Loss Rate Constants Research Articles

Nucleoside Cation Radicals: Generation, Radical-Induced Hydrogen Atom Migrations, and Ribose Ring Cleavage in the Gas Phase.

Nucleoside ions that were furnished on ribose with a 2'-O-acetyl radical group were generated in the gas phase by multistep collision-induced dissociation of precursor ions tagged with radical initiator groups, and their chemistry was investigated in the gas phase. 2'-O-Acetyladenosine cation radicals were found to undergo hydrogen transfer to the acetoxyl radical from the ribose ring positions that were elucidated using specific deuterium labeling of 1'-H, 2'-H, and 4'-H and in the N-H and O-H exchangeable positions, favoring 4'-H transfer. Ion structures and transition-state energies were calculated by a combination of Born-Oppenheimer molecular dynamics and density functional theory and used to obtain unimolecular rate constants for competitive hydrogen transfer and loss of the acetoxyl radical. Migrations to the acetoxyl radical of ribose hydrogens 1'-H, 2'-H, 3'-H, and 4'-H were all exothermic, but product formation was kinetically controlled. Both Rice-Ramsperger-Kassel-Marcus (RRKM) and transition-state theory (TST) calculations indicated preferential migration of 4'-H in a qualitative agreement with the deuterium labeling results. The hydrogen migrations displayed substantial isotope effects that along with quantum tunneling affected the relative rate constants and reaction branching ratios. UV-vis action spectroscopy indicated that the cation radicals from 2'-O-acetyladenosine consisted of a mixture of isomers. Radical-driven dissociations were also observed for protonated guanosine, cytosine, and thymidine conjugates. However, for those nucleoside ions and cation radicals, the dissociations were dominated by the loss of the nucleobase or formation of protonated nucleobase ions.

In vitro modeling of the post-ingestion mobilization and bioaccessibility of pesticides sorbed to soil and house dust

Soils and dusts can act as sinks for semivolatile lipophilic organic compounds and children ingest relatively large amounts of both soils and dusts. Following intake, sorbed chemicals may desorb (mobilize) and become available for intestinal absorption (bioaccessible). When chemicals are not degraded in the digestive tract, mobilization can approximate bioaccessibility. Alternatively, when gastrointestinal degradation of mobilized chemicals does occur, it can be useful to separate mobilization from bioaccessibility.In this study we used synthetic digestive fluids in a sequential, three-compartment (saliva, gastric, and intestinal) in vitro assay to construct mobilization and bioaccessibility models for 16 pesticides (log Kow 2.5–6.8) sorbed to 32 characterized soils and house dusts. To address the potential loss of mobilized pesticides due to absorption, the assays were repeated using a solid phase sorbent (tenax) added to the digestive fluid immediately after addition of the intestinal fluid components.We found that pesticide mobilization was predicted by pesticide log Kow and the carbon content of the soils and dusts. Pesticide loss measurably reduced the bioaccessibility of most pesticides, and bioaccessibility was largely predicted by log Kow and pesticide loss rate constants. Introduction of the sink increased mobilization by x̄ = 4 ± 6% (soil) and x̄ = 9 ± 7% (dust) while bioaccessibility increases were x̄ = 41 ± 21% (soil) and x̄ = 24 ± 12% (dust).The physicochemical properties of the soils, dusts, and pesticides used in this study successfully predicted the in vitro mobilization and bioaccessibility of the pesticides. This suggests that modeling of pesticide mobilization and bioaccessibility could reduce uncertainty in exposure and risk assessments.

On the intrinsic reaction rate of polyethylene pyrolysis and its interplay with mass transfer

An attempt to determine the the intrinsic kinetic of polyolefins pyrolysis is presented. For this, pyrolysis experiments of polyethylene were performed in a screen heater reactor, where the effects of mass and heat transfer on the pyrolysis process are minimized. First-order mass loss rate constants obtained at 500 °C were circa 0.5 and 1.3 s−1 for HDPE and LDPE, respectively, which are significantly higher than the majority of the values reported in the literature. At 450 °C, the mass loss rate of HDPE was lower, circa 10−2 s−1. Additionally, we have observed that the interplay between mass transfer and the depolymerization reactions can be used to steer the product distribution. For instance, in a 50 g scale batch reactor, the product obtained at 420 °C (only oil) is much lighter than at 500 °C (mixture of oil and wax), which can be attributed to the much lower evaporation rate of larger cracking products at 420 °C as a result of which these fragments crack further in the reacting liquid phase.

