A comparison of the effectiveness of TiO2 photocatalysis and UVA photolysis for the destruction of three pathogenic micro-organisms.

TiO2 photocatalysis has demonstrated efficacy as a treatment process for water contaminated with chemical pollutants. When exposed to UVA light TiO2 also demonstrates an effective bactericidal activity. The mechanism of this process has been reported to involve attack by valence band generated hydroxyl radicals. In this study when three common bacterial pathogens, Escherichia coli, Salmonella enterica serovar Enteritidis and Pseudomonas aeruginosa, were exposed to TiO2 and UVA light a substantial decrease in bacterial numbers was observed. Control experiments in which all three pathogens were exposed to UVA light only resulted in a similar reduction in bacterial numbers. Moreover, exposure to UVA light alone resulted in the production of a smaller than average colony phenotype among the surviving bacteria, for all three pathogens examined, a finding which was not observed following treatment with UVA and TiO2. Small slow growing colonies have been described for several pathogenic bacteria and are referred to as small colony variants. Several studies have demonstrated an association between small colony variants and persistent, recurrent and antibiotic resistant infections. We propose that the production of small colony variants of pathogenic bacteria following UVA treatment of drinking water may represent a health hazard. As these small colony variants were not observed with the UVA/TiO2 system this potential hazard is not a risk when using this technology. It would also appear that the bactericidal mechanism is different with the UVA/TiO2 process compared to when UVA light is used alone

Preface to the special issue on selected papers from the Second International Conference on Semiconductor Photochemistry SP-2

This article gives an overview of the Second International Conference on Semiconductor Photochemistry, SP-2

Photocatalytic reactors for environmental remediation: a review.

Research in the field of photocatalytic reactors in the past three decades has been an area of extensive and diverse activity with an extensive range of suspended and fixed film photocatalyst configurations being reported. The key considerations for photocatalytic reactors, however, remain the same; effective mass transfer of pollutants to the photocatalyst surface and effective deployments and illumination of the photocatalyst. Photocatalytic reactors have the potential versatility to be applied to the remediation of a range of water and gaseous effluents. Furthermore they have also been applied to the treatment of potable waters. The scale-up of photocatalytic reactors for waste and potable water treatment plants has also been demonstrated. Systems for the reduction of carbon dioxide to fuel products have also been reported. This paper considers the main photocatalytic reactor configurations that have been reported to date

Development of a doped titania immobilised thin film multi tubular photoreactor.

This paper describes a novel doped titania immobilised thin film multi tubular photoreactor which has been developed for use with liquid, vapour or gas phase media. In designing photocatalytic reactors measuring active surface area of photocatalyst within the unit is one of the critical design parameters. This dictate greatly limits the applicability of any semi-conductor photocatalyst in industrial applications, as a large surface area equates to a powder catalyst. This demonstration of a thin film coating, doped with a rare earth element, novel photoreactor design produces a photocatalytic degradation of a model pollutant (methyl orange) which displayed a comparable degradation achieved with P25 TiO2. The use of lanthanide doping is reported here in the titania sol gel as it is thought to increase the electron hole separation therefore widening the potential useful wavelengths within the electromagnetic spectrum. Increasing doping from 0.5% to 1.0% increased photocatalytic degradation by ∼17% under visible irradiation. A linear relationship has been seen between increasing reactor volume and degradation which would not normally be observed in a typical suspended reactor system

Destruction of cyanobacterial toxins by semiconductor photocatalysis.

The rapid destruction of microcystin, a cyanobacterial toxin, using a titanium dioxide photocatalyst is observed; the process is extremely efficient with high concentrations of toxin completely undetectable within 10-40 min, depending on the initial concentration

Removal of microcystins from a waste stabilisation lagoon: evaluation of a packed-bed continuous flow TiO2 reactor.

Photocatalysis has been shown to successfully remove microcystins (MC) in laboratory experiments. Most research to date has been performed under ideal conditions in pure or ultrapure water. In this investigation the efficiency of photocatalysis using titanium dioxide was examined in a complex matrix (waste stabilisation lagoon water). A flow-through photocatalytic reactor was used for the photocatalytic removal of four commonly-occurring microcystin analogues (MC-YR, MC-RR, MC-LR, and MC-LA). Up to 51% removal for single MC analogues in waste lagoon water was observed. Similar removal rates were observed when a mixture of all four MC analogues was treated. Although treatment of MC-containing cyanobacterial cells of Microcystis aeruginosa resulted in no decline in cell numbers, or viability with the current reactor design and treatment regime, the photocatalytic treatment did improve the overall quality of waste lagoon water. This study demonstrates that, despite the presence of natural organic matter, the microcystins could be successfully degraded in a complex environmental matrix

A study of the kinetic solvent isotope effect on the destruction of microcystin-LR and geosmin using TiO2 photocatalysis.

