Gary Low

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.

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.

Investigating the effect of UV light pre-treatment on the oxygen activation capacity of Au/TiO2

The potential for applying UV light pre-treatment to enhance the oxygen activation capacity of Au/TiO2 under ambient conditions was examined. Catalytic formic acid oxidation in an aqueous environment was employed as the test reaction. Pre-illuminating Au/TiO2 with UV light can amplify the catalytic formic acid oxidation rate by up to four times with the degree of enhancement governed by system parameters such as Au loading, pre-illumination time, and initial formic acid loading. X-ray photoelectron spectroscopy, photoluminescence spectroscopy and electrochemical assessment of the Au/TiO2 indicated light pre-illumination invokes photoexcited electron transfer from the TiO2 support to the Au deposits. The Au deposits then utilise the additional electrons to catalyse molecular oxygen activation and promote the oxidation reaction. Scanning tunneling spectroscopy analysis and first principle calculations indicated the Au deposits introduced new electronic states above the TiO2 valence band. The new electronic states were most intense at the Au–TiO2 interface suggesting the Au deposit:TiO2 perimeter may be the key region for oxygen activation. The current study has demonstrated that pre-illuminating Au/TiO2 with light can be used to augment reactions where oxygen activation is a critical component, such as for the oxidation of organic pollutants and for the oxygen reduction reaction in fuel cells or energy storage systems.