Measurement of Henry's law and liquid-phase loss rate constants of peroxypropionic nitric anhydride (PPN) in deionized water and in n-octanol

Abstract. The Henry's law solubility (HS) and liquid-phase loss rate constants (kl) of the tropospheric trace gas constituents peroxyacetic nitric anhydride (PAN; CH3C(O)O2NO2, commonly known as peroxyacetyl nitrate) and peroxypropionic nitric anhydride (PPN; C2H5C(O)O2NO2, also known as peroxypropionyl nitrate) in deionized (DI) water and of PPN in n-octanol were measured using a flow bubble apparatus at temperatures between 5.0 and 25.0 ∘C. For PAN in DI water, the observed values for HS,aq are consistent with the literature, whereas the solubility of PPN in DI water is slightly lower than literature values, ranging from HScp(PPN)aq = (1.49 ± 0.05) M atm−1 at 25.0 ∘C to HScp(PPN)aq = (7.01 ± 0.25) M atm−1 at 5.0 ∘C (stated uncertainties are at the 1σ level). The data are best described by ln⁡(HScp(PAN)aq/[Matm-1]) = -(17.8±0.3) + (5620±85)/T and ln⁡(HScp(PPN)aq/[Matm-1]) = -(19.5±1.7) + (5955±480)/T, where T is in kelvin. For n-octanol, the PPN solubility ranges from HScp(PPN)oct = (88±5)Matm-1 at 25.0 ∘C to HScpoct = (204±16)Matm-1 at 5.0 ∘C and is best described by ln⁡(HScp(PPN)oct/[Matm-1]) = -(6.92±0.75) + (3390±320)/T. n-Octanol–water partition coefficients (KOW) for PPN were determined for the first time, ranging from 59 ± 4 at 25.0 ∘C to 29 ± 3 at 5.0 ∘C. Observed loss rate constants in DI water are consistent with recent literature and larger than the thermal dissociation rates for both PAN and PPN, consistent with a hydrolysis mechanism, whereas kl values in n-octanol are significantly smaller than gas-phase dissociation rate constants, likely owing to a “cage effect” in the organic liquid. The results imply that uptake of either PAN or PPN on cloud water and organic aerosol is negligible but that uptake of PPN may constitute an overlooked source of peroxy radicals in organic aerosol.

The reactivity of pyridine in cold interstellar environments: The reaction of pyridine with the CN radical

The recent detection of cyclic species in cold interstellar environments is an exciting discovery with yet many unknowns to be solved. Among them, the presence of aromatic heterocycles in space would act as an indirect evidence of the presence of precursors of nucleotides. The seeming absence of these species in the observations poses a fascinating conundrum that can be tackled with computational insights. Whilst many arguments can be given to explain the absence of heterocycles in space, one of the possible scenarios involves fast chemical conversion and formation of new species to be detected. We have tested this hypothesis for the reaction of pyridine with the CN radical to find possible scenarios in which the detectability of pyridine, as an archetypical heterocycle, could be enhanced or diminished via chemical conversions. Using a combination of ab-initio characterization of the reactive potential energy surface and kinetic and chemical simulations, we have established that pyridine does react very fast with CN radicals, estimating that the studied reactions is between 2.5–4.5 times faster in pyridine than in benzene, with a total loss rate constant of 1.33 × 10–9 cm3s−1 at 30 K, with an almost null temperature dependence in the (30–150) K range. Addition reactions forming 1,2,3-cyanopyridine are favored over abstraction reactions or the formation of isocyanides. Besides, for 1 and 2-cyanopyridine there is an increase in the total dipole moment with respect to pyridine, which can help in their detection. However, the reaction is not site specific, and equal amounts of 1,2,3-cyanopyridine are formed during the reaction, diluting the abundance of all the individual pyridine derivatives.

