Complete Mineralization of 2,4-Dichlorophenoxyacetic Acid in a Reduction and Oxidation Synergistic Platform (ROSP)

Emissions of 2,4-dichlorophenoxyactic acid (2,4-D), one of the most widely used herbicides, pose serious risks to human and ecosystem health. Although biodegradation and abiotic chemical methods are available for 2,4-D removal, continuous treatment systems for 2,4-D removal are rare. We evaluated a Reduction and Oxidation Synergistic Platform (ROSP) that sequentially links an H2-based membrane catalyst-film reactor (H2-MCfR) and an O2-based membrane biofilm reactor (O2-MBfR). In continuous operation of the ROSP, 50 & mu;M (or 11 ppm) of 2,4-D was completely mineralized with a total HRT of 12 h, yielding an effluent concentration < 0.2 & mu;M 2,4-D (or < 44 ppb). In the H2-MCfR, over 80% of the influent 2,4-D was reductively hydrodechlorinated to phenoxyacetic acid (POA). The produced POA was the primary substrate supporting the co-oxidation of 2,4-D in the O2-MBfR. The biofilm community structure and genes encoding enzymes for POA and 2,4-D co-oxidation were identified through shallow metagenomic sequencing of the biofilm DNA samples. The dominant bacterial genus, Variovorax, and various mono-/dioxygenase-encoding genes related to aromatic ring hydrolysis, hydroxylation, and ring cleavage were associated with the removal of 2,4-D. In summary, reductive-dehalogenation products from the H2-MCfR become the primary substrate to enable the co-oxidation of the halogenated organic pollutants in the O2-MBfR, which does not require the addition of other organic materials. Therefore, the ROSP eliminates the need for an added organic substrate, which greatly reduces the demand for O2 and minimizes CO2 emissions.