Impact of phosphate addition on PFAS treatment performance for drinking water.

Adding new unit operations to drinking water treatment systems requires consideration of not only efficacy for its design purpose but also costs, water quality characteristics, impact on overall regulatory compliance, and impact of other treatment unit operations. Here, pilot study results for ion exchange (IX) and granular activated carbon (GAC) are presented for a utility with both per- and polyfluoroalkyl substances (PFAS) and volatile organic contaminant removal needs. Specifically, the impact of upstream air stripping and phosphate addition on PFAS treatment performance was evaluated. Modeling was used to fit the IX and GAC pilot data and predict performance under different scenarios. GAC performance was generally consistent for treating water before or after the air stripper, but the addition of phosphate prior to air-stripping resulted in a loss of 15%-25% capacity for some PFAS on IX media, demonstrating the need to consider the entire treatment train before implementing PFAS removal unit operations.

Strong Base Anion Exchange Selectivity of Nine Perfluoroalkyl Chemicals Relevant to Drinking Water.

Selectivity with respect to chloride (KPFAS∕C) was determined for nine drinking water relevant perfluoroalkyl and polyfluoroalkyl substances (PFAS): perfluoro-2-propoxypropanoic acid (GenX), five perfluoroalkyl carboxylic acids (PFCAs), and three perfluoroalkyl sulfonic acids (PFSAs). Three single-use strong base anion exchange gel resins were investigated, targeting drinking water relevant equilibrium PFAS liquid concentrations (≤500 ng/L). Except for the longest carbon chain PFCA (perfluorodecanoic acid) and PFSA (perfluorooctanesulfonic acid) studied, PFAS followed traditional ion exchange theory (law of mass action), including increasing equilibrium PFAS liquid concentrations with increasing equilibrium chloride liquid concentrations. Overall, KPFAS∕C values were (i) similar among resins for a given PFAS, (ii) 1-5 orders of magnitude greater than the selectivity of inorganic anions (e.g., nitrate) previously studied, (iii) 2 orders of magnitude greater for the same carbon chain length PFSA versus PFCA, (iv) found to proportionally increase with carbon chain length for both PFSAs and PFCAs, and (v) similar for GenX and perfluorohexanoic acid (six-carbon PFCA). A multisolute competition experiment demonstrated binary isotherm-determined KPFAS∕C values could be applied to simulate a multisolute system, extending work previously done with only inorganic anions to PFAS. Ultimately, estimated KPFAS∕C values allow future extension and validation of an open-source anion exchange column model to PFAS.

Determination of Cyanotoxins and Prymnesins in Water, Fish Tissue, and Other Matrices: A Review.

Harmful algal blooms (HABs) and their toxins are a significant and continuing threat to aquatic life in freshwater, estuarine, and coastal water ecosystems. Scientific understanding of the impacts of HABs on aquatic ecosystems has been hampered, in part, by limitations in the methodologies to measure cyanotoxins in complex matrices. This literature review discusses the methodologies currently used to measure the most commonly found freshwater cyanotoxins and prymnesins in various matrices and to assess their advantages and limitations. Identifying and quantifying cyanotoxins in surface waters, fish tissue, organs, and other matrices are crucial for risk assessment and for ensuring quality of food and water for consumption and recreational uses. This paper also summarizes currently available tissue extraction, preparation, and detection methods mentioned in previous studies that have quantified toxins in complex matrices. The structural diversity and complexity of many cyanobacterial and algal metabolites further impede accurate quantitation and structural confirmation for various cyanotoxins. Liquid chromatography-triple quadrupole mass spectrometer (LC-MS/MS) to enhance the sensitivity and selectivity of toxin analysis has become an essential tool for cyanotoxin detection and can potentially be used for the concurrent analysis of multiple toxins.

The Impacts of Potassium Permanganate and Powdered Activated Carbon on Cyanotoxin Release.

