Ken J. Friesen

Mechanistic aspects regarding the direct aqueous environmental photochemistry of phenol and its simple halogenated derivatives. A review

We have reviewed the mechanistic aspects regarding the direct aqueous phase environmental photochemistry of phenol and its simple halogenated derivatives. These compounds are important industrial and natural products, are ubiquitous in aquatic systems, and their acute and chronic toxicity makes their environmental fate of interest. Work over the past two decades has unified the photochemistry of phenol and its simple halogenated derivatives. In general, three photochemical pathways dominate in aqueous solution depending on the nature of the substrate: (1) photoionization, (2) photochemical aryl-halogen bond homolysis, and (3) photochemical aryl-halogen bond heterolysis. Photoionization typically results in an array of biaryl radical coupling products which are only relevant for highly concentrated waste streams. Photolytic aryl-halogen bond homolysis will primarily give photoreduction products where reducing agents such as dissolved organic matter or reduced metal cations are present, and radical coupling products in highly concentrated waste streams. The 2- and 4-substituted halophenols may undergo photochemical aryl-halogen bond heterolysis upon irradiation to give an aryl cation. The aryl cation can be attacked by water to give the corresponding hydroxylated derivative, or may deprotonate to generate alpha- and gamma-ketocarbenes, respectively. Following their formation, the singlet alpha-ketocarbenes may undergo Wolff rearrangements to cyclopentadiene-ketenes that are subsequently hydrolyzed to cyclopentadiene carboxylic acids. The triplet alpha- and gamma-ketocarbenes are attacked by oxygen and hydrolyzed to give benzoquinones, directly hydrolyzed to yield hydroquinones, reduced to give phenols, or could take part in coupling reactions in highly concentrated waste streams to give dimers and hydroxybiaryl complexes. Additional studies in natural water samples are required to assess the relative importance of these direct irradiation mechanisms relative to indirect photolysis and other abiotic and biotic degradation and environmental partitioning pathways across the continuum of marine, freshwater, and wastewater biogeochemical signatures.

Computational approaches may underestimate pK a values of longer-chain perfluorinated carboxylic acids: Implications for assessing environmental and biological effects

Acidity constants were calculated using the semiempirical PM6 pK a estimation method for all C2 through C9 perfluoroalkyl carboxylate (PFCA) congeners and the straight-chain C10 through C13 isomers. According to the PM6 estimates, the linear congeners within each PFCA homologue group have the highest pK a values by up to 6 units depending on the degree of branching in the perfluoroalkyl chain. In general, the higher the degree of branching in the perfluoroalkyl chain within a homologue group, the lower the estimated pK a value. When the branching is closest to the terminal carboxylate group, the effect on the calculated pK a is greatest. Although the PM6 calculated pK a values agree well with previously reported estimates for selected linear PFCA congeners using the SPARC and COSMOtherm approaches, all computational approaches only show good agreement with reported experimental values for short chain PFCAs (C2 through C5). Increasing divergences are observed between calculated and experimental results by up to several pK a units as the perfluoroalkyl chain length increases beyond C5. The findings demonstrate a need for additional experimental pK a measurements for an expanded set of both linear and branched PFCA congeners to confirm previous experimental reports that are potentially in error, and upon which to calibrate existing computational methods and environmental, toxicological, and waste treatment method models.Ammonia (NH(3)) emission from livestock manures used in agriculture reduces N uptake by crops and negatively impacts air quality. This laboratory study was conducted to evaluate NH(3)emission from different livestock manures applied to two soils: Candler fins sand (CFS; light-textured soil, pH 6.8 and field capacity soil water content of 70 g kg(-1)) from Lake Alfred, Florida and Ogeechee loamy sand (OLS; medium-textured soil, pH 5.2 and field capacity soil water content of 140 g kg(-1)) from Savannah, Georgia. Poultry litter (PL) collected from a poultry farm near Douglas, Georgia, and fresh solid separate of swine manure (SM) collected from a farm near Clinton, North Carolina were used. Each of the soil was weighed in 100 g sub samples and amended with either PL or SM at rates equivalent to either 0, 2.24, 5.60, 11.20, or 22.40 Mg ha(-1) in 1L Mason jars and incubated in the laboratory at field capacity soil water content for 19 days to monitor NH(3) volatilization. Results indicated a greater NH(3) loss from soils amended with SM compared to that with PL. The cumulative NH(3)volatilization loss over 19 days ranged from 4 to 27% and 14 to 32% of total N applied as PL and SM, respectively. Volatilization of NH(3) was greater from light-textured CFS than that from medium-textured OLS. Volatilization loss increased with increasing rates of manure application. Ammonia volatilization was lower at night time than that during the day time. Differences in major factors such as soil water content, temperature, soil type and live stock manure type influenced the diurnal variation in volatilization loss of NH(3) from soils. A significant portion (> 50%) of cumulative NH(3) emission over 19 d occurred during the first 5-7 d following the application of livestock manures. Results of this study demonstrate that application of low rates of livestock manure (< or = 5.60 Mg ha(-1)) is recommended to minimize NH(3) emissions.

