Assessment of Microwave/UV/O3 in the Photo-Catalytic Degradation of Bromothymol Blue in Aqueous Nano TiO2 Particles Dispersions
© The Author(s) 2010
Received: 15 June 2010
Accepted: 1 July 2010
Published: 18 July 2010
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© The Author(s) 2010
Received: 15 June 2010
Accepted: 1 July 2010
Published: 18 July 2010
In this study, a microwave/UV/TiO2/ozone/H2O2 hybrid process system, in which various techniques that have been used for water treatment are combined, is evaluated to develop an advanced technology to treat non-biodegradable water pollutants efficiently. In particular, the objective of this study is to develop a novel advanced oxidation process that overcomes the limitations of existing single-process water treatment methods by adding microwave irradiation to maximize the formation of active intermediate products, e.g., OH radicals, with the aid of UV irradiation by microwave discharge electrodeless lamp, photo-catalysts, and auxiliary oxidants. The results of photo-catalytic degradation of BTB showed that the decomposition rate increased with the TiO2 particle dosages and microwave intensity. When an auxiliary oxidant such as ozone or hydrogen peroxide was added to the microwave-assisted photo-catalysis, however, a synergy effect that enhanced the reaction rate considerably was observed.
Azo dye is the most widely used one of those synthesized organic dyes, whose market share is about 50% of the whole dye market. The high market share of azo dye is due to its relatively low production cost and easy supply of raw materials. When discharged, however, it causes unpleasant deep color and is reduced to toxic amines. Therefore, wastewater treatment is necessary after a use of azo dye.
The treatment of wastewater containing dyes is difficult. Generally, adsorption using activated carbon and biological treatment using microorganisms are used to remove organic pollutants such as dyes contained in waste water. However, these methods do not easily remove the complex aromatic compounds with various substitutions contained in dye wastewater and causes generation of large amount of sludge and solid waste leading to high treatment cost. Oxidation has been widely used to convert toxic non-biodegradable materials into biodegradable forms. Conventional oxidation processes using ozone or hydrogen peroxide (H2O2), however, have limits in treating a number of different kinds of pollutants, calling for a more efficient oxidation process. Traditional methods (for example adsorption on activated carbons ) only transfer contaminations from one phase to another. The most promising way for removing dyes is photo-catalysis, because this process decomposes the end dyes to water and carbon dioxide . Application of TiO2 photo-catalyst in water treatment has recently been investigated widely [3, 4]. There are still many problems yet to be solved, however, in the application of TiO2 photo-catalyst in the treatment of non-biodegradable materials. First, photo-catalysis has usually been used in air pollutants treatment because it is suitable for treatment of low-concentration pollutants. Concentrations of water pollutants, however, are much higher than those of air pollutants. Thus, their treatment by photo-catalysis is difficult compared to that of air pollutants. Second, polluted water often contains mixture of hydrophilic and hydrophobic materials. Therefore, it is not easy for the pollutants to be adsorbed on the photo-catalyst surface. Third, polluted water has high turbidity, hence low transparency, hindering activation of photo-catalysts by ultraviolet (UV) rays. Fourth, some materials are not easily degraded by photo-catalysis only. Fifth, the amount of oxygen available for photo-catalytic oxidation is very low in water compared to in air. Due to these reasons, photo-catalytic oxidation of water pollutants has not received large attention thus far. Recently, researches have been conducted actively to improve oxidative degradation performance by adding microwave irradiation as an effort to utilize TiO2 photo-catalyst in water treatment more efficiently [5–10].
In this study, a microwave/UV/TiO2/ozone/H2O2 hybrid process system, in which various techniques that have been used for water treatment are combined, is evaluated to develop an advanced technology to treat non-biodegradable water pollutants efficiently. In particular, the objective of this study is to develop a novel advanced oxidation process that overcomes the limitations of existing single-process water treatment methods by adding microwave irradiation to maximize the formation of active intermediate products, e.g., OH radicals, with the aid of UV irradiation by MDEL, photo-catalysts, and auxiliary oxidants.
