Solar-based detoxification of phenol and p-nitrophenol by sequential TiO2 photocatalysis and photosynthetically aerated biological treatment

Simulated solar UV/TiO2 photocatalysis was efficient to detoxify a mixture of 100 mg phenol l-1 and 50 mg p-nitrophenol (PNP) l-1 and allow the subsequent biodegradation of the remaining pollutants and their photocatalytic products under photosynthetic aeration with Chlorella vulgaris. Photocatalytic degradation of phenol and PNP was well described by pseudo-first order kinetics (r2>0.98) with removal rate constants of 1.9×10-4 and 2.8×10-4 min-1, respectively, when the pollutants were provided together and 5.7×10-4 and 9.7×10-4 min-1, respectively, when they were provided individually. Photocatalytic pre-treatment of the mixture during 60 h removed 50±1% and 62±2% of the phenol and PNP initially present but only 11±3% of the initial COD. Hydroquinone, nitrate and catechol were identified as PNP photocatalytic products and catechol and hydroquinone as phenol photocatalytic products. Subsequent biological treatment of the pre-treated samples removed the remaining contaminants and their photocatalytic products as well as 81-83% of the initial COD, allowing complete detoxification of the mixture to C. vulgaris. Similar detoxification efficiencies were recorded after biological treatment of the irradiated mixture with activated sludge microflora or with an acclimated consortia composed of a phenol-degrading Alcaligenes sp. and a PNP-degrading Arthrobacter sp., although the acclimated strains biodegraded the remaining pollutants faster. Biological treatment of the non-irradiated mixture was inefficient due to C. vulgaris inhibition. © 2007 Elsevier Ltd. All rights reserved.