Second-order Rate Constants For Reactions Research Articles

A test of the potentialities of the APEX software (Aqueous Photochemistry of Environmentally occurring Xenobiotics). Modelling the photochemical persistence of the herbicide cycloxydim in surface waters, based on literature kinetic data

Based on literature kinetic data, it is shown that the photochemical transformation of cycloxydim (CXD) in surface waters would mainly occur by direct photolysis. Reaction of CXD with 1O2 and with the triplet states of chromophoric dissolved organic matter would only account for a small fraction (⩽2%) of total phototransformation. The second-order reaction rate constant between CXD and OH is not known. However, estimates based on data analysis with the APEX software and on diffusion-control kinetics in water indicate that OH would account for less than 10% of CXD phototransformation. The photochemical half-life time of CXD in mid-latitude summertime would vary in the day–month range, depending on water chemistry and depth. Transformation would be favoured in shallow waters with low dissolved organic carbon.

Degradation of atenolol by UV/peroxymonosulfate: Kinetics, effect of operational parameters and mechanism

Photoactivation of peroxymonosulfate (PMS) with UV (254nm) irradiation was used to generate the SO4--based advanced oxidation process, which was adopted to degrade atenolol (ATL) in water. The second-order reaction rate constants of ATL with HO and SO4- were determined, and the effects of operational parameters (dose of PMS, solution pH, HCO3-, humic acids (HA), and N2 bubbling) were evaluated as well. Finally the main transformation intermediates were identified and possible degradation pathways were proposed. The results showed that there was a linear positive correlation between the degradation rate of ATL and specific dose of PMS (1–16M PMS/M ATL). Increasing solution pH from 3 to 9 promoted elimination of ATL due to the pH-dependent effect of PMS photodecomposition, while further pH increase from 9 to 11 caused slowing down of degradation because of apparent conversion of HO to SO4-. 1–8mM HCO3- exerted no more than 5.3% inhibition effect on ATL destruction, suggesting HCO3- was a weak inhibitor. Absorption (or complexation) and photosensitized oxidation induced by HA improved ATL degradation during the first minute of degradation process, whereas photon competition and radical scavenging effects became the leading role afterward. Bubbling with nitrogen enhanced the degradation rate due to the stripping of dissolved oxygen. Hydroxylation of aromatic ring, cleavage of ether bond, oxidation of primary and secondary amine moieties, and dimerization were involved in the degradation mechanism of ATL by UV/PMS.

The α-Effect in Nucleophilic Substitution Reactions of Y-Substituted-Phenyl Diphenylphosphinates with HOO-and OH-

Second-order rate constants (<TEX>$k_{HOO^-}$</TEX>) for the nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with <TEX>$HOO^-$</TEX> in <TEX>$H_2O$</TEX> have been measured spectrophotometrically. The <TEX>${\alpha}$</TEX>-nucleophile <TEX>$HOO^-$</TEX> is 10-70 times more reactive than the reference nucleophile <TEX>$OH^-$</TEX> although the former is ca. <TEX>$4pK_a$</TEX> units less basic than the latter, indicating the <TEX>${\alpha}$</TEX>-effect is operative. The Bronsted-type plot for the reactions of 4a-4i with <TEX>$HOO^-$</TEX> is linear with <TEX>${\beta}_{lg}=-0.51$</TEX>, a typical <TEX>${\beta}_{lg}$</TEX> value for reactions which were reported to proceed through a concerted mechanism. The Yukawa-Tsuno plot is also linear with <TEX>${\rho}=1.40$</TEX> and r = 0.47, indicating that a negative charge develops partially on the O atom of the leaving group, which can be delocalized to the substituent Y through resonance interactions. Thus, the reactions have been proposed to proceed through a concerted mechanism. The magnitude of the <TEX>${\alpha}$</TEX>-effect (i.e., the <TEX>$k_{HOO^-}/k_{HO^-}$</TEX> ratio) decreases linearly as the leaving-group basicity increases. It has been concluded that solvation effect is not solely responsible for the <TEX>${\alpha}$</TEX>-effect found in this study but the transition-state stabilization through an intramolecular H-bonding interaction is also responsible for the <TEX>${\alpha}$</TEX>-effect.