Insights into the implication of halogen ions on the photoactivity of dissolved black carbon for the degradation of pharmaceutically active compounds

Although dissolved black carbon (DBC) is an important photosensitized substance and plays an irreplaceable role in the photodegradation of organic pollutants in freshwater, the photochemical activity of DBC in certain halogen ion enriched environments, such as estuaries, is unknown. Therefore, a halogen ions riched water environment was simulated in this study to investigate the effects of halogen ions on the photoactivity of DBC for the degradation of carbamazepine and 17β-estradiol as two representative pharmaceutically active compounds (PhACs). The results showed that the quantum yield coefficients of excited triplet state DBC (3DBC*) decreased along with halogen ions as compared to pure water, attributing to the ionic strength (IS) effect of halogen ions that inhibited the charge transfer (CT) of 3DBC* in its high/low-energy states. Nevertheless, no obvious change of singlet oxygen (1O2) quantum yield was observed in the presence of DBC with/without halogen ions. The 3DBC* formation rate (FT), loss rate constant (ks’), and steady-state concentration ([T]ss) with/without halogen ions were measured by a trimethylphenol probe method, suggesting that the elevation of [T]ss in the presence of halogen ions is attributed to the inhibition of ks’ by IS effect. In addition, we observed that the indirect photolysis of carbamazepine and 17β-estradiol were both inhibited by halogen ions. Radical scavenging experiments demonstrated that halogen ions were the dominant competitor to carbamazepine for 3DBC*, resulting in the retardation of indirect photolysis; whereas for 17β-estradiol, the presence of halogen ions inhibited DBC-mediated photodegradation due to the quenching of 3DBC* with halogen ions. These findings are helpful in understanding the photochemistry of DBC in estuaries and high salinity seawater and its effect on the phototransformation of PhACs.

Investigation of the limonene photooxidation by OH at different NO concentrations in the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber)

Abstract. The oxidation of limonene by the hydroxyl (OH) radical and ozone (O3) was investigated in the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) in experiments performed at different nitric oxide (NO) mixing ratios from nearly 0 up to 10 ppbv. For the experiments dominated by OH oxidation, the formaldehyde (HCHO) yield was experimentally determined and found to be (12 ± 3), (13 ± 3), and (32 ± 5) % for experiments with low (∼ 0.1 ppbv), medium (∼ 0.3 ppbv), and high NO (5 to 10 ppbv), respectively. The yield in an ozonolysis-only experiment was (10 ± 1) %, which agrees with previous laboratory studies. The experimental yield of the first-generation organic nitrates from limonene–OH oxidation is calculated as (34 ± 5) %, about 11 % higher than the value in the Master Chemical Mechanism (MCM), which is derived from structure–activity relationships (SARs). Time series of measured radicals, trace-gas concentrations, and OH reactivity are compared to results from zero-dimensional chemical box model calculations applying MCM v3.3.1. Modeled OH reactivity is 5 to 10 s−1 (25 % to 33 % of the OH reactivity at the start of the experiment) higher than measured values at the end of the experiments under all chemical conditions investigated, suggesting either that there are unaccounted loss processes of limonene oxidation products or that products are less reactive toward OH. In addition, model calculations underestimate measured hydroperoxyl radical (HO2) concentrations by 20 % to 90 % and overestimate organic peroxyl radical (RO2) concentrations by 50 % to 300 %. The largest deviations are found in low-NO experiments and in the ozonolysis experiment. An OH radical budget analysis, which uses only measured quantities, shows that the budget is closed in most of the experiments. A similar budget analysis for RO2 radicals suggests that an additional RO2 loss rate constant of about (1–6) × 10−2 s−1 for first-generation RO2 is required to match the measured RO2 concentrations in all experiments. Sensitivity model runs indicate that additional reactions converting RO2 to HO2 at a rate constant of about (1.7–3.0) × 10−2 s−1 would improve the model–measurement agreement of NOx, HO2, and RO2 concentrations and OH reactivity. Reaction pathways that could lead to the production of additional OH and HO2 are discussed, which include isomerization reactions of RO2 from the oxidation of limonene, different branching ratios for the reaction of RO2 with HO2, and a faster rate constant for RO2 recombination reactions. As the exact chemical mechanisms of the additional HO2 and OH sources could not be identified, further work needs to focus on quantifying organic product species and organic peroxy radicals from limonene oxidation.

