Major Reaction Intermediates Research Articles

Kinetics and degradation mechanism of clofibric acid and diclofenac in UV photolysis and UV/H2O2 reaction

In this study, we investigated the removal of the selected pharmaceuticals, clofibric acid and diclofenac, using UV photolysis and UV/H2O2 reactions. The degradation of diclofenac during UV photolysis reaction was faster than that of clofibric acid. Diclofenac was mainly removed by photolysis reaction; clofibric acid was more sensitive to OH radical. More effective removals of clofibric acid and diclofenac were observed when H2O2 was added in the UV photolysis reaction. The competition kinetics showed that the second-order rate constant between the OH radical and the pharmaceuticals was 5.57 × 109 M−1 s−1 for clofibric acid and 2.45 × 109 M−1 s−1 for diclofenac, respectively. The major reaction intermediates during the UV photolysis and UV/H2O2 reactions of clofibric acid were 4-chlorophenol and hydroquinone, and these intermediates were degraded as phenol with further reaction.

Removal of glyphosate from water by electrochemically assisted MnO2 oxidation process

Glyphosate, one of the organophosphate herbicides, has been widely used in the word. The removal of glyphosate was comparatively investigated by MnO2 oxidation, electrochemical oxidation, and electrochemically assisted MnO2 oxidation (electro-MnO2) processes. The effects of MnO2 dosage, current density, and solution pH on glyphosate removal and Mn2+ release were examined. The results indicated that the removal of glyphosate by MnO2 oxidation favored acidic pH conditions and a large portion of Mn2+ ions were released from MnO2. With the electro-MnO2 process using RuO2/TiO2 coated titanium mesh as both anode and cathode, glyphosate removal was significantly promoted and most of the released Mn2+ ions were oxidatively reverted to MnO2, which in turn enhanced the removal of glyphosate. Solution pH exerted an insignificant effect on glyphosate removal in the electro-MnO2 process. Major reaction intermediates including sarcosine, glycine, and PO43- were indentified in the initial phase of the MnO2 oxidation and electro-MnO2 processes. Glycine was further decomposed to glycolic acid and NH3–N in the MnO2 oxidation process; in contrast, glycine was oxidized into oxamic acid, glycolic acid and N-contained intermediates, and N-contained intermediates could be finally oxidized into acetic acid, NH3–N and NO3-–N in the electro-MnO2 process.

Removal of benzotriazole from solution by BiOBr photocatalysis under simulated solar irradiation

In this paper, two-dimensional bismuth oxybromide (BiOBr) nanoplates and three-dimensional BiOBr microspheres were prepared via facile co-precipitation methods with Bi(NO3)3·5H2O as the Bi source. The resulting BiOBr catalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), high-resolution transmission electron microscopy (HRTEM), N2 adsorption–desorption and ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) techniques, and were used to degrade benzotriazole under simulated solar light irradiation. The BiOBr catalysts were of pure tetragonal phase, and their band gaps were about 2.7eV. Nearly 90% of benzotriazole was stepwisely eliminated in the presence of the synthesized BiOBr catalysts after 3h of irradiation compared to the removal efficiency of about 50% by P25 under identical conditions. The superior photocatalytic performance of BiOBr can be attributed to its special nanostructure and optical abilities. With the help of various radical scavengers, the degradation mechanism of benzotriazole by BiOBr was found to be mainly driven by the direct hole and HO radicals oxidation. Four major reaction intermediates identified by gas chromatography–mass spectrometry (GC–MS) analysis confirmed the role of direct hole and HO radicals during the oxidation process, and a tentative photodegradation pathway was proposed. The research showed promising results for the application of the obtained BiOBr materials to benzotriazole removal from wastewater.

