Rate Constants For Reduction Research Articles

5,7,3′,4′-Hydroxy substituted flavonoids reduce the heme of cytochrome c with a range of rate constants

Flavonoids are antioxidants known to be abundant in edible plants. Seven 5,7,3′,4′-tetrahydroxy substituted flavonoids representing each major flavonoid class were used as cytochrome c reductants to systematically investigate the redox role of their C-rings. Additional examples of flavonoids and benzenediols were investigated to better understand the role of the B-ring. Pseudo-first order rate constants (kobs) and apparent bimolecular rate constants (kapp) values were calculated from spectroscopic measurements. Of the seven flavonoids tested, five yielded measurable observed reduction rate constants. Butein (a chalcone) had the highest apparent bimolecular rate constant (kapp), followed by taxifolin (a flavanonol), catechin (a flavanol), eriodictyol (a flavanone), and luteolin (a flavone). Quercetin (a flavonol) and cyanidin (an anthocyanidin), however, reduced cytochrome c but kapp rate constants were unable to be calculated. Neither this trend nor trends in observed rate constants correlated with flavonoid pKa, solvent accessible surface area, polar surface area, reduction potential, antioxidant ability, resonance, or radical scavenging efficiency. Weak correlation, however, was found with degrees of freedom and the number of redox involved electrons. While some cytochrome c reduction rates have been reported, this study is the first to systematically investigate the role of the structure of the flavonoid C-ring across a full set of flavonoids with identical B-rings.

RNA Polymerase: Step-by-Step Kinetics and Mechanism of Transcription Initiation.

To determine the step-by-step kinetics and mechanism of transcription initiation and escape by E. coli RNA polymerase from the λPR promoter, we quantify the accumulation and decay of transient short RNA intermediates on the pathway to promoter escape and full-length (FL) RNA synthesis over a wide range of NTP concentrations by rapid-quench mixing and phosphorimager analysis of gel separations. Experiments are performed at 19 °C, where almost all short RNAs detected are intermediates in FL-RNA synthesis by productive complexes or end-products in nonproductive (stalled) initiation complexes and not from abortive initiation. Analysis of productive-initiation kinetic data yields composite second-order rate constants for all steps of NTP binding and hybrid extension up to the escape point (11-mer). The largest of these rate constants is for incorporation of UTP into the dinucleotide pppApU in a step which does not involve DNA opening or translocation. Subsequent steps, each of which begins with reversible translocation and DNA opening, are slower with rate constants that vary more than 10-fold, interpreted as effects of translocation stress on the translocation equilibrium constant. Rate constants for synthesis of 4- and 5-mer, 7-mer to 9-mer, and 11-mer are particularly small, indicating that RNAP-promoter interactions are disrupted in these steps. These reductions in rate constants are consistent with the previously determined ∼9 kcal cost of escape from λPR. Structural modeling and previous results indicate that the three groups of small rate constants correspond to sequential disruption of in-cleft, -10, and -35 interactions. Parallels to escape by T7 RNAP are discussed.

Separation of Phosphorus and Manganese from Steelmaking Slag by Selective Reduction

Steelmaking slag contains considerable amount of manganese (Mn) and phosphorus (P), and recycling them is an important issue. Since Mn is an alloy added to enhance the steel quality, while P is a harmful impurity in the steel product, they need to be separated for recycling. We have already found the possibility of separating Mn and P from steelmaking slag by selectively reducing P2O5 and FeO while suppressing the reduction of MnO. As a fundamental study for the selective reduction of P from steelmaking slag, in this study, we vary the slag basicity, graphite mixing ratio, temperature, and crucible type to find the optimum condition. Artificial steelmaking slag with various compositions is synthesized and mixed with graphite powder in an Al2O3 or a MgO crucible. The reduction is conducted at 1673 K, 1773 K, and 1873 K for 20 to 80 minutes. As a result, carbon saturated metal is formed from the slag, and the reduced metal and slag were separated easily. We find that a decrease in slag basicity suppressed the reduction rate of Mn while enhancing that of P. The activation energy for the reduction rate constant of P is found to be close to that of the diffusion of P in slag.

Electrochemical Removal of Chromium (VI) from Wastewater

The removal of hexavalent chromium has attracted much attention as it is a hazardous contaminant. Electrochemical reduction technology was applied to remove chromium (VI) from wastewater. The mechanisms and parameters that affect the reduction process were investigated. The results showed that the reduction efficiency was significantly affected by the concentration of H2SO4, current density, and reaction temperature. The reduction efficiency was up to 86.45% at an H2SO4 concentration of 100 g/L, reaction temperature of 70 °C, current density at 50 A/m2, reaction time at 180 min, and stirring rate of 500 rpm. The reduction process of chromium (VI) followed a pseudo-first-order equation, and the reduction rate constant could be expressed as Kobs = k [H2SO4]1·[j]4·exp−4170/RT.

