Second-order Rate Constants For Reactions Research Articles

Development of Flow-Injection Spin-Trapping ESR Methods for Kinetic Study of the O2−• Radical Scavenging Reaction with Mixtures of Phenolic Antioxidants

AbstractThe apparent second-order rate constants (aks) of the reaction between the O2−• radical and mixed solutions (aAox) of caffeic acid (CA) and 4-hydroxy-coumaric acid (4CA) were estimated in this study; this was achieved by a flow-injection spin-trapping ESR (FI-ESR) method that employed 5,5-dimethyl-pyrroline-N-oxide (DMPO) as the spin-trapping reagent. The aAox solutions were prepared by varying the mixing molar ratio of CA (a = [CA]/([CA] + [4CA])). The FI-ESR signal intensities of the DMPO spin-adduct of the O2−• radical (DMPO/O2) were monitored in the absence (I0) and presence (I) of aAox to estimate the inhibition ratio Y (%) (Y = 100 × I/I0), and 50% inhibitory dose of aAox (aID50). Further, the aks values of aAox were evaluated using these aID50 values through a competitive reaction kinetics treatment, which was conducted by using the second-order rate constant of the spin-trapping reaction (k1). The value obtained for the aks of aAox showed excellent agreement with the calculated values (akscal) that were estimated from the molar ratio (a) weighted mean of the ks values of CA (CAks) and 4CA (4CAks): akscal = aCAks + (1 − a)4CAks. These results demonstrate that CA and 4CA contribute independently to the O2−• radical scavenging reactions, and that the aks values of aAox completely satisfy the additive property depending on the mixing molar ratio (a). The O2−• radical scavenging capacities of aAox were also evaluated as equivalent concentrations of CA (Y[CA]eq.) using the observed inhibition ratio Y (%), and the 50% inhibitory dose of CA (CAID50): Y[CA]eq. = CAID50 × (100 − Y)/Y. The estimated Y[CA]eq. values of aAox agreed with the calculated equivalent concentrations of CA (k[CA]eq.) that were obtained on the basis of the additive property established for aAox and aks: k[CA]eq. = [CA] + (4CAks/CAks) × [4CA].

Novel thermodynamic model for vapor-liquid equilibrium of CO2 in aqueous solution of 4-(ethyl-methyl-amino)-2-butanol with designed structures

The aim of this project was to develop a thermodynamic model for the vapour-liquid equilibrium of an amine-water-carbon dioxide system and to evaluate the CO2 capture performance of aqueous EMAB solution. The structurally modified tertiary amine 4-(ethyl-methyl-amino)-2-butanol (EMAB) was chosen because of its molecular structure-activity relationship. Measurements were made of its equilibrium CO2 solubility, viscosity, dissociation constant, and second-order reaction rate constant, and the heat of absorption was then calculated. Additionally, the novel semi empirical model (Extended Cf model) and the novel rigorous thermodynamic model (fugacity-activity model) which take into consideration the association of the partial molar property and the equation of state were developed to predict the equilibrium CO2 solubility. The models can provide reasonable prediction results with AARDs of 2.3% and 3.68%, respectively. Furthermore, the thermodynamic and dynamic properties of EMAB were analyzed and evaluated by a comprehensive method and compared with some new tertiary amines and traditional commercial amines. The study concludes that EMAB has good CO2 capture performance and is expected to be one of the most promising alternative amines for post combustion CO2 capture technology.

