Influence of pH and chloride anion on the photocatalytic degradation of organic compounds: Part I. Effect on the benzamide and para-hydroxybenzoic acid in TiO2 aqueous solution

In the present work, we have studied the influence of pH (in the range 3–11) and chloride anions (in the range 0–0.8 mol/l) on the photocatalytic degradation (PCD), with titanium dioxide (TiO2), in aqueous solution of two aromatic compounds: benzamide (BA) and para-hydroxybenzoic acid (4-HBZ). No significant adsorption is detected for the BA; further the pH input has a very low effect on this photodegradation rate. However, the pH effect on the adsorption of 4-HBZ on TiO2 surface is significant and this is a factor to take into account in relation the photocatalytic decomposition of organic pollutants in water. For this compound, the degradation rate increases when the pH decreases. The effect of the pH on the degradation rate of 4-HBZ is not only dependent on the TiO2 charge surface but also on the formation of hydroxyl free radicals. At low chloride concentration, the behaviours of BA and 4-HBZ are sensitively different ([Cl−]<0.02 mol/l). 4-HBZ is only weakly dependent on the chloride concentration. By contrast, the degradation of BA is strongly affected by the chloride ions concentration.