Mathematical Modeling and Analysis of the Kinetics of Thermal Inactivation of Enzyme

A theoretical model of Illeova et al. (2003) thermal inactivation of urease is discussed. Analytical expressions pertaining to the molar concentrations of the native and denatured enzyme are obtained in terms of second-order reaction rate constant. Simple and closed form of theoretical expression pertains to the temperature are also derived. In this paper, homotopy analysis method (HAM) is used to obtain approximate solutions for a nonlinear ordinary differential equation. The obtained approximate result in comparison with the numerical ones is found to be in satisfactory agreement.

Unstationary Film Model for the Determination of Absolute Gas-Liquid Kinetic Rate Constants: Ozonation of Acid Red 27, Acid Orange 7, and Acid Blue 129

A method for the determination of absolute kinetic rate constants is proposed using an unstationary film model. This methodology avoids the experimental determination of parameters like the enhancement factor or the Hatta number which are usually model-dependent. The mathematical model is general for gas-liquid systems with irreversible second order reactions. An optimization procedure based on artificial neural networks is used to estimate the initial guess of the parameters and the subsequent application of Gauss-Newton algorithm for the final nonlinear parameter estimation. The model is tested with the ozonation reaction of Acid Red 27, Acid Orange 7 and Acid Blue 129. The second-order kinetic rate constants for the direct reaction with O3 are 1615 ± 93, 609 ± 83, and 49 ± 2 M−1s−1, respectively.

Kinetics and Modeling of Degradation of Ionophore Antibiotics by UV and UV/H2O2

Ionophore antibiotics (IPAs), one of the major groups of pharmaceuticals used in livestock industry, have been found to contaminate agricultural runoff and surface waters via land application of animal manures as fertilizers. However, limited research has investigated the means to remove IPAs from water sources. This study investigates the degradation of IPAs by using ultraviolet (UV) photolysis and UV combined with hydrogen peroxide (UV/H2O2) advanced oxidation process (AOP) under low-pressure (LP) UV lamps in various water matrices. Three widely used (monensin, salinomycin, and narasin) and one model (nigericin) IPAs exhibit low light absorption in the UV range and degrade slowly at the light intensity of 3.36 × 10(-6) Einstein·L(-1)·s(-1) under UV photolysis conditions. However, IPAs react with hydroxyl radicals produced by UV/H2O2 at fast reaction rates, with second-order reaction rate constants at (3.49-4.00) × 10(9) M(-1)·s(-1). Water matrix constituents enhanced the removal of IPAs by UV photolysis but inhibited UV/H2O2 process. A steady-state kinetic model successfully predicts the impact of water constituents on IPA degradation by UV/H2O2 and determines the optimal H2O2 dose by considering both energy consumption and IPA removal. LC/MS analysis of reaction products reveals the initial transformation pathways of IPAs via hydrogen atom abstraction and peroxidation during UV/H2O2. This study is among the first to provide a comprehensive understanding of the degradation of IPAs via UV/H2O2 AOP.

Degradation of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid by TiO2 photocatalysis: Kinetics, photoproducts and comparison to structurally related compounds

The wide occurrence of sunscreen agent micropollutants in natural environment received extensive attention in recent years due to their potential endocrine disrupting effect. The present study focuses on the kinetics and mechanism of photocatalytic degradation of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid (PBSA) in illuminated TiO2 suspensions. Photocatalysis of PBSA was systematically investigated under different process conditions and water matrices. Experimental results demonstrated that PBSA photocatalytic reactions followed pseudo-first-order kinetics. Radical scavenging experiments indicated that hydroxyl radical (HO) is the predominant reactive species responsible for an appreciable degradation of PBSA. Second-order rate constant of PBSA-HO reaction was determined to be 5.8x109M−1s−1 by competition kinetics method. Major intermediates included hydroxylated products, benzamide, hydroxylated benzamidine, hydroxylated 2-pheny-1H-benzimidazole as well as phenylimidazolecarboxylic derivatives which were elucidated by means of high performance liquid chromatograph–mass spectrometry (HPLC–MS) technique. Four carboxylic acids, oxalic, malonic, acetic and maleic acids, were detected during PBSA photocatalysis by HPLC–UV analysis. Ion chromatography (IC) results revealed that the sulfonic group of PBSA was primarily converted to sulfate ion while nitrogen atoms were released predominantly as ammonium and to a lesser extent as nitrate. The reduction of TOC processed much more slowly compared to PBSA degradation, however, approximately 80% TOC was removed after 720min irradiation. A comparison of photocatalytic degradation of PBSA and structurally related compounds revealed that the 5-sulfonic moiety in PBSA had negligible effect on the photocatalysis of 2-pheny-1H-benzimidazole while 2-phenyl substituent stabilized the benzimidazole ring system to photocatalytic degradation.

