Photocatalytic degradation of dyes in wastewater using TiO2/UV in a flat-plate reactor

The photocatalyst TiO₂ with UV irradiation was used to degrade dyes in textile effluent in a flat-plate photoreactor. A test system was built with the reactor area of 1 x 0.3m2, UV light of six 36W-blacklight. TiO₂ powder P25 with BET surface area 50±15m2/g, average primary particle size 21 nm, purity> 99.5% and content of 83.9% anatase and 16.1 % rutile was used as the photocatalyst. A number of dyes commonly present in dyeing wastewater were tested in this study. The different operating parameters, such as dosage of photocatalyst, the structure of the reactor, flow rates through the flat-plate reactor, UV radiation intensity and tilted angle of the reactor, were investigated. The results showed that the photocatalytic process could efficiently remove most of the colour contained in the dyeing wastewater. It was experimentally observed that first-order kinetics was adequate for characterising the process. The flow rate and the tilted angle had some influence on the film thickness of the fluid in the reactor and the empirical correlation between the film thickness of the fluid and these two parameters was developed. The photoreaction rate was mainly determined by the film thickness of the fluid on the reactor surface and the dosage of the photocatalyst. Optimum operating parameters of the system were found to be at the film thickness of about 1.4mm and a TiO₂ dosage of 1 gIL. The higher the UV intensity, the faster the reaction rate was. The results of these experiments showed that this method has the great potential for colour removal from wastewater at commercial scale.

To overcome the common difficulty of separating the used TiO₂ suspension after treatment precipitation followed with filtration was used in this study to determine the separation efficiencies. On the other hand, TiO₂ in a small pillar shape was also studied for photocatalytic degradation of textile dye effluent. The pillar pellet was made in Oegussa Company, Germany ranging from 2.5 to 5.3mm long and with a diameter of 3.7mm. It was almost pure TiO₂ (83.2% anatase and 16.8% rutile), with a S-content of <20 ppm and a CI content of the order of 0.1 wt. %. No further elements are present in contents above 0.05 wt.%. The TiO₂ pillars were placed on the flat-plate reactor that was divided by the rectangular slots and irradiated under UV light when the treated solution went through the reactor. Four dyes and their mixtures were tested. The results showed that the photocatalytic process under this configuration efficiently remove the colour from textile dyeing effluent, and pillar shape TiO₂ photocatalyst was not dissolved in water and very easy to be separated from solution, enabling it to be reused many times. The first-order kinetics was adequate for characterising the photocatalytic degradation process and the photocatalytic performance was comparable to TiO₂ powder. It is believed that the TiO₂ pellet would be a preferable form of photocatalyst in applications for textile effluent treatment process, and other wastewater treatment processes.

Solar photocatalytic oxidation (PCO) process with a flat-plate reactor

An experimental rig with a flat-plate solar reactor was built to study the effectiveness of degradation using the reactive methylene blue as sensitive objective. The factors that affect the degradation performance, such as dosages of photocatalyst (Ti0₂), initial concentration of reactive methylene blue, flow rate through the flat-plate reactor, solar UV radiation intensity and decolourising efficiency of the solution, have been investigated. The results showed that the solar PCO process with a Flat-plate Reactor could degrade the methylene blue and decolour in methylene blue solution efficiently.

Removal of VOCs by photocatalysis process using adsorption enhanced TiO2–SiO2 catalyst

Volatile organic compounds (VOCs) exist widely in both the indoor and outdoor environment. The main contributing sources of VOCs are motor vehicle exhaust and solvent utilization. Some VOCs are toxic and carcinogenic to human health, such as benzene. In this study, TiO₂–SiO₂ based photocatalysts were synthesized using the sol–gel method, with high surface areas of 274.1–421.1 m²/g obtained. Two types of pellets were used as catalysts in a fixed-bed reactor installed with a UV black light lamp. Experiments were conducted to compare their efficiencies in degrading the VOCs. Toluene was used as the VOC indicator. When the toluene laden gas stream passed through the photocatalytic reactor, the removal efficiencies were determined using a FTIR multi-gas analyser, which was connected to the outlet of the reactor to analyse the toluene concentrations. As the TiO₂–SiO₂ pellets used have a high adsorption capacity, they had dual functions as a photocatalyst and adsorbent in the hybrid photocatalysis and adsorption system. The experiments demonstrated that the porous photocatalyst with very high adsorptive capacity enhanced the subsequent photocatalysis reactions and lead to a positive synergistic effect. The catalyst can be self-regenerated by photocatalytic oxidation of the adsorbed VOCs. When the UV irradiation and feeding gas is continuous, a destruction efficiency of about 25% was achieved over a period of 20 h. Once the system was designed and operated into adsorption/regeneration mode, a higher removal efficiency of about 55% was maintained. It was found that the catalyst pellets with a higher surface area (421 m²/g) achieved higher conversion efficiency (100%) for a longer period than those with a lower surface area. A full spectrum scan was carried out using a Bio-rad Infrared spectrometer, finding that the main components of the treated gas stream leaving the reactor, along with untreated toluene, were CO₂ and water. The suspected intermediates of aliphatic hydrocarbons and CO were found in minimal amounts or were non detectable. The kinetic rate constants were calculated from the experimental results, it appeared that the stronger adsorption capacity, i.e. larger specific surface area, the higher conversion efficiency would be achieved.