QSAR modeling for the ozonation of diverse organic compounds in water

The ozonation-based advanced oxidation process is a promising treatment technology for wastewater with micropollutants. The second-order reaction rate constant (kO3) of ozone (O3) with organic compounds is an important index for estimating removal efficiency of organic pollutants in engineered treatment; however, the experimental kO3 values are currently only available for hundreds of chemicals. In this study, two quantitative-structure activity relationship (QSAR) models were developed to predict kO3 of various organic chemicals with multiple linear regression (MLR) and support vector machine (SVM) methods. The built QSAR models cover a large dataset (136 chemicals) and more structurally diverse chemicals as compared to the existing models. The MLR model possesses satisfactory goodness-of-fit (R2tr = 0.734), robustness (Q2LOO = 0.700, Q2BOOT = 0.772) and predictive ability (R2ext = 0.797, Q2ext = 0.794), and the SVM model also has good fitness (R2tr = 0.862) and predictability (R2ext = 0.782, Q2ext = 0.775). The applicability domain of the models has been extended and includes chemicals (especially some emerging pollutants) that are rarely covered in many previous models. The underlying molecular structural factors influencing ozonation are revealed. The energy of the highest occupied molecular orbital (EHOMO) and the phenol/enol/carboxyl OH group (O-057) are the two most important molecular structural factors governing the reactivity of organic compounds with ozone. The developed models can serve as a prescreening tool for the removal prediction of organic pollutants by ozone.

Sulfate radical-based oxidation of an alcohol ethoxylate (Brij30®) by the PS/UV-C process.

In this study, sulfate radical-based oxidation of an alcohol ethoxylate (AE) was explored by the persulfate (PS)/UV-C process. Poly(oxyethylene)(4)laurylether, commercially known as Brij30®, was used as a model AE. PS/UV-C oxidation of aqueous Brij30® (8-20 mg/L) was performed at initial PS concentrations varying between 0.50 and 6.50 mM and at initial pH values of 3.0 and 6.0. Results indicated that an increase in both initial PS and Brij30® concentrations did not have a significant effect on Brij30® removal efficiency and that Brij30® abatements of more than 90% could be achieved after 60 min of treatment time. Total organic carbon (TOC) removals were significantly improved with increasing initial PS concentrations for both initial pH values. On the other hand, an increase in initial Brij30® concentration had a negative effect on mineralization. By employing the competitive kinetic method, the second-order reaction rate coefficient of Brij30® with the sulfate radical (SO4 •-) was determined as 1.62 × 109 ± 3.5 × 107 M-1s-1. The second-order reaction rate constant of TOC, originating from Brij30® and reaction intermediates, was found to be 9.09 × 105 ± 2.91 × 105 M-1s-1 and 1.13 × 106 ± 0.46 × 106 M-1s-1 for pH values of 6.0 and 3.0, respectively. Toxicity of PS/UV-C treated aqueous Brij30® solutions towards Vibrio fischeri was also investigated to determine the possible toxic behavior of oxidation products.

Formation and kinetic studies of manganese(IV)-oxo porphyrins: Oxygen atom transfer mechanism of sulfide oxidations

Visible light irradiation of photo-labile porphyrin-manganese(III) chlorates or bromates (2) produced manganese(IV)-oxo porphyrins [MnIV(Por)(O)] (Por = porphyrin) (3) in three porphyrin ligands. The same oxo species 3 were also formed by chemical oxidation of the corresponding manganese(III) precursors (1) with iodobenzene diacetate, i.e. PhI(OAc)2. The systems under study include 5,10,15,20-tetra(pentafluorophenyl)porphyrin‑manganese(IV)-oxo (3a), 5,10,15,20-tetra(2,6-difluorophenyl)porphyrin‑manganese(IV)-oxo (3b), and 5,10,15,20-tetramesitylporphyrin‑manganese(IV)-oxo (3c). As expected, complexes 3 reacted with thioanisoles to produce the corresponding sulfoxides and over-oxidized sulfones. The kinetics of oxygen atom transfer (OAT) reactions of these generated 3 with aryl sulfides were studied in CH3CN solutions. Second-order rate constants for sulfide oxidation reactions are comparable to those of alkene epoxidations and activated CH bond oxidations by the same oxo species 3. For a given substrate, the reactivity order for the manganese(IV)-oxo species was 3a > 3b > 3c, consistent with expectations on the basis of the electron-withdrawing capacity of the porphyrin macrocycles. Free-energy Hammett analyses gave near-linear correlations with σ values, indicating no significant positive charge developed at the sulfur during the oxidation process. The mechanistic results strongly suggest [MnIV(Por)(O)] reacts as a direct OAT agent towards sulfide substrates through a manganese(II) intermediate that was detected in this work. However, an alternative pathway that involves a disproportionation of 3 to form a higher oxidized manganese(V)-oxo species may be significant when less reactive substrates are present. The competition product studies with the Hammett correlation plot confirmed that the observed manganese(IV)-oxo species is not the true oxidant for the sulfide oxidations catalyzed by manganese(III) porphyrins with PhI(OAc)2.