Enhanced photodegradation of dimethoxybenzene isomers in/on ice compared to in aqueous solution

Abstract. Photochemical reactions of contaminants in snow and ice can be important sinks for organic and inorganic compounds deposited onto snow from the atmosphere and sources for photoproducts released from snowpacks into the atmosphere. Snow contaminants can be found in the bulk ice matrix, in internal liquid-like regions (LLRs), or in quasi-liquid layers (QLLs) at the air–ice interface, where they can readily exchange with the firn air. Some studies have reported that direct photochemical reactions occur faster in LLRs and QLLs than in aqueous solution, while others have found similar rates. Here, we measure the photodegradation rate constants for loss of the three dimethoxybenzene isomers under varying experimental conditions, including in aqueous solution, in LLRs, and at the air–ice interface of nature-identical snow. Relative to aqueous solution, we find modest photodegradation enhancements (3- and 6-fold) in LLRs for two of the isomers and larger enhancements (15- to 30-fold) at the air–ice interface for all three isomers. We use computational modeling to assess the impact of light absorbance changes on photodegradation rate enhancements at the interface. We find small (2–5 nm) bathochromic (red) absorbance shifts at the interface relative to in solution, which increases light absorption, but this factor only accounts for less than 50 % of the measured rate constant enhancements. The major factor responsible for photodegradation rate enhancements at the air–ice interface appears to be more efficient photodecay: estimated dimethoxybenzene quantum yields are 6- to 24-fold larger at the interface compared to in aqueous solution and account for the majority (51 %–96 %) of the observed enhancements. Using a hypothetical model compound with an assumed Gaussian-shaped absorbance peak, we find that a shift in the peak to higher or lower wavelengths can have a minor to substantial impact on photodecay rate constants, depending on the original location of the peak and the magnitude of the shift. Changes in other peak properties at the air–ice interface, such as peak width and height (i.e., molar absorption coefficient), can also impact rates of light absorption and direct photodecay. Our results suggest our current understanding of photodegradation processes underestimates the rate at which some compounds are broken down, as well as the release of photoproducts into the atmosphere.

Wall losses of oxygenated volatile organic compounds from oxidation of toluene: Effects of chamber volume and relative humidity

Vapor wall losses can affect the yields of secondary organic aerosol. The effects of surface-to-volume (S/V) ratio and relative humidity (RH) on the vapor-wall interactions were investigated in this study. The oxygenated volatile organic compounds (OVOCs) were generated from toluene-H2O2 irradiations. The average gas to wall loss rate constant (kgw) of OVOCs in a 400 L reactor (S/V = 7.5 m−1) is 2.47 (2.41 under humid conditions) times higher than that in a 5000 L reactor (S/V = 3.6 m−1) under dry conditions. In contrast, the average desorption rate constant (kwg) of OVOCs in 400 L reactor is only 1.37 (1.20 under humid conditions) times higher than that in 5000 L reactor under dry conditions. It shows that increasing the S/V ratio can promote the wall losses of OVOCs. By contrast, the RH effect on kgw is not prominent. The average kgw value under humid conditions is almost the same as under dry conditions in the 400 L (5000 L) reactor. However, increasing RH can decrease the desorption rates. The average kwg value under dry conditions is 1.45 (1.27) times higher than that under humid conditions in the 400 L (5000 L) reactor. The high RH can increase the partitioning equilibrium timescales and enhance the wall losses of OVOCs.

