Kinetics of O·-and O3·- in alkaline aqueous solutions

Abstract

In aerated solutions containing molecular oxygen, O·-radicals effectively react with O2 yielding ozonide radical ions, O3·- In the absence of O·- scavengers, O3·- mainly decays by unimolecular reaction yielding O·- and O2 and by further reaction with O·-. The decay rate of O3·- is therefore extremely sensitive to the presence of small quantities of O·- scavengers. In the present communication we have generated O3·- radicals by two different methods: photolysis of strong alkaline (pH> 12.7) solutions of either H2O2 or S2O8=. A detailed kinetic study shows a first order decay of O3·- generated after photolysis of hydrogen peroxide, while a more complex kinetics is observed when O3·- is formed following photolysis of S2O8=. These observations indicate the involvement of different reaction mechanisms. In order to obtain a complete interpretation of the experimental data, ab-initio kinetic computer simulations were done.On one hand, the decay kinetics of O3·- generated from alkaline photolysis of H2O2 in the presence and absence of scavengers was well fitted by the ab-initio kinetic computer simulations. From the analysis of the mechanism, information on the reaction kinetics of the hydroxyl radicals with different substrates can be obtained. The possibility of retrieving information on the reaction kinetics and efficiency of these substrates as 0·- and HO· scavengers, is exemplified for cyanide and carbonate ions.On the other hand, the decay kinetics of O3·-generated from alkaline photolysis of K2S2O8 can only be understood if the participation of reaction intermediates other than SO4·- O·-/ HO· and O2·- are considered. Alternative reaction mechanisms are discussed.