TiO 2 speciation precluding mineralization of 4-tert-butylpyridine. Accelerated mineralization via Fenton photo-assisted reaction

Abstract

TiO 2 mediated photodegradation of 4-tert-butylpyridine (from now on tBP) in the presence of H 2O 2 was absent for all practical purposes, because only 3% of the stoichiometric amount of CO 2 was generated under mercury lamp irradiation (120 W). This study has the objective of rationalizing why in the presence of TiO 2 and light the tBP molecule does not degrade, but it readily undergoes mineralization in homogeneous solution in the presence of Fe 3+/H 2O 2. The adsorption of tBP on TiO 2 was observed to be ≤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 TiO 2 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 Fe 3+. In a typical run tBP (7.4 mM) in the presence of H 2O 2 (5.8×10 −2 M) and Fe 3+ (1.5×10 −2 M) in aqueous solution was seen to mineralize in a little more than 2 h. The same solution irradiated in the presence of TiO 2 instead of Fe 3+ was seen to generate only about 3% of the stoichiometric amount of CO 2. The mineralization products of tBP were identified as CO 2, NH + 4 and NO − 3. 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 H 2O 2. Aqueous solutions of tBP generate CO 2 in the presence of H 2O 2 and Fe 3+ 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 TiO 2 photocatalytically mediated processes.