Electron Microscopic Detection of Single Membrane Proteins by a Specific Chemical Labeling.

SummaryElectron microscopy (EM) is a technology that enables visualization of single proteins at a nanometer resolution. However, current protein analysis by EM mainly relies on immunolabeling with gold-particle-conjugated antibodies, which is compromised by large size of antibody, precluding precise detection of protein location in biological samples. Here, we develop a specific chemical labeling method for EM detection of proteins at single-molecular level. Rational design of α-helical peptide tag and probe structure provided a complementary reaction pair that enabled specific cysteine conjugation of the tag. The developed chemical labeling with gold-nanoparticle-conjugated probe showed significantly higher labeling efficiency and detectability of high-density clusters of tag-fused G protein-coupled receptors in freeze-fracture replicas compared with immunogold labeling. Furthermore, in ultrathin sections, the spatial resolution of the chemical labeling was significantly higher than that of antibody-mediated labeling. These results demonstrate substantial advantages of the chemical labeling approach for single protein visualization by EM.

Comparison of UVC and UVC/persulfate processes for tetracycline removal in water

The comparative study on the elimination of tetracycline (TC) utilizing UVC and UVC/PS processes was systematically conducted. The TC degradation via UVC irradiation and UVC/PS treatment well fitted pseudo first-order kinetics. Hydroxyl radicals (OH) and sulfate radicals (SO4−) were identified as the primary reactive species in UVC/PS process. Based on competition kinetic method, the second-order reaction rate constants of TC reacting with OH and SO4− were calculated as 4.6 × 109 and 2.2 × 109 M−1 s−1, respectively. The contributions of UVC photolysis, OH and SO4− to TC degradation were calculated as 5.91%, 61.97% and 28.11% at pH 7.0, respectively. The TC degradation in UVC and UVC/PS process was highly pH-dependent and enhanced with the increase of solution pH. Water matrix constituents played the multiple roles in TC degradation, including radical sensitizer, radical quencher and light absorber. Natural organic matters, CO32– and HCO3– enhanced the degradation during UVC photolysis, whereas the opposite phenomenon occurred in UVC/PS process. Cl− presented the inhibitory effect, while TC degradation can be accelerated by NO3– both in UVC and UVC/PS processes. The TC degradation in UVC/PS process can be classified into the hydroxylation, demethylation, decarbonylation, photocyclinzation, dehydration and cleavage of CN bond. After UVC and UVC/PS pretreatment, the decreased formation of dichloroacetic acid and trichloroacetic acid and the increased production of trichloromethane have been found. The TC removal was evaluated in terms of electrical energy per order, and its value can be considerably reduced via the combination of UVC and PS. Except for ultrapure water, the removal of TC from real waters by UVC/PS treatment could also be desirable.

Bioconjugation with Aminoalkylhydrazine for Efficient Mass Spectrometry-Based Detection of Small Carbonyl Compounds.

