Reductive Dehalogenation of Herbicides Catalyzed by Pd 0 NPs in a H 2 -Based Membrane Catalyst-Film Reactor.

More food production required to feed humans will require intensive use of herbicides to protect against weeds. The widespread application and persistence of herbicides pose environmental risks for nontarget species. Elemental-palladium nanoparticles (Pd 0 NPs) are known to catalyze reductive dehalogenation of halogenated organic pollutants. In this study, the reductive conversion of 2,4-dichlorophenoxyacetic acid (2,4-D) was evaluated in a H 2 -based membrane catalyst-film reactor (H 2 -MCfR), in which Pd 0 NPs were in situ-synthesized as the catalyst film and used to activate H 2 on the surface of H 2 -delivery membranes. Batch kinetic experiments showed that 99% of 2,4-D was removed and converted to phenoxyacetic acid (POA) within 90 min with a Pd 0 surface loading of 20 mg Pd/m 2 , achieving a catalyst specific activity of 6.6 ± 0.5 L/g-Pd-min. Continuous operation of the H 2 -MCfR loaded with 20 mg Pd/m 2 sustained >99% removal of 50 μM 2,4-D for 20 days. A higher Pd 0 surface loading, 1030 mg Pd/m 2 , also enabled hydrosaturation and hydrolysis of POA to cyclohexanone and glycolic acid. Density functional theory identified the reaction mechanisms and pathways, which involved reductive hydrodechlorination, hydrosaturation, and hydrolysis. Molecular electrostatic potential calculations and Fukui indices suggested that reductive dehalogenation could increase the bioavailability of herbicides. Furthermore, three other halogenated herbicides─atrazine, dicamba, and bromoxynil─were reductively dehalogenated in the H 2 -MCfR. This study documents a promising method for the removal and detoxification of halogenated herbicides in aqueous environments.