Donia Beydoun

Role of Nanoparticles in Photocatalysis

The aim of this review paper is to give an overview of the development and implications of nanotechnology in photocatalysis. The topics covered include a detailed look at the unique properties of nanoparticles and their relation to photocatalytic properties. Current applications of and research into the use of nanoparticles as photocatalysts has also been reviewed. Also covered is the utilization of nanoparticles in doped, coupled, capped, sensitized and organic–inorganic nanocomposite semiconductor systems, with an effort to enhance photocatalytic and/or optical properties of commonly used semiconductor materials. The use of nanocrystalline thin films in electrochemically assisted photocatalytic processes has been included. Finally, the use of nanoparticles has made a significant contribution in providing definitive mechanistic information regarding the photocatalytic process.

Photocatalytic oxidation of organics in water using pure and silver-modified titanium dioxide particles

The simultaneous photocatalytic degradation of organic compounds and reduction of silver ions in titanium dioxide suspensions at pH 3.0-3.5 has been studied. The organic compounds of interest were sucrose and salicylic acid. The presence of silver ions in TiO 2 suspensions was found to enhance the photooxidation of high loadings (2001) μgC) of sucrose. However, for low sucrose loadings (100 μg C), pure TiO 2 particles performed as well as modified TiO 2 particles. An optimum silver ion loading of 2.0 at.% Ag + was observed for the mineralisation of 2000 μg C sucrose. At this silver ion loading, the mineralisation of 2000 μg C sucrose was enhanced by a factor of approximately 4.0 (based on 90% overall oxidation rates). In contrast, the addition of silver ions to TiO 2 suspensions did not have any significant effect on the photocatalytic mineralisation of salicylic acid to carbon dioxide, for both low and high loadings of salicylic acid in the suspension. It was also observed that pure TiO 2 particles performed as well as silver-modified TiO 2 particles for the degradation of 2000 μg C salicylic acid. The higher activity of silver ion-modified titanium dioxide suspensions for sucrose mineralisation is predominantly due to the presence of small silver particles on the titania surface, rather than due to the trapping of electrons during the reduction of silver ions. Approximately 50% of the initial mass of silver ions added to TiO 2 suspensions were reduced to metallic silver deposits in the presence of sucrose and salicylic acid mineralisation at the 2.0 at.% Ag + loading. Nanosize silver deposits on TiO 2 particles act as sites of electron accumulation where the reduction of adsorbed species such as oxygen occur. The enhanced reduction of oxygen through better electron-hole separation in Ag/TiO 2 particles compared to pure TiO 2 particles increases the rate of sucrose mineralisation. Therefore, it is proposed that the rate-limiting step in the sucrose photooxidation reaction is the transfer of electrons to dissolved oxygen molecules, whereas in the case of salicylic acid degradation and mineralisation, the rate-limiting step is the attack of salicylic acid molecules and its degradation intermediate products by holes and hydroxyl radicals. Hence silver deposits on TiO 2 particles are not beneficial for the photocatalytic degradation and mineralisation of salicylic acid but are advantageous for the mineralisation of sucrose.

Occurrence and prevention of photodissolution at the phase junction of magnetite and titanium dioxide

A stable magnetic photocatalyst was prepared by coating a magnetic core with a layer of photoactive titanium dioxide. A direct deposition of titanium dioxide onto the surface of magnetic iron oxide particles proved ineffective in producing a stable magnetic photocatalyst, with high levels of photodissolution being observed with these samples. This observed photodissolution is believed to be due to the dissolution of the iron oxide phase, induced by the photoactive the titanium dioxide layer due to electronic interactions at the phase junction in these magnetic photocatalysts. The introduction of an intermediate passive SiO2 layer between the titanium dioxide phase and the iron oxide phase inhibited the direct electrical contact and hence prevented the photodissolution of the iron oxide phase. The magnetic photocatalyst is for use in slurry-type reactors from which the catalyst can be easily recovered by the application of an external magnetic field.

Preparation of nanosized crystalline TiO2 particles at low temperature for photocatalysis

Nanocrystalline titanium dioxide (TiO2) particles were prepared by a modified alkoxide method under acidic conditions at temperatures ranging from 60°C to 90°C. The reaction temperature was used to control the crystalline phase of the TiO2 particles. At 60°C and 75°C rutile was formed whilst at 90°C anatase and brookite were formed.The photocatalytic activity of the prepared particles was tested for the degradation of sucrose. The photocatalytic activities of the prepared nanosized TiO2 were compared to those obtained from Degussa P-25 TiO2 as well as TiO2 crystalline samples prepared using the conventional sol–gel/heat treatment method. At low organic concentrations, Degussa P-25 exhibited higher photocatalytic behaviour than all the prepared particles while, at high organic concentrations, the nanosized TiO2 particles prepared at low temperature displayed an activity comparable to Degussa P-25 but much higher than the heat treated sample. The formation of excess intermediates during the degradation of higher sucrose loadings is believed to hinder the photoactivity of Degussa P-25, while the prepared TiO2 particles are able to maintain their activity for the degradation of the intermediates of sucrose.

