Hydrated electron (e aq –) treatment processes show great potential in remediating recalcitrant water contaminants, including perfluoroalkyl and polyfluoroalkyl substances (PFAS). However, treatment ...efficacy depends upon many factors relating to source water composition, UV light source characteristics, and contaminant reactivity. Here, we provide critical insights into the complex roles of solution parameters on contaminant abatement through application of a UV-sulfite kinetic model that incorporates first-principles information on e aq – photogeneration and reactivity. The model accurately predicts decay profiles of short-chain perfluoroalkyl acids (PFAAs) during UV-sulfite treatment and facilitates quantitative interpretation of the effects of changing solution composition on PFAS degradation rates. Model results also confirm that the enhanced degradation of PFAAs observed under highly alkaline pH conditions results from changes in speciation of nontarget e aq – scavengers. Reverse application of the model to UV-sulfite data collected for longer chain PFAAs enabled estimation of bimolecular rate constants (k 2, M–1 s–1), providing an alternative to laser flash photolysis (LFP) measurements that are not feasible due to the water solubility limitations of these compounds. The proposed model links the disparate means of investigating e aq – processes, namely, UV photolysis and LFP, and provides a framework to estimate UV-sulfite treatment efficacy of PFAS in diverse water sources.
Advanced reduction processes (ARPs) that generate hydrated electrons (eaq –; e.g., UV-sulfite) have emerged as a promising remediation technology for recalcitrant water contaminants, including per- ...and polyfluoroalkyl substances (PFASs). The effectiveness of ARPs in different natural water matrices is determined, in large part, by the presence of non-target water constituents that act to quench eaq – or shield incoming UV photons from the applied photosensitizer. This study examined the pH-dependent quenching of eaq – by ubiquitous dissolved carbonate species (H2CO3*, HCO3 –, and CO3 2–) and quantified the relative importance of carbonate species to other abundant quenching agents (e.g., H2O, H+, HSO3 –, and O2(aq)) during ARP applications. Analysis of laser flash photolysis kinetic data in relation to pH-dependent carbonate acid–base speciation yields species-specific bimolecular rate constants for eaq – quenching by H2CO3*, HCO3 –, and CO3 2– ( k H 2 C O 3 * = 2.23 ± 0.42 × 109 M–1 s–1, k H C O 3 − = 2.18 ± 0.73 × 106 M–1 s–1, and k C O 3 2 − = 1.05 ± 0.61 × 105 M–1 s–1), with quenching dominated by H2CO3* (which includes both CO2(aq) and H2CO3) at moderately alkaline pH conditions despite it being the minor species. Attempts to apply previously reported rate constants for eaq – quenching by CO2(aq), measured in acidic solutions equilibrated with CO2(g), overpredict quenching observed in this study at higher pH conditions typical of ARP applications. Moreover, kinetic simulations reveal that pH-dependent trends reported for UV-sulfite ARPs that have often been attributed to eaq – quenching by varying H+ can instead be ascribed to variable acid–base speciation of dissolved carbonate and the sulfite sensitizer.
Recent reports demonstrate that technologies generating hydrated electrons (eaq−; e.g., UV-sulfite) are a promising strategy for destruction of per- and polyfluoroalkyl substances, but fundamental ...rate constants are lacking. This work examines the kinetics and mechanisms of eaq− reactions with ultra-short chain (C2–C4) fluorocarboxylates using experimental and theoretical approaches. Laser flash photolysis (LFP) was used to measure bimolecular rate constants (k2; M−1 s−1) for eaq− reactions with thirteen per-, and for the first time, polyfluorinated carboxylate structures. The measured k2 values varied widely from 5.26 × 106 to 1.30 × 108 M−1s−1, a large range considering the minor structural changes among the target compounds. Molecular descriptors calculated using density functional theory did not reveal correlation between k2 values and individual descriptors when considering the whole dataset, however, semiquantitative correlation manifests when grouping by similar possible initial reduction event such as electron attachment at the α-carbon versus β- or γ-carbons along the backbone. From this, it is postulated that fluorocarboxylate reduction by eaq− occurs via divergent mechanisms with the possibility of non-degradative pathways being prominent. These mechanistic insights provide rationale for contradictory trends between LFP-derived k2 values and apparent degradation rates recently reported in UV-sulfite constant irradiation treatment experiments.
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•eaq−-ARPs show promise in destroying PFAS, but rate constants are lacking.•LFP was used to measure rate constants of eaq− reactions with poorly studied PFAS.•Kinetic values range over two orders of magnitude despite structural similarities.•DFT provided mechanistic insight into the initial reduction of PFAS by eaq−•Non-degradative pathways are possible and pose major concerns for ARP efficiency.
The synthesis, characterization, and photophysical properties of 4- and 6-coordinate Bi3+ coordination complexes are reported. Bi(bzq)3 (1) and Bi(bzq)2Br (2) (bzq = benzohquinoline) are ...synthesized by reaction of 9-Li-bzq with BiCl3 and BiBr3, respectively. Absorption spectroscopy, electrochemistry, and DFT studies suggest that 1 has 42% Bi 6s character in its highest-occupied molecular orbital (HOMO) as a result of six σ* interactions with the bzq ligands. Excitation of 1 at 450 nm results in a broad emission feature at 520 nm, which is rationalized as a metal-to-ligand charge transfer (MLCT) and phosphorescent emission resulting from bismuth-mediated intersystem crossing (ISC) to a triplet excited state. This excited state revealed a 35 μs lifetime and was quenched in the presence of oxygen. These results demonstrate that useful optoelectronic properties of Bi3+ can be accessed through hypercoordination with covalent organobismuth interactions that mimic the electronic structure of lead perovskites.
Lignin valorization approaches, which are critical to biorefining, often involve depolymerization to aromatic monomers. Alkaline oxidation has long held promise as a lignin depolymerization strategy, ...but requires high concentrations of base, typically NaOH, much of which must be neutralized to recover lignin-derived aromatic monomers. This consumption of base and associated waste generation incurs high cost and negative environmental impacts. In this work, we demonstrate that Sr(OH)
2
and Ba(OH)
2
perform comparably to NaOH in terms of total aromatic monomer yields in the aqueous aerobic alkaline depolymerization of corn stover lignin, and that up to 90% of these reversibly-soluble bases can be recovered
via
precipitation and filtration. Process modeling suggests that the use of Sr(OH)
2
could reduce the cost of alkaline oxidation by 20-60% compared to NaOH, depending on lignin loading. In contrast, the energy required to regenerate the Sr largely offsets potential improvements in sustainability over Na-promoted alkaline oxidation, though the sustainability comparison is likely sensitive to the lignin composition and could be improved by further optimization of the regeneration step.
When excess base is required to drive desired reactions, such as in lignin alkaline oxidation, Sr(OH)
2
can offer a reversibly-soluble alternative to NaOH that allows simple recycle of the excess base with concomitant cost and environmental benefits.
