Photocatalytic decomposition of humic acids on TiO₂. Part I: Discussion of adsorption and mechanism

In this paper we present the results of the photocatalytic degradation of humic acids (HAs) in aqueous solution. Bench scale experiments were carried out using titanium oxide as photocatalyst. We studied the adsorption of HAs on TiO₂ surface and we describe the kinetics of their photocatalytic degradation. The adsorption study of HA at three pH solution (1.9/7.5/11) by FT-IR DRIFT technique indicated that at acidic pH, HA are adsorbed on TiO₂ mainly by carboxylate groups. We determined the isotherms of HA and it was shown that Langmuir model cannot be applied in this case. However, if the asymptotic values of the isotherms were assumed for these analytical methods the close values were obtained. The kinetics of degradation was followed with TOC and pH. There are no important pH evolutions. It was obtained 88% of TOC removal after 6h of irradiation with optimum TiO₂ loading 1.0 g/l. We observed the presence of two domains of HA degradation kinetics. In the first step, slight TOC decreased possibly due to the photodepolymerization of adsorbed HA on TiO₂. In the second one, the photodegradation followed pseudo-first-order kinetics. Organic intermediates were determined by high performance liquid chromatography (HPLC) (PDA-detector) analysis as well as biological oxygen demand (BOD₅). It was shown that photocatalysis process improved the biodegradability of HA nevertheless mineralization process mainly occurred and there was no large amount of easy biodegradable species present in the reaction solution.