Fate and removal of aromatic organic matter upon a combined leachate treatment process

• Iron-carbon micro-electrolysis coupled with Fenton enhanced the oxidation efficiency of aromatic organics. • Coagulation process decreased the antioxidant organic components. • Biodegradation process removed electron-accepting moieties more efficiently. • The aromaticity, complexation and mobility of the leachate reduced after treatment. Mature leachates contain extremely recalcitrant aromatic organic matter, and require appropriate treatment before discharge. The combined process for treating mature landfill leachate is a common and proper due to its inexpensiveness and high performance. Identification of refractory components and insights into the physicochemical properties transformations of the organics are essential for the development of efficient treatment process. Optical, three-dimensional chromatography and electrochemical techniques were applied to characterize aromatic organic matter in leachate from a pilot landfill leachate treatment plant. Results showed that the combination of biological contact oxidation, iron-carbon micro-electrolysis, Fenton, coagulation and integrated activated sludge system could efficiently remove 95.5% COD and 88.3% DOC in leachate organic matter, together with a simultaneous reduction in its molecular weight (MW), aromaticity, polarity, and humification degree. The low MW proteinaceous, carboxylic acid and amine-containing fractions were preferentially removed in biological contact oxidation process, whereas the hydrophilic humic substances with the MW > 4.0 kDa were the main species against biological treatment. Conversely, the advanced oxidation processes (AOPs) could efficiently destroy high MW, aromatic rings and conjugated moieties fractions, resulting in an increase in the leachate biodegradability. The oxygen-containing moieties generated in the AOPs were bridged by the flocculants to promote precipitate during the coagulation process, and the polymerized metastable organic fractions formed during the process were partly biodegraded in the integrated activated sludge system. This study highlights the potential of combined optical-electrochemical measurements to monitor the intrinsic reactivity of aromatic organic matter and guide the practical leachate treatment.