ACKNOWLEDGMENTS The authors acknowledge Hanbin Ma and Jun Yu for valuable input on the circuit design, Deena Modeshia and Maurice Mourad for assisting with the characterisation studies, Felicity Sartain for project management of this work, and the Deanship of Scientific Research at King Abdulaziz University for the support of this project (T/80/429).

Figure 15. (a) Measurements at different volumes in the reactor vessel. (b) Graph to show the observed decay rate against volume showing an exponential slowdown of the rate as the volume of polluted water increases.

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Although energy efficient, due to UV-LEDs, the reactor design would need to be enhanced in order to reach performances of a suspension based system.21 To enhance the reactor’s cleaning capacity the geometric arrangement of light source, liquid and catalyst or periodic illumination has to be optimized.14 Another possibility is to increase the ratio of coated surface to water volume, introducing coated light guides.22

REFERENCES

(1) Corcoran, E.; Nellemann, C.; Baker, E.; Bos, R.; Osborn, D.; Savelli, H. Sick Water? The Central Role of Wastewater Management in Sustainable Development; United Nations Environment Programme, UN-HABITAT, GRID-Arendal: Arendal, Norway, 2010; www.grida.no. (2) Cheremisinoff, N. P. Handbook of Water and Wastewater Treatment Technologies; Butterworth-Heinemann: Oxford, 2002. (3) Matsuo, T., Hanaki, K., Takizawa, S.; Satoh, H. Advances in Water and Wastewater Treatment Technology; Molecular Technology, Nutrient Removal, Sludge Reduction and Environmental Health: Elsevier Science BV: Amsterdam, 2001.

4. CONCLUSIONS We have assembled a cheap, robust, and small closed circulating water system and have integrated a sensor unit that can measure the concentration of chemicals in a water stream. We 3307



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Surface versus lattice mechanisms. J. Phys. Chem. B 2005, 109, 977− 980. (25) Li, G. H.; Gray, K. A. The solid−solid interface: Explaining the high and unique photocatalytic reactivity of TiO2-based nanocomposite materials. Chem. Phys. 2007, 339, 173−187. (26) Gan, X.; Li, X.; Gao, X.; Zhuge, F.; Yu, W. ZnO nanowire/TiO2 nanoparticle photoanodes prepared by the ultrasonic irradiation assisted dip-coating method. Thin Solid Films 2010, 518, 4809−4812. (27) Zhang, J.; Xu, Q.; Feng, Z.; Li, M.; Li, C. Importance of the relationship between surface phases and photocatalytic activity of TiO2. Angew. Chem., Int. Ed. 2008, 47, 1766 −1769. (28) Baiocchi, C.; Brussino, M. C.; Pramauro, E.; Prevot, A. B.; Palmisano, L.; Marci, G. Characterization of methyl orange and its photocatalytic degradation products by HPLC/UV−VIS diode array and atmospheric pressure ionization quadrupole ion trap mass spectrometry. Int. J. Mass Spectrom. 2002, 214, 247−256. (29) Dvininov, E.; Joshi, U. A.; Darwent, J. R.; Claridge, J. B.; Xu, Z.; Rosseinsky, M. J. Room temperature oxidation of methyl orange and methanol over Pt-HCa2Nb3O10 and Pt-WO3 catalysts without light. Chem. Commun. 2011, 47, 881−883. (30) Ollis, D. F. Kinetic disguises in heterogeneous photocatalysis. Top. Catal. 2005, 35, 217−223. (31) Emeline, A. V.; Ryabchuk, V. K.; Serpone, N. Dogmas and misconceptions in heterogeneous photocatalysis. Some enlightened reflections. J. Phys. Chem. B 2005, 109, 18515−18521. (32) Ollis, D. F. Kinetics of liquid phase photocatalyzed reactions: An illuminating approach. J. Phys. Chem. B 2005, 109, 2439−2444. (33) Mills, A.; Wang, J. S.; Ollis, D. F. Kinetics of liquid phase semiconductor photoassisted reactions: Supporting observations for a pseudo-steady-state model. J. Phys. Chem. B 2006, 110, 14386−14390. (34) Stefanov, B. I.; Kaneva, N. V.; Puma, G. L.; Dushkin, C. D. Novel integrated reactor for evaluation of activity of supported photocatalytic thin films: Case of methylene blue degradation on TiO2 and nickel modified TiO2 under UV and visible light. Colloid Surface A. 2011, 382, 219−225. (35) Wang, Z.; Liu, J.; Dai, Y.; Dong, W.; Zhang, S.; Chen, J. Dimethyl sulfide photocatalytic degradation in a light-emitting-diode continuous reactor: Kinetic and mechanistic study. Ind. Eng. Chem. Res. 2011, 50, 7977−7984.

