Second-order Rate Constants For Reactions Research Articles

In-situ electrochemical generation of permanganate for the treatment of atrazine

A novel process involving the simultaneous electrochemical oxidation and electrosynthesis of permanganate oxidant has been explored for the treatment of the triazine organic herbicide, atrazine. The electrochemical synthesis of permanganate in neutral pH conditions using low concentration manganese (Mn2+), analogous to levels found in some raw groundwater sources, and their subsequent effect on atrazine degradation were studied in bench-scale experiments. Permanganate synthesis was found to be largely unaffected by the operating current density (10, 40 and 80 mA cm−2) during electrolysis, indicating as mass transport controlled process. Under the same operating conditions, hydroxyl radical mediated oxidation was observed to degrade atrazine from an initial concentration of 9.27 µM (2 mg L−1), to 6.22, 4.88 and 2.36 µM after 120 min of electrolysis for 10, 40 and 80 mA cm−2 conditions. When 55 µM (3.0 mg L−1) Mn2+ was added to the water matrix, atrazine degradation increased, yielding final concentrations of 5.80, 3.66 and 2.17 µM, respectively. Atrazine degradation was found to be accurately described by pseudo-first-order reaction kinetics, with and without the enhanced oxidation by permanganate generation, as the concentration of hydroxyl radicals remained constant and comparatively high throughout electrolysis. Finally, the yielded second-order reaction rate constants of electrochemically generated permanganate, and dosed potassium permanganate, with atrazine were 9.79 and 8.35 M−1 s−1, respectively, whereby the latter degradation mechanism was kinetically limited and the former was under mass transfer control due to an extremely low permanganate-atrazine ratio. Finally, four primary oxidation by-products were observed to form in the reactions, including deethylatrazine, deisopropylatrazine and deethyldeisopropylatrazine.

Reactivity of Chlorine Radicals (Cl• and Cl2•-) with Dissolved Organic Matter and the Formation of Chlorinated Byproducts.

Chlorine radicals, including Cl• and Cl2•-, can be produced in sunlight waters (rivers, oceans, and lakes) or water treatment processes (e.g., electrochemical and advanced oxidation processes). Dissolved organic matter (DOM) is a major reactant with, or a scavenger of, Cl• and Cl2•- in water, but limited quantitative information exists regarding the influence of DOM structure on its reactivity with Cl• and Cl2•-. This study aimed at quantifying the reaction rates and the formation of chlorinated organic byproducts produced from Cl• and Cl2•- reactions with DOM. Laser flash photolysis experiments were conducted to quantify the second-order reaction rate constants of 19 DOM isolates with Cl• (kDOM-Cl•) and Cl2•- (kDOM-Cl2•-), and compare those with the hydroxyl radical rate constants (kDOM-•OH). The values for kDOM-Cl• ((3.71 ± 0.34) × 108 to (1.52 ± 1.56) × 109 MC-1 s-1) were orders of magnitude greater than the kDOM-Cl2•- values ((4.60 ± 0.90) × 106 to (3.57 ± 0.53) × 107 MC-1 s-1). kDOM-Cl• negatively correlated with the weight-averaged molecular weight (MW) due to the diffusion-controlled reactions. DOM with high aromaticity and total antioxidant capacity tended to react faster with Cl2•-. During the same experiments, we also monitored the formation of chlorinated byproducts through the evolution of total organic chlorine (TOCl) as a function of chlorine radical oxidant exposure (CT value). Maximum TOCl occurred at a CT of 4-8 × 10-12 M·s for Cl• and 1.1-2.2 × 10-10 M·s for Cl2•-. These results signify the importance of DOM in scavenging chlorine radicals and the potential risks of producing chlorinated byproducts of unknown toxicity.

Development of a free radical-based kinetics model for the oxidative degradation of chlorazol black in aqueous solution using periodate photoactivated process