A comparative evaluation of the photocatalytic and optical properties of nanoparticulate ZnO synthesised by mechanochemical processing

In this study, mechanochemical processing has been used to manufacture a nanoparticulate powder of ZnO with a controlled particle size and minimal hard agglomeration. The suitability of this ZnO powder for use as either a photocatalyst or an optically transparent UV-filter was evaluated by comparing its optical and photocatalytic properties with those of three commercially available powders that were synthesised by chemical precipitation and flame pyrolysis. The ZnO powder synthesised by mechanochemical processing was found to exhibit high optical transparency and low photocatalytic activity per unit of surface area, which indicates that it is suitable for use in optically transparent UV-filters.

Tailoring the photocatalytic activity of nanoparticulate zinc oxide by transition metal oxide doping

The successful use of nanoparticulate ZnO in applications such as UV-screening agents or photocatalyst for the destruction of chemical waste requires the development of techniques for controlling its photocatalytic activity. In this study, we have investigated transition metal doping as a means of achieving this goal. Powders of ZnO, Mn_xZn_1−xO, and Co_xZn_1−xO were synthesised by a three-stage process consisting of high-energy mechanical milling, heat treatment, and washing. The photocatalytic activity of these powders was evaluated using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It was found that the photocatalytic activity of Co_xZn_1−xO progressively decreased with the doping level. In contrast, the activity of Mn_xZn_1−xO initially increased with doping up to a level of 2 mol% and thereafter declined. These results demonstrate that doping with transition metal oxides can be used to tailor the photocatalytic properties of nanoparticulate ZnO.

The photostability of wool doped with photocatalytic titanium dioxide

Photoyellowing of wool is a serious problem for the wool industry. This study assessed the role of photocatalytic nanocrystalline titanium dioxide (P-25) as a potential antagonist or catalyst in the photoyellowing of wool. Untreated, bleached and bleached and fluorescent-whitened wool slivers were processed into fine wool powders for the purpose of even and intimate mixing with the TiO2 nanoparticles in the solid state. Pure wool and wool/TiO2 mixtures were then compressed into solid discs for a photoyellowing study under simulated sunlight and under UVB and UVC radiations. Yellowness and photo-induced chemiluminescence (PICL) measurements showed that nanocrystalline TiO2 could effectively reduce the rate of photoyellowing by inhibiting free radical generation in doped wool, and that a higher concentration of TiO2 contributed to a lower rate of photooxidation and reduced photoyellowing. Hence nanocrystalline TiO2 acts primarily as a UV absorber on wool in dry conditions and not as a photocatalyst.

Controlled synthesis of TiO2 hierarchical nanofibre structures via electrospinning and solvothermal processes : photocatalytic activity for degradation of methylene blue

The present article describes a new titanium oxide‐based (TiO2) photocatalyst that shows promise for acceleration of dye degradation. A hierarchical TiO2 nanostructure comprising nanorods on‐nanofibres has been prepared using a sol–gel route and electrospinning. Calcination of electrospun nanobre mats was performed in air at 500 °C. The TiO2 nanofibre surface was then exploited as a ‘seeding ground’ to grow TiO2 nanorods by a solvothermal process in NaOH. The nanofibres had a diameter of approximately 100 nm while the nanorods were evenly distributed on the nanofibre surface with a mean diameter of around 50–80 nm. The hierarchical nanostructure showed enhanced photocatalytic activity when compared to pure TiO2 nanofibres. This improved efficiency in degrading methylene blue through the photocatalytic process was attributed to the larger specific surface area of the TiO2 nanostructures, as well as high surface‐to‐volume ratio and higher reactive surface resulting in enhanced surface adsorption and interfacial redox reaction.

A review on the application of photocatalytic materials on textiles

Incorporation of functional additives is one of the most important ways of functionalizing textiles. Among the additives, photocatalyst has been used extensively in recent years. This paper reviews the current research status on application of photocatalysts in fibrous materials. The review concentrates on novel methods for coating various photocatalysts on textiles and properties of the photocatalyst-modified textiles. The fundamental limitations and future prospects of this research are also discussed.

