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 μM (or 11 ppm) of 2,4-D was completely mineralized with a total HRT of 12 h, yielding an effluent concentration < 0.2 μM 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.