Structural dependence of PFAS oxidation in a boron doped diamond-electrochemical system.

Driven by long-term persistence and adverse health impacts of legacy perfluorooctanoic acid (PFOA), production has shifted towards shorter chain analogs (C4, perfluorobutanoic acid (PFBA)) or fluorinated alternatives such as hexafluoropropylene oxide dimer acid (HFPO-DA, known as GenX) and 6:2 fluorotelomer carboxylic acid (6:2 FTCA). Yet, a thorough understanding of treatment processes for these alternatives is limited. Herein, we conducted a comprehensive study using an electrochemical approach with a boron doped diamond anode in Na2SO4 electrolyte for the remediation of PFOA common alternatives, i.e., PFBA, GenX, and 6:2 FTCA. The degradability, fluorine recovery, transformation pathway, and contributions from electro-synthesized radicals were investigated. The results indicated the significance of chain length and structure, with shorter chains being harder to break down (PFBA (65.6 ± 5.0%) < GenX (84.9 ± 3.3%) < PFOA (97.9 ± 0.1%) < 6:2 FTCA (99.4 ± 0.0%) within 120 min of electrolysis). The same by-products were observed during the oxidation of both low and high concentrations of parent PFAS (2 and 20 mg L-1), indicating that the fundamental mechanism of PFAS degradation remained consistent. Nevertheless, the ratio of these by-products to the parent PFAS concentration varied which primarily arises from the more rapid PFAS decomposition at lower dosages. For all experiments, the main mechanism of PFAS oxidation was initiated by direct electron transfer at the anode surface. Sulfate radical (SO4•-) also contributed to the oxidation of all PFAS, while hydroxyl radical (•OH) only played a role in the decomposition of 6:2 FTCA. Total fluorine recovery of PFBA, GenX, and 6:2 FTCA were 96.5%, 94.0%, and 76.4% within 240 min. The more complex transformation pathway of 6:2 FTCA could explain its lower fluorine recovery. Detailed decomposition pathways for each PFAS were also proposed through identifying the generated intermediates and fluorine recovery. The proposed pathways were also assessed using 19F Nuclear Magnetic Resonance (NMR) spectroscopy.

Methanol oxidation on Au(332): an isothermal pulsed molecular beam study.

Isothermal molecular beam experiments on the methanol oxidation over the stepped Au(332) surface were conducted under well-defined ultra-high vacuum conditions. In the measurements, a continuous flux of methanol at excess in the gas phase and pulses of atomic oxygen were provided to the surface kept at 230 K. The formation of the partial oxidation product methyl formate under the applied conditions was evidenced by time-resolved mass spectrometry, and accumulation of formate species, which resulted in a deactivation of the surface for methyl formate formation, was followed by in situ Infrared Reflection Absorption Spectroscopy measurements. The results suggest a different reactivity of oxygen accumulated during the oxygen pulses and atomic oxygen for the competing reaction pathways in the oxidation of methanol to the desired partial and the unwanted overoxidation products.

Major mechanistic differences between the reactions of hydroxylamine with phosphate di- and tri-esters.

Hydroxylamine reacts as an oxygen nucleophile, most likely via its ammonia oxide tautomer, towards both phosphate di- and triesters of 2-hydroxypyridine. But the reactions are very different. The product of the two-step reaction with the triester TPP is trapped by the NH2OH present in solution to generate diimide, identified from its expected disproportionation and trapping products. The reaction with H3N(+)-O(-) shows general base catalysis, which calculations show is involved in the breakdown of the phosphorane addition-intermediate of a two-step reaction. The reactivity of the diester anion DPP(-) is controlled by its more basic pyridyl N. Hydroxylamine reacts preferentially with the substrate zwitterion DPP(±) to displace first one then a second 2-pyridone, in concerted S(N)2(P) reactions, forming O-phosphorylated products which are readily hydrolysed to inorganic phosphate. The suggested mechanisms are tested and supported by extensive theoretical calculations.

Automated monitoring the kinetics of homogeneous and heterogeneous chemical processes using a smartphone.

Heterogeneous chemical processes occupy a pivotal position in many fields of applied chemistry. Monitoring reaction kinetics in such heterogeneous systems together with challenges associated with ex-situ analytical methodologies can lead to inaccurate information about the nature of the catalyst surfaces as well as information about the steps involved. The present work explores the possibility of kinetic measurements of chemical reactions and adsorption processes of homogeneous and heterogeneous systems through the variation of RGB intensities of digital images using a smartphone combined with a program written in Python to accelerate and facilitate data acquisition. In order to validate the method proposed, the base promoted hydrolysis of 4-nitrophenyl acetate was initially investigated. The rate constants obtained through RGB analysis (0.01854 min-1) is almost identical to that using traditional UV-Vis spectroscopy (0.01848 min-1). The proposed method was then applied to monitor the kinetics of three heterogeneous processes: (1) reduction of 4-nitrophenolate in the presence of dispersed Pd/C; (2) decomposition of methyl orange with TiO2; and (3) adsorption of rhodamine on montmorillonite. In general, the method via digital images showed high reproducibility and analytical frequency, allowing the execution of simultaneous analyses, with an accuracy comparable to UV-Vis spectrophotometry. The method developed herein is a practical and valuable alternative for obtaining kinetic data of heterogeneous reactions and processes where a color change is involved, bypassing sampling collection and processing which decreases analytical frequency and may lead to data errors.