Adsorption and Photocatalytic Decomposition of theβ-Blocker Metoprolol in Aqueous Titanium Dioxide Suspensions: Kinetics, Intermediates, and Degradation Pathways

This study reports the photocatalytic degradation of theβ-blocker metoprolol (MET) using TiO2suspended as catalyst. A series of photoexperiments were carried out by a UV lamp, emitting in the 250–400 nm range, providing information about the absorption of radiation in the photoreactor wall. The influence of the radiation wavelength on the MET photooxidation rate was investigated using a filter cutting out wavelengths shorter than 280 nm. Effects of photolysis and adsorption at different initial pH were studied to evaluate noncatalytic degradation for this pharmaceutical. MET adsorption onto titania was fitted to two-parameter Langmuir isotherm. From adsorption results it appears that the photocatalytic degradation can occur mainly on the surface of TiO2. MET removed by photocatalysis was 100% conditions within 300 min, while only 26% was achieved by photolysis at the same time. TiO2photocatalysis degradation of MET in the first stage of the reaction followed approximately a pseudo-first-order model. The major reaction intermediates were identified by LC/MS analysis such as 3-(propan-2-ylamino)propane-1,2-diol or 3-aminoprop-1-en-2-ol. Based on the identified intermediates, a photocatalytic degradation pathway was proposed, including the cleavage of side chain and the hydroxylation addition to the parent compounds.

Photodegradation of naproxen and its photoproducts in aqueous solution at 254 nm: A kinetic investigation

The kinetics of photodegradation of the non steroidal anti-inflammatory drug naproxen (+)-S-2-(6-methoxynaphthalen-2-yl)propanoic acid, an emerging organic pollutant, was studied in aqueous solutions under deaerated and aerated conditions. The photolysis experiments were carried out under monochromatic irradiation (λ = 254 nm) at pH = 7.0 and T = 25 °C.Simplified reaction schemes of photodegradation of naproxen are proposed in absence and in presence of oxygen respectively. The schemes take into account the photolysis of naproxen and its photoproducts and the reactions of the measured species with oxygen dissolved in the liquid bulk. According to these schemes, two kinetic models were developed which correlate the experimental data, for runs performed in absence and in presence of oxygen, with a fair accuracy and allowed to estimate the best values for the unknown kinetic parameters.The calculated quantum yield of direct photolysis of naproxen under deaerated media is in good agreement with the one previously reported. Under aerated conditions, the generation of singlet oxygen has also been taken into account.The obtained results, under the adopted conditions, indicated a marked influence of dissolved oxygen on the photodegradation rates of naproxen and the relative distribution of the major reaction intermediates.

A Novel Cryo-Reduction Method to Investigate the Molecular Mechanism of Nitric Oxide Synthases

Nitric oxide synthases (NOSs) are hemoproteins responsible for the biosynthesis of NO in mammals. They catalyze two successive oxidation reactions. The mechanism of oxygen activation is based on the transfer of two electrons and two protons. Despite structural analogies with cytochromes P450, the molecular mechanism of NOS remains yet to be elucidated. Because of extremely high reaction rates, conventional kinetics methods failed to trap and characterize the major reaction intermediates. Cryo-reduction methods offer a possibility to circumvent this technological lock, by triggering oxygen activation at cryogenic temperatures by using water radiolysis. However, this method is not adapted to the NOS mechanism because of the high instability of the initial Fe(II)O2 complex (extremely fast autoxidation and/or reaction with the cofactor H4B). This imposed a protocol with a stable Fe(II)O2 complex (observed only for one NOS-like protein) and that excludes any redox role for H4B. A relevant approach to the NOS mechanism would use H4B to provide the (second) electron involved in oxygen activation; water radiolysis would thus provide the first electron (heme reduction). In this context, we report here an investigation of the first electron transfer by this alternative approach, i.e., the reduction of native NOS by water radiolysis. We combined EPR and resonance Raman spectroscopies to analyze NOS reduction for a combination of different substrates, cofactor, and oxygen concentrations, and for different NOS isoforms. Our results show that cryo-reduction of native NOS is achieved for all conditions that are relevant to the investigation of the NOS mechanism.