The effect of preparation method on the photoactivity of crystalline titanium dioxide particles

Abstract The present study shows that acidic conditions during the hydrolysis and condensation of titanium isopropoxide as well as a high water to alkoxide ratio result in crystalline TiO2 precipitates. Nanosized rutile and anatase particles were prepared at relatively low temperatures ( Surface cleanliness of the prepared particles was found to be highly influential on the photoactivity of the catalyst. The extent and type of washing was found to affect the surface properties of the particles. X-ray diffractometry, photon correlation spectroscopy (PCS), Brunauer–Emmett–Teller (BET) surface area analysis and transmission electron microscopy (TEM) were used to characterise the powders.

Effects of organic hole scavengers on the photocatalytic reduction of selenium anions

Abstract The photocatalytic reduction of selenium anions, selenate (Se(VI)) and selenite (Se(IV)) to elemental selenium (Se) over UV-illuminated TiO2 was performed using formic acid, acetic acid, methanol, ethanol, sucrose and salicylic acid as the organic hole scavengers. Photoreduction was only observed in the presence of formic acid, methanol or ethanol. The fastest rate of Se ions photoreduction was observed in the presence of formic acid followed by methanol and ethanol. This was attributed to the ability of formate ions to adsorb onto TiO2 in the presence of Se ions, its fast mineralisation rate and its ability to form reducing radicals quickly. For the methanol and ethanol systems, these two organic compounds could not compete with Se ions for the TiO2 surface and were not easily mineralised. The photocatalytic reduction of Se ions observed in the presence of these two organic compounds was attributed to their ability to form reducing radicals. When formic acid was used, optimum pH values at pH 3.5 and 4.0 was encountered for the Se(VI) and Se(IV) photoreduction, respectively. When methanol and ethanol were used as the hole scavenger in the pH range of 2.2–4.0, the greatest extent of Se ions photoreduction was encountered at pH 2.2. This suggests the different role of formic acid and methanol/ethanol in the photoreduction of Se ions.

Effect of formate and methanol on photoreduction/removal of toxic cadmium ions using TiO2 semiconductor as photocatalyst

Photoreduction/removal ofcadmium was studied at pH 7 using TiO 2 Degussa as photocatalyst, and either formate or methanol as hole scavengers. In the absence oforganic additives, approximately 60% of30 ppm cadmium was f to be removed f the solution by adsorption. Addition offormate resulted in the photoreduction ofcadmium to its metallic form. No cadmium reduction was observed when methanol was added as the hole scavenger. Zeta potential measurements ofthe catalyst suspensions and studies on the extent oforganic additive adsorption and mineralisation were used to prove the photoreduction process. It was found that the adsorption of both cadmium

Landfill Management, Leachate Generation, and Leach Testing of Solid Wastes in Australia and Overseas

Abstract Disposal of waste to landfill remains the most common means of waste management worldwide. The most serious environmental impact of waste disposal to landfill is contamination of local groundwater by the generated leachate. One measure designed to prevent this occurring is the classification of the nature of the waste (hazardous or nonhazardous) in order to determine the most appropriate means of disposal. Of the numerous tests developed to classify solid waste, the toxicity characteristic leaching procedure (TCLP) is the most commonly used in Australia. This test was developed in the United States. Since its dissemination, the TCLP has been subject to extensive scrutiny regarding its effectiveness as a compliance tool for waste classification, particularly in view of the advances in waste management practices over the past decade. Within Australia, concerns also exist regarding its applicability for Australian conditions. This review on landfill practices and waste classification found the TCLP to be limited in its usefulness as a regulatory tool within Australia. Moreover, this review identified various gaps in the knowledge pertaining to landfills, leachate generation, and waste classification that necessitate action to improve waste management practices in the future.

Studies on the Mineralization and Separation Efficiencies of a Magnetic Photocatalyst

A bench-scale system was used to demonstrate the application of a recently developed magnetic titanium dioxide photocatalyst for the degradation of organics in aqueous systems. The integrated water treatment system included a photoreactor and magnetic separator for the recovery of the magnetic photocatalyst particles. Methylene blue (MB) was used as a test pollutant. The mineralization and bleaching efficiencies of methylene blue using the magnetic photocatalyst were found to be comparable to those of Degussa P25. The magnetic photocatalyst also demonstrated high magnetic separation efficiency. The recycled photocatalyst particles were found to have the same separation efficiency as the fresh catalyst, although the mineralization and bleaching efficiencies were lower.

Evaluating the applicability of a modified toxicity characteristic leaching procedure (TCLP) for the classification of cementitious wastes containing lead and cadmium

Having the toxicity characteristic leaching procedure (TCLP) as a starting point, this study examined the effect of the various leaching parameters on the leaching of Pb and Cd from cementitious wastes. Using modified TCLP procedures, the parameters investigated were the acid concentration, leaching duration, particle size of the crushed waste, liquid to solid ratio, and the acid type. The main finding was that the final leachate pH controls the leachability of metals due to its influence on their solubility. The high alkalinity of cementitious waste buffers the leachate at a pH where most metals become insoluble. The TCLP was found to result in an unrealistic condition for cementitious wastes due to the high resultant leachate pH.