This paper presents the first modeling study on UV/periodate (IO4–) advanced oxidation process. A reaction mechanism consisting in 45 chemical reactions including a number of radicals (IO3•, IO4•, •OH, O•–, HO2•, O2•–, O3•–) and non-radical intermediates/products (O(3 P), O3, IO3–, H2O2, HO2–, I2O6 and I2O8) has been developed to assess the reactive species generation, use and distribution during the oxidation of one model azo dye, chlorazol black (CB), by the periodate photoactivated process. The model fitted excellently the CB degradation data over a wide range of solution pH and initial IO4– concentration. Unavailable second-order rate constants of several important reactions were optimized using the genetic algorithm. Those include O3 + IO3• → IO4• + O2 (k = 1 × 108 M−1s−1), 2IO4• → I2O8 (k = 4.68 × 108 M−1s−1), reaction of CB with •OH (k = 1 × 109 M−1s−1), IO3• (k = 2.63 × 108 M−1s−1), IO4• (k = 0 M−1s−1), O(3 P) (k = 0 M−1s−1) and O3 (k = 0.574 M−1s−1). Besides, the first order kinetic constant of periodate photolysis, IO4– + hv → IO3• + O•– and IO4– + hv → IO3– + O(3 P), were respectively (0.5–4.16)×10-3 s−1 and ∼2 × 10-4 s−1, indicating that the radical pathway for IO4– photolysis is predominately the IO3–-releasing path. OH and IO3 were found to play the key role in the CB degradation. The concentration of radicals increased with pH decrease and initial periodate concentration rise, favoring higher degradation rate at acidic conditions and higher IO4– dosages. The selectivity analysis showed that the contribution of these species is ∼ 79 % for •OH and ∼ 21 % for IO3•, under various conditions of pH and [IO4–]0. This study would helpfully provide some practical indications for the application of UV/periodate AOP

Advanced oxidation mechanism of UV photolysis of electrochemically generated free bromine

ABSTRACT In recent times, some researchers have successfully demonstrated the efficacy of UV photolysis of electrochemically generate free chlorine (UV/electro-chlorine) as for an advanced oxidation process. Since bromine as well as chlorine is an element belonging to halogen, it is expected that UV photolysis of electrochemically generated free bromine (UV/electro-bromine) also shows an advanced oxidation effect. To elucidate the feasibility of UV/electro-bromine system, its advanced oxidation mechanism was investigated using radical probes of 1,4-dioxane and nitrobenzene. In contrast to the UV/electro-chlorine system, the advanced oxidation effect of UV/electro-bromine system was inhibited under acidic conditions due to the accumulation of photochemically inert Br2. The most abundant radical in UV/electro-bromine system was dibromine radical anion (Br2˙−) and the second-order reaction rate constant of Br2˙− with 1,4-dioxane was estimated to be 2.4 × 105 M−1 s−1. As a result of the abundance and the reactivity of Br2˙−, it was the main contributor to 1,4-dioxane degradation. On the other hand, nitrobenzene was mainly decomposed by direct UV photolysis because Br2˙− does not react with nitrobenzene. The contribution of hydroxyl radical (HO˙) to 1,4-dioxane degradation was much lower than that of Br2˙− because its concentration was 4–5 order of magnitude lower than that of Br2˙−. However, the HO˙ concentration elevated with a decrease in the concentration of bromide ion (Br−). Consequently, the reactivity of Br2˙− with pollutants and the Br− concentration have critical impacts on the advanced oxidation performance of UV/electro-bromine system.

Photochemical persistence of sulfa drugs in aqueous medium: kinetic study and mathematical simulations.

This study aimed at investigating the photochemical behavior of sulfa drugs containing five and six-membered heterocyclic substituents (sulfamethoxazole (SMX) and sulfadiazine (SDZ), respectively), in an aqueous medium. Despite their importance, studies devoted to the use of photochemical models to predict the environmental phototransformation of pollutants in surface waters, by combining laboratory results and natural aquatic systems parameters, are still scarce in the scientific literature. In this work, the second-order reaction rate constants of SDZ and SMX with hydroxyl radicals (●OH), singlet oxygen (1O2), and triplet excited states of chromophoric dissolved organic matter (3CDOM*) were experimentally determined at pH 7, using the competition kinetics approach. The results show that ●OH and 3CDOM* are the key species involved in sulfonamide degradation, with anionic SMX, most prevalent at pH 6-9, being degraded much slower than the anionic form of SDZ. Moreover, SDZ and SMX photodegradation in natural water samples (spring-fed natural pond, public supply reservoir, and sea water) was significantly enhanced relative to depletion in pure water. Finally, from mathematical simulations of the sunlight-driven sulfonamide degradation, half-life times were predicted for these drugs varying from less than 2 to about 90days, depending on the water depth, concentration of key species (DOC, HCO3-, NO2-, CO32-) in natural aqueous systems, as well as on the particular heterocyclic substituent.