Oxidation of volatile organic vapours in air by solid potassium permanganate

Volatile organic compounds (VOCs) may frequently contaminate groundwater and pose threat to human health when migrating into the unsaturated soil zone and upward to the indoor air. The kinetic of chemical oxidation has been investigated widely for dissolved VOCs in the saturated zone. But, so far there have been few studies on the use of in situ chemical oxidation (ISCO) of vapour phase contaminants. In this study, batch experiments were carried out to evaluate the oxidation of trichloroethylene (TCE), ethanol, and toluene vapours by solid potassium permanganate. Results revealed that solid potassium permanganate is able to transform the vapour of these compounds into harmless oxidation products. The degradation rates for TCE and ethanol were higher than for toluene. The degradation process was modelled using a kinetic model, linear in the gas concentration of VOC [ML−3] and relative surface area of potassium permanganate grains (surface area of potassium permanganate divided by gas volume) [L−1]. The second-order reaction rate constants for TCE, ethanol, and toluene were found to be equal to 2.0×10−6cms−1, 1.7×10−7cms−1, and 7.0×10−8cms−1, respectively.

Pesticide Photolysis in Prairie Potholes: Probing Photosensitized Processes

Prairie pothole lakes (PPLs) are glacially derived, ecologically important water bodies found in central North America and represent a unique setting in which extensive agriculture occurs within wetland ecosystems. In the Prairie Pothole Region (PPR), elevated pesticide use and increasing hydrologic connectivity have raised concerns about the impact of nonpoint source agricultural pollution on the water quality of PPLs and downstream aquatic systems. Despite containing high dissolved organic matter (DOM) levels, the photoreactivity of the PPL water and the photochemical fate of pesticides entering PPLs are largely unknown. In this study, the photodegradation of sixteen pesticides was investigated in PPL waters sampled from North Dakota, under simulated and natural sunlight. Enhanced pesticide removal rates in the irradiated PPL water relative to the control buffer pointed to the importance of indirect photolysis pathways involving photochemically produced reactive intermediates (PPRIs). The steady-state concentrations of carbonate radical, hydroxyl radical, singlet oxygen, and triplet-excited state DOM were measured and second-order rate constants for reactions of pesticides with these PPRIs were calculated. Results from this study underscore the role of DOM as photosensitizer in limiting the persistence of pesticides in prairie wetlands through photochemical reactions.

Assessing the occurrence of the dibromide radical (Br2−[rad]) in natural waters: Measures of triplet-sensitised formation, reactivity, and modelling

The triplet state of anthraquinone-2-sulphonate (AQ2S) is able to oxidise bromide to Br/Br2−, with rate constant (2–4)⋅109M−1s−1 that depends on the pH. Similar processes are expected to take place between bromide and the triplet states of naturally occurring chromophoric dissolved organic matter (3CDOM*). The brominating agent Br2− could thus be formed in natural waters upon oxidation of bromide by both OH and 3CDOM*. Br2− would be consumed by disproportionation into bromide and bromine, as well as upon reaction with nitrite and most notably with dissolved organic matter (DOM). By using the laser flash photolysis technique, and phenol as model organic molecule, a second-order reaction rate constant of ~3⋅102L(mg C)−1s−1 was measured between Br2− and DOM. It was thus possible to model the formation and reactivity of Br2− in natural waters, assessing the steady-state [Br2−]≈10−13–10−12M. It is concluded that bromide oxidation by 3CDOM* would be significant compared to oxidation by OH. The 3CDOM*-mediated process would prevail in DOM-rich and bromide-rich environments, the latter because elevated bromide would completely scavenge OH. Under such conditions, OH-assisted formation of Br2− would be limited by the formation rate of the hydroxyl radical. In contrast, the formation rate of 3CDOM* is much higher compared to that of OH in most surface waters and would provide a large 3CDOM* reservoir for bromide to react with. A further issue is that nitrite oxidation by Br2− could be an important source of the nitrating agent NO2 in bromide-rich, nitrite-rich and DOM-poor environments. Such a process could possibly account for significant aromatic photonitration observed in irradiated seawater and in sunlit brackish lagoons.

