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Using photocatalytic degradation as a demonstration of an advanced oxidation process.

This article describes an experiment which uses a relatively new wastewater treatment process to illustrate concepts in heterogeneous catalysis and reaction engineering

The new program in environmental engineering within chemical engineering at the University of Waterloo needs new laboratory experiments that combine classical chemical engineering concepts with emerging environmental technologies. The experiment described in this article uses a relatively new wastewater treatment process to demonstrate important concepts in heterogeneous catalysis and reaction engineering.

Photocatalysis is one of a series of pollutant degradation techniques known as advanced oxidative processes. Advanced oxidation processes (AOPs), which include such techniques as Fenton's reagent oxidations, ultraviolet (UV) photolysis and sonolysis, are capable of degrading organic contaminants at ambient temperature and pressure. Thus, AOPs can provide cost savings over conventional treatment methods especially for dilute pollutants. AOPs may also have an advantage over biological treatment for waste streams containing toxic or bio-inhibitory contaminants.

In photocatalysis, light energy from a UV-A light source ([Lambda] 350-400 nm) excites an electron from the valence band of the catalyst to the conduction band (Figure 1 shows a schematic representation of the photocatalytic process). This promotion of an electron leaves a region of positively charged space on the titanium dioxide surface that is termed a "fixed valence band hole". The promoted electron can either recombine with a hole or may be accepted by an electron acceptor and removed from the valence band. The usual electron acceptor is dissolved oxygen in aqueous systems and the reaction is as follows:

[Mathematical Expression Omitted]

The superoxide radical ([O.sub.2]) in aqueous solution can undergo further reactions which result in the formation of hydroxyl radicals (O[H.sup.*]). The fixed valance band hole ([[h.sub.cb.].sup.+]) can react with hydroxide ion to form hydroxyl radicals in the following reaction:

[Mathematical Expression Omitted]

The hydroxyl radicals have high oxidizing potential (second only to that of fluorine) and hence can attack most organic structures, causing oxidation and leading to complete mineralization of carbon in the system (all carbon converted to C[O.sub.2] or C[[O.sub.3].sup.2-] species).

Titanium dioxide is the most commonly used semi-conductor in photocatalysis as a result of its suitable band-gap, its non-toxic nature and its resistance to photo-corrosion.

The rate of photocatalytic degradation is governed by the concentration of the contaminant being degraded, the contaminant temperature, the light intensity reaching the catalyst surface and the presence or absence of electron acceptors (e.g., hydrogen peroxide) or radical scavengers (e.g., bicarbonate species).

Materials and methods

The organic dye degraded is quinoline acid yellow (QAY) obtained from Aldrich Chemical Company. This inexpensive, water-soluble, non-toxic dye absorbs strongly at 412 nm such that concentrations of between 35[[micro]meter] and 2[[micro]meter] (12.3 mg/L and 0.7 mg/L) could be accurately analyzed by spectrometry and application of Beer's law.

The apparatus used for experimentation is shown in Figure 2. A reaction vessel coated with a thin layer of titanium dioxide is filled with the aqueous dye solution and the 9W UV-A lamp (a Philips PL-S9W/10 fluorescent lamp, Microlites Scientific, Scarborough ON) is switched on once the solution reaches the desired temperature. To maintain a constant temperature, the reaction vessel is submerged in a constant temperature bath since much of the energy of the UV-A lamp is dissipated as heat. A magnetic stir bar is required to prevent the system from becoming mass transfer limited. The use of a flow-through cuvette for spectrometric analysis eliminates the error caused by removing and repositioning the cuvette which, in the experience of the authors, can be significant.

The titanium dioxide catalyst (Degussa P25) is immobilized on the vessel walls by first mixing a 1% by weight slurry (in de-ionized water) of P25 in the reaction vessel for about 2 h. The slurry is poured off and the vessel dried for 1 h at 100 [degrees] C. The temperature in the oven is then increased to 200 [degrees] C and the vessel is left in the oven for 1 h. After cooling, another 1% slurry of P25 is mixed in the reaction vessel for 2 h and the same drying procedure is used. De-ionized water is then placed in the vessel and the water agitated for 24 h to remove any loose P25 from the surface of the reactor. Attempting to use more than two coatings did not improve the rate of QAY degradation and simply led to more loose P25 removal from the surface during the 24 h agitation period. The double coating of P25 has proven to be durable, losing only about 10% of its activity in 400 h of experimentation.

After a run, the vessel is cleaned by first rinsing with de-ionized water. A second rinse using either acetone or methanol is required to remove any water insoluble intermediates from the reactor. Three or four de-ionized water rinses are then used to remove the organic solvent. Finally, the UV-A lamp is switched on for 15 min in the empty reactor vessel to fully oxidize any remaining organics to carbon dioxide. Neglecting this cleaning procedure resulted in up to a 20% reduction in the observed reaction rate with each subsequent degradation run.