Reevaluation of the contributions of reactive intermediates to the photochemical transformation of 17β-estradiol in sewage effluent

Photodegradation of the natural steroid 17β-estradiol (E2), an endocrine-disrupting hormone that has been widely detected in aquatic environments, was investigated in wastewater effluents at various pH ranges under simulated solar irradiation. The rate of E2 degradation in the sewage effluents was stable at pH 6.0-7.0 but suddenly increased from pH 8.0-10.0. The second-order reaction rate constants of E2 with 3EfOM* and CO3•− were measured to increase 11.0-fold and 18.0-fold from pH 6.0 to 10.0, respectively. Two main reasons are proposed for this sharp increase. First, the change in the ionization state of E2 made it susceptible to oxidation by triplet-state effluent organic matter (3EfOM*) and carbonate radicals (CO3•−). Second, the steady-state concentration of CO3•− increased with increasing pH. Indirect photolysis was suggested to be the main degradation pathway in the sewage effluents, and 3EfOM* was proposed to play a major role at pH 8.0-9.0, while CO3•− played a significant role at pH 10.0. In this study, EfOM was shown for the first time to inhibit the oxidation of E2 initiated by 3EfOM* and CO3•−. Thus, we suggest that EfOM plays a dual role in the photodegradation of E2: EfOM can not only be activated as 3EfOM* to degrade E2 but also can inhibit the degradation of E2 by reducing the E2 oxidation intermediate back to E2. The estrogenic activity of the photodegradation products was also studied. The in vitro estrogenic activity of E2 solutions decreased approximately as fast as the E2 photodegradation occurred in the effluent water at various pH values, suggesting that solar photodegradation in sewage effluents reduces the risk of endocrine disruption in waters impacted by E2 and subject to continuing inputs. The results of this study are important for predicting the environmental fate of endocrine-disrupting chemicals and developing methods for their removal from aquatic environments.

Photochemistry of dissolved organic matter extracted from coastal seawater: Excited triplet-states and contents of phenolic moieties

Coastal seawater constitutes an important ecosystem receiving inputs of organic micropollutants (OMPs) such as sulfa antibiotics from land-based sources or mariculture activities. It is necessary to investigate photodegradation of OMPs in coastal seawaters for assessing their environmental fate and risks. However, effects of coastal seawater dissolved organic matter (S-DOM) on OMPs photodegradation are largely unknown, given that chemical compositions of S-DOM are different from those of freshwater DOM. Herein, photochemical characteristics of S-DOM extracted from Dalian coastal seawaters were investigated by simulating photochemical experiment adopting sulfachloropyridazine as a case. Results show that S-DOM accelerates the photodegradation mainly through excited triplet-state DOM (3DOM*) with an apparent rate constant (4.43 × 108 M−1 s−1) ten folds of that of freshwater DOM, which is mainly due to much lower phenol contents detected in the S-DOM (0.022 mg-Gallic acid mg-C−1). The S-DOM impacted by mariculture can photogenerate more high-energy 3DOM* than those less impacted by mariculture, further contributing to the high 3DOM* reactivity. The study shows that to accurately predict photolytic persistence of OMPs in field water bodies, it is of significance to determine the second-order reaction rate constants between 3DOM* and target OMPs using DOM extracted from relevant water bodies.

Degradation of chlorpyrifos formulation in water by the UV/H2O2 process: Lumped kinetic modelling of total organic carbon removal

The degradation of a commercial chlorpyrifos (CP) formulation solution under the UV/H2O2 process was predicted through a lumped kinetics modelling approach (LKA). The proposed models are based on different reaction schemes that include the hydroxyl radical reaction rate with the Total Organic Carbon (TOC). TOC represents the concentration of the mixture of CP and “unknown organic matter”, both components present in CP formulation. TOC is one of the most practical parameter that can be used to quantify organic pollution in a complex matrix. A one-step model was selected and a second-order reaction rate constant between hydroxyl radical and TOC was obtained by optimization: 1.75 × 107 M−1 s−1. According to computed performance metrics, the chosen model well fitted the concentrations evolution of the reactants (TOC and H2O2). The model could be useful to predict the behaviour of reaction systems with different configuration for wastewater treatment applications.

