Solar driven photocatalysis using iron and chromium doped TiO2 coupled to moving bed biofilm process for olive mill wastewater treatment

TiO2 based-photocatalysts doped with Fe and/or Cr was evaluated as pre- and post-treatment method of a moving bed biofilm reactor (MBBR) as possible solution for the treatment of real olive oil mill wastewater (OMW). Photocatalysts were synthesized by wet chemical method and their chemical-physical properties were accurately investigated through X-ray diffraction (XRD), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and specific surface area measurements. UV-Vis DRS measurements evidenced that the simultaneous doping of TiO2 lattice with Fe and Cr improves the optical absorption into the visible region leading to a narrow band gap (2.1 eV) with respect to undoped TiO2 (3.2 eV) and Fe-doped TiO2 (2.8 eV) while, Cr-doped TiO2 showed the lowest bandgap value (1.9 eV). XRD patterns and Raman spectra showed that anatase is the predominant crystalline phase for all the prepared photocatalysts and Fe and Cr ions were effectively inserted into the TiO2 lattice. The TiO2 doping with Cr did not change the average crystallites size that was equivalent to that of TiO2 (8 nm), whereas, for Fe-doped TiO2, it was lower than the others and equal to 6 nm. The specific surface area values of doped catalysts were higher than TiO2 and the value for Fe-Cr-codoped TiO2 resulted to be 97 m(2) g(-1). Photocatalytic treatment of OMW was evaluated in terms of total polyphenols (TPHs), Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD5) and biodegradability (as BOD5/COD ratio) in 3 h of treatment under simulated solar light irradiation. Fe-doped TiO2 showed the highest TPHs removal for both untreated and biologically treated OMW. When H2O2 was added (optimum dosage 0.5 g L-1), Fe-Cr-TiO2 resulted in the best photocatalytic performance with TPHs removal as high as 97% and increased biodegradability (0.22 to 0.33), making the effluent suitable for the subsequent biological process. When photocatalytic process was investigated as post-treatment of OMW, Fe-TiO2 showed the best activity and the addition of 0.5 g L-1 of H2O2 was sufficient to make the effluent in compliance with Italian regulation for effluent disposal into surface water