The photo-catalyst was Degussa P-25 TiO2 (specific surface area 53 m2g−1 by the BET method, particle size 20–30 nm by TEM, composition 83% anatase and 17% rutile by X-ray diffraction). In this study, the photo-catalytic activity of TiO2 nano particle was investigated with the photo-catalytic decomposition of bromothymol blue (hereafter BTB) in its aqueous solution. BTB was chosen since it does not show strong absorption (and photo-decomposition) of UV-A light. High purity grade BTB was purchased from Daejung Chem. Co. Ltd. Initial concentration of BTB was about 3.0 × 10−5 mol/l, and 1,000 ml of solution was circulated into the quartz reactor tube (230 mm length, 40 mm diameter) by a flow rate of 300 cc/min. Double distilled water was employed in these studies to make a solution for the degradation experiments. The decomposition rate was evaluated from the change in BTB concentration at the reactor outlet as a function of irradiation time. The concentration of BTB was measured by the absorbance at 420 nm using a spectrophotometer (UV-1601, Shimadzu).
In this study, a short wavelength electromagnetic wave UV is emitted by MDEL upon the irradiation of microwave. Therefore, the intensity of UV increases with the microwave power. UV, which carries intense energy, is used for exciting photo-catalyst. It can also contribute to degrading BTB directly. It was not possible to figure out the detailed mechanism how microwave took part in the degradation of BTB. Nevertheless, it can be inferred from the experimental result, which showed higher degradation efficiency at higher microwave intensity, that microwave contributed to degradation of BTB indirectly by increasing UV intensity. The thermal and non-thermal effects of microwave are also presumed to have contributed directly to the degradation reaction.
As is shown in Fig. 8, the decomposition reaction seldom took place when only microwave was irradiated (M). The rate constant for the case M was much lower than the ozone addition only case (O) even with the smallest ozone addition amount of 0.75 g/hr, for which the rate constant was 0.0584 min−1. When microwave irradiation and ozone addition were applied at the same time (MO), the rate constant (0.0588 min−1) was almost same as that of the case O. Thus, microwave irradiation does not seem to play a significant role in the decomposition reaction without photo-catalysis. For the case of microwave-assisted UV-TiO2 photo-catalysis using MDEL (MUP), the rate constant (0.0547 min−1) was significantly higher than that of the microwave only case (M), but it was a little lower than the ozone only case (O). When the microwave-assisted UV-TiO2 photo-catalysis was applied on top of ozone addition (MUPO), the decomposition rate constant was very high (0.1550 min−1), which was even larger than the sum of the rate constants for the cases of MO and MUP. When hydrogen peroxide was added as the auxiliary oxidant, instead of ozone, to the microwave-assisted UV-TiO2 photo-catalysis (MUPH), the decomposition rate still remained very high; the decomposition rate constant was 0.1954 min−1 with addition of 1.1632 × 10−2 mol hydrogen peroxide. The results of MUPO and MUPH indicate that there is a synergy effect when an auxiliary oxidant such as ozone or hydrogen peroxide is added to the microwave-assisted UV-TiO2 photo-catalytic decomposition reaction.
Microwave, a kind of electromagnetic wave with a very short wavelength, excites polar molecules to cause them to rotate and vibrate back and forth rapidly: e.g., water molecules vibrate about 2.45 × 109 times per second upon microwave irradiation. The original objective of this study was to enhance the decomposition reaction rate by exciting pollutant molecules using microwave irradiation. According to the experimental results shown above, the effect of excitement of pollutant molecules was negligible. When an auxiliary oxidant such as ozone or hydrogen peroxide was added to the microwave-assisted photo-catalysis, however, a synergy effect that enhanced the reaction rate considerably was observed. This result suggests that microwave irradiation may enhance the production of active intermediate products, e.g., OH radicals, by activating the auxiliary oxidants. However, it is difficult to examine this hypothesis quantitatively using the limited experimental results obtained in this study. It is required to design a new experimental system and conduct more quantitative investigation into this question in the future.
The results of photo-catalytic degradation of BTB showed that the decomposition rate increased with the TiO2 particle dosages.
For degradation of BTB, the decomposition rate increased with microwave intensity, from analysis of the effect of microwave intensity, how microwave participates in the degradation reaction.
When microwave irradiation was used to assist the UV-TiO2 photo-catalysis by MDEL together with ozone injection, the decomposition rate increased significantly.
The H2O2 addition to reactant solution increases the photo-catalytic degradation rate to a maximum, but further addition of H2O2 above this level decreases the efficiency.
This result suggests that there is a synergy effect when the constituent techniques are applied together and that the additional irradiation of microwave can play a very important role in photo-catalysis of organic water pollutants.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0007412).
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