Quasi-equilibrium and non-equilibrium adsorption in heterogeneous photocatalysis

Heterogeneous photocatalysis is a promising alternative for the removal of organic and inorganic pollutants in water. Some chemical species display a strong and slow surface adsorption kinetics reacting under non-equilibrium while others adsorb quickly and moderately with adsorption taking place under quasi-equilibrium. In the specific case of a given pollutant once the intrinsic kinetic constants are evaluated, one can establish a photochemical efficiency factor (PTEF), representing the efficiency of using absorbed photon energy in chemical transformations. Deviations between equilibrium and the more rigorous non-equilibrium adsorption models can be assessed with a PTEFnon-eq/PTEFeq parameter. In the case of phenol photoconversion it is observed that the PTEFnon-eq/PTEFeq ratio varies in a limited range (0.5-2) and this is consistent with quasi-equilibrium adsorption assumptions. On the other hand, for the photocatalytic conversion of methylene blue, 2-chlorophenol and pyrogallol, the PTEFnon-eq/PTEFeq ratio changes from 3 to 18. This makes, in these cases, adsorption at quasi-equilibrium unsuitable given it leads to a severe underestimation of energy efficiency factors. As a result, this study emphasizes the critical importance of properly modeling photocatalytic degradation processes in terms of adsorption, reaction, kinetic parameters as well as for the evaluation of energy efficiency factors. (c) 2007 Published by Elsevier Ltd.