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Title: Photolytic and photocatalytic (Ti02) destruction of halogenated pyridines in aqueous solution
Author: Stapleton, David Robert
Awarding Body: The University of Leeds
Current Institution: University of Leeds
Date of Award: 2007
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Abstract:
The effects of direct photolysis and combined photolysis/photocatalysis (254 nm) on the removal of the 2-halogenated pyridines, 3-chloropyridine, 2 & 3-hydroxypyridine and pyridine were investigated in aqueous solution (400 ml, 2.64 mMol L-1 , 293-323 K). Complete removal of all substrates and primary products was observed under all conditions studied, aside from a. photostable. product formed in the photolysis of 2-IPY. The degradation of the halogenated pyridines showed a strong temperature dependence. The order of rate of degradation was found to be: 2-FPY > 2-BPY > 2-CPY > 2-IPY >3-CPY The differences in the rates of reaction between the halogenated pyridines are attributed to UV absorbance (and quantum yield), electronegativity of halogen, mechanism of reaction and .'t. structure of primary product. The effects of dissolved oxygen, pH, agitation (swirl-flow) and tertbutanol were examined. None of these parameters had a significant effect on the degradation ofthe substrate. For 2-chloropyridine (2~CPY), 2-bromopyridine (2-BPY) and 2-iodopyridine (2-IPY), 2- hydroxypyridine (2-HPY) was identified as the primary product. Similarly, 3-hydroxypyridine (3- HPY) was identified in the degradation of 3-chloropyridine (3-CPY), however to a lesser extent. The formation and destruction of 2-HPY was measured under a variety experimental conditions and mathematical simulation was conducted [Levenspiel 1999]. All results were compared with the degradation of 2-HPY as the initial substrate. All variables studied had a significant effect on the degradation of 2-HPY. The degradation rates of 2-HPY and 3-HPY were considerably lower than the respective halogenated pyridines (5-10 times longer). This is attributed to lower UV absorbance and the absence of the photohydrolysis reaction observed in the case of the halogenated pyridines. In all cases the degradation of the substrate molecule was found to fit pseudo first order rate kinetics. The corresponding activation energy calculations were performed for each ofthese halogenated substrates. Experiments were performed using supported Ti02~ prepared by ... the coating methods published by Gelover et aI, 2004 and Yun et aI, 2004. No. significant effect was observed in the . . degradation rates measured compared with direct photolysis alone. A slurry combined photolytic/photocatalytic system was employed using powder catalyst, Degussa P-25. Light blocking effects (i.e. suppression of direct photolysis through solution turbidity) were found to be more prominent than the photocatalytic effects in the degradation of 2-CPY. Changes in pH and catalyst concentration, as well as the use of tert-butanol (as an OHo radical scavenge) had a significant impact of the degradation profiles of the substrate and/or primary intermediate. The primary photocatalytic degradation mechanism of the substrate was found to be electron transfer and the greater susceptibility of2-fluoropyridine (2-FPY) to photocatalysis as opposed to 2-CPY is attributed to displacement of hydroxyl radicals from the catalyst surface by F [Minero et aI, 2000a]. Kinetic analysis of the combined photolytic/photocatalytic system showed'that at low catalyst concentrations «0.05 g L·1 ), a non-linear relationship between catalyst concentration and the contribution of photocatalysis to the overall degradation rate exists. At higher catalyst concentrations, this relationship becomes linear, indicating a shift from the light blocking of direct photolysis to a fall in the photocatalytic degradation efficiency due to the inability of the light to penetrate the solution. Arange ofphotolytic and photocatalytic degradation products were identified using 2-FPY, 2-CPY and 3-CPY as the substrates. 2-HPY was not formed in sigIl:ificant quantities in the direct photolysis of2-FPY. It is believed that a one step reaction occurs to form the primary degradation product of 2-HPY. Chlorination, ring opening, ring contraction and photooxidation reactions were also observed. In the case of 3-CPY, several photodimer reactions were also identified. In the photodegradation or' 2-IPY, a stable product was observed, 'believed to be formed through the excitation of the liberated iodine ion. The photocatalytic degradation pathway of 2-FPY and 2CPY were found t~ differ. This was attributed to C-Halogen bond scission, which is postulated to OCcur in the case of2-FPY, but not 2-CPY. TOC removal was 'observed In hath·· photolytic and photocatalytic experiments. In the photolytic case, extended periods oftime was required to achieve this (around 900 minutes). In the case of aerated photolytic experiments, it is. believed that stripping (i.e. loss of VOCs through evaporation) was largely responsible for an observed increase in TOC reduction. The addition· of Ti02 (0.02-1 g Lot) had no significaf;lt effect on TOe removal rates. This was again attributed to stripping effects. It was also found that the TOC reduction of 2-HPY (when taken as the initial substrate) was significantly slower than 2-CPY, despite 2-HPY being the primary degradation product of2-CPY.
Supervisor: Not available Sponsor: Not available
Qualification Name: The University of Leeds, 2007 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.485194  DOI: Not available
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