Bioconjugation through oxime or hydrazone formation is a versatile strategy for covalent labeling of biomolecules in vitro and in vivo. In this work, a mass spectrometry-based method was developed for the bioconjugation of small carbonyl compounds (CCs) with an aminoalkylhydrazine to form stable hydrazone conjugates that are readily detectable with electrospray ionization mass spectrometry (ESI-MS). Out of all hydrazine reagents tested, 2-(dimethylamino)ethylhydrazine (DMAEH) was selected for further analysis due to the fastest reaction rates observed. A thorough study of the reaction kinetics between structurally varied short-chain CCs and DMAEH was performed with the second-order reaction rate constants spanning in the range of 0.23–208 M–1 s–1. In general, small aldehydes reacted faster than the corresponding ketones. Moreover, a successful reaction monitoring of a deoxyribose-5-phosphate aldolase-catalyzed reversible retro–aldol cleavage of deoxyribose was demonstrated. Thus, the developed method shows potential also for ESI-MS-based enzyme kinetics studies.

Radical chemistry of diethyl phthalate oxidation via UV/peroxymonosulfate process: Roles of primary and secondary radicals

The UV/peroxymonosulfate (PMS) process forms hydroxyl radicals (OH) and sulfate radicals (SO4−) to degrade micro-pollutants. Both these two radicals can be converted to secondary radicals (e.g. Cl2−, CO3− and Cl) by effects of water matrix components. By using laser flash photolysis, the second-order rate constants for reactions of three PAEs with OH, SO4−, Cl2−, Cl and CO3− were determined as (3.5–4.4) × 109, (4.9–5.6) × 108, (1.1–1.3) × 107, (1.8–2.0) × 1010 and <1 × 106 M−1s−1, respectively. Then diethyl phthalate (DEP) was selected as the target compound to investigate the radical chemistry of its degradation during the 254 nm UV/PMS process. Multiple effects of water matrix components in DEP degradation were investigated. Alkaline condition, chloride, bicarbonate and natural organic matter (NOM) inhibited DEP degradation while their effects were radical specific. SO4− could better withstand the negative effect of alkaline condition, bicarbonate and NOM than OH, while OH was less affected by chloride than SO4−. Cl accounted for 12% of DEP degradation in the presence of 1 mM chloride. Cl2−, CO3− and 1O2 hardly contributed to DEP oxidation but they might be involved in the further oxidation of transformation intermediates. Transient-state intermediates and steady-state products were identified by time-resolved spectroscopy and GC-MS, respectively, from which the degradation pathways of DEP in different water matrices were proposed.

Kinetics of oxidation of iron(II) ions with lipid hydroperoxides

AbstractOne of the pathways for excessive production of free radicals is the decomposition of lipid hydroperoxides catalyzed by iron. A number of hydroperoxides of unsaturated fatty acids (LOOH), some prepared in our laboratory and others extracted from biological materials, were used to determine the rate constants of Fe2+ oxidation by measuring the formation rate of Fe3+ ions in the presence of simple unidentate ligands, chloride, and thiocyanate as the [FeCl]2+ and [FeNCS]2+ complexes, in a deoxygenated dichloromethane:methanol (2:1, v/v) mixture. The rates of Fe2+ oxidation with prepared LOOHs via the [FeNCS]2+ complex were approximately the same‐the average second‐order reaction rate constant was 1390 ± 340 dm3 mol−1 s−1; the rate constants of LOOHs from different biological materials were in the same range. The rates measured as the [FeNCS]2+ complex were somewhat higher than the rates measured as the [FeCl]2+ complex, indicating that ligands could interact in the transition state, thus affecting the disruption of the intermediate complex. Since there were no significant differences in the activation thermodynamic parameters for reactions within the reaction series of studied hydroperoxides, it was assumed that the oxidation proceeded by an inner sphere mechanism, considering that the breakdown of the successor inner sphere complex with the homolytic cleavage of peroxide bonds of hydroperoxides forming reactive alkoxyl radicals was the rate‐limiting step. Based on this research, an indirect spectrophotometric method for quantitative determination of LOOH was reestimated. The microprocedure proposed for the lipid hydroperoxide assay could be applied to follow the early stages of lipid peroxidation processes in real biological samples.

