TiO2 speciation precluding mineralization of 4-tert-butylpyridine.: Accelerated mineralization via Fenton photo-assisted reaction

TiO2 mediated photodegradation of 4-tert-butylpyridine (from now on tBP) in the presence of H2O2 was absent for ail practical purposes, because only 3% of the stoichiometric amount of CO2 was generated under mercury lamp irradiation (120 W). This study has the objective of rationalizing why in the presence of TiO2 and light the tBP molecule does not degrade, but it readily undergoes mineralization in homogeneous solution in the presence of Fe3+/H2O2. The adsorption of tBP on TiO2 was observed to be less than or equal to 1% and was also found to be a function of the suspension pH. The low adsorption observed was due to the lack of electrostatic attraction between the ionic species of tBP and on the TiO2 surface in well defined pH regions. To explain this effect, the speciation of the two reactants was modeled as a function of pH. Fenton photo-enhanced reactions led to full mineralization of tBP in relatively short times following an induction period. No complex intermediate was observed between tBP and Fe3+. In a typical run tBP (7.4 mM) in the presence of H2O2 (5.8 x 10(-2) M) and Fe3+ (1.5 x 10(-2) M) in aqueous solution was seen to mineralize in a little more than 2 h. The same solution irradiated in the presence of TiO2 instead of Fe3+ was seen to generate only about 3% of the stoichiometric amount of CO2. The mineralization products of tBP were identified as CO2, NH4+ and NO3-. The timing of the oxidant addition during the degradation process was shown to either reduce the time of tBP degradation or minimize the required amount of H2O2. Aqueous solutions of tBP generate CO2 in the presence of H2O2 and Fe3+ in the dark and under light after an induction period. The mineralization due to Fenton reactions in homogeneous media was observed to follow pseudo-first order decay kinetics. This work describes and explains the absence of reaction limitations during the photodegradation of tBP in Fenton photo-assisted reactions as compared to TiO2 photocatalytically mediated processes. (C) 2000 Elsevier Science Ltd. All rights reserved.