Chlorination and bromination of olefins: Kinetic and mechanistic aspects

Hypochlorous acid (HOCl) is typically assumed to be the primary reactive species in free available chlorine (FAC) solutions. Lately, it has been shown that less abundant chlorine species such as chlorine monoxide (Cl2O) and chlorine (Cl2) can also influence the kinetics of the abatement of certain organic compounds during chlorination. In this study, the chlorination as well as bromination kinetics and mechanisms of 12 olefins (including 3 aliphatic and 9 aromatic olefins) with different structures were explored. HOCl shows a low reactivity towards the selected olefins with species-specific second-order rate constants <1.0 M−1s−1, about 4-6 orders of magnitude lower than those of Cl2O and Cl2. HOCl is the dominant chlorine species during chlorination of olefins under typical drinking water conditions, while Cl2O and Cl2 are likely to play important roles at high FAC concentration near circum-neutral pH (for Cl2O) or at high Cl− concentration under acidic conditions (for Cl2). Bromination of the 12 olefins suggests that HOBr and Br2O are the major reactive species at pH 7.5 with species-specific second-order rate constants of Br2O nearly 3-4 orders of magnitude higher than of HOBr (ranging from <0.01 to >103 M−1s−1). The reactivities of chlorine and bromine species towards olefins follow the order of HOCl < HOBr < Br2O < Cl2O ≈ Cl2. Generally, electron-donating groups (e.g., CH2OH- and CH3-) enhances the reactivities of olefins towards chlorine and bromine species by a factor of 3-102, while electron-withdrawing groups (e.g., Cl-, Br-, NO2-, COOH-, CHO-, -COOR, and CN-) reduce the reactivities by a factor of 3-104. A reasonable linear free energy relationship (LFER) between the species-specific second-order rate constants of Br2O or Cl2O reactions with aromatic olefins and their Hammett σ+ was established with a more negative ρ value for Br2O than for Cl2O, indicating that Br2O is more sensitive to substitution effects. Chlorinated products including HOCl-adducts and decarboxylated Cl-adduct were identified during chlorination of cinnamic acid by high-performance liquid chromatography/high resolution mass spectrometry (HPLC/HRMS).

Kinetics and mechanism of sulfate radical- and hydroxyl radical-induced degradation of Bisphenol A in VUV/UV/peroxymonosulfate system

The degradation of Bisphenol A (BPA) in the vacuum ultraviolet/ ultraviolet/ peroxymonosulfate (VUV/UV/PMS) process based on a mini-fluidic VUV/UV photoreaction system has been comprehensively investigated focusing on the degradation performances and kinetics in this study. With 20 min reaction at pH 7.0 in VUV/UV/PMS process, the removal efficiency and the apparent degradation rate constant (kapp) of BPA were > 95.2 % and 0.145 min−1 (R2 = 0.9935). In addition, the impacts of operational parameters and co-existing constituents on BPA degradation are systematically evaluated. Importantly, the effective degradation of BPA (> 96.1 % at least) achieved at the test pH conditions (5–11) after 30 min reaction and achieved the highest BPA removal rate at pH = 11 (0.243 min−1, R2 = 0.9952). The electron paramagnetic resonance and radical scavenger experiments confirmed that both hydroxyl radical (OH) and sulfate radical (SO4−) are the predominant reactive radicals in VUV/UV/PMS process. The second-order reaction rate constants of BPA with OH and SO4− under different pH conditions were determined to be 0.74–1.86 × 1010 M−1 s−1 and 2.92–5.01 × 109 M−1 s−1 via the competition kinetics. Meanwhile, the relative contribution rate of OH decreased from 97.2 %–33.1 % with the increasement of pH from 5.0 to 11.0, while the relative contribution rate of SO4− rose from 1.7 % to 48.9 %. The present study demonstrated that the VUV/UV/PMS process, as a new advanced oxidation process, has potential applications for micropollutants removal in water.

100th Anniversary of Macromolecular Science Viewpoint: Integrated Membrane Systems.