First kinetic data of the CO substitution in fac-[Re(L,L′-Bid)(CO)3(X)] complexes (L,L′-Bid = acacetylacetonate or tropolonate) by tertiary phosphines PTA and PPh3: Synthesis and crystal structures of water-soluble rhenium(I) tri- and dicarbonyl complexes with 1,3,5-triaza-7-phosphaadamantane (PTA)

fac-[Re(OH2)3(CO)3]+ reacts with acetylacetone (AcacH) at room temperature to yield fac-[Re(Acac)(CO)3(OH2)] (1) under aqueous conditions 1 was isolated and reacted with the monodentate ligand, 1,3,5-triaza-7-phosphaadamantane (PTA), to form fac-[Re(Acac)(CO)3(PTA)] (2) in a 9:1 ratio of water and methanol. The isolated complex 2 was further reacted with an excess of PTA ligand in a 9:1 ratio of water and methanol to yield the highly soluble cis-trans-[Re(Acac)(CO)2(PTA)2] (3). The complexes have been characterized by spectroscopic techniques IR, 1H, 13C and 31P NMR, as well as single crystal X-ray crystallography for 2 and 3. From the crystallographic results of 3, the formation of the cis-trans conformation was confirmed, while the 31P NMR results further established 3 as the sole product formed. A 31P NMR kinetic study, involving the methanol substitution of fac-[Re(CO)3(Acac)(CH3OH)] (4) and fac-[Re(CO)3(Trop)(CH3OH)] (TropH = tropolone) with PTA as entering monodentate ligand was also undertaken. The second-order rate constants (k1) for the forward reactions at 25.0 °C were respectively obtained as (35.9 ± 0.2) x 10−3 M−1 s−1 and (25.1 ± 0.8) M−1 min−1. In a preliminary 31P NMR kinetic investigation of the carbonyl substitution in fac-[Re(Acac)(CO)3(PPh3)] by PPh3, an estimated equilibrium constant (Kest) of ≈ 5.09 was calculated for the reversible reaction.

Investigation on kinetics of carbon dioxide absorption in aqueous solutions of monoethanolamine + 1, 3-diaminopropane

ABSTRACTEnvironmental concerns from global warming and climate change demand carbon dioxide separation from post-combustion gases. Important parameters are involved in choosing the suitable solvent for carbon dioxide separation, including the reaction rate of carbon dioxide and the solvent. In this paper, the kinetics of carbon dioxide (CO2) absorption in aqueous solutions of Monoethanolamine (MEA) + 1,3-Diaminopropane (DAP), a diamine containing two primary amino group, was developed. The measurements were performed in a stirred cell with a horizontal gas-liquid interface in the temperature range of 313.15–333.15 K and aqueous solutions of 10 wt% MEA + 5 wt% DAP and 12.5 wt% MEA + 2.5 wt% DAP. Experiments were conducted in an isothermal batch reactor with a horizontal gas-liquid interface under pseudo-first-order conditions, enabling the determination of the overall kinetic rate constant from the pressure drop method. Second-order reaction rate constants of CO2 absorption in amine solutions were estimated using the calculated initial absorption rate. It was found that the rate constants in MEA+ DAP solutions were greater than in MEA solutions which means that DAP increases the reaction rate.

Disparate effects of DOM extracted from coastal seawaters and freshwaters on photodegradation of 2,4-Dihydroxybenzophenone

Dissolved organic matter (DOM) plays an important role in degradation of organic pollutants by photochemically-produced reactive intermediates (RIs), such as excited triplet-states of DOM (3DOM*), singlet oxygen (1O2) and hydroxyl radical (·OH). However, it is not clear whether DOM extracted from coastal seawaters (CS-DOM) and DOM derived from freshwaters (FW-DOM) exhibit similar effects on photodegradation of organic micropollutants. Herein, 2,4-dihydroxybenzophenone (BP-1) was adopted as a model compound to probe the effects of different DOM on photodegradation kinetics of organic micropollutants. Results show that the CS-DOM promotes the photodegradation of BP-1 mainly via the pathway involving 3DOM*; while 3DOM*, 1O2 and ·OH are responsible for BP-1 photodegradation in the presence of the FW-DOM. Compared with the FW-DOM, the CS-DOM undergoes more photobleaching, and contains less aromatic C=C and C=O functional groups. Although 3DOM* formation quantum yields for the CS-DOM are relatively higher than those for the FW-DOM, the CS-DOM has lower rates of light absorption, leading to lower steady-state RI concentrations for the CS-DOM. BP-1 photodegradation in the presence of the CS-DOM is faster than in the presence of the FW-DOM, due to higher second-order reaction rate constants between BP-1 and CS-3DOM* and fewer antioxidants contained in the CS-DOM.