Reaction kinetics of selected micropollutants in ozonation and advanced oxidation processes

Second-order reaction rate constants of micropollutants with ozone (kO3) and hydroxyl radicals (kOH) are essential for evaluating their removal efficiencies from water during ozonation and advanced oxidation processes. Kinetic data are unavailable for many of the emerging micropollutants. Twenty-four micropollutants with very diverse structures and applications including endocrine disrupting compounds, pharmaceuticals, and personal care products were selected, and their kO3 and kOH values were determined using bench-scale reactors (at pH 7 and T = 20 °C). Reactions with molecular ozone are highly selective as indicated by their kO3 values ranging from 10−2–107 M−1 s−1. The general trend of ozone reactivity can be explained by micropollutant structures in conjunction with the electrophilic nature of ozone reactions. All of the studied compounds are highly reactive with hydroxyl radicals as shown by their high kOH values (108–1010 M−1 s−1) even though they are structurally very diverse. For compounds with a low reactivity toward ozone, hydroxyl radical based treatment such as O3/H2O2 or UV/H2O2 is a viable alternative. This study contributed to filling the data gap pertaining kinetic data of organic micropollutants while confirming results reported in the literature where available.

Chemoassay Screening of DNA-Reactive Mutagenicity with 4-(4-Nitrobenzyl)pyridine – Application to Epoxides, Oxetanes, and Sulfur Heterocycles

Organic electrophiles have the potential to covalently attack DNA bases, and thus initiate mutagenic and carcinogenic processes. In this context, aromatic nitrogen sites of the DNA bases are often particularly nucleophilic, with guanine N7 being one of the most favored sites of adduct formation with electrophilic xenobiotics. Employing 4-(4-nitrobenzyl)pyridine (NBP) as model nucleophile with a respective aromatic ═N- unit, a new kinetic variant of a photometric chemoassay for sensing the DNA reactivity of organic compounds is introduced and applied to 21 three- and four-membered oxygen and sulfur heterocycles (15 epoxides, two thiiranes, three oxetanes, and one thietane). Besides six unreactive compounds (oxetanes, thietane, and aliphatic epoxides with six or more side-chain carbons), second-order rate constants of the electrophile-NBP reaction, k(NBP), were obtained for 15 compounds, ranging from (1.16 ± 0.05)·10⁻³ to (36.5 ± 0.6)·10⁻³ L mol⁻¹ min⁻¹ in a methanol/tris-HCl buffer (16/84 v/v) reaction medium. Solvolysis as confounding factor was addressed by determining respective first-order rate constants k(solv). Analysis of the k(NBP) values resulted in structure-reactivity relationships, and comparison with literature data from the Ames test bacterial strains TA100, TA1535, and TA97 (Salmonella typhimurium) as well as from WP2 uvrA (Escherichia coli) revealed significant log-log relationships between the mutagenic potency of the heterocycles and their reactivity toward NBP. The latter demonstrates the potential of the NBP chemoassay as a nonanimal component of integrated testing strategies for REACH, enabling an efficient screening of organic electrophiles with respect to their DNA reactivity and associated mutagenicity and carcinogenicity.

Different photolysis kinetics and photooxidation reactivities of neutral and anionic hydroxylated polybrominated diphenyl ethers