Congener-specific numbering systems for the environmentally relevant C4 through C8 perfluorinated homologue groups of alkyl sulfonates, carboxylates, telomer alcohols, olefins, and acids, and their derivatives

We introduce a congener-specific numbering system for the C4 through C8 perfluorinated homologue groups of alkyl sulfonates, carboxylates, telomer alcohols, olefins, and acids, and their derivatives. Increasing length of the carbon chain beyond C3 leads to a corresponding rapid increase in the number of potential isomers (C4 = 4, C5 = 8, C6 = 17, C7 = 39, and C8 = 89 congeners). There is a need for clear and unambiguous chemical shorthand to ensure accuracy and consistency in the future perfluorinated alkyl substance (PFA) literature, and to correct previous misconceptions that may have restricted research efforts into developing full-congener PFA analysis. If adopted by the research community, introduction of a numbering system at this relatively early stage of investigations into the congener-specific analysis, environmental behavior, and toxicology of PFAs would not require an arduous and difficult reassignment of historical structures and naming conventions presented in the prior art. Many PFA congeners are chiral, necessitating a consideration of their enantiospecific environmental behavior and toxicology.

Homogeneous degradation of 1,2,9,10-tetrachlorodecane in aqueous solutions using hydrogen peroxide, iron and UV light

The homogeneous degradation of the polychlorinated n-alkane, 1,2,9,10-tetrachlorodecane (T4C10), was studied in aqueous solutions of hydrogen peroxide, including Fenton and photo-Fenton reaction conditions. All solutions were adjusted to a pH of 2.8 and an ionic strength of 0.1 M NaClO4 prior to photolysis. T4C10 (2 x 10(-6) M) was substantially degraded by the H2O2/UV system (1.0 x 10(-2) M H2O2), with 60% disappearance in 20 min of irradiation in a photoreactor equipped with 300 nm lamps of light intensity 3.6 x 10(-5) Ein L(-1) min(-1) (established by ferrioxalate actinometry). The reaction produced stoichiometric amounts of chloride ion indicating complete dechlorination of the chlorinated n-alkane. T4C10 degraded very slowly under Fenton (Fe2+/H2O2/dark) and Fenton-like (Fe3+/H2O2/dark) conditions. However, when the same solutions were irradiated, T4C10 degraded more rapidly than in the H2O2/UV system, with 61% disappearance in 10 min of exposure. The rapid degradation is related to the enhanced degradation of hydrogen peroxide to oxidizing *OH radicals under photo-Fenton conditions. Degradation was inhibited in both the H2O2/UV and photo-Fenton systems by the addition of KI and tert-butyl alcohol due to *OH scavenging.

Estimated bioconcentration factors (BCFs) for the C4 through C8 perfluorinated alkylsulfonic acid (PFSA) and alkylcarboxylic acid (PFCA) congeners

Bioconcentration factors (BCFs) were estimated for all congeners in each of the C4 through C8 homologue groups for perfluorinated alkylsulfonic acids (PFSAs) and alkylcarboxylic acids (PFCAs). Predictive equations were developed between molecular areas and volumes using optimized gas-phase geometries from the AM1 and PM3 semiempirical computational basis sets and previously determined BCFs for representative straight-chain members of each contaminant class. Molecular area approaches for estimating PFSA and PFCA BCFs resulted in more variability both between and within homologue groups than the use of molecular volumes as proxies for hydrophobicity of the perfluoroalkyl chain. An increasing degree of perfluoroalkyl chain branching within each homologue group reduces the estimated BCF, suggesting that the more linear PFSA and PFCA congeners will display the highest BCFs in aquatic organisms.The degradation of the main azadirachtoids on tomatoes was studied after greenhouse treatment. These experiments were carried out at 1 and 5x the concentration recommended by the manufacturer. In all experiments the deposition of azadirachtin A (AZA-A) was below the maximum residue level (MRL). Even if at the highest dose, AZA-A half-life time calculated as pseudo first order kinetic was 1.2 days in agreement with the recommended preharvest interval (PHI) of 3 days. Experiments with a model system showed that sunlight photodegradation is the main factor influencing the rate of disappearance of AZA-A after greenhouse treatment while tomato epicuticular waxes doubled the photodegradation rate of AZA-A in a commercial formulation.

Estimated congener specific gas-phase atmospheric behavior and fractionation of perfluoroalkyl compounds: Rates of reaction with atmospheric oxidants, air-water partitioning, and wet/dry deposition lifetimes

A quantitative structure-activity model has been validated for estimating congener specific gas-phase hydroxyl radical reaction rates for perfluoroalkyl sulfonic acids (PFSAs), carboxylic acids (PFCAs), aldehydes (PFAls) and dihydrates, fluorotelomer olefins (FTOls), alcohols (FTOHs), aldehydes (FTAls), and acids (FTAcs), and sulfonamides (SAs), sulfonamidoethanols (SEs), and sulfonamido carboxylic acids (SAAs), and their alkylated derivatives based on calculated semi-empirical PM6 method ionization potentials. Corresponding gas-phase reaction rates with nitrate radicals and ozone have also been estimated using the computationally derived ionization potentials. Henrys law constants for these classes of perfluorinated compounds also appear to be reasonably approximated by the SPARC software program, thereby allowing estimation of wet and dry atmospheric deposition rates. Both congener specific gas-phase atmospheric and air-water interface fractionation of these compounds is expected, complicating current source apportionment perspectives and necessitating integration of such differential partitioning influences into future multimedia models. The findings will allow development and refinement of more accurate and detailed local through global scale atmospheric models for the atmospheric fate of perfluoroalkyl compounds.

