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

Cloete, Valeska (1999-12)

Thesis (M Sc) -- University of Stellenbosch, 1999.

Thesis

ENGLISH ABSTRACT: For decades the production of potable water, from surface and ground waters, was based mainly on 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 of potentially harmful disinfection by-products in treated water. This problem, together with increased quantities of pollution in raw and surface waters, has necessitated research in alternative 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 heterogeneous photocatalysis. A semiconductor photocatalyst is required for the production of highly reactive hydroxyl radicals in aqueous medium for the photocatalytic oxidation of the relevant organic pollutants. A novel photocatalytic reactor (developed at the University of Stellenbosch) was employed to investigate the efficiency of heterogeneous photocatalysis for the production of potable water. Ti02 was used as semiconductor catalyst in the investigation. Various factors were optimised by means of statistical experimental design for the optimal destruction of NOM in the 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 NOM removal, 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 this method. Secondly, the reduction of the total organic carbon content as determined by a low temperature total carbon analyser, was investigated. The smaller molecules produced by the photocatalytic process before complete removal, are still registered as total organic carbon content, and therefore presents a wrong picture. Finally, a protocol was developed whereby the trihalomethane formation potential (THMFP) of photocatalytically treated water could be monitored. This procedure was based on simulating the chlorination process, followed by the extraction of the formed trihalomethanes by means of the technique of solid phase microextraction (SPME) and the consequent quantitative analysis of these compounds by means of a gas chromatograph coupled to a mass spectrometer. This proved to be a superior method in comparison with the other methods.

AFRIKAANSE OPSOMMING: Drinkwater was die afgelope paar dekades hoofsaaklik geproduseer deur en proses wat bestaan uit en kombinasie van koagulasie- en flokkulasie-prosesse vir die verwydering van natuurlike organiese materiaal (NOM) teenwoordig in grond- en oppervlakwater. As gevolg van onvolledige verwydering van NOM uit die waterbronne, het potensiele skadelike ontsmettingsneweprodukte gevorm in die behandelde water. Hierdie probleem, tesame met die toenemende hoeveelheid besoedeling in rou- en oppervlakwaters, het navorsing genoodsaak om alternatiewe prosesse, vir die suiwering van water in oorleg met bestaande metodes, te ondersoek. en Proses wat die afgelope ruk baie aandag geniet het,· is gebaseer op heterogene fotokatalise. en Halfgeleier fotokatalisator is nodig vir die generering van hoogs aktiewe hidroksielradikale in waterige medium wat fotokatalitiese oksidasie van die gegewe organiese besoedelingstowwe tot gevolg het. en Nuwe fotokatalitiese reaktor (ontwikkel aan die Universiteit van Stellenbosch) is gebruik om ondersoek in te stel na die effektiwiteit van heterogene fotokatalise vir die verskaffing van drinkwater. Ti02, is gebruik as halfgeleier katalisator tydens hierdie ondersoek. Verskeie faktore is met behulp van statistiese eksperimentele ontwerp geoptimiseer om optimale toestande vir die vemietiging van NOM uit rou water vas te stel. Die effektiwiteit van die vemieting van NOM, deur middel van die fotokatalitiese proses, was moeilik om te monitor. Drie verskillende metodes is vir die monitering ondersoek. Eerstens is die algemeen aanvaarde metode, waar die afname in UV absorpsie (by 254 nm) en maatstaf is vir die NOM verwydering, SODS toegepas in die flokkulasie proses, ondersoek. Die fotokatalitiese proses reduseer egter eers die NOM tot kleiner molekuh3re spesies voor die finale vernietiging deur oksidasie. Hierdie kleiner molekule absorbeer nie lig by 254 nm nie en kan dus nie deur hierdie metode waargeneem word nie. Tweedens is die afname in totale organiese koolstof inhoud bepaal deur en lae temperatuur totale organiese koolstof inhoud analiseerder. Die kleiner molekule wat deur die fotokatalitiese proses geproduseer is voor die totale verwydering daarvan, word wei geregistreer as deel van die totale organiese koolstof inhoud en skep dus 'n verkeerde indruk. Laastens is 'n protokol, vir die bepaling van die trhalometaan formasie potensiaal (THMFP) van die fotokatalities behandelde water, ontwikkel. Hierdie metode is gebaseer op die simulasie van die chlorineringsproses, gevolg deur die ekstraksie van die trihalometaanverbindings deur middel van die tegniek van soliede fase mikroekstraksie (SFME) en die daarmee gepaardgaande kwantitatiewe analise van die verbindings deur middel van 'n gaschromatograaf gekoppel aan 'n massaspektrometer. Hierdie metode is by verre beter in vergelyking met die ander metodes.

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