Vacuum-UV oxidation (H2O-VUV) with a xenon excimer flow-trough lamp at 172 nm:: Use of methanol as actinometer for VUV intensity measurement and as reference compound for OH-radical competition kinetics in aqueous systems

During the past decade the intensive development of novel incoherent xenon excimer light sources emitting in the vacuum-UV region of the electromagnetic spectrum at lambda(max) of 172 nm led to a renaissance of investigations that deal with the direct photolysis of water. Upon 172 nm excitation water molecules homolyze with formation of hydroxyl radicals and hydrogen atoms. In the presence of organic substrates and dissolved molecular oxygen a complex series of interrelated radical reactions is initiated leading to the oxidative degradation of the organic compounds and finally to their complete mineralization, i.e. to the diminution of the total organic carbon (TOC) content of the solution. This advanced oxidation process (AOP) is usually designated as H2O-VUV. It opens up new and alternative possibilities for water and air treatment with extraordinary technical and analytical potentials. Therefore, the VUV induced oxidative degradation of methanol in aqueous solution using a xenon excimer flow-through lamp was investigated as a function of the initial methanol concentration. A distinguished kinetic scheme is used to explain the overall reaction kinetics of this H2O-VUV process that demonstrates pseudo first-order rate constants for low initial methanol concentrations and a pseudo zero-order rate description for high methanol concentrations. The high concentration domain (75 mmol L-1 < [methanol] < 250 mmol L-1) allows the determination of the absorbed photon flow and subsequently the calculation of the electrical efficiency of the excimer lamp. This actinometric concept was first introduced by the group of A. M. Braun (Karlsruhe, Germany). On the other hand, the domain of low methanol concentrations ([methanol] < 1 mmol L-1) was effectively used as a reference system for competition kinetics to determine second-order hydroxyl radical rate constants with specific target molecules. The latter concept was introduced by the group of J. R. Bolton (London, Ontario) for use with the H2O2-UV AOP. In the present investigation it was validated with the H2O-VUV AOP by using target compounds with well known hydroxyl radical rate constants. This work describes an attempt to interconnect both kinetic concepts.