Modelling the natural photodegradation of water contaminants - A kinetic study on the light-induced aerobic interactions between riboflavin and 4-hydroxypyridine

Within our current studies on the natural photodegradation of representativ e aquatic environmental contaminants, the visible Light irradiation of the model compound 4-hydroxypyridine (4-OHP), in air-equilibrated aqueous solut ion and in the presence of riboflavin (Rf), has been studied employing stat ionary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. The r esults indicate that 4-OHP - a compound inert towards the attack of singlet molecular oxygen (O-2((1)Delta (g))) generated by Rose Bengal sensitizatio n - quenches excited singlet and triplet states of Rf, with rate constants of 2.4 x 10(9) M-1 s(-1) and 1.2 x 10(7) M-1 s(-1), respectively, The photo decomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved 4-OHP: at greater than o r equal to 20 mM limited photodecomposition occurs due to the quenching of excited singlet Rf, while at ca. 5 mM triplet Rf is photogenerated and subs equently quenched either by oxygen, giving rise to O-2((1)Delta (g)), or by 4-OHP yielding semireduced Rf through an electron transfer process. Supero xide dismutase inhibition of the oxygen uptake and flash photolysis data in dicate that superoxide anion is generated, likely by the reaction of the an ion radical from Rf with disserved oxygen, also yielding neutral, ground st ate Rf. The final result is that both 4-OHP and Rf are photodegraded, proba bly through oxidation with superoxide anion. (C) 2001 Elsevier Science B.V. All rights reserved.