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/PTEFeqPTEFnon-eq/PTEFeq parameter. In the case of phenol photoconversion it is observed that the PTEFnon-eq/PTEFeqPTEFnon-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/PTEFeqPTEFnon-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.