Heterogeneous photo-Fenton oxidation of benzoic acid in water: Effect of operating conditions, reaction by-products and coupling with biological treatment

The heterogeneous photo-Fenton oxidation of benzoic acid, a precursor of several organic pollutants found in agro-industrial effluents, was studied in model aqueous solutions. UVA irradiation was provided by a 125 W medium pressure mercury lamp, while a nanocomposite material of crystalline iron oxides supported over mesostructured SBA-15 was used as the catalyst. Experiments were conducted at benzoic acid initial concentrations between 25 and 450 mg/L, catalyst concentrations between 0.3 and 1.2 g/L and hydrogen peroxide concentrations between 20% and 100% of the stoichiometric amount needed for complete mineralization. Conversion, which was found to be first order regarding benzoic acid concentration, generally increased with increasing the concentration of Fenton's reagents and decreasing substrate concentration. HPLC analysis showed that oxidation was accompanied by the formation of several by-products; of these, the three monohydroxybenzoic acids as well as oxalic acid were successfully identified and quantified. By-products were more resistant to oxidation than benzoic acid since COD reduction was generally lower than substrate conversion. Catalyst stability was assessed measuring the extent of iron leaching in the reaction mixture and was found to be excellent as dissolved iron never exceeded 5% relative to the initial iron content. The aerobic biodegradability of benzoic acid before and after photo-Fenton oxidation was assessed by shake flask tests. Chemical oxidation enhanced the biodegradability of benzoic acid although the oxidized solution was more ecotoxic to marine bacteria than the original one. The feasibility of coupling chemical and biological oxidation was also assessed for an actual olive oil mill effluent.