Hydroxylated polybrominated diphenyl ethers (HO-PBDEs) pose potential ecological risks due to their endocrine disrupting effects and extensive sources. It is of great importance to know their environmental transformation for the purpose of ecological risk assessment. Photodegradation is an important transformation pathway of HO-PBDEs. As HO-PBDEs ionize in natural waters, the photochemical reactivities of both neutral and anionic HO-PBDEs need to be unveiled. In this study, six HO-PBDEs were selected as model compounds. Their direct photolysis rate constants (kd), quantum yields (Φ), light absorptions (A), second-order reaction rate constants with 1O2 (k1O2) and OH (kOH) at different pH were determined, as well as these parameters for the neutral and anionic HO-PBDEs. The kd, Φ, A, k1O2 and kOH for the anions are much higher than those for the neutral molecules and vary with the bromination degree. Molecular parameters computed with the density functional theory (DFT) were employed to construct structure–reactivity equations. The ether bond strength, the frontier molecular orbital energy and the charge distribution were found to be the intrinsic structural characters governing the photochemical reactivities. The half-lives range from 0.7 to 60.1h for the photodegradation including the direct photolysis and the reactions with 1O2 and OH in surface waters at 45°N latitude under the continuous solar irradiation of sunny noon on 15 July. Direct photolysis is the dominant pathway. The photochemical reactivities of other HO-PBDEs at a given pH can also be estimated based on the structure–reactivity equations, which is important for the ecological risk assessment of HO-PBDEs.

Photochemistry of Cp′Mn(CO)2(NHC) (Cp′ = η5-C5H4Me) Species: Synthesis, Time-Resolved IR Spectroscopy, and DFT Calculations

UV irradiation of Cp′Mn(CO)3 (Cp′ = η5-C5H4Me) in the presence of the free N-heterocyclic carbenes IEt2Me2, IiPr2Me2, IMes, and IPr affords the NHC dicarbonyl complexes Cp′Mn(CO)2(NHC) (1–4). Time-resolved infrared spectroscopy in alkane solution reveals that 1–4 photodissociate CO to generate Cp′Mn(CO)(NHC) (1-CO, 2-CO, 3-CO, 4-CO), which exhibit solvent-independent second-order rate constants (kCO) for reaction with CO. These observations are consistent with 1-CO to 4-CO being stabilized by intramolecular agostic interactions with the NHCs rather than intermolecular alkane coordination. Density functional theory calculations provide support for this hypothesis and locate a series of agostic structures varying from δ-agostic (1-CO, 2-CO), to ε-agostic (3-CO), to ϕ-agostic (4-CO). The atoms-in-molecules approach is used to characterize these species, along with the γ-agostic interaction seen in the CpMn(CO)(PPh3) analogue (5-CO), and shows that these species are distinguished primarily by the magnitude of the electron density at the agostic ring critical point.

Ozonation Kinetics Of o-phenylphenol in Aqueous Solutions

This study investigated the oxidation and reaction kinetics of biocide o-phenylphenol (o-PP) during ozonation. The second-order rate constants for direct reaction of molecular ozone with o-PP were determined in homogenous system using classical and competition kinetics. Obtained values of the second-order rate at pH 2 are equal to (3.79 ± 0.23) × 102 M−1 s−1 and (4.42 ± 0.64) × 102 M−1 s−1, for the two methods, respectively. The rate constant for the dissociated form of o-PP was also determined. It was found that the rate constants for the reaction of o-PP with ozone increase significantly with increasing pH.

PH Effect on Ozonation of Ampicillin: Kinetic Study and Toxicity Assessment

Ampicillin (AP) is a penicillin-type antibiotic and one of the most widely used bacteriostatic antibiotics in human and veterinary medicine. A kinetic study was performed under different pH conditions (5, 7.2, and 9) to determine the degradation efficiency of AP by ozonation. The second-order rate constants for the direct reaction of AP with ozone were measured to be 2.2 ˜5.4×105 M−1s−1 under the pH conditions tested. The rate constants were greater at higher pH. The potential toxicity of the AP intermediates formed after ozonation under the various pH conditions were examined using a bioluminescence assay on Vibrio fischeri species. The biodegradability of the AP degraded products was also determined by measuring the BOD5/COD of the ozonated samples under the different pH conditions. A lower biodegradability and acute toxicity was observed at the lowest pH (pH 5). These results suggest that higher pH conditions are needed for the removal of AP by ozonation in order to mitigate the residual toxicity that can remain even after complete removal of the parent compound by ozonation.