We have previously reported the effectiveness of TiO2 photocatalysis in the destruction of species generated by cyanobacteria, specifically geosmin and microcystin-LR. In this paper we report an investigation of factors which influence the rate of the toxin destruction at the catalyst surface. A primary kinetic solvent isotope effect of approximately 1.5 was observed when the destruction was performed in a heavy water solvent. This is in contrast to previous reports of a solvent isotope effect of approximately 3, however, these studies were undertaken with a different photocatalyst material. The solvent isotope effect therefore appears to be dependent on the photocatalyst material used. The results of the study support the theory that the photocatalytic decomposition occurs on the catalyst surface rather than in the bulk of the solution. Furthermore it appears that the rate determining step is not oxygen reduction as previously reported

Processes influencing the destruction of microcystin-LR by TiO2 photocatalysis.

We have previously reported the effectiveness of TiO2 photocatalysis in the destruction of the cyanotoxin microcystin-LR [P.K.J. Robertson, L.A. Lawton, B. Münch, J. Rouzade, J. Chem. Soc., Chem. Commun., 4 (1997) 393; P.K.J. Robertson, L.A. Lawton, B. Münch, B.J.P.A. Cornish, J. Adv. Oxid. Technol., in press]. In this paper we report an investigation of factors which influence the rate of the toxin destruction at the catalyst surface. A primary kinetic isotope effect of approximately 3 was observed when the destruction was performed in a heavy water solvent. Hydroxylated compounds were observed as products of the destruction process. No destruction was observed when the process was investigated under a nitrogen atmosphere

Mixing regime simulation and cellulose particle tracing in a stacked frame photocatalytic reactor.

To sustainably meet the global energy demand, unconventional methods to produce renewable energy must emerge. Biofuels from cellulose (via fermentable sugar production) mediated via photocatalysis provides an alternative to conventional fossil fuels. In order to effectively drive photocatalytic processes an effective reactor design is required, the design of which is influenced by a number of key factors such as the catalyst to reactant ratio and residence time, catalyst illumination time, light penetration and distribution for the system, mass transfer limitations (mixing) and product recovery. In this study we use COMSOL Multiphysics® to simulate and assess one of the mentioned parameters – mixing regime of cellulose particles in a Stacked Frame Photocatalysis Reactor (SFPR). In the reactor design, we compare two mixers: a ‘plus’ shaped magnetic stirrer bar and an 8 blade Rushton impeller. The simulations reveal that the Rushton impeller offers a radial mixing pattern with a higher fluid velocity of 1.2m/s when compared to the stirrer bar that offers a fluid velocity of 0.9m/s. Cellulose particle tracing simulations confirm that the particle dispersion is superior in the case of the Rushton impeller as the vorticity generated during the mixing push the particles to the reactor's walls. Since the particles are forced towards the walls, there is a probability of more particles being illuminated than in the case of no or improper mixing

Remediation of oily wastewater from an interceptor tank using a novel photocatalytic drum reactor.

A novel photocatalytic reactor has been developed to remediate oily wastewaters. In the first instance degradation rates of model organic compounds, methylene blue (MB) and 4-chlorophenol (4-CP) were determined. The experimental set-up investigated a 1:10 w/v catalyst to organic solution volume, 30 g catalyst, 300 mls MB (10 μM) or 4-CP (100 μM). The catalyst investigated was a pellet catalyst to improve separation of the remediated volume from the catalyst following treatment. MB concentration decreased by 93% after 15 mins irradiation whilst 4-CP concentration decreased by 94% following 90 mins irradiation. Oily waste water (OWW) from an interceptor tank typically containing diesel oils was obtained from Sureclean, an environmental clean-up company. The OWW was treated using the same conditions as MB and 4-CP, the model organic compounds. Levels of total organic carbon (TOC) and total petroleum hydrocarbon (TPH) were used to monitor the efficacy of the photocatalytic reactor. TOC reduced by 45% following two 90 mins treatment cycles. TPH reduced by 45% following 90 mins irradiation and by a further 25% during a second stage of treatment. This reactor can be used as a polishing technique assembled within a wastewater treatment plant. Allowing for more than one pass through the reactor improves its efficiency. Presented at the 2009 International workshop on urbanisation, land use, land degradation and environment (ULE 2009), 28 September - 1 October 2009, Denizli, Turkey