The UV-sulfite reductive treatment using hydrated electrons (e aq − ) is a promising technology for destroying perfluorocarboxylates (PFCAs, C n F 2n+1 COO − ) in any chain length. However, the C−H bonds formed in the transformation products strengthen the residual C−F bonds and thus prevent complete defluorination. Reductive treatments of fluorotelomer carboxylates (FTCAs, C n F 2n+1 −CH 2 CH 2 −COO − ) and sulfonates (FTSAs, C n F 2n+1 − CH 2 CH 2 −SO 3 − ) are also sluggish because the ethylene linker separates the fluoroalkyl chain from the end functional group. In this work, we used oxidation (Ox) with hydroxyl radicals (HO•) to convert FTCAs and FTSAs to a mixture of PFCAs. This process also cleaved 35−95% of C−F bonds depending on the fluoroalkyl chain length. We probed the stoichiometry and mechanism for the oxidative defluorination of fluorotelomers. The subsequent reduction (Red) with UV-sulfite achieved deep defluorination of the PFCA mixture for up to 90%. The following use of HO• to oxidize the H-rich residues led to the cleavage of the remaining C−F bonds. We examined the efficacy of integrated oxidative and reductive treatment of n = 1−8 PFCAs, n = 4,6,8 perfluorosulfonates (PFSAs, C n F 2n+1 −SO 3 − ), n = 1−8 FTCAs, and n = 4,6,8 FTSAs. A majority of structures yielded near-quantitative overall defluorination (97−103%), except for n = 7,8 fluorotelomers (85−89%), n = 4 PFSA (94%), and n = 4 FTSA (93%). The results show the feasibility of complete defluorination of legacy PFAS pollutants and will advance both remediation technology design and water sample analysis.

The C−F bond is one of the strongest single bonds in nature. Although microbial reductive dehalogenation is well known for the other organohalides, no microbial reductive defluorination has been documented for perfluorinated compounds except for a single, nonreproducible study on trifluoroacetate. Here, we report on C−F bond cleavage in two C 6 perand polyfluorinated compounds via reductive defluorination by an organohalide-respiring microbial community. The reductive defluorination was demonstrated by the release of F − and the formation of the corresponding product when lactate was the electron donor, and the fluorinated compound was the sole electron acceptor. The major dechlorinating species in the seed culture, Dehalococcoides, were not responsible for the defluorination as no growth of Dehalococcoides or active expression of Dehalococcoides-reductive dehalogenases was observed. It suggests that minor phylogenetic groups in the community might be responsible for the reductive defluorination. These findings expand our fundamental knowledge of microbial reductive dehalogenation and warrant further studies on the enrichment, identification, and isolation of responsible microorganisms and enzymes. Given the wide use and emerging concerns of fluorinated organics (e.g., perand polyfluoroalkyl substances), particularly the perfluorinated ones, the discovery of microbial defluorination under common anaerobic conditions may provide valuable insights into the environmental fate and potential bioremediation strategies of these notorious contaminants.

This study investigates structure−reactivity relationships within branched per-and polyfluoroalkyl substances (PFASs) undergoing cobalt-catalyzed reductive defluorination reactions. Experimental results and theoretical calculations reveal correlations among the extent of PFAS defluorination, the local C−F bonding environment, and calculated bond dissociation energies (BDEs). In general, BDEs increase in the following order: tertiary C−F bonds < secondary C−F bonds < primary C−F bonds. A tertiary C−F bond adjacent to three fluorinated carbons (or two fluorinated carbons and one carboxyl group) has a relatively low BDE that permits an initial defluorination to occur. Both a biogenic cobalt−corrin complex (B 12 ) and an artificial cobalt−porphyrin complex (Co-PP) are found to catalytically defluorinate multiple C−F bonds in selected PFASs. In general, Co-PP exhibits an initial rate of defluorination that is higher than that of B 12 . Neither complex induced significant defluorination in linear perfluorooctanoic acid (PFOA; no tertiary C−F bond) or a perfluoroalkyl ether carboxylic acid (tertiary C−F BDEs too high). These results open new lines of research, including (1) designing branched PFASs and cobalt complexes that promote complete defluorination of PFASs in natural and engineered systems and (2) evaluating potential impacts of branched PFASs in biological systems where B 12 is present.

Chlorate (ClO3 –) is an undesirable byproduct in the chlor-alkali process. It is also a heavily used chemical in various industrial and agricultural applications, making it a toxic water pollutant worldwide. Catalytic reduction of ClO3 – into Cl– by H2 is of great interest to both emission control and water purification, but platinum group metal catalysts are either sluggish or severely inhibited by halide anions. Here, we report on the facile preparation, robust performance, and mechanistic investigation of a MoO x –Pd/C catalyst for aqueous ClO3 – reduction. Under 1 atm H2 and room temperature, the Na2MoO4 precursor is rapidly immobilized from aqueous solution onto Pd/C as a mixture of low-valent Mo oxides. The catalyst enables complete reduction of ClO3 – in a wide concentration range (e.g., 1 μM to 1 M) into Cl–. The addition of Mo to Pd/C not only enhances the catalytic activity by >55-fold, but also provides strong resistance to concentrated salts. To probe the reaction mechanisms, we conducted a series of kinetic measurements, microscopic and X-ray spectroscopic characterizations, sorption experiments, tests with other oxyanion substrates, and a comparative study using dissolved Mo species. The catalytic sites are the reduced MoO x species (primarily MoIV), showing selective and proton-assisted reactivity with ClO3 –. This work demonstrates a great promise of using relatively abundant metals to expand the functionality of hydrogenation catalysts for environmental and energy applications.

Supported palladium (Pd) catalysts have been extensively studied for water purification applications. However, this technology is primarily challenged by the high cost of Pd and the lack of optimization of catalyst formulations. In this report, we demonstrate a convenient approach to prepare and optimize Pd catalysts for the reduction of toxic oxyanions (bromate, chlorate, and perchlorate). Water-dissolved Na2PdCl4 was quickly adsorbed in the suspension of activated carbon within 5 min and reduced into Pd0 nanoparticles in situ within another 5 min under 1 atm H2 at 20 °C. In terms of both material characterizations and reaction kinetics, the Pd catalysts prepared with the new method show no significant difference from those prepared by the conventional method (involving multiple-step high-temperature procedures) and from benchmark commercial Pd catalysts. With the very simple approach to control, evaluate, and optimize Pd content in the catalyst, we elucidate the relationships among the Pd content, Pd0 particle size, and catalytic activity. We further showcase that the precious metals in previously reported Re–Pd/C and Mo–Pd/C catalysts can be saved up to 80% without sacrificing the activity. The new and convenient catalyst preparation method will significantly enhance the cost-effectiveness of reductive catalysis technologies for water purification.