Degradation of tetracycline by medium pressure UV-activated peroxymonosulfate process: Influencing factors, degradation pathways, and toxicity evaluation

This study employed the medium pressure UV/peroxymonosulfate (MPUV/PMS), a new sulfate radical-based advanced oxidation process, to eliminate tetracycline (TTC) in water. At pH = 3.7, initial TTC concentration of 11.25 μM, PMS dosage of 0.2 mM and UV dose of 250 mJ cm−2, 82% of TTC was degraded by MPUV/PMS. The second-order reaction rate constants of TTC with SO4− and OH were found to be 1.4 × 1010 M−1 s−1 and 6.0 × 109 M−1 s−1, respectively. Radical quenching experiments indicated that OH played the major role in the degradation of TTC. Higher PMS dosage (0.1–1.0 mM) and higher pH (3–11) could accelerate the TTC removal. Besides, the presence of Cl− (0.1–5.0 mM) and CO32− (0.05–0.5 mM) could also promote the reaction. Eight transformation products (TPs) were identified, and the potential degradation pathways mainly involved hydroxylation, demethylation and decarbonylation processes. The variation in the genotoxicity was investigated using the umu-test, and the results indicate that the genotoxicity of TTC after the MPUV/PMS treatment significantly increased during the initial stage. In addition, the ecotoxicity and mutagenicity of TTC and its TPs were predicted using quantitative structure-activity relationship (QSAR) analysis, and the results revealed that some TPs could have equivalent and even higher toxicity than TTC. MPUV/PMS showed better performance in TTC degradation in real waters than in Milli-Q water. MPUV/PMS is concluded to be an efficient method for removing TTC, but more attention should be paid to the changes of toxicity during this process.

Aqueous-Phase Oxidation of Terpene-Derived Acids by Atmospherically Relevant Radicals.

Terpene-derived acids formed through the atmospheric gas-phase oxidation of terpenes are able to efficiently undergo a phase transfer into the aqueous phase. The subsequent aqueous-phase oxidation of such compounds has not been intensely studied. Accordingly, the aqueous-phase second-order rate constants of the oxidation reactions of cis-pinonic acid (CPA) and (+)-camphoric acid (+CA) with hydroxyl radicals (•OH), nitrate radicals (NO3•), and sulfate radicals (SO4•-) were investigated as a function of temperature and pH in the present study. For CPA and +CA the following •OH reaction rate constants at T = 298 K are determined: ksecond(CPA, pH<2) = (2.8 ± 0.1) × 109 L mol-1 s-1, ksecond(CPA, pH>8) = (2.7 ± 0.3) × 109 L mol-1 s-1, ksecond(+CA, pH<2) = (2.1 ± 0.1) × 109 L mol-1 s-1, ksecond(+CA, pH=5.3) = (2.7 ± 0.3) × 109 L mol-1 s-1, ksecond(+CA, pH>8) = (2.7 ± 0.1) × 109 L mol-1 s-1. In order to assess the atmospheric impact of the aqueous-phase oxidation of such compounds, atmospheric aqueous-phase lifetimes were calculated for two model scenarios based on CAPRAM 3.0i. The aqueous-phase oxidation under remote conditions emerges to be the most favored pathway with lifetimes of 5 ± 1 h.