Development of Mathematical Models for Predicting Mass Transfer and Changes in Quality Properties of Falafel Prepared from Faba Bean (Vicia faba L.) by Air Frying Technique

Air Frying technology is an innovative technique which could produce healthy fried food materials using hot air. The current work aimed to model the kinetic changes in mass transfer, texture and colour attributes and determining the temperature dependency of the rate constants of the mass transfer, texture and colour traits of the air-fried falafel during the air frying process. To achieve these objectives, five mathematical equations were applied to forecast the mass transfer through frying process. The findings of this study indicated that the air-fried falafel moisture content declined as the frying time increases at varied frying temperatures. The outputs of nonlinear regression analysis showed that the moisture loss of air-fried falafel could be satisfactorily fitted to the modified Page model. The outcomes discovered that the alterations in the texture and the total colour difference (ΔE) of air-fried falafel throughout frying process have linearly increased at varied temperatures and were properly fitted to the zero-order kinetic equation. Meanwhile, temperature dependence of the rate constant of moisture loss, texture and colour change was sufficiently described by the Arrhenius-type model. The increase in rate constants was detected because of the growth of frying temperature. The activation energy values of mass transfer, hardness and ΔE were 70, 21.8 and 0.167 kJ/mol, respectively. The information on kinetic modelling the quality changes during air frying of falafel could be beneficial for helping air fried falafel manufactures with the optimum conditions and quality as well.

Kinetic modelling of dissolution dynamic nuclear polarisation 13 C magnetic resonance spectroscopy data for analysis of pyruvate delivery and fate in tumours.

Dissolution dynamic nuclear polarisation (dDNP) of 13 C-labelled pyruvate in magnetic resonance spectroscopy/imaging (MRS/MRSI) has the potential for monitoring tumour progression and treatment response. Pyruvate delivery, its metabolism to lactate and efflux were investigated in rat P22 sarcomas following simultaneous intravenous administration of hyperpolarised 13 C-labelled pyruvate (13 C1 -pyruvate) and urea (13 C-urea), a nonmetabolised marker. A general mathematical model of pyruvate-lactate exchange, incorporating an arterial input function (AIF), enabled the losses of pyruvate and lactate from tumour to be estimated, in addition to the clearance rate of pyruvate signal from blood into tumour, Kip , and the forward and reverse fractional rate constants for pyruvate-lactate signal exchange, kpl and klp . An analogous model was developed for urea, enabling estimation of urea tumour losses and the blood clearance parameter, Kiu . A spectral fitting procedure to blood time-course data proved superior to assuming a gamma-variate form for the AIFs. Mean arterial blood pressure marginally correlated with clearance rates. Kiu equalled Kip , indicating equivalent permeability of the tumour vasculature to urea and pyruvate. Fractional loss rate constants due to effluxes of pyruvate, lactate and urea from tumour tissue into blood (kpo , klo and kuo , respectively) indicated that T1 s and the average flip angle, θ, obtained from arterial blood were poor surrogates for these parameters in tumour tissue. A precursor-product model, using the tumour pyruvate signal time-course as the input for the corresponding lactate signal time-course, was modified to account for the observed delay between them. The corresponding fractional rate constant, kavail , most likely reflected heterogeneous tumour microcirculation. Loss parameters, estimated from this model with different TRs, provided a lower limit on the estimates of tumour T1 for lactate and urea. The results do not support use of hyperpolarised urea for providing information on the tumour microcirculation over and above what can be obtained from pyruvate alone. The results also highlight the need for rigorous processes controlling signal quantitation, if absolute estimations of biological parameters are required.

Degradation of organic filter 2-Phenylbenzidazole-5-Sulfonic acid by light-driven free chlorine process: Reactive species and mechanisms

This study investigated the degradation of organic filter 2-phenylbenzidazole-5-sulfonic acid (PBSA) under ultraviolet and simulated sunlight activated free chlorine (UV/FC and SS/FC) in different water matrices. A mechanistic study revealed that ozone, HO•, Cl• and ClO• were responsible for PBSA degradation, and reactive chlorine species (RCS) played a major role. The developed mathematical model was utilized to estimate the rate constants of PBSA with representative radical species and predict the kinetics of PBSA attenuation well. Observed PBSA loss rate constants (kobs) decrease with pH during UV/FC due to the high quantum yield and weak radical scavenging ability of hypochlorite (pKa = 7.5), while there is little difference under SS/FC. Furthermore, formation of reactive oxidants (HO•, Cl•, ClO• and ozone) during FC photolysis as a function of pH (5–10) were quantified with probe compounds. In seawater matrix, kobs for PBSA degradation under UV/FC (0.0109 min−1) was two times lower than that under SS/FC (0.0244 min−1) because the highly abundant HCO3–, Br- and Cl- exhibited a stronger inhibition (76%) for the former. A screening test showed that NaBr and NaBr + NaHCO3 during SS/FC displayed the reinforcement effects, primarily due to the interaction between reactive bromine species and the imidazole moiety in PBSA. Based on these identified products, transformation pathways of PBSA included hydroxylation, ring cleavage and desulfonate. This work provided mechanistic insight into PBSA degradation by reactive species formed through FC photoactivation, and suggest a promise of SS/FC for efficient abatement of organic filters-caused seawater pollution for realistic application.