Oxidation of formaldehyde over Pd/Beta catalyst

The role of catalyst support in the complete oxidation of HCHO has been investigated over the metal oxide- and zeolite-supported Pd catalysts. The 0.25Pd/Beta catalyst exhibited the highest intrinsic activity for the complete oxidation of HCHO among other monometallic catalysts, mainly due to the high HCHO adsorption capacity of Beta zeolite support and the fast surface reaction for HCHO oxidation on Pd as determined by TPD and TPSR studies, respectively. Formate and dioxymethylene have been identified as major reaction intermediates by an in situ FTIR study. These reaction intermediates are the most abundant surface species on the Pd/Beta catalyst during HCHO oxidation, so their oxidation is the rate determining step of the complete oxidation of HCHO. When Mn was incorporated into the 0.25Pd/Beta catalyst, the HCHO oxidation activity was further improved, especially in the low temperature region; HCHO was completely oxidized to CO2 over the 0.25Pd/20Mn/Beta catalyst at 40°C and 50,000h−1. The enhanced oxidation activity of the 0.25Pd/20Mn/Beta catalyst could be understood by a kinetic synergism between MnOx and Pd for the oxidation of HCHO. Oxygen adsorbs on and diffuses through MnOx along an oxygen concentration gradient to Pd, while Pd consumes oxygen via HCHO oxidation to create the oxygen concentration gradient in MnOx on the surface of the Beta zeolite. A possible reaction pathway has been proposed that could elucidate the enhanced HCHO oxidation activity over the bimetallic 0.25Pd/20Mn/Beta catalyst by the extended Mars–van Krevelen mechanism involving direct and indirect surface reactions over the bimetallic Pd–MnOx catalyst system.

Photocatalytic degradation of tetrabromobisphenol A by mesoporous BiOBr: Efficacy, products and pathway

Tetrabromobisphenol A (TBBPA) is the most widely used brominated flame retardant around the world. In the present study, for the first time we explored the photocatalytic oxidation of TBBPA by nanoarchitectural BiOBr microspheres. BiOBr photocatalysts were synthesized via solvothermal method with various preparation conditions, and the optimal synthesis condition was determined according to their characterization results and rhodamine B (RhB) removal efficiencies. The prepared optimal BiOBr was mesoporous material of pure tetragonal phase, which removed nearly 100% of RhB from solution after 60min simulated solar light irradiation. The BiOBr was adopted to decompose TBBPA, which was almost totally eliminated after 15min in the UV–vis/BiOBr system. The kinetic analysis indicated that the reaction rate constants were 0.388 and 0.101min−1 for BiOBr and P25 TiO2, respectively, suggesting that BiOBr effectively and kinetically enhanced the decomposition of TBBPA. Eight major reaction products or intermediates were identified by HPLC–MS and HPLC–SIR–MS, and a tentative degradation pathway of TBBPA was proposed. The mechanism originally illustrated that both hydroxylation and debromination played important roles in TBBPA transformation. The exceptional efficiency of mesoporous BiOBr in removing TBBPA represents a promising technique for treatment of TBBPA-containing wastewater or remediation of TBBPA-contaminated environmental matrices.

Intermediates and transport phenomena in two‐temperature synthesis of ZnGeP2

AbstractHigh quality semiconducting ternary compound ZnGeP2 was synthesized by a modified two‐temperature technique using high purity elemental zinc, germanium and phosphorus as the starting materials. Transport phenomena of zinc and phosphorus vapors and the major reaction intermediates, taking place in ZnGeP2 formation, were studied by interrupting the synthesis process using quenching technique as well as by adjusting the temperatures of cold and hot zones. The powder X‐ray diffraction analysis showed that the major reaction intermediates were ZnP2, Zn3P2, and GeP, which proportions were changed at the different temperature stages. ZnP2 was formed in the temperature gradient region and ZnGeP2 was formed in the hot zone when the temperature of the hot zone was higher than 900 °C. The 520‐1040 °C temperature profile was chosen for the ZnGeP2 synthesis and charge amount per run reached 200 g. The powder X‐ray diffraction pattern of the synthesized ZnGeP2 compound was in agreement with the standard pattern of ZnGeP2. These results demonstrated that the synthesized ZnGeP2 compound was a single phase. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Abatement of methidathion and carbaryl from aqueous solutions using organic photocatalysts

Degradation of two pesticides, methidathion and carbaryl, has been studied using organic photocatalysts, namely triphenylpyrylium (TPP +), triphenylthiapyrylium (TPTP +) and acridine yellow G (AYG). Experiments carried out using a solar simulator showed that methidathion was more easily photo-oxidised than carbaryl, in all cases. TPP + and TPTP + exhibited a better performance than AYG. Detoxification of solutions was related with the primary abatement of the pesticides, as indicated by respirometry studies. Major reaction intermediates were identified by GC–MS, indicating early removal of the carbamate or phosphorated moiety. Photophysical experiments (fluorescence and laser flash photolysis) proved the involvement of an electron-transfer mechanism, most probably from a ground state complex formed between substrate and catalyst.

