Role of humic acids in the TiO2-photocatalyzed degradation of tetrachloroethene in water

The effect of humic acids on the TiO2-mediated photocatalytic degradation of tetrachloroethene (PCE) was kinetically investigated at different pH and initial substrate concentrations. The process occurs through two parallel paths: a major oxidative route leading to mineralization and a reductive route leading to the formation of dichloroacetic acid (DCAA), also undergoing photodegradation. The rate of PCE decomposition was found to decrease in the presence of humic acids adsorbed on the semiconductor surface, while the concentration of the intermediate dichloroacetic acid increased. This is a consequence of the scavenging action of humic acids toward photoproduced surface oxidant species, which makes conduction band electrons more easily available for interface reactions. Kinetic studies on the effect of humic acids in the TiO2-mediated photodegradation of dichloroacetic acid showed that the progressively greater accumulation of this highly toxic intermediate, observed with increasing humic acids content, was a consequence of both an increase in the rate of its production from PCE and a decrease in the rate of its oxidative photodegradation.