Analysis of Solid-State Reaction Mechanisms with Two-Dimensional Fourier Transform Infrared Correlation Spectroscopy.

The utility of two-dimensional generalized correlation spectroscopy (2D-COS) for tracking complex solid-state reactions is demonstrated using infrared spectra acquired during a photochemically induced decomposition reaction. Eleven different thin films, consisting of six monometallic and five bimetallic 2-ethylhexanoate complexes, were tracked as a function of photolysis time. Overlapping peaks in the infrared fingerprint region are readily discriminated using 2D-COS, enabling individual vibrational components to be used to distinguish whether carboxylate ligands are free/ionic or bound in a chelating, bridging, or monodentate fashion. This classification enables the decomposition mechanism to be tracked for all 11 samples, revealing that ligands bound in monodentate and bridging fashions are first converted to chelates before being lost as volatile products for all samples. The magnitude of the measured first-order rate constants for loss of chelated ligands is found to correlate linearly to the asymmetric stretching frequency of monodentate ligands but exhibits a V shape when plotted against the electronegativity of the metal center. We propose that loss of chelated ligands proceeds via C-O scission for highly electronegative transition metals but M-O scission for transition metals with low electronegativity. These results establish 2D-COS as a powerful tool to deconvolute and correlate individual components, enabling mechanistic analysis of complex chemical reactions.

Effect of Various Parameters on the Thermal Stability and Corrosion of CO2-Loaded Tertiary Amine Blends

In this study, the thermal stability and corrosivity of various CO2-loaded tertiary amine blends in both aqueous and non-aqueous form in stainless steel cylinders were studied for combined acid gas and water removal. The thermal stability was measured from the remaining amine concentration and the corrosivity was measured from the amount of various metals in blends using titration and inductively coupled plasma mass spectrometry (ICP-MS), respectively. The experimental data were used to calculate the rate constants of amine group loss. The developed model represented the experimental data very well. Solvent change from H2O to triethylene glycol (TEG) in blends decreased the thermal stability and vice versa for corrosivity. The amine stability was increased when contact with stainless steel was reduced. An increase in the amine concentration or CO2 loading at constant temperature decreased the thermal stability and vice versa for corrosivity.

Termite Control of Leaf Litter Decomposition of Eight Selected Plant Species of Sudano-guinea Savannahs of Ngaoundere, Cameroon

Few studies on effects of termites on litter decomposition have been done in African savannahs, particularly in the Adamawa savannahs of Cameroon. In the framework of management of resource quality to restore or improve soil fertility of farming systems of Sudano-guinea savannahs of Ngaoundere, Cameroon, study on termites’ control of leaf litter decomposition of eight plant species was conducted on the field. The selected plant species are Bixa orellana, Erythrina sigmoïdea, Ficus polita, Maytenus senegalensis, Mucuna stans, Piliostigma thonningii, Vitex madiensis and Vitellaria paradoxa. Leaf litter samples were incubated in situ using litterbags of 2 mm mesh during 24 weeks in two plots out of canopy, corresponding to two treatments, with and without termites. Experimental design was split-plot with three replications. Collected data was carried out on litter dry mass remaining (LMR). Results showed total mass loss at the end of incubation time (24 weeks) and decomposition rate constants (k) differed significantly among plant species for the two treatments. The values ranged respectively from 23.05% and 0.012 week-1 in V. madiensis to 61.93% of initial dry mass and 0.046 week-1 in P. thonningii for treatment without termites and from 43.88% and 0.022 week-1 in B. orellana to 91.51% and 0.095 week-1 in P. thonningii for treatment with termites. These macro organisms fasted litter decomposition in all plant species, with intensity variation according to species. Litter mass loss and decomposition rate constant (k) correlated with litter thickness, density, area and specific area mass, and these relationships were influenced by the presence of termites. Globally litter decomposition was influenced by termite activities and resource quality. These results contributed to understand litter decomposition process in the sudano-guinea savannahs of Ngaoundere in order improve soil fertility, nutrient cycling and some plant species domestication.

