Methods for establishing the efficiency of the photocatalytic destruction of humic substances in drinking water

[摘要] ENGLISH ABSTRACT: For decades the production of potable water, from surface and ground waters, was based mainlyon the process of coagulation and flocculation for the removal of natural organic matter (NOM).The incomplete removal of NOM from these waters has however resulted in the formation ofpotentially harmful disinfection by-products in treated water. This problem, together withincreased quantities of pollution in raw and surface waters, has necessitated research inalternative methods to be used in conjunction with existing methods for water purification.A water treatment process, which has received much attention in recent times, is heterogeneousphotocatalysis. A semiconductor photocatalyst is required for the production of highly reactivehydroxyl radicals in aqueous medium for the photocatalytic oxidation of the relevant organicpollutants. A novel photocatalytic reactor (developed at the University of Stellenbosch) wasemployed to investigate the efficiency of heterogeneous photocatalysis for the production ofpotable water. Ti02 was used as semiconductor catalyst in the investigation. Various factorswere optimised by means of statistical experimental design for the optimal destruction of NOM inthe raw water.Monitoring the NOM destruction efficiency by the photocatalytic process proved to be difficult.Three analytical methods for monitoring were investigated.Firstly, the widely accepted method of reduction of UV (at 254 nm) as a measure of NOMremoval, as applied in the flocculation process, was investigated. The photocatalytic process,however, reduces NOM to smaller molecular species before complete removal by oxidation.These smaller molecules do not absorb at 254 nm and therefore cannot be detected by thismethod.Secondly, the reduction of the total organic carbon content as determined by a low temperaturetotal carbon analyser, was investigated. The smaller molecules produced by the photocatalyticprocess before complete removal, are still registered as total organic carbon content, andtherefore presents a wrong picture. Finally, a protocol was developed whereby the trihalomethane formation potential (THMFP) ofphotocatalytically treated water could be monitored. This procedure was based on simulatingthe chlorination process, followed by the extraction of the formed trihalomethanes by means ofthe technique of solid phase microextraction (SPME) and the consequent quantitative analysis ofthese compounds by means of a gas chromatograph coupled to a mass spectrometer. Thisproved to be a superior method in comparison with the other methods.

[发布机构] Stellenbosch University