(4) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (5) Hashimoto, K.; Irie, H.; Fujishima, A. TiO2 photocatalysis, a historical overview and future prospects. Jpn. J. Appl. Phys. 2005, 44, 8269−8285. (6) Gaya, U. I.; Abdullah, A. H. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problems. J. Photochem. Photobiol. C 2008, 9 (1), 1−12. (7) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, 2891−2959. (8) The Food Standard Agency: Current EU approved additives and their E Numbers: http://www.food.gov.uk/safereating/chemsafe/ additivesbranch/enumberlist, 2010 (Accessed September 2011). (9) Feitz, A. J.; Boyden, B. H.; Waite, T. D. Evaluation of two solar pilot-scale fixed-bed photocatalytic reactors. Water Res. 2000, 34, 3927−3932. (10) Braham, R. J.; Harris, A. T. Review of major design and scale-up considerations for solar photocatalytic reactors. Ind. Eng. Chem. Res. 2009, 48, 8890−8905. (11) Chong, M. N.; Jin, B.; Chow, C. W.; Saint, C. Recent developments in photocatalytic water treatment technology: A Review. Water Res. 2010, 44, 2997−3027. (12) Friedmann, D.; Mendive, C.; Bahnemann, D. TiO2 for water treatment: Parameters affecting the kinetics and mechanisms of photocatalysis. Appl. Catal. B 2010, 99, 398−406. (13) Brown, G. T.; Darwent, J. R. Methyl-orange as a probe for photooxidation reactions of colloidal TiO2. J. Phys. Chem. 1984, 88, 4955−4959. (14) Wang, W. Y.; Ku, Y. Photocatalytic degradation of Reactive Red 22 in aqueous solution by UV−LED radiation. Water Res. 2006, 40, 2249−2258. (15) Ghosh, J. P.; Langford, C. H.; Achari, G. Characterization of an LED based photoreactor to degrade 4-chlorophenol in an aqueous medium using coumarin (C-343) sensitized TiO2. J. Phys. Chem. A 2008, 112, 10310−10314. (16) Ghosh, J. P.; Sui, R.; Langford, C. H.; Achari, G.; Berlinguette, C. P. A comparison of several nanoscale photocatalysts in the degradation of a common pollutant using LEDs and conventional UV light. Water Res. 2009, 43, 4499−4506. (17) Natarajan, T. S.; Natarajan, K.; Bajaj, H. C.; Tayade, R. J. Energy efficient UV-LED source and TiO2 nanotube array-based reactor for photocatalytic application. Ind. Eng. Chem. Res. 2011, 50, 7753−7762. (18) Wurtele, M. A.; Kolbe, T.; Lipsz, M.; Kulberg, A.; Weyers, M.; Kneissl, M.; Jekel, M. Application of GaN-based ultraviolet-C light emitting diodes-UV LEDs-for water disinfection. Water Res. 2011, 45, 1481−1489. (19) Mori, M.; Hamamoto, A.; Takahashi, A.; Nakano, M.; Wakikawa, N.; Tachibana, S.; Ikehara, T.; Nakaya, Y.; Akutagawa, M.; Kinouchi, Y. Development of a new water sterilization device with a 365 nm UV-LED. Med. Biol. Eng. Comput. 2007, 45, 1237−1241. (20) Tayade, R. J.; Natarajan, T. S.; Bajaj, H. C. Photocatalytic degradation of methylene blue dye using ultraviolet light emitting diodes. Ind. Eng. Chem. Res. 2009, 48, 10262−10267. (21) Natarajan, T.S.; Thomas, M.; Natarajan, K.; Bajaj, H.C.; Tayade, R.J.; , Study on UVLED/TiO2 process for degradation of rhodamine B dye, Chem. Eng. J. 169 2011, 169, 126−134. (22) Kalithasan, N.; Natarajan, T. S.; Bajaj, H. C.; Tayade, R. J. Photocatalytic reactor based on UV-LED/TiO2 coated quartz tube for degradation of dyes. Chem. Eng. J. 2011, 178, 40−49. (23) Doll, T. E.; Frimmel, F. H. Kinetic study of photocatalytic degradation of carbamazepine, clofibric acid, iomeprol, and iopromide assisted by different TiO2 materials-determination of intermediates and reaction pathways. Water Res. 2004, 38, 955−964. (24) Hurum, D. C.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. Recombination pathways in the Degussa P25 formulation of TiO2: 3308


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