Laboratory-scale photomineralisation of n-alkanes in aqueous solution, by photocatalytic membranes immobilising titanium dioxide

Abstract

Kinetics of photocatalytic oxidation of methane, ethane, n-heptane, n-decane, and n-dodecane, to yield intermediates, and photomineralisation of intermediates, to yield carbon dioxide and water, was studied in aqueous solution, by a laboratory-scale photoreactor and photocatalytic membranes immobilizing 30 ± 3 wt.% of TiO2, in the presence of stoichiometric hydrogen peroxide as oxygen donor. The whole volume of irradiated solution was 4.000 ± 0.005 L, the ratio between this volume and the geometrical apparent surface of the irradiated side of the photocatalytic membrane was 3.8 ± 0.1 cm, and the absorbed power 0.30 W/cm (cylindrical geometry). A kinetic model was used, by which mineralisation of substrate to CO2 was supposed to occur, by kinetic constants k1, through one single intermediate, mediating the behaviour of all the numerous real intermediates formed in the path from the substrate to CO2 (kinetic constants of formation of the latter being k2). A competitive Langmuirian adsorption of both substrate and " intermediate" was also supposed to be operative, as expressed by apparent adsorption constants K1 and K2, possessing a, partly at least, kinetic significance. By Langmuir-Hinshelwood treatment of initial rate data, starting values of the k and K couples were obtained, from which, by a set of differential equations, the final optimised parameters, k1 and K1, k2 and K2, were calculated, able fit the whole photomineralisation curve, and not only its initial segment, as the Langmuirian parameters do. The parameters of present work are critically compared with those obtained in two preceding set of studies relative to n-alkanoic acids and to n-alkanols. They are interpreted on the basis of a closer behaviour of hydrocarbons to alkanols, from the photocatalytic point of view, than to carboxylic acids are. Discussion of limiting effective quantum yields, and their comparison with maximum, theoretical values, are also carried out.