Theoretical and Numerical Analysis of Electrocatalytic Processes at Conducting Polymer Modified Electrodes

A theoretical model of an electrocatalytic processes taking place at conducting polymer modified electrodes is discussed. In this model the diffusion of solution species, charge carriers and chemical reaction within the film are taken into account. The model involves the system of non-linear non-steady-state reaction diffusion equations. Analytical expressions pertaining to the concentrations are obtained in terms of second-order reaction rate constant. Also simple theoretical expression of transient current is derived. In this paper, a powerful analytical method, called Homotopy analysis method (HAM) is used to obtain approximate solutions for a non-linear partial differential equation. The obtained approximate solution in comparison with the numerical ones is found to be in satisfactory agreement.

State-Specific Reactions of Cu+(1S, 3D) with CH3X and CF3X (X = Cl, Br, I): Exploring the Influence of Dipole Orientation on Association and C–X Bond Activation

The reactions of gas-phase Cu(+)((1)S) and Cu(+)((3)D) with CF(3)X and CH(3)X (X = Cl, Br, and I) have been examined experimentally using the drift cell technique at 3.5 Torr in He at room temperature. State-specific product channels and overall bimolecular rate constants for depletion of the two Cu(+) states were determined using electronic state chromatography. The results showed that Cu(+)((1)S) participates exclusively in association with all of these neutrals, whereas, depending on the neutral, Cu(+)((3)D) initiates up to three bimolecular processes, resulting in the formation of CuX(+), CuC(H/F)(3)(+), and C(H/F)(3)X(+). Possible structures for the singlet association products were explored using density functional methods. These calculations indicated that Cu(+) preferentially associates with the labile halogen (Cl, Br, I) with all neutrals except CF(3)Cl, for which a "backside" geometry occurs in which Cu(+)((1)S) is weakly bound to the -CF(3) end of the molecule. All products observed on the triplet reaction surface can be understood in terms of either known or calculated thermochemical requirements. Product distributions and overall reaction efficiencies for C-X bond activation (X = Br, I) through Cu(+)((3)D) suggest that the orientation of the neutral dipole has little or no effect in controlling access to specific product channels. Likewise, second-order rate constants for reactions with X = Br and I indicate efficient depletion of Cu(+)((3)D) and do not exhibit the dramatic variations in reaction efficiency previously observed with CH(3)Cl and CF(3)Cl. These results suggest that C-X bond activation proceeds through a bond-insertion mechanism as opposed to direct abstraction.

The effects of ligand decomposition on the pseudo first-order profile of a ligand substitution reaction: a “silent killer” in the background

The effect of ligand decomposition on the apparent rate constant for a ligand substitution reaction where the reverse reaction is negligible is examined. When the ligand concentration remains in excess for the entirety of the reaction, the data can be fitted to a modified form of the familiar pseudo first-order rate expression. However rapid ligand decomposition may result in the ligand concentration not remaining in excess for the duration of the reaction. Data simulations show that in this latter case the data still fit remarkably well to a first-order rate equation. Plots of the apparent rate constant versus the initial ligand concentration are also linear. However the reaction will not proceed to completion despite the reverse reaction being insignificant. Furthermore intercepts in the plots of apparent rate constant versus the initial ligand concentration are obtained, in addition to misleading values of the second-order rate constants for the ligand substitution reaction. We show that kinetic coupling between the decomposition of the ligand and its complex formation reaction may easily lead to falsified conclusions. Thus, the first-order appearance of a kinetic trace does not guarantee real first-order behavior.

Investigation of the reaction of hexabromocyclododecane with polysulfide and bisulfide in methanol/water solutions

Reactions of hexabromocyclododecane (HBCD), a widely used brominated flame retardant, with reduced sulfur species (e.g., polysulfide and bisulfide) were investigated in well-defined solutions under anoxic conditions. It is likely that reactions of HBCD with reduced sulfur species such as polysulfides and bisulfide present in anoxic subregions of coastal water bodies and sediments could have a significant impact on the fate of HBCD. The second-order reaction rate constant of HBCD with polysulfides in 80% methanol/20% water at 40°C is 2.2 (±0.3)×10−2M−1s−1. The second-order reaction rate constant of HBCD with bisulfide is 8.9 (±2.8)×10−4M−1s−1 under the same conditions. The formation of two products was observed with either of the two reduced sulfur species. The experiments also indicate that the γ-isomer of HBCD is reacting significantly faster with reduced sulfur species than the α-isomer.