Experimental Study of OH-Initiated Heterogeneous Oxidation of Organophosphate Flame Retardants: Kinetics, Mechanism, and Toxicity.

The environmental risks and health impacts associated with particulate organophosphate flame retardants (OPFRs), which are ubiquitous in the global atmosphere, have not been adequately assessed due to the lack of data on the reaction kinetics, products, and toxicity associated with their atmospheric transformations. Here, the importance of such transformations for OPFRs are explored by investigating the reaction kinetics, degradation chemical mechanisms, and toxicological evolution of two OPFRs (2-ethylhexyl diphenyl phosphate (EHDP) and diphenyl phosphate (DPhP)) coated on (NH4)2SO4 particles upon heterogeneous OH oxidation. The derived reaction rate constants for the heterogeneous loss of EHDP and DPhP are (1.12 ± 0.22) × 10-12 and (2.33 ± 0.14) × 10-12 cm3 molecules-1 s-1, respectively. Using recently developed real-time particle chemical composition measurements, particulate products from heterogeneous photooxidation and the associated degradation mechanisms for particulate OPFRs are reported for the first time. Subsequent cytotoxicity analysis of the unreacted and oxidized OPFR particles indicated that the overall particle cytotoxicity was reduced by up to 94% with heterogeneous photooxidation, likely due to a significantly lower cytotoxicity associated with the oxidized OPFR products relative to the parent OPFRs. The present work not only provides guidance for future field sampling for the detection of transformation products of OPFRs, but also strongly supports the ongoing risk assessment of these emerging chemicals and most critically, their products.

Understanding the Impact of Relative Humidity and Coexisting Soluble Iron on the OH-Initiated Heterogeneous Oxidation of Organophosphate Flame Retardants.

The current uncertainties in the reactivity and atmospheric persistence of particle-associated chemicals present a challenge for the prediction of long-range transport and deposition of emerging chemicals such as organophosphate flame retardants, which are ubiquitous in the global environment. Here, the OH-initiated heterogeneous oxidation kinetics of organophosphate flame retardants (OPFRs) coated on inert (NH4)2SO4 and redox-active FeSO4 particles were systematically determined as a function of relative humidity (RH). The derived reaction rate constants for the heterogeneous loss of tricresyl phosphate (TCP; kTCP) and tris(2-butoxyethyl) phosphate (TBEP; kTBEP) were in the range of (2.69-3.57) × 10-12 and (3.06-5.55) × 10-12 cm3 molecules-1 s-1, respectively, depending on the RH and coexisting Fe(II) content. The kTCP (coated on (NH4)2SO4) was relatively constant over the investigated RH range while kTBEP was enhanced by up to 19% with increasing RH. For both OPFRs, the presence of Fe(II) enhanced their k by up to 53% over inert (NH4)2SO4. These enhancement effects (RH and Fe(II)) were attributed to fundamental changes in the organic phase state (higher RH lowered particle viscosity) and Fenton-type chemistry which resulted in the formation of reactive oxygen species, respectively. Such findings serve to emphasize the importance of ambient RH, the phase state of particle-bound organics in general, and the presence of coexisting metallic species for an accurate description of the degradation kinetics and aging of particulate OPFRs in models used to evaluate their atmospheric persistence.