Membranes fabricated from self-assembled materials are one recent example of how polymer science has been leveraged to advance membrane technology. Due to their well-defined nanostructures, the performance of membranes made from these materials is pushing the boundaries of size-selective filtration. Still, there remains a need for higher performance and more selective membranes. The advent of functional membrane platforms that rely on mechanisms beyond steric hindrance (e.g., charge-selective membranes and membrane sorbents) is one approach to realize improved solute-solute selectivity and further advance membrane technology. To date, the lab-scale demonstration of these platforms has often relied on fabrication schemes that require extended processing times. However, in order to translate lab-scale demonstrations to larger-scale implementation, it is critical that the rate of the functionalization scheme is reconciled with membrane manufacturing rates. In this viewpoint, it is postulated that substrates lined by reactive moieties that are amenable to postfabrication modification would enable the production of membranes with controlled nanostructures while providing access to a diverse array of pore wall chemistries. A comparison of reaction and manufacturing rates suggests that mechanisms that exhibit second-order reaction rate constants of at least 1 M-1 s-1 are needed for roll-to-roll processing. Furthermore, for mechanisms that exhibit rate constants greater than 300 M-1 s-1, it may be possible to integrate multiple functional domains over the membrane surface such that useful properties emerge. These multifunctional systems can expand the capabilities of membranes when the patterned chemistries interact at the heterojunctions between domains (e.g., Janus and charge-patterned mosaic membranes) or if they exhibit cooperative responses to external operating conditions (e.g., membrane pumps).

Effects of dissolved organic matter on photochemicaltransformation of organic micropollutants in natural waters

<p indent="0mm">Organic micropollutants (OMPs) are ubiquitous in natural water bodies and can have negative impacts to health of aquatic ecosystems and human beings. It is of importance to investigate fate of the OMPs so as to assess their environmental risks. Photodegradation is an important pathway to transform the OMPs in natural water bodies, including direct photolysis and indirect photodegradation induced by photosensitizers. Dissolved organic matter (DOM) is an important photosensitizer that is ubiquitous in water bodies. DOM plays an important role in photodegradation of the OMPs especially for those recalcitrant to direct photolysis. On one hand, DOM can produce photochemically produced reactive intermediates (PPRIs), e.g., hydroxyl radical (HO^•), singlet state oxygen (¹O₂), and excited triplet-state DOM (³DOM*), after absorbing solar irradiation. The PPRIs are capable of inducing indirect photodegradation of the various OMPs. On the other hand, DOM can exhibit inhibition effects (light screening effects and quenching effects) on photodegradation of the OMPs. Chromophores in DOM can compete photons with the OMPs and thus inhibit direct photolysis of some OMPs, exhibiting the light screening effects. Antioxidants like phenolic moieties in DOM can quench the formed PPRIs and reduce the reactive intermediates of OMPs to the parent OMPs, manifesting the quenching effects. Generally, the effects of DOM on photodegradation of the OMPs depend on sources of DOM. DOM can derive from autochthonous (microbial) and allochthonous (terrestrial) sources based on its forming processes. Many previous studies on DOM mainly focused on formation pathways of PPRIs, reactivity of ³DOM* from different sources, and effects of different DOM on photodegradation of the OMPs. These efforts in early phase laid a foundation for emerge of the discipline of environmental (aquatic) photochemistry. In recent decades, many studies focused on photochemical properties of autochthonous and allochthonous DOM extracted from freshwater bodies. Very limited studies focused on seawater DOM (S-DOM). S-DOM may undergo longer period of photobleaching than freshwater DOM. Due to sources of riverine inputs, atmospheric depositions, wastewater discharges and mariculture activities, S-DOM in coastal seawaters can be quite different from S-DOM in distant seas or DOM in inland freshwater bodies. Therefore, S-DOM may potentially exhibit photochemical properties different from that of freshwater DOM. As seawater is a final sink of many OMPs, it is necessary to investigate photochemical property of S-DOM and its effects on photodegradation of the OMPs. Some recent studies did prove that S-DOM and freshwater DOM has different photochemical reactivities towards some OMPs. Data on environmental photodegradation kinetics are necessary for assessing fates, persistence and ecological risks of the OMPs. However, translation and extrapolation of reaction rates from environment to environment and laboratory to environment is a challenging task. Underwater downward sunlight attenuation in field water bodies should be taken into consideration when estimating photolytic persistence of the OMPs. More research efforts are needed to develop prediction models on photodegradation kinetics of OMPs in different field water bodies, and to develop prediction models (such as quantitative structure-activity relationship models that can predict second-order reaction rate constants between OMPs and PPRIs) on photodegradation kinetics parameters of the vast and ever-increasing number of OMPs (or environmental organic chemicals). In this review, the recent progresses in aquatic photochemistry of DOM were summarized into three aspects, mechanisms for photogeneration of PPRIs from DOM of different sources, effects of DOM from different sources on photodegradation of OMPs, and photodegradation kinetics prediction models of OMPs in natural water bodies. In addition, potential differences in the effects of freshwater DOM and S-DOM on photodegradation of OMPs were discussed. Based on the research progresses, future efforts in the field of aquatic environmental photochemistry were suggested.