Fabrication and visible-light photocatalytic behavior of perovskite praseodymium ferrite porous nanotubes

Perovskite praseodymium ferrite (PrFeO3) porous nanotubes are prepared by electrospinning of the precursor solution into nanofibers, subsequently by annealing the precursor fibers at a low temperature (e.g. 40 °C) and finally by calcination at a high temperature. The low temperature annealing treatment is found to play a key role in the formation of porous nanotube. The porous tubes show a perovskite-type PrFeO3 crystal characteristic with high optical absorption in the UV-visible region and an energy band gap of 1.97 eV. When compared with PrFeO3 porous nanofibers and PrFeO3 particles, the PrFeO3 porous nanotubes show better visible-light photo-catalytic ability to degrade Rhodamine B in aqueous phase because of the increased surface area and more active catalytic sites on the inner walls and outer surfaces.

Degradation of organic dyes by P25-reduced graphene oxide: influence of inorganic salts and surfactants

Treatment of coloured effluent treatment is a major issue for the textile industry. In this study, catalyst P25-graphene was prepared and applied for degrading dye from an aqueous solution. Three types of dyes were selected to determine the feasibility of the catalyst for the dye degradation, including sulphonic, azoic, and fluorescent dyes. P25-graphene catalyst showed good ability to degrade all selected dyes. The influence of inorganic salts and surfactants on the photocatalytic degradation of rhodamine B using catalyst P25-graphene was also investigated. The degradation of rhodamine B was suppressed by the presence of NaCl, but the effect of Na2SO4 was negligible. The degradation of rhodamine B was significantly suppressed by all three types of surfactant, namely anionic, cationic and non-ionic surfactants. NMR technique was used to investigate the mechanisms associated with this suppression.

Wettability and photodegradation activity of sol-gel dip-coated zinc oxide films

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Preparation of CuO/SiO2 and photocatalytic activity by degradation of methylene blue

A photocatalyst based on CuO/SiO2 was prepared, and evaluated for the degradation of methylene blue in aqueous medium. The photocatalyst was obtained by calcination method of copper salt, in the presence of silica. The characterization by XRD, FTIR, and TPR techniques confirmed the formation of CuO as active phase. SEM studies showed CuO deposited on the surface of SiO2. By ESI-MS, it was demonstrated that the degradation of methylene blue occurs through successive hydroxylations. Photodegradation assays showed that CuO/SiO2 was efficient for degradation, and that the material worked better in the presence of UV light.

Photocatalytic decomposition of methylene blue via Fenton mechanisms by silicon wafer doped with Au and Cu: a theoretical and experimental study

In this work, we studied the photocatalytic and the structural aspects of silicon wafers doped with Au and Cu submitted to thermal treatment. The materials were obtained by deposition of metals on Si using the sputtering method followed by fast heating method. The photocatalyst materials were characterized by synchrotron-grazing incidence X-ray fluorescence, ultraviolet-visible spectroscopy, X-ray diffraction, and assays of H(2)O(2) degradation. The doping process decreases the optical band gap of materials and the doping with Au causes structural changes. The best photocatalytic activity was found for thermally treated material doped with Au. Theoretical calculations at density functional theory level are in agreement with the experimental data.

Anelastic spectroscopy in TiO2

Titanium dioxide (rutile) has a lot of interesting and useful features and hence is widely utilized for application. It has been used as white pigment, photocatalyst, biocompatible material and semiconductor material used in solar battery. In semiconducting TiO2 oxygen vacancies are said to play an important role in the electrical conduction. Measurements of the elastic energy loss and modulus (anelastic spectroscopy) as a function of temperature can distinguish among the different atomic jumps, which occur in the various phases or at different local ordering. In this paper, it is reported anelastic relaxation measurements in TiO2 samples using a torsion pendulum operating in frequencies around 40Hz, in the temperature range between -173°C to 330°C with heating rate of 1°C/min. The results shown a reduction in the elasticity modulus with the increase in the corn starch content used for this consolidation.

Enhanced photocatalytic reduction of Hg(II) in aqueous medium by 2-aminothiazole-modified TiO2 particles

This work describes the synthesis and characterization of 2-aminothiazole-modified titania and its application on Hg (II) photoreduction in aqueous medium. Infrared spectroscopy confirmed the chemical modification of the titania matrix. The number of 2-aminothiazole groups attached to the titania was determined by Kjeldahl's method. The photocatalytic experiments were carried out in a cylindrical photoreactor thermostatted at 298 K. The resulting modified photocatalyst 2-aminothiazole titania (TiAT) revealed an enhance in the Hg (II) photoreduction capacity at studied pH values (3, 7 and 9). In addition, sorption studies showed that the photocatalyst TiAT presented a lower equilibrium time and a higher sorption capacity of Hg(II) ion, demonstrating that sorption plays a fundamental role in the photoreduction mechanism. ©2006 Sociedade Brasileira de Química.