Bench-scale trials were performed to: (1) Expose Microcystis aeruginosa cells to potassium permanganate doses of 1, 3 and 5 mg/L, at contact times of 15, 30 and 90 minutes, pH levels of 7 and 9, and turbidities of 0.1, 5 and 20 NTU; (2) Compare the impacts of oxidation alone and oxidation plus powdered activated carbon for the final 60 minutes of contact time and (3) evaluate the impact of these treatment conditions on extracellular microcystins, extra- plus intracellular (combined) microcystins, cell membrane integrity and chlorophyll-a concentrations. Toxin releases from the cells were observed at both pH levels. The greatest toxin releases were observed at the lowest KMnO4 doses. The toxin releases were accompanied by relatively stable total cell counts, increases in membrane compromised cells and decreases in chlorophyll-a. The application of 10 mg/L PAC resulted in extracellular toxin concentrations that were markedly lower than those observed in oxidant-only situations.

Mechanistic Insights into the Hydrolysis of Organophosphorus Compounds by Paraoxonase-1: Exploring the Limits of Substrate Tolerance in a Promiscuous Enzyme.

We designed, synthesized and screened a library of analogs of the organophosphate pesticide metabolite paraoxon against a recombinant variant of human serum paraoxonase-1. Alterations of both the aryloxy leaving group and the retained alkyl chains of paraoxon analogs resulted in substantial changes to binding and hydrolysis, as measured directly by spectrophotometric methods or in competition experiments with paraoxon. Increases or decreases in the steric bulk of the retained groups generally reduced the rate of hydrolysis, while modifications of the leaving group modulated both binding and turnover. Studies on the hydrolysis of phosphoryl azide analogs as well as amino-modified paraoxon analogs, the former being developed as photo-affinity labels, found enhanced tolerance of structural modifications, when compared with O-alkyl substituted molecules. Results from computational modeling predict a predominant active site binding mode for these molecules which is consistent with several proposed catalytic mechanisms in the literature, and from which a molecular-level explanation of the experimental trends is attempted. Overall, the results of this study suggest that while paraoxonase-1 is a promiscuous enzyme, there are substantial constraints in the active site pocket, which may relate to both the leaving group and the retained portion of paraoxon analogs.

Mechanism and stereoselection in a Y-catalyzed transacylation reaction. A computational modeling study.

Density functional theory calculations were performed to evaluate the proposed mechanism of a yttrium-salen complex-catalyzed acylation of secondary alcohols using an enol acetate as the acyl-transfer agent. A key step in the proposed mechanism is an intramolecular nucleophilic reaction between the coordinated alcohol and enol ester, brought into close proximity by the yttrium catalyst. The use of the BP86 pure density functional for reproduction of the geometry of the yttrium complex was validated by comparison with the experimental crystal structure. Mapping of the free energy surface of the reaction followed, employing both the BP86 and B3LYP functionals, and results suggest that the proposed mechanism (Org. Lett. 2002, 4, 1607-1610) is a reasonable pathway for the reaction. Reproduction of the experimentally observed enantioselectivity of the reaction was subsequently attempted, with diastereomeric transition states identified for the turnover-limiting step in the reaction, the intramolecular nucleophilic attack of the alcohol into the enol ester reagent. The predicted experimental enantioselectivities were successfully reproduced for the three ligands studied, although the energetic predictions did not perfectly correlate with the experimental enantioselectivities.

Computational Modeling of Human Paraoxonase 1: Preparation of Protein Models, Binding Studies, and Mechanistic Insights.

The enzyme human paraoxonase 1 (huPON1) has demonstrated significant potential for use as a bioscavenger for treatment of exposure to organophosphorus (OP) nerve agents. Herein we report the development of protein models for the human isoform derived from a crystal structure of a chimeric version of the protein (pdb ID: 1V04) and a homology model derived from the related enzyme diisopropylfluorophosphatase (pdb ID: 1XHR). From these structural models, binding modes for OP substrates are predicted, and these poses are found to orient substrates in proximity to residues known to modulate specificity of the enzyme. Predictions are made with regard to the role that residues play in altering substrate binding and turnover, in particular with regard to the stereoselectivity of the enzyme, and the known differences in activity related to a natural polymorphism in the enzyme. Potential mechanisms of action of the protein for catalytic hydrolysis of OP substrates are also evaluated in light of the proposed binding modes.

