Combining chemical with biological oxidation for efficient treatment of chloronitrobenzene in groundwater
Date
2019
Authors
Amiri, Samia, author
Blotevogel, Jens, advisor
Sale, Thomas C., committee member
DiVerdi, Joseph A., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Chloronitrobenzene (CNB) is a chloronitroaromatic compound widely used in the synthetic production of pharmaceuticals, pesticides, dyes, lumber preservatives, and many other industrial products. CNB has been recognized as a toxic organic contaminant to humans and is recalcitrant to microbial biodegradation in anoxic environments. When receptors are threatened by CNB-contaminated groundwater, regulators may demand immediate remedial approaches, such as advanced oxidation processes (AOPs). While AOPs are effective for the removal of many organic contaminants from water, these techniques are often costly, especially when complete mineralization is the goal. In this study, it was hypothesized that chemical oxidation for the primary purpose of ring cleavage followed by biological oxidation of the generated intermediates is more cost-effective than relying on AOPs only for complete mineralization. Electrochemical oxidation via hydroxyl radicals was chosen as model AOP and performed at various applied potentials and for different treatment durations. Liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (LC/QToF-MS) revealed that the aromatic ring in CNB is rapidly hydroxylated and cleaved to form dicarboxylic products including muconic acid, succinic acid, malic acid, and maleic acid. Further electrochemical oxidation of these dicarboxylates was slower by about two orders of magnitude. To evaluate the universal biodegradability of the generated intermediates, the electrochemically oxidized samples of CNB were then exposed to a microbial culture enriched from a rhizosphere soil. Results showed that the dicarboxylic ring opening products biodegraded under anoxic conditions within 7 days while aromatic species including CNB, chloronitrophenol, chlorohydroquinone and dihydroxybenzoquinone persisted over 28 days of biological treatment. A comprehensive cost analysis considering both capital costs (electrodes) and operational costs (electric energy) revealed that the most efficient treatment strategy is to apply electrochemical oxidation at a low applied potential around 6 V until complete cleavage of the aromatic ring is achieved. Beyond that, advanced oxidation of the readily biodegradable ring cleavage products becomes uneconomical. Consequently, the coupling of chemical oxidation for persistent parent compounds with biodegradation of transformation intermediates is an efficient approach for the treatment of groundwater contaminated with CNB and likely other aromatic contaminants.