Landfill leachate treatment by solar-driven AOPs

Sanitary landfill leachate resulting from the rainwater percolation through the landfill layers and waste material decomposition is a complex mixture of high-strength organic and inorganic compounds which constitutes serious environmental problems. In this study, different heterogeneous (TiO 2/UV, TiO 2/H 2O 2/UV) and homogenous (H 2O 2/UV, Fe 2+/H 2O 2/UV) photocatalytic processes were investigated as an alternative for the treatment of a mature landfill leachate. The addition of H 2O 2 to TiO 2/UV system increased the reduction of the aromatic compounds from 15% to 61%, although mineralization was almost the same. The DOC and aromatic content abatement is similar for the H 2O 2/UV and TiO 2/H 2O 2/UV processes, although the H 2O 2 consumption is three times higher in the H 2O 2/UV system. The low efficiency of TiO 2/H 2O 2/UV system is presumably due to the alkaline leachate solution, for which the H 2O 2 becomes highly unstable and self-decomposition of H 2O 2 occurs. The efficiency of the TiO 2/H 2O 2/UV system increased 10 times after a preliminary pH correction to 4. The photo-Fenton process is much more efficient than heterogeneous (TiO 2, TiO 2/H 2O 2/UV) or homogeneous (H 2O 2/UV) photocatalysis, showing an initial reaction rate more than 20 times higher, and leading to almost complete mineralization of the wastewater. However, when compared with TiO 2/H 2O 2/UV with acidification, the photo-Fenton reaction is only two times faster. The optimal initial iron dose for the photo-Fenton treatment of the leachate is 60 mg Fe 2+ L −1, which is in agreement with path length of 5 cm in the photoreactor. The kinetic behaviour of the process (60 mg Fe 2+ L −1) comprises a slow initial reaction, followed by a first-order kinetics ( k = 0.020 L kJ UV - 1 , r 0 = 12.5 mg kJ UV - 1 ), with H 2O 2 consumption rate of k H2O2 = 3.0 mmol H 2O 2 kJ UV - 1 , and finally, the third reaction period, characterized by a lower DOC degradation and H 2O 2 consumption until the end of the experiment, presumably due to the formation of low-molecular-weight carboxylic groups. A total of 306 mM of H 2O 2 was consumed for achieving 86% mineralization (DOC final = 134 mg L −1) and 94% aromatic content reduction after 110 kJ UV L −1, using an initial iron concentration of 60 mg Fe 2+ L −1.