Quantitative structure–activity relationships for estimating the aryl hydrocarbon receptor binding affinities of resveratrol derivatives and the antioxidant activities of hydroxystilbenes

Quantitative structure–activity relationships (QSARs) were developed for the aryl hydrocarbon receptor (AhR) binding affinity of non-fluorinated and fluorinated cis and trans 3,4′,5-substituted resveratrol derivatives. Lower quality QSAR fits were found when all compounds were modeled together, in contrast to strong correlations with Hammett substituent constants and atomic charges for separate non-fluorinated/fluorinated and cis/trans groupings. The collective findings suggest little promise in developing new resveratrol derivatives with significantly higher AhR binding affinity beyond the range already mapped by the available experimental data sets. The results also suggest that future QSAR searches for AhR binding affinity among resveratrol derivatives will likely need to proceed in a more constrained class-by-class fashion owing to the potentially different mechanistic implications from changes in the types/positions of aromatic substitution and the cis/trans geometrical isomerism. The antioxidant activity of hydroxystilbenes can be accurately modeled using high-quality empirical univariate relationships with various molecular descriptors. In comparison, correlations between theoretical bond dissociation enthalpies and ionization energies of the molecular and dissociated forms yielded lower quality QSAR fits with the antioxidant behavior.

Linear free energy relationship based estimates for the congener specific relative reductive defluorination rates of perfluorinated alkyl compounds

Linear free energy relationships (LFERs) were developed to estimate the congener specific relative rates of reductive defluorination for a suite of perfluorinated compound (PFC) classes. The LFERs were based on the semiempirically calculated lowest unoccupied molecular orbital energy (ELUMO) using gas and aqueous phase computations with the PM6 and RM1 methods. PFC classes in the modeling effort included the C1 through C8 perfluoroalkyl sulfonates (PFSAs), carboxylates (PFCAs), sulfonyl fluorides (PFSFs), sulfonamides and their derivatives (SAs), and the perfluorotelomer alcohols (PFTAls), olefins (PFTOls), and acids (PFTAcs). Gas and aqueous phase calculations using the PM6 method predict that branched PFSA, PFCA, and PFSF congeners will have more rapid reductive defluorination kinetics than their linear counterparts. The RM1 method predicts that only PFSFs will display intra-homologue dependent branching effects. For the PFSAs and PFSFs, both the PM6 and RM1 methods predict no significant difference in mean rates of reductive defluorination between the homologue groups. For the PFCAs, the PM6 method suggests no significant difference in inter-homologue mean rates of reductive defluorination, whereas the RM1 method predicts a significant increase with a lengthening perfluoroalkyl chain. All approaches used suggest that the intrahomologue variability in reduction rates increases with increasing chain length for PFSAs, PFCAs, and PFSFs, implying that the larger homologue groups in these classes will see a more rapid linearization of the congener profiles under reducing conditions than their lower homologue counterparts. Chain length has a negligible effect on the estimated rates of SA reductive defluorination, but a significant role for the fluorotelomer derivatives. Ratios of rates between the C8:C1 straight chain telomeric congeners are expected to range up to 200-fold depending on the computational combination. The kinetics for reductively defluorinating PFC starting materials will likely be 2 to 3 orders of magnitude more rapid than for most of the partially defluorinated degradation products. Significant quantities of partially defluorinated PFCs are thus expected to be observed under steady state conditions during reductive treatment processes, leading to a potentially significant reservoir of these compounds residing in reducing environmental and biological systems.

Photochemical oxidation of short-chain polychlorinated n-alkane mixtures using H2O2/UV and the photo-Fenton reaction

The photochemical oxidation of a series of short-chain polychlorinated n-alkane (PCA) mixtures was investigated using H2O2/UV and modified photo-Fenton conditions (Fe3

Kinetics of sunlight photodegradation of 2,3,4,7,8-pentachlorodi-benzofuran in natural water

The aquatic photodegradation of the environmental contaminant 2,3,4,7,8-pentachlorodibenzofuran (P5CDF) was studied under midsummer sunlight conditions at 50‡N latitude both in distilled water-acetonitrile solutions and in lake water. The observed net photodegradation rate of P5CDF in natural water was 240-fold faster than the rate of direct photolysis in distilled water-acetonitrile solutions. The difference in rates of photodegradation is attributed to the action of sensitizers present in natural waters.