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.

This work details the early events in the reductive defluorination of perfluoroalkyl substances (PFASs) and presents a straightforward methodology for predicting the reduction behavior of the perfluoroalkyl acids (PFAAs) using electronic structure calculations. Electron attachment to linear perfluorocarboxylic acids generally occurs at the α-carbon and is energetically not correlated to chain length, contrary to the case for linear perfluoroalkanesulfonates, where electrons generally insert into other positions. Perfluorooctanesulfonate and perfluorooctanoic acid, two widely studied and scrutinized PFAAs, are therefore predicted to be reduced through diverging pathways. Our protocol can predict the standard reduction potentials of PFAAs, provides a rational basis for probing reaction intermediates, establishes free energy relationships, and accounts for PFASs' inherent structural diversity beyond the linear substrates.

Historically, development of catalysts for treatment of nitrate-contaminated water has focused on supported Pd-based catalysts, but high costs of the Pd present a barrier to commercialization. As part of an effort to develop lower cost hydrogenation catalysts for water treatment applications, we investigated catalysts incorporating Ru with lower cost. Pseudo-first-order rate constants and turnover frequencies were determined for carbon-and alumina-supported Ru and demonstrated Ru's high activity for hydrogenation of nitrate at ambient temperature and H 2 pressure. Ex situ gas pretreatment of the catalysts was found to enhance nitrate reduction activity by removing catalyst surface contaminants and exposing highly reducible surface Ru oxides. Ru reduces nitrate selectively to ammonium, and no aqueous nitrite intermediate is observed during reactions. In contrast, reactions initiated with nitrite yield a mixture of two endproducts, with selectivity shifting from ammonium towards N 2 at increasing initial aqueous nitrite concentrations. Experimental observation and Density Functional Theory calculations

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Perfluoroalkyl carboxylic acids (PFCAs) are ubiquitous contaminants known for their bioaccumulation, toxicological harm, and resistance to degradation. Remediating PFCAs in water is an ongoing challenge with existing technologies being insufficient or requiring additional disposal. An emergent approach is using activated persulfate, which degrades PFCAs through sequential scission of CF 2 equivalents yielding shorter-chain homologues, CO 2 and F − . This transformation is thought to be initiated by single electron transfer (SET) from the PFCA to the activate oxidant, SO 4•− . A pronounced pH effect has been observed for thermally activated persulfate PFCA transformation. To evaluate the role of pH during SET, we directly determined absolute rate constants for perfluorobutanoic acid and trifluoroacetic acid oxidation by SO 4•− in the pH range of 0.5−4.0 using laser flash photolysis. The average of the rate constants for both substrates across all pH values was 9 ± 2 × 10 3 M −1 s −1 (±2σ), implying that acid catalysis of thermal persulfate activation may be the primary culprit of the observed pH effect, instead of pH influencing the SET step. In addition, density functional theory was used to investigate if SO 4•− protonation might enhance PFCA transformation kinetics. We found that when calculations include explicit water molecules, direct SO 4•− protonation does not occur.