Diastereoselective intramolecular pinacol couplings of sulfinyl iron(0) diene complexes.

3-(2-Formylaryl)-1-sulfinyl-(1Z,3E)-pentadien-5-al iron tricarbonyl complexes were prepared to examine the feasibility and diastereoselectivity of intramolecular pinacol couplings on such substrates. It was found that the pinacol coupling, promoted by VCl(3) x (THF)(3)/Zn, proceeded in good yield and with high diastereoselectivity (>23:1 dr), provided the 2-formylaryl unit remained unsubstituted at the aryl C3 position (ortho to the formyl group). In these latter cases the pinacol coupling was diastereorandom. A 3-formyl-4-(2-formylaryl)-1-sulfinyl-(1Z,3E)-butadiene iron tricarbonyl complex also underwent diastereoselective pinacol coupling (22:1 dr). 3-(3-Formylindolyl)-1-sulfinyl-(1Z,3E)-pentadien-5-al iron tricarbonyl complexes were also prepared, though pinacol coupling of these substrates proceeded in, at best, modest yield for two of the four examples tested. All cases described herein represent the first intramolecular pinacol couplings performed on the periphery of an iron(0) diene tricarbonyl complex.

Photomodulated chiral induction in helical azobenzene oligomers.

Appending L-alanine to the terminal positions of a helical azobenzene oligomer produced a P helical bias, which increased with oligomer length. Irradiation gave rise to E-->Z isomerization of the terminal azo linkages, which displaced the stereogenic center of L-Ala from the helix backbone and suppressed chiral induction. Theoretical simulations of the CD spectrum of the P helical conformation are in qualitative agreement with the experimental spectra.

Oxidative dehalogenation of perhalogenated benzenes by cytochrome P450 compound I.

Resolution of the identity PBE (RI-PBE) and B3LYP density functional theory calculations are used to understand the cytochrome P450-catalyzed, Compound I-mediated oxidation of perchlorobenzenes, perfluorobenzenes, their phenols, and mixed chlorofluorobenzenes to form benzoquinones. Addition of Compound I to the chlorine-bearing carbon of perchlorobenzenes and perchlorophenols results in an apparently barrierless 1,2-shift of the chlorine atom to form hexachlorocyclohexadienones and hydroxypentachlorocyclohexadienones, respectively. Hexachlorocyclohexadienone has a significant electron affinity, and its radical anion expels chloride in a facile manner to give the pentachlorophenoxyl radical. Deprotonation of hydroxypentachlorocyclohexadienones results in the expulsion of chloride and provides a direct route to the production of tetrachloroquinones. Barrier heights for Compound I addition to fluorine-bearing carbons of hexafluorobenzene and pentafluorophenol are comparable to those computed for oxidation of benzene via an analogous reaction path. In contrast to the chlorinated cases, fluorine migration to cyclohexadienones occurs with a moderate barrier. Additionally, gas-phase elimination of fluoride from the hexafluorocyclohexadienone radical anion and deprotonated hydroxypentafluorocyclohexadienone are not facile. Rather, consideration of implicit and explicit solvent is required to achieve favorable thermochemistry for fluoride elimination and generation of the experimentally observed products. Finally, the theoretical approach described herein is predictive of the experimentally observed preferential elimination of fluorine from chloropentafluorobenzene and 1,3,5-trichloro-2,4,6-trifluorobenzene. These studies illustrate the effectiveness of P450 Compound I as an oxidant of halogenated aromatic hydrocarbons, which are persistent environmental contaminants, and the potential utility of such computational methods for predicting P450 metabolism.

Simultaneous and stereoselective formation of planar and axial chiralities in enantiopure sulfinyl iron diene complexes.

[reaction: see text] Enantiopure (1Z,3E)-1-sulfinyl dienes bearing an o-dithianylphenyl group can be prepared and complexed with (bda)Fe(CO)(3) to afford the corresponding sulfinyl diene iron(0) tricarbonyl complexes. This diastereoselective complexation introduces planar and axial chirality simultaneously, with a high degree of facial selectivity as well as atropselectivity. Dynamic kinetic resolution is likely to be the origin of the atropselectivity.