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Doctoral Thesis
DOI
10.11606/T.46.2016.tde-11052016-114923
Document
Author
Full name
Amilcar Machulek Junior
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2007
Supervisor
Committee
Quina, Frank Herbert (President)
Baader, Josef Wilhelm
Freire, Renato Sanches
Guardani, Roberto
Nogueira, Raquel Fernandes Pupo
Title in Portuguese
Estudos mecanísticos da origem da inibição da reação foto-Fenton por íons cloreto
Keywords in Portuguese
Fenol
Foto-Fenton
Fotoquímica
Ion cloreto
Reações químicas
Abstract in Portuguese
O objetivo principal deste estudo foi determinar a origem da inibição do processo foto-Fenton [Fe(II)/Fe(III), H2O2, luz UV] pelo íon cloreto. Um estudo das reações primárias da etapa fotocatalítica do processo foto-Fenton por fotólise por pulso de laser na presença de NaCl mostrou que a inibição reflete: i) fotólise competitiva dos complexos Fe(Cl)2+ e Fe(Cl)2+; ii) captura do radical hidroxila (dependente do pH) pelo íon cloreto. Esses dois processos formam o ânion radical menos reativo Cl2•- em lugar do radical HO•-, provocando uma progressiva inibição da reação de degradação com a diminuição do pH. Modelagem cinética destes resultados previa que a manutenção do pH em 3,0 durante a fotodegradação evitaria a formação do Cl2•-, o que foi confirmada através de experimentos de fotodegradação do fenol e da gasolina em meio aquoso na presença de NaCl. Por outro lado, na degradação do fenol pela reação térmica de Fenton [Fe(II)/Fe(III), H2O2], o radical hidroxila não parece ter um papel muito importante. A degradação térmica não foi inibida pela presença de íon cloreto e a cinética de mineralização do fenol pela reação térmica de Fenton é indistinguível da degradação do fenol pelo processo foto-Fenton inibido por NaCl. Isso sugere que a reação proposta por Hamilton, isto é, a redução de Fe(III) a Fe(II) por catecol (o principal intermediário inicial da oxidação do fenol) na presença de H2O2, é o mecanismo principal de catálise da reação térmica de Fenton no nosso sistema.
Title in English
Mechanistic studies of the origin of the inhibition of the photo-Fenton reaction by chloride ions
Keywords in English
Chemical reations
Chloride ion
Phenol
Photo-Fenton
Photochemistry
Abstract in English
The principal objective of the present study was to determine the origin of the inhibition of the photo-Fenton process [Fe(II)/Fe(III), H2O2, UV light] by chloride ion. A laser flash photolysis study of the primary reactions involved in the photocatalytic step of the photo-Fenton process in the presence of NaCl demonstrated that the inhibition reflects: i) competitive photolysis of the complexes Fe(Cl)2+ and Fe(Cl)2+; ii) pH-dependent sequestration of the hydroxyl radical by chloride ion. Both processes form the less reactive radical anion Cl2•- instead of HO, resulting in a progressive inhibition of the degradation with decreasing reaction pH. Kinetic modeling of these results predicted that maintaining the pH at 3.0 throughout the reaction should prevent the formation of Cl2•-, a prediction confirmed experimentally for the photodegradation of phenol and gasoline. In contrast, in the degradation of phenol via the thermal Fenton reaction [Fe(II)/Fe(III), H2O2], the hydroxyl radical does not appear to play a very important role. Thus, the thermal degradation is not inhibited by chloride ion and the kinetics of mineralization of phenol by the thermal Fenton reaction are indistinguishable from those for degradation of phenol in the photo-Fenton reaction inhibited by NaCl. This suggests that the reaction proposed by Hamilton, i.e., reduction of Fe(III) to Fe(II) by catechol (the major initial intermediate in the oxidation of phenol) in the presence of H2O2, is the principal catalytic mechanism operative in the thermal Fenton reaction in our system.
 
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Publishing Date
2016-05-11
 
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