Comprehensive Kinetic Model for the Degradation of Methyl tert-Butyl Ether by an Ozone/UV Process

The paper presents a comprehensive kinetic model that describes the degradation of methyl tert-butyl ether (MTBE) in an ozone/UV process. First, the degradation pathways for MTBE were proposed on the basis of major reaction intermediates identified during the oxidation of MTBE. The yield for each major reaction intermediate was determined by fitting experimental data to the model prediction. Accordingly, 43, 20, 15, 11, and 6% of MTBE oxidized resulted in the generation of TBF, MMP, TBA, acetone, and methyl acetate, respectively. TBF oxidation resulted in the generation of HiBA, acetone, and TBA as primary intermediates at 47, 24, and 20%, respectively. During the oxidation of TBA, 66% resulted in the generation of HiBA and 34% in acetone. The kinetic model was verified using different sets of experimental data by varying the initial concentration of MTBE, influent ozone gas concentration, and incident UV light intensity. The model predicted well the degradation of MTBE by an ozone/UV process. In addition...

Two-temperature synthesis of ZnGeP2

We describe a modified two-temperature process for reproducible high-volume (up to 500 g) synthesis of the nonlinear-optical semiconductor ZnGeP2, which enables the preparation of nominally stoichiometric material. The major reaction intermediates in the two-temperature ZnGeP2 synthesis are ZnP2, Zn3P2, GeP, and Ge. Using x-ray diffraction, we refined the interplanar spacings in the tetragonal structure of ZnGeP2 and fcc structure of GeP and indexed peaks missing in the PDF cards 33-1471 (ZnGeP2) and 21-353 (GeP).

Photocatalytic degradation of azo dye Metanil Yellow: Optimization and kinetic modeling using a chemometric approach

The photocatalytic degradation of an azo dye Metanil Yellow was carried out in aqueous solution using TiO2 as photocatalyst under UV irradiation. The decolorization and degradation kinetics were investigated and both followed a pseudo first order kinetic according to Langmuir–Hinshelwood model. Using HPLC/DAD and GC/MS analyses, more than 10 major reaction intermediates were identified and a tentative degradation pathway was proposed. Furthermore, ion chromatography (IC) and TOC measurements revealed a complete mineralization of Metanil Yellow into CO2, N2, H2O and inorganic ions (NH4+, NO3− and SO42−).On the other hand, an experimental design based on the surface response methodology was applied to assess the individual and interaction effects of several operating parameters (dye concentration, TiO2 concentration, pH, light flux, etc.) on the treatment efficiency (dye removal time). Based on the experimental design data, a semi-empirical expression was obtained, permitting to predict and optimize the dye removal time. This model was very consistent with experiment results (correlation factor: 99.5%). Moreover, additional experimental results obtained under near optimal conditions were found to be very close to the predicted values.This work demonstrates well the utility and benefits of the experimental design approach for screening and modeling the reaction parameters. Furthermore, it contributes significantly to the improvement and better understanding of photocatalytic processes.

The catalytic cycle of a thiamin diphosphate enzyme examined by cryocrystallography

Enzymes that use the cofactor thiamin diphosphate (ThDP, 1), the biologically active form of vitamin B(1), are involved in numerous metabolic pathways in all organisms. Although a theory of the cofactor's underlying reaction mechanism has been established over the last five decades, the three-dimensional structures of most major reaction intermediates of ThDP enzymes have remained elusive. Here, we report the X-ray structures of key intermediates in the oxidative decarboxylation of pyruvate, a central reaction in carbon metabolism catalyzed by the ThDP- and flavin-dependent enzyme pyruvate oxidase (POX)3 from Lactobacillus plantarum. The structures of 2-lactyl-ThDP (LThDP, 2) and its stable phosphonate analog, of 2-hydroxyethyl-ThDP (HEThDP, 3) enamine and of 2-acetyl-ThDP (AcThDP, 4; all shown bound to the enzyme's active site) provide profound insights into the chemical mechanisms and the stereochemical course of thiamin catalysis. These snapshots also suggest a mechanism for a phosphate-linked acyl transfer coupled to electron transfer in a radical reaction of pyruvate oxidase.

