The photocatalytic degradation of protocatechuic acid, a biorecalcitrant polyphenolic compound typically
found in olive processing and wine distillery waste waters, has been investigated in aqueous heterogeneous
solutions containing semiconductor powders (TiO2, ZnO) as photocatalysts, both in the presence of
artificial and natural illumination. The disappearance of the organic molecule follows, approximately a
pseudo-first kinetic order according to the Langmuir-Hinshelwood model. Various commercial
photocatalysts have been compared with respect to their overall efficiency as well as the production of
CO2. Among them, ZnO shows the highest photocatalytic activity. The effect of H2O2 on the reaction rate
has been ascertained.