Comparative study on ferrate oxidation of BPS and BPAF: Kinetics, reaction mechanism, and the improvement on their biodegradability

Bisphenol S (BPS) and bisphenol AF (BPAF) were increasingly consumed and these compounds are resistant to environmental degradation. Herein, ferrate oxidation of BPS and BPAF was investigated, and biodegradability of the oxidation products was examined. The second-order reaction rate constants of ferrate with BPS and BPAF were 1.3 × 103 M−1s−1 and 3 × 102 M−1s−1, respectively, at pH 7.0, 25 °C. In the oxidation process, some BPS molecules dimerized, while other BPS molecules were oxidized through oxygen-transfer process, leading to the formation of hydroxylation products and benzene-ring cleavage products. The dominant reaction of BPAF with ferrate was oxygen-transfer process, and BPAF was degraded into lower molecular weight products. The variation of assimilable organic carbon (AOC) suggested that the biodegradability of BPAF and BPS was largely improved after ferrate oxidation. Compared with the BPS oxidation products, the BPAF oxidation products were easier to be bio-consumed. Pure culture test showed that BPAF inhibited the growth of Escherichia coli, while ferrate oxidation completely eliminated this toxic effect. Co-existing humic acid (HA, 1 mg C/L to 5 mg C/L) decreased the removal of BPS and BPAF with ferrate. Compared with BPAF, more oxidation intermediates formed in the ferrate oxidation of BPS may be reduced by HA to the parent molecular. Thus, the inhibition effect of HA on the ferrate oxidation of BPS was more obvious than that on BPAF.

Aqueous chlorination of benzodiazepines diazepam and oxazepam: Kinetics, transformation products and reaction pathways

The pharmaceutical benzodiazepine diazepam and its metabolite oxazepam have been detected in wastewater treatment plants effluents, surface water and treated drinking water, but their transformation during chlorination process is not well understood. We investigated the reactions of diazepam and oxazepam with free available chlorine using a high-resolution mass analyzer and theoretical calculation to elucidate the fate of benzodiazepines during water chlorination process. The obtained apparent second-order rate constants (kapp) for chlorine reaction with diazepam and oxazepam varied from 0.01 to 1.7 M−1 s−1 and 0.7 to 31.6 M−1 s−1 in the pH range of 5.5–10.0, respectively. Under typical wastewater disinfection conditions of neutral pH values, free chlorine concentrations of 5 mg L−1 and contact times of up to 2 h, the corresponding half-lives for diazepam (∼180 min) and oxazepam (∼61 min) suggest that diazepam will be partly transformed during disinfection. Conversely, oxazepam will be considerably transformed during wastewater disinfection. The pH-dependency of kapp for diazepam could be explained by the reactions between neutral diazepam and HClO species. The kinetic pattern for oxazepam can be well described by species-specific reactions involving oxazepam or Cl2 and Cl2O species. In total, fifteen and eight transformation products were identified for chlorination of diazepam and oxazepam, respectively. The C-3 of 1,4-benzodiazepine structure was the main site of attack, leading predominantly to the oxidation and then cleavage of the C(3)-N(4) bond, as well as diazepine ring contractions. Based on mass balance estimation, the main chlorination product for diazepam and oxazepam are 7-chloro-1-methyl-5-phenyl-1,3-dihydro-2H-1,4-benzodiazepin-2,3-dione and 6-chloro-4-phenyl-2(1H)-quinazolinone, respectively.

Direct and indirect photolysis of the antibiotic enoxacin: kinetics of oxidation by reactive photo-induced species and simulations.