Theoretical evaluation of different factors affecting the HO2 uptake coefficient driven by aqueous-phase first-order loss reaction

The heterogeneous loss on aerosols is an important sink of HO2, affecting the radical chemistry and cycling, and thus it plays a key role in the atmospheric photochemistry. Gaining a reasonable HO2 uptake coefficient (γHO2) would be of great importance in evaluating the heterogeneous loss rate of HO2 on aerosols. This work was motivated by the large variance of reported HO2 mass accommodation coefficients (αHO2) in laboratory studies (0.1–1), which can cause consequent bias in the parameterized HO2 uptake coefficient (γHO2). We conducted a theoretical analysis of the roles of several key factors or parameters in determining γHO2 on a sphere droplet with adjustable Cu2+ ion concentration including αHO2, aqueous-phase acidity, the first-order loss-rate constant KI value, and the aqueous phase production of HO2. The results intuitively demonstrate that utilizing a single γHO2 value for aerosols of different sizes, compositions or hygroscopic states is unsafe in atmospheric models. The theoretical analysis indicated that for a single aerosol experiencing hygroscopic growth, γHO2 decreased with increasing aerosol size, because of the increased gas phase diffusion resistance and dilution of aqueous-phase HO2 consuming ions. Aerosol pH and metal abundance influence γHO2 by determining the aqueous-phase loss-rate constants, and these two factors were found to be only predominant for large particles/droplets (Rp > 1 μm). For small and middle size aerosols, the mass accommodation process plays the determining role in controlling HO2 uptake. Considering ambient aerosols rarely grow to cloud droplet size on sunny days when photochemical budget of HO2 radicals is of more concern, it is crucial to adopt appropriate αHO2 in models, as arbitrarily choosing the αHO2 value can lead to large bias when simulating HO2 heterogeneous process on ambient aerosols.

The Impact of pH and Irradiation Wavelength on the Production of Reactive Oxidants during Chlorine Photolysis

Chlorine photolysis is an advanced oxidation process which relies on photolytic cleavage of free available chlorine (i.e., hypochlorous acid and hypochlorite) to generate hydroxyl radical, along with ozone and a suite of halogen radicals. Little is known about the impact of wavelength on reactive oxidant generation even though chlorine absorbs light within the solar spectrum. This study investigates the formation of reactive oxidants during chlorine photolysis as a function of pH (6-10) and irradiation wavelength (254, 311, and 365 nm) using a combination of reactive oxidant quantification with validated probe compounds and kinetic modeling. Observed chlorine loss rate constants increase with pH during irradiation at high wavelengths due to the higher molar absorptivity of hypochlorite (p Ka = 7.5), while there is no change at 254 nm. Hydroxyl radical and chlorine radical steady-state concentrations are greatest under acidic conditions for all tested wavelengths and are highest using 254 and 311 nm irradiation. Ozone generation is observed under all conditions, with maximum cumulative concentrations at pH 8 for 311 and 365 nm. A comprehensive kinetic model generally predicts the trends in chlorine loss and oxidant concentrations, but a comparison of previously published kinetic models reveals the challenges of modeling this complex system.

Model-based exploration of the variability in lake trout (Salvelinus namaycush) bioaccumulation factors: The influence of physiology and trophic relationships.

Because dietary consumption of fish is often a major vector of human exposure to persistent organic pollutants (POPs), much effort is directed toward a quantitative understanding of fish bioaccumulation using mechanistic models. However, many such models fail to explicitly consider how uptake and loss rate constants relate to fish physiology. We calculated the bioaccumulation factors (BAFs) of hypothetical POPs, with octanol-water partition coefficients ranging from 104.5 to 108.5 , in lake trout (Salvelinus namaycush) with a food-web bioaccumulation model that uses bioenergetics to ensure that physiological parameters applied to a species are internally consistent. We modeled fish in 6 Canadian lakes (Great Slave Lake, Lake Ontario, Source Lake, Happy Isle Lake, Lake Opeongo, and Lake Memphremagog) to identify the factors that cause the BAFs of differently sized lake trout to vary between and within lakes. When comparing differently sized lake trout within a lake, larger fish tend to have the highest BAF because they allocate less energy toward growth than smaller fish and have higher activity levels. When comparing fish from different lakes, the model finds that diet composition and prey energy density become important in determining the BAF, in addition to activity and the amount of total energy allocated to growth. Environ Toxicol Chem 2019;38:831-840. © 2019 SETAC.