UV-A and UV-C induced photolytic and photocatalytic degradation of aqueous ciprofloxacin and moxifloxacin: Reaction kinetics and role of adsorption

UV-A (485 mu W cm(-2)) and UV-C (389 mu W cm(-2)) induced photolysis and TiO2-P25 mediated heterogeneous photocatalysis are investigated as advanced oxidation technologies for the removal of fluoroquinolone (FQ) antibiotics ciprofloxacin (CIP) and moxifloxacin (MOX) in aqueous solution. Experiments performed in a thermostated (298 K) lab scale batch reactor show that pH is of main importance for FQ degradation kinetics with both processes. Whereas apparent first-order kinetics of UV-A photolysis were slow for both FQs in the entire investigated pH range (k(1,CIP) <= 0.015 min(-1): k(1,MOX) <= 0.006 min(-1) at 3 <= pH <= 10), UV-C photolysis was faster with maximum k-values obtained at pH 7 and pH 10 for CIP (k(1,CIP)=0.072 min(-1)) and MOX (k(1,MOX) = 0.058 min(-1)), respectively. The highest removal rates, however, are obtained at pH 7 in the presence of TiO2 (0.5 g L-1) as a photocatalyst (k(1,CIP),(UV-A) = 0.137 min(-1), k(1,CIP),(UV-C) = 0.163 min(-1): k(1,MOX),(UV-A) = 0.227 min(-1): k(1,MOX),(UV-C) = 0.236 min(-1)). Both the difference in reaction kinetics between photolysis and heterogeneous photocatalysis and the observed pH dependency indicate that surface reactions are of main importance during TiO2/UV photocatalysis. A positive relationship is noticed between the photocatalytic FQ degradation rate and the fraction of FQ adsorbed onto the catalyst surface, with the latter being strongly pH dependent. Adsorption experiments reveal that FQ adsorption is favoured at neutral pH. Explanations are proposed based on the amphoteric nature of the FQ molecules and the pH dependent catalyst surface charge. Based on reported pKa values and experimental adsorption data, partition ratios are calculated for the different FQ species. These indicate that mainly the single positively charged and zwitter FQ ion participate in the adsorption process, explaining the highest photocatalytic degradation at pH 7. (C) 2010 Elsevier B.V. All rights reserved.