The purpose of this study was to investigate the aqueous phase photochemical behavior of enoxacin (ENO), an antibiotic selected as a model pollutant of emerging concern. The second-order reaction rate constants of ENO with hydroxyl radicals (HO●) and singlet oxygen (1O2) were determined at pH3, 7, and 9. Also, the rate constants of the electron transfer reaction between ENO and triplet states of chromophoric dissolved organic matter (3CDOM*) are reported for the first time, based on anthraquinone-2-sulfonate (AQ2S) as CDOM proxy. The sunlight-driven direct and indirect ENO degradation in the presence of dissolved organic matter (DOM) is also discussed. The results show that direct photolysis, which occurs more rapidly at higher pH, along with the reactions with HO● and 3AQ2S*, is the key pathway involved in ENO degradation. The ENO zwitterions, prevailing at pH7, show kENO, HO●, kENO,1O2, and kENO,3AQ2S* of (14.0 ± 0.8) × 1010, (3.9 ± 0.2) × 106, and (61.5 ± 0.7) × 108Lmol-1s-1, respectively, whose differences at pH3, 7, and 9 are due to ENO pH-dependent speciation and reactivity. These k values, along with the experimental ENO photolysis quantum yield, were used in mathematical simulations for predicting ENO persistence in sunlit natural waters. According to the simulations, dissolved organic matter and water depth are expected to have the highest impacts on ENO half-life, varying from a few hours to days in summertime, depending on the concentrations of relevant waterborne species (organic matter, NO3-, NO2-, HCO3-).

Mechanistic consideration of the photochemical transformation of domoic acid (algal toxin) in DOM-Rich brackish water

Domoic acid (DA) is a neurotoxin generated by several diatom species in harmful algae blooms (HABs). We report the photo-induced transformation products (TPs) and degradation mechanisms of DA in dissolved organic matter (DOM)-rich freshwater and brackish water. High-resolution quadrupole time-of-flight mass spectrometry (QTOF-MS) and the multivariate statistical strategy orthogonal partial least-squares discriminant analysis (OPLS-DA) identified 36 and 23 potential TPs in DOM-rich freshwater and brackish water, respectively. The main reactive sites of DA are the conjugated double bond and proline ring. Isomerization is the predominant transformation pathway induced by excited-state triplet DOM (3DOM∗). The second-order rate constant of the isomerization reaction was measured as (3.8 ± 0.2) × 108 M−1 s−1. The inverse correlation between the dissolved oxygen (DO) concentration and the rate of photo-induced DA isomerization was revealed. Furthermore, under halide-present conditions, halide radicals are mainly responsible for the differentiation of products by quenching hydroxyl radicals and generating unique organic peroxide products. Our results indicated that halide radicals could be important in the photochemical transformation of organic contaminants in high saline environments.

Behavior of Ionic Liquids Around Charged Metal Complexes: Investigation of Homogeneous Electron Transfer Reactions Between Metal Complexes in Ionic Liquids

The second-order electron transfer reaction between the photo-excited triplet state of [Zn(TPP)]* (TPP = 5,10,15,20-tetraphenylporphyrin) and [Co(sep)]3+ (sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) was investigated in three ionic liquids (ILs, 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with R = butyl, pentyl, and hexyl) and in acetonitrile. Results of electrochemical and kinetic measurements indicated that ILs dissociate in the vicinity of charged metal complexes and at electrodes, although the dissociated anionic and cationic components of the ILs seem to exist as pairs around the metal complexes. Second-order rate constants for the electron transfer reaction are 1.88 × 109, 3.65 × 107, 2.63 × 107, and 2.01 × 107 kg·mol−1·s−1 in acetonitrile and in the butyl, pentyl and hexyl ILs, respectively, at 298 K, after correction of the contribution of diffusion. The average slope of the plot of the logarithmic second-order rate constants observed in acetonitrile and ILs against the logarithmic viscosity of each solvent was − 0.84. However, the slope of the same plot was much steeper (− 4.1) when data for only the three ILs were used. Detailed analyses of the experimental results on the basis of the Latner–Levin cross relation and the Marcus theory lead to the conclusion that the solvent properties such as the dielectric constant and refractive index around the polarized/charged transition states are different from those for the bulk ILs: observed self-exchange rate constants did not exhibit the Pekar factor dependence when dielectric constants and refractive indices for bulk ILs are used.