The destruction of the cyanobacterial toxin microcystin-LR by semiconductor photocatalysis.
Cornish, Benjamin J. P. A.
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CORNISH, B.J.P.A. 2000. The destruction of the cyanobacterial toxin microcystin-LR by semiconductor photocatalysis. Robert Gordon University, PhD thesis.
In fresh waters where cyanobacteria (blue-green algae) flourish, dense growths known as blooms occur. Such blooms present a threat to human and animal health as many of these cyanobacteria produce toxins. One such group of toxins are the microcystins which are hepatotoxic resulting in haemoraging and tumour promotion in the liver. There have been several reports of human poisonings resulting from the presence of cyanotoxins in potable waters, some of which have resulted in fatalities. The most frequently cited cyanotoxin in these poisonings has been microcystin-LR, which has prompted the World Health Organisation (WHO) to set a guideline for the recommended safe level of this toxin in drinking water of 1 mgl-1. Removal of microcystin-LR from potable waters has proven to be inefficient using conventional water treatment techniques such as coagualtion, filtration and chemical oxidation using chlorine. While activated carbon adsorption and membrane filtration have been shown to physically remove microcystin-LR from water the toxin is not destroyed. Recently the use of photocatalysis was shown to rapidly degrade microcystin-LR even at high concentrations. The process involves the illumination of a titanium dioxide catalyst with ultraviolet (UV) light to produce highly oxidising hydroxyl radicals in solution. While several researchers have demonstrated the process's effectiveness in degrading the toxin none have determined the fate of the compound, or if the toxicity related to microcystin-LR has been removed. This study was carried out to determine if photocatalytic oxidation of microcystin-LR was suitable as a treatment method for potable water supplies. Analysis of treated toxin samples by high performance liquid chromatography (HPLC) with photo-diode array detection (PDA) and mass spectroscopy established that the toxin was not completely degraded during photocatalysis. A simple toxicity assessment however indicated that by-products were non-toxic. Using the data from this work a proposed pathway for toxin destruction was produced giving the speculative identity of some of the by-products. The use of hydrogen peroxide to enhance UV mediated destruction of microcystin-LR has been previously reported. There have also been reports of the enhancement of photocatalytiC reaction in the presence of this oxidant. The work carried out in this study demonstrated that the destruction of microcystin-LR by photocatalysis was both more rapid and more efficient when hydrogen peroxide was present in the system. The use of a fixed film flow reactor was also investigated for microcystin-LR destruction. While degradation of the toxin occurred it was demonstrated that batch reactors were more efficient as a treatment method. The effectiveness of the photocatalytic process on microcystin-RR, -LW and -LF was also investigated. While destruction of a" the variants occurred during photocatalytic treatment each microcystin demonstrated different rates and efficiencies of photooxidation. It was concluded from this study that photocatalysis is a promising treatment method for the removal of microcystin-LR and other variants from potable waters. Further research however is required to assess if the tumour promoting effects of microcystin-LR are rendered inactive and to determine the behaviour of the toxins degradation in natural water supplies. The study also allowed for speculation as to how the degradation of the toxin occurred during the photocatalytic process.