Oxidation kinetics and effect of pH on the degradation of MTBE with Fenton reagent

The fundamentals of the Fenton reagent-based degradation of low concentrations of methyl tert-butyl ether (MTBE) in batch reactors under initially anaerobic conditions are discussed in this work. The objective of the study was to quantitatively verify the feasibility of MTBE degradation with Fenton reagent under such conditions. The conclusions may be potentially helpful to further develop an effective in situ treatment of MTBE-contaminated groundwater. Initial MTBE concentrations ([MTBE](0)) of 11.4 and 22.7 microM (approximately 1.0 and 2.0 mg/L, respectively) were treated with Fenton reagent (FR) using a [FR](0):[MTBE](0) molar ratio of 10:1. FR was used in equimolar mixture of Fe(2+)and H(2)O(2) (i.e., [Fe(2+)](0):[H(2)O(2)](0)=1:1). This analysis considers the hydroxyl radicals (OH(*)) produced by FR as the main species responsible for the degradation processes. The effects of [MTBE](0) and pH on the oxidation kinetics were investigated. Under these conditions it was observed that: (i) MTBE was degraded at high extent (90-99%) after 1h of reaction time, (ii) MTBE mineralization was low in all cases and reached only 31.7% at the best conditions, and (iii) In all cases, most of MTBE degradation occurred during the initial 3-5 min of reaction. During this brief initial phase, MTBE transformation followed pseudo-first order kinetics, while the subsequent phase exhibited a sharp drop in degradation rate and had almost negligible contribution to the overall degradation. Experiments performed at acidic pH exhibited the best degradation results, while at neutral pH the degradation rates dropped significantly. Other parameters included in this analysis were: TOC reduction and total concentration of compounds containing the tert-butyl group in their structure (tBC). These compounds were analyzed because of the concerns related to their potential toxicity. Tert-butyl formate (TBF), Tert-butyl alcohol (TBA), acetone and methyl acetate were identified and quantified as major reaction intermediates.

UV Photolysis of Trichloroethylene: Product Study and Kinetic Modeling

Direct UV photolysis of trichloroethylene (TCE) in dilute aqueous solution generated chloride ions as a major end product and several reaction intermediates, such as formic acid, di- and monochloroacetic acids, glyoxylic acid, and, to a lesser extent, mono- and dichloroacetylene, formaldehyde, dichloroacetaldehyde, and oxalic acid. Under prolonged irradiation, these byproducts underwent photolysis, and a high degree of mineralization (approximately 95%) was achieved. TCE decays through the following major pathways: (1) TCE + h nu --> ClCH=C*Cl + Cl*; (2) TCE (H2O) + h nu --> ClCH(OH)-CHCl2; (3) TCE + h nu --> HC[triple bond]CCl + Cl2; (4) TCE + h nu --> ClC[triple bond]CCl + HCl; (5) TCE + Cl* --> Cl2HC-C*Cl2. A kinetic model was developed to simulate the destruction of TCE and the formation and fate of byproducts in aqueous solution under irradiation with polychromatic light. By fitting the experimental data, the quantum yields for the four photolysis steps were predicted as phi(1) = 0.13, phi(2) = 0.1, phi(3) = 0.032, and phi(4) = 0.092, respectively. The reaction mechanism proposed for the photodegradation of TCE accounts for all intermediates that were detected. The agreement between the computed and experimental patterns of TCE and reaction products is satisfactory given the complexity of the reaction mechanism and the lack of photolytic kinetic parameters that are provided in the literature.

