ReviewRemoval of microorganisms and their chemical metabolites from water using semiconductor photocatalysis
Introduction
Since the first reported studies on the ability of titanium dioxide photocatalysis to destroy microbial pathogens in water [1], [2], [3], numerous studies have been undertaken to further the knowledge and understanding of this process [4], [5], [6], [7], [8], [9], [10], [11], [12]. Early insights into the bactericidal mechanism of action of TiO2 photocatalysis were provided by Matsunaga et al. [1] who demonstrated the direct oxidation of intracellular coenzyme A in the bacteria Lactobacillus acidophilus and E. coli and in the yeast Saccharomyces cerevisia. This resulted in the inhibition of respiratory activity and eventual cell death. Later work by Saito et al. [13] showed that cell death was accompanied by a rapid leakage of potassium ions along with the slow release of bacterial protein and RNA from Streptococcus sobrinus AHT. Results from transmission electron micrographs of treated S. sobrinus AHT showed that cell death was due to a significant disorder of cell membranes and cell wall decomposition. Further evidence for cell membrane involvement in the photocatalytic killing process was then demonstrated by several other groups [14], [15], [16], [17], [18], [19].
This review will focus on recent developments in the area of photocatalytic destruction of microorganisms in water and on the challenges recent findings have presented for the real application of this technology to water treatment. In addition the use of semiconductor photocatalysis for the destruction of toxins released from cyanobacteria will also be considered. The World Health Organisation (WHO) has stated that microbial hazards are the primary concern for drinking water quality in both developed and underdeveloped countries. The greatest risk is associated with the consumption of water contaminated with human or animal faeces as these are a source of pathogenic bacteria, viruses, fungi, protozoa and helminths. Therefore the use of any technology which can improve the quality of drinking water will provide significant worldwide health benefits [20].
Section snippets
Basic processes in semiconductor photocatalysis
The basic principle of semiconductor photocatalysis relies on the formation of an electron-hole pair upon the absorption of a photon with energy equal or bigger than the semiconductor's band-gap. These two highly reactive entities are consequently involved in all subsequent coupled reactions oxidizing and reducing the suitable species in the system concomitantly.
The photogeneration of an exciton is a well established process both in direct and in indirect semiconductors [21]. Photogenerated
Photocatalytic destruction of micro-organisms
The usefulness of photocatalysis for the disinfection of water has been shown by the destructive effects it has on a wide range of microorganisms i.e. bacteria [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43] viruses [44], [45], [46], [47], fungi [48], [49], [50], [51], [52] and protozoa [53], [54], [55].
Many studies have examined the role of experimental variables on the response of microorganisms to photocatalytic treatment. These have included the effects of aeration, pH,
Photocatalytic destruction of cyanotoxins
In addition to the challenges of pathogenic micro-organisms contaminating water there has been a growing problem in terms of the occurrence of cyanobacterial blooms, resulting from factors such as increased nutrient levels in water from intensive farming [89], [90]. There are a number of genera of cyanobacteria including Microcystis, Oscillatoria, Anabaena and Aphanizomenon which generated neurotoxins including anatoxin-a, anatoxin-a(s) and saxitoxins or hepatotoxins such as microcystins or
Conclusions
Semiconductor photocatalysis has proven to be an effective method for the removal of micro-organisms and cyanotoxins from water. Over the past decades there have been a number of attempts to develop an understanding of the precise mechanism of the photocatalytic oxidation of bacterial species. While significant progress has been made there is still much research that needs to be undertaken before the process is fully understood. With respect to cyanotoxins such as microcystins, TiO2
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