Dissertation (PhD)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: This study investigates the electrochemical oxidation of phenol. Phenol is a major toxin
and water pollutant. In addition, during water treatment it reacts with chlorine to produce
carcinogenic chlorophenols. lts treatment down to trace levels is therefore of increasing
concern.
For this purpose, dynamically stable anodes for the breakdown of phenols to carbon
dioxide or other less harmful substances were developed and characterized. The anodes
were prepared from mixed oxides of tin (Sn) and the precious metals ruthenium (Ru),
tantalum (Ta) and iridium (Ir), which in tum were prepared using sol-gel techniques. This
involved dip-coating the aqueous salts of the respective metals onto titanium substrates
and heating to temperatures of several hundreds of degree Celsius.
The properties of these mixed oxide thin films were investigated and characterized using
thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), atomic force
microscopy (AFM), elemental dispersive energy X-ray analysis (EDX), X-ray diffraction
(XRD), Rutherford backscattering spectrometry (RBS), particle induced X-ray emission
(PIXE) and electrochemical measurements.
A variety of different electrode materials including Til Sn02-Ru02-Ir02, Ti/Ta20s-Ir02
and Ti/RhOx-Ir02 were developed and tested for their potential as oxidation catalysts for
organic pollutants in wastewaters. Depending on the anode type, phenol was found to be electrochemically degraded, to different extents, on these surfaces during electrolysis. It
was however found that the oxidation rate not only depended on the chemical
composition but also on the oxide morphology revealed, resulting from the preparation
procedure. The Ti/SnOz-Ru02-Ir02 film was found to be the most efficient surface for the
electrolytic breakdown of phenol. This film oxidized phenol at a potential of 200 mV vs
Ag/AgC!.
The activity of the catalytic systems was evaluated both on the basis of phenol removal
efficiency as well as the kinetics of these reactions. Phenol removal efficiency was more
than 90% for all the film surfaces prepared and the rate of the reaction followed first
order kinetics. A pathway for the electrochemical degradation of phenol was derived
using techniques such as HPLC to identify the breakdown products. These pathway
products included the formation of benzoquinone and the further oxidation of
benzoquinone to the carboxylic acids malic, malonic and oxalic. / AFRIKAANSE OPSOMMING: Die onderwerp van hierdie studie is die elektrochemiese oksidasie van fenol deur nuwe
gemengde-oksied elektrodes. Fenol is 'n belangrike gifstof en besoedelingsmiddel in
water. Daarbenewens kan fenolook met chloor reageer tydens waterbehandeling om
sodoende karsinogeniese chlorofenole te vorm. Dit is dus belangrik dat metodes
ondersoek word wat die konsentrasie van fenol in water verminder.
Hierdie studie behels die bereiding en karakterisering van nuwe dinamiese stabiele
anodes (DSA) vir die afbreek van fenol tot koolstofdioksied en ander minder gevaarlike
verbindings. Hierdie nuwe anodes is berei vanaf die gemengde-okside van die edelmetale
tin (Sn), ruthenium (Ru), tantalum (Ta) en iridium (Ir), met behulp van sol-gel tegnieke.
Die finale stap in die bereiding behels kalsinering van die oksides by temperature van "n
paar honderd grade Celsius. Hierdie nuwe elektrodes is later gebruik om die oksidasie
van fenol te evalueer.
Die gemengde-oksied dunlae/anodes IS d.m.v. die volgende analitiesetegnieke
gekarakteriseer: termiese-gravimetriese analise (TGA), skandeerelektronmikroskopie
(SEM), atoomkragmikroskopie (AFM), elementverstrooiingsenergie- X-straalanalise
(EDX), X-straaldiffraksie (XRD), Rutherford terug-verstrooiingspektroskopie (RBS),
partikel-geinduseerde X-straal emissie (PIXE), en elektrochemiese metings.
'n Verskeidenheid elektrodes van verskillende materiale is berei en hul potensiaal as
oksidasie-kataliste vir organiese besoedelingsmiddels in afloopwater bepaal. Hierdie
elektrodes het die volgende ingesluit: Ti/Sn02-Ru02-Ir02, Ti/Ta20s-Ir02 en Ti/RhOx-Ir02.
Gedurende elektrolise is fenol elektrochemies afgebreek tot verskillende vlakke,
afhangende van die tipe elektrode. Die oksidasietempo het egter nie alleen van die
chemiese samestelling van die elektrode afgehang nie, maar ook van die morfologie van
die okside, wat op hulle beurt van die voorbereidingsprosedure afgehang het.
Daar is bevind dat die Ti/Sn02-Ru02-Ir02 elektrode die mees effektiewe oppervlakke vir
die afbreek van fenol is. Hier het die oksidasie van fenol by 'n potensiaal van 200 mV
plaasgevind.
Die aktiwiteite van die katalitiese sisteme IS bepaal op grond van hulle
fenolverwyderingsdoeltreffendheid. Die kinetika van die reaksies is ook bepaal. Al die
elektrodes het >90% fenolverwyderingsdoeltreffendheid getoon en die reaksietempos was
van die eerste-orde.
Deur van analitiese tegnieke soos hoëdrukvloeistofchromatografie (HPLC) gebruik te
maak is die afbreekprodukte van fenol geïdentifiseer en 'n skema vir die elektrochemiese
afbreek van fenol uitgewerk. Daar is bevind dat bensokinoon gevorm het, wat later
oksidasie ondergaan het om karboksielsure te vorm.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/49947 |
| Date | 12 1900 |
| Creators | Makgae, Mosidi Elizabeth |
| Contributors | Crouch, A. M., Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xiv, 132 p. : ill. |
| Rights | Stellenbosch University |
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