Kinetics and reaction pathways for the transformation of 4-tert-butylphenol by ferrate(VI)

4-tert-butylphenol (4-tBP) is a phenolic endocrine disrupting chemical that has attracted great attention due to its wide occurrence, environmental persistence, and possible toxic effects. In this study, we systematically investigated the transformation of 4-tBP in ferrate (VI) oxidation process. The second-order reaction rate constant (kapp) of Fe(VI) with 4-tBP decreases with solution pH, and the kapp value was determined as 295 M−1·s−1 at pH 8.0. The removal efficiency of 4-tBP was slightly decreased by Mg2+ and HCO3–, while accelerated at varying degrees by the presence of Cu2+ and humic acid. Product analysis revealed that 4-tBP was mainly transformed into hydroxylation products, benzene-ring cleavage products, dimers and higher polymerization products via oxygen atom transfer, ring-opening of the benzene ring and radical coupling reaction. Furthermore, initial reactions of 4-tBP were rationalized by theoretical analysis of atom partial charges, frontier electron densities, and spin densities. Nearly complete removal of 4-tBP (20 μM) was achieved after 5 min of reaction in both ultrapure water and natural waters, demonstrating the feasibility of this Fe(VI) oxidation method in treating phenols-contaminated waters.

The role of chlorine oxide radical (ClO•) in the degradation of polychoro-1,3-butadienes in UV/chlorine treatment: kinetics and mechanisms

Polychoro-1,3-butadienes (CBDs) were widely found in aqueous environment and resistant to conventional water treatment. In this study, the abatement of CBDs during UV/chlorine treatment was investigated. In comparison to UV irradiation alone, free chlorine addition brought benefits for the reduction of tetra-CBDs (TCBDs), but to lesser extent for penta-CBDs (PCBDs), and virtually no benefit for hexa-CBD (HCBD). At a UV dose of 128 mJ cm−2 and a chlorine dose of 10 mg L−1, about 71.7–97.8% CBDs were degraded by UV/chlorine treatment within 10 min. UV irradiation contributed 32.8%–97.6%, HO• contributed 2.6%–14.4%, and reactive chlorine species (RCS) contributed less than 0.5%–42.3% to CBDs degradation. The percentages of RCS contribution generally followed the order of TCBDs (except (Z,Z)-1,2,3,4-TCBD) > PCBDs > HCBD. The chlorine oxide radical (ClO•) was the dominant RCS contributing to the degradation of CBDs. The second-order reaction rate constants of ClO• with CBDs (kClO•,CBDs) were at ∼ 107 M−1s−1 except (Z,Z)-1,2,3,4-TCBD and HCBD (<106 M−1s−1). kClO•,CBDs generally decreased with increasing numbers of chlorine atoms and was also affected by the positions of chlorine atoms in CBDs. A distinct reaction pathway of ClO•, with (Z)-1,1,2,3,4-PCBD as a representative CBD, was proposed. Photoisomers of CBDs from Z or E configuration were observed at lower concentrations in UV/chlorine treatment than under UV irradiation alone due to the radical-involved oxidation, but more organic acids including oxalic acid were observed. In a natural water sample, UV/chlorine treatment also exhibited a good performance in abatement of TCBDs and PCBDs, but not in abatement of HCBD.

Visible light generation of chromium(V)-oxo salen complexes and mechanistic insights into catalytic sulfide oxidation

Visible light irradiation of the photo-labile salen-chromium(III) chlorate or bromate precursors produced salen-chromium(V)-oxo complexes that were spectroscopically and kinetically indistinguishable from those formed by chemical oxidation of chromium(III) salens with PhI(OAc)2. The photochemistry observed in this work is ascribed to the heterolytic cleavage of the O-X bond in the apical counterion that results in a two-electron oxidation of the metal to form the chromium(V)-oxo species. Second-order rate constants for oxidation reactions of 3 with organic substrates were determined under pseudo-first order condition, and particularly low level of reactivity for sulfide oxidations was observed. In this study, chromium(III) salen complexes effectively catalyzed the oxidation of aryl sulfides into sulfoxides with PhI(OAc)2 in the presence of a small amount of water. The competition product studies with the Hammett correlation plot indicated that the observed chromium(V)-oxo species is not likely to serve as the major oxidant for the sulfide oxidations catalyzed by chromium(III) salens with PhI(OAc)2.

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

Abstract The 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.