Thermal decomposition (pyrolysis) of urea in an open reaction vessel

A study was done of the thermal decomposition of urea under open reaction vessel conditions by thermogravimetric analysis (TGA), high performance liquid chromatography (HPLC), Fourier transform-infrared (FT-IR), and an ammonium ion-selective electrode (ISE). Both evolved gases and urea residue were analyzed, and profiles of substances present versus temperature are given. Major reaction intermediates are also identified. Plausible reaction schemes based on product distribution in relation to temperature are proposed. Our data indicate that at temperatures in excess of 190°C, cyanuric acid (CYA), ammelide and ammeline are produced primarily from biuret. Biuret itself is a result of prior reaction of cyanic acid, HNCO, with intact urea. Cyanic acid is primarily a result of urea decomposition at temperatures in excess of 152°C. CYA and ammelide first appear at approximately 175°C, but the reaction rate is very slow. At temperatures in excess of 193°C, alternate reactions involving the decomposition of biuret substantially increases reaction rates. Several parallel processes compete for the production of products. Production of CYA, ammeline and ammelide appears complete at 250°C, after which sublimation and eventual decomposition of products occurs.

Sonolysis of 2,4-Dichlorophenoxyacetic Acid in Aqueous Solutions. Evidence for •OH-Radical-Mediated Degradation

2,4-dichlorophenoxyacetic acid (2,4-D) undergoes efficient degradation when an O2- or Ar-saturated aqueous solution is subjected to high-frequency (640 kHz) sonolysis. 2,4-dichlorophenol, hydroquinone, and catechol are major reaction intermediates common to various experimental conditions. The similarities between the reaction intermediates of sonolytic and radiolytic reactions indicate •OH radical as the primary reactive species responsible for 2,4-D degradation. Very little 2,4-D degradation occurs if the sonolysis is carried out in the presence of the •OH radical scavenger tert-butyl alcohol, also indicating that little or no pyrolysis of the compound occurs. Most of the 2,4-D eventually forms oxalic acid, which, unlike other •OH-mediated oxidation methods, is not easily mineralized with high-frequency ultrasound.

Pyrolysis and Oxidation of Phenol

Pathways of aromatic fuel oxidation have revealed the importance of phenol chemistry and motivated this study to obtain the very first stable intermediate species profiles for the atmospheric pressure oxidation of phenol near 1170 K over a range of equivalence ratios, 0.64−1.73. Under these conditions, cyclopentadiene was found to be a major reaction intermediate. Other major species observed included carbon monoxide, carbon dioxide, acetylene, benzene, 1,3-butadiene, ethene, and methane. Minor species were allene, methylacetylene, propene, ethane, methylcyclopentadiene, and naphthalene. The reaction intermediates are consistent not only with the findings of a cyclopentadiene oxidation study previously performed in this laboratory but also with reactions in the postulated mechanism of phenol oxidation. A complementary study of phenol pyrolysis indicated that carbon monoxide, cyclopentadiene, and benzene were major reaction intermediates, also consistent with an earlier study of phenol pyrolysis in this la...

Integration of chemical and biological oxidation processes for water treatment: Review and recommendations

AbstractThe literature of studies which used a combination of chemical and biological degradation (usually oxidative) of organic contaminants in water is reviewed. Beneficial effects of such two‐step treatments are commonly reported; these results, primarily from laboratory studies, suggest potential advantages for water treatment via process integration rather than single technology processing.Four wastewater contaminant types are identified which can benefit from combined processes: 1.) recalcitrant compounds 2.) biodegradable wastes with small amounts of recalcitrant compounds 3.) inhibitory compounds and 4.) intermediate dead‐end products. The design key for such two‐step systems lies in choosing processes that complement each other and lead to a synergistic effect. Predicting this performance outcome requires knowledge of the physical, chemical and biological properties of the major reaction intermediates and their susceptibility to degradation by each process.Economic, physical and technological limitations of the individual processes should be recognized for design of more effective and economical integrated processes. The ultimate treatment goal, whether specific pollutant removal or reduction of a global parameter such as TOC, must be known so that appropriate and complementary processes can be utilized. More work is needed concerning the degradation kinetics within the combined process, from initial attack of the primary compound through dynamics of intermediates and on to total mineralization.