Effect of key operating parameters on phenols degradation during H2O2-assisted TiO2 photocatalytic treatment of simulated and actual olive mill wastewaters

The treatment of a synthetic solution containing 13 organic compounds typically found in olive mill wastewaters (OMW) by means of H2O2-promoted TiO2 photocatalysis was investigated concerning the effect of major operating factors, namely catalyst type and loading, H2O2 dosage and method of addition, irradiation intensity, solution pH and initial substrate concentration on process efficiency. The latter was assessed in terms of individual compounds degradation (followed by high performance liquid chromatography), total phenols (measured according to the Folin-Ciocalteu protocol), chemical oxygen demand and total organic carbon decrease, decolorization and ecotoxicity. Of the various catalysts screened, Degussa Aeroxide P25 was found to be highly active for TPh degradation and therefore subsequent studies were performed using this catalyst. Experiments with the synthetic effluent at 650 mg/L initial TPh concentration, H2O2 concentration in the range 0.073-0.118 mol/L, 0.75 g/L catalyst loading and 400 W irradiation power led to complete TPh and color removal and 80% mineralization after 60 min of reaction. The rate of degradation decreased with decreasing intensity from 400 to 250 to 9 W and remained unchanged decreasing initial TPh concentration from 650 to 488 to 325 to 163 mg/L indicating a zero order reaction regarding TPh. Changing the starting solution pH from 3.5 to 6 to 8 had only marginal effect on degradation which also generally decreased with decreasing catalyst loading and H2O2 concentration. The process was applied to treat actual OMW which were first filtered and diluted several times with water. Efficiency strongly depended on effluent composition and concentration; a 100-fold dilution was needed to make the effluent amenable to photocatalytic treatment. (c) 2007 Elsevier B.V. All rights reserved.