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

Abstract

Within our current studies on the natural photodegradation of representative aquatic environmental contaminants, the visible light irradiation of the model compound 4-hydroxypyridine (4-OHP), in air-equilibrated aqueous solution and in the presence of riboflavin (Rf), has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. The results indicate that 4-OHP — a compound inert towards the attack of singlet molecular oxygen (O 2( 1 Δ g)) generated by Rose Bengal sensitization — quenches excited singlet and triplet states of Rf, with rate constants of 2.4×10 9 M −1 s −1 and 1.2×10 7 M −1 s −1 , respectively. The photodecomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved 4-OHP: at ≥20 mM limited photodecomposition occurs due to the quenching of excited singlet Rf, while at ca. 5 mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O 2( 1 Δ g), or by 4-OHP, yielding semireduced Rf through an electron transfer process. Superoxide dismutase inhibition of the oxygen uptake and flash photolysis data indicate that superoxide anion is generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf. The final result is that both 4-OHP and Rf are photodegraded, probably through oxidation with superoxide anion.