Safety

Both the Degussa P25 and the QAY are nontoxic, non-flammable and non-corrosive and pose only a minor risk of respiratory irritation when handled in solid form. The only identified physical hazard associated with this experiment comes from the radiation of the ultraviolet light source. The radiation field strength at 30 cm from the reaction vessel is about 300 W/[cm.sup.2] which corresponds to less than 45% of the Occupational Health and Safety Act's exposure limit for an eight hour day at a wavelength of 350 nm (and is about one-seventh of the intensity experienced during a sunny summer day). It is still advisable that UV-resistant glasses be worn while using ultraviolet light.

Suitable topics for Experimental Demonstration

To date, the experimental set-up has been successfully tested by several undergraduate student groups. Following is a short description of the types of phenomena that they have examined.

The effect of concentration on the rate of degradation can be expressed in terms of a rate law. QAY degradation exhibits Langmuir-Hinshelwood (LH) kinetics at concentrations between 2 [[micro]meter] and 30 [[micro]meter]. The constants (k, K) of the LH model, given by:

Rate = kC/1 + KC

can be found by using the approximation:

Rate = dC/dt [approximately equal to] [Delta]C/[Delta]t

Thus, by measuring the concentration as a function of time, using the approximation above and plotting 1/Rate vs. 1/C it is possible to estimate the kinetic (k) and binding (K) parameters of the LH model. The values for the binding and kinetic constants are a function of many factors in the experimental setup including light intensity reaching the catalyst surface, quality of the de-ionized water used and the catalyst surface to reactor volume ratio. Our experimental results show that a reasonable exposure time (20 minutes), at 30 [degrees] C with a 9W UV-A source and a cylindrical vessel (volume 350 mL, ID 7 cm) is required to degrade QAY to 50% of its original concentration.

The rate of QAY degradation increases with temperature according to the Arrhenius equation:

k = [A.sub.0] exp {-[E.sub.a]/RT}

The activation energy ([E.sub.a]) observed is approximately 30 kJ/mol, which is typical for reactions involving radicals. Although the binding constant (K) is temperature dependent, experiments have shown that it is difficult to detect this effect without working across a broader temperature range than would be possible in a two or three hour laboratory period.

The light intensity reaching the catalyst surface also influences the rate of reaction. As light intensity increases from zero, the rate of reaction increases linearly. At an intensity of about 1700 [[micro]watts]/[cm.sup.2], the rate of recombination of holes and electrons becomes significant and above this threshold the rate of reaction only increases with the square-root of intensity. By varying the distance between the light source and the catalyst or by using a filtering material with a known transmittance at [approximately]365 nm, it is possible to investigate the relationship between initial reaction rate and light intensity.

The addition of auxiliary electron acceptor (e.g., [H.sub.2][O.sub.2]) reduces the rate of recombination of holes and electrons and hence increases the rate of photocatalytic degradation. Adding a 0.35% by weight hydrogen peroxide solution increased the QAY degradation rate by between 15 to 25 percent over that found without hydrogen peroxide. Tests with no catalyst present confirm that with the UV-A light used there was no homogeneous photolysis (ie., non-catalytic degradation) of the QAY.

Conclusions

The photocatalytic degradation of QAY provides a fascinating system to study both standard chemical engineering concepts such as rate law constant estimation and determination of activation energy. It also provides an opportunity to examine specific photocatalytic reaction parameters such as the effect of electron acceptor concentrations and light intensity at the catalyst surface.

Acknowledgement

The Philips PL-S9W/10 fluorescent lamp was kindly contributed by Microlites Scientific, Scarborough, ON.

Reference

Turchi, C.S. and D.F. Ollis. 'Photocatalytic Degradation of Organic Water Contaminants: Mechanisms Involving Hydroxyl Radical Attack', Journal of Catalysis, 122(1): 178-192, 1990.

Terrence L. Koehler is a MASc student researching the photocatalytic degradation of volatile organics in air. Robert R. Hudgins, FCIC, is a Professor specializing in heterogeneous catalysis and reactor engineering and William A. Anderson, MCIC, is an Assistant Professor researching in the area of photocatalysis and advanced oxidation processes for water and air treatment. All are with the Department of Chemical Engineering at the University of Waterloo, Waterloo, ON.
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Author:Koehler, Terrence L.; Hudgins, Robert R.; Anderson, William A.
Publication:Canadian Chemical News
Date:Mar 1, 1996
Words:1619
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