Ligand mediated reduction of c-type cytochromes by Fe(II): Kinetic and mechanistic insights

Cytochrome c (c-Cyts), as a key protein involved in extracellular electron transfer, plays an important role in the microbial Fe(II)-oxidizing process. In this study, due to their similar structure with the outer membrane protein of microorganisms, the equine c-Cyts was employed as a model protein to examine the impact of different organic ligands on the redox kinetics between Fe(II) and c-Cyts using the rapid stopped-flow spectrometer. Our results suggest that the presence of organic ligands will significantly accelerate the reaction between c-Cyts and Fe(II), with reaction rates obtained in the following order: EDTA > citric acid (Cit) > NTA > malonic acid (Mal) > glycine amino acid (Gly) > acetate acid (Ace) > control. It was also found from the kinetic modeling results that the reduction rate of c-Cyts by Fe(II)-ligands was closely correlated to the complex constant between Fe(II) and each ligand, and the reduction rate constants of c-Cyts by Fe(II)L species (L represents “ligands”). Further analysis on the ligand structure indicates that the reaction rate constants are highly correlated with the complex constants of Fe(II)-ligand and the reduction rate constants of c-Cyts by Fe(II)L species (k3) for Group I (Gly, NTA, and EDTA) and Group II (Cit, Mal, and Ace). The results of midpoint potentials of all Fe(II)-ligands obtained through cyclic voltammetry (CV) experiments were ranked as Ace < Gly < NTA < control < Mal < EDTA < Cit, indicating that the redox potentials of Fe(II) complexes in the same group with similar structures were positively correlated with Fe(II)-ligand complex constants. The reorganization energy change of the electron-transfer reaction resulting from the different Fe(II)-ligand structures may contribute significantly more than does the Gibbs free energy (ΔG) to the reaction kinetics of Fe(II)-ligand and c-Cyts.

A Kinetic Study on Combustible Coastal Debris Pyrolysis via Thermogravimetric Analysis

Coastal debris has recently emerged as a serious environmental pollution problem. Coastal debris can be treated using pyrolysis because it consists mainly of combustible materials like plastics (e.g., polyethylene (PE), polypropylene (PP), nylon) and wood. In this study, the pyrolysis characteristics of coastal debris were fully utilized by applying their basic data to fuel production. The initial temperature increased from 330 °C to 380 °C for the nylon fishing net coastal debris sample, from 405.01 °C to 430.08 °C for the PE fishing net coastal debris sample, from 395.01 °C to 419.96 °C for the PP rope coastal debris sample, and from 114.95 °C to 115.02 °C for the wood (bamboo) coastal debris sample. The activation rate of the global activation energy and the pre-exponential factors rose with the increasing heating rate, complementing the reduction rate constant due to the larger growth of the exponential term due to the kinetic models used.

RNA-mediated, green synthesis of palladium nanodendrites for catalytic reduction of nitroarenes

Palladium (Pd)-catalyzed reactions mostly show structure sensitivity: i.e., the selectivity and activity of the reactions are highly dependent on the arrangement of Pd atoms. In this regard, branched Pd nanoparticles show enhanced catalytic performance owing to the presence of low coordinated Pd atoms. In this paper, a novel solution-phase synthesis of flower-like Pd nanodendrites using ribonucleic acid (RNA) as a capping agent and ascorbic acid as a reducing agent was described. On the other hand, the co-use of polyvinylpyrrolidone (PVP) and potassium bromide (KBr) instead of RNA at the same synthesis conditions led to cuboid nanoparticles, while the sole use of ascorbic acid resulted in faceted nanoparticles. The formation of nanodendritic morphology was attributed to the RNA-assisted growth through particle attachment. This scenario was supported by TEM analysis that demonstrated the aggregation of small particles to form larger nanoparticles at the onset of the reaction. The shape and size of the nanoparticles could be readily tuned by the RNA content used. XPS confirmed the formation of metallic Pd nanoparticles. The presence of crystalline planes of {1 1 1}, {2 0 0}, {2 2 0}, {3 1 1} and {2 2 2} was demonstrated by XRD and SAED analyses. The Pd nanodendrites were used for the reduction of p-nitrophenol (PNP) and 2,4,6-trinitrotoluene (TNT), and reduction rate constants (k) were calculated as 1.078 min−1 (normalized rate constant, knor = 59.66 mmol−1 s−1) for PNP and 0.3181 min−1 (knor = 17.6 mmol−1 s−1) for TNT with the corresponding turnover frequencies (TOFs) as 16.06 and 40.80 h−1, respectively.

Facile construction of reduced graphene oxide supported three-dimensional polyaniline/WO2.72 nanobelt-flower as a full solar spectrum light response catalyst for efficient photocatalytic conversion of bromate

As a carcinogenic byproduct in drinking water treatment, bromate has raised global concerns on environmental and health hazard, calling for effective treatments. In the current work, a novel reduced graphene oxide supported polyaniline/WO2.72 nanobelt-flower (RGO/PANI/WO2.72) ternary composite was prepared through a solvent volatilization method for photocatalytic reduction of bromate. The prepared sample was characterized, and the influence of aqueous pH, ions and dissolved oxygen on the bromate reduction was explored. As expected, the introduction of RGO and PANI on the WO2.72 exhibited great synergistic effects on the separation of photogenerated carriers. The calculated reduction rate constant of RGO/PANI/WO2.72 was about six times as high as that of pure WO2.72. Specially, the prepared photocatalyst possessed strong optical absorption in a broad range of 250–2500 nm, and thus displaying excellent catalytic performance in utilization of all solar spectrum energy. Moreover, the RGO/PANI/WO2.72 exhibited stable photocatalytic activity in cycling test. Considered holistically, the present study offered a valuable approach for rational construction of heterogeneous structure in the development of bromate-catalyzed reduction.

Biological Reduction of Nitrate and Perchlorate in Soil Microcosms: An Electron Donor Comparison of Glycerol, Emulsified Oil, and Mulch Extract

AbstractDiverse technologies have been developed and tested for their efficacy in remediating perchlorate‐contaminated surface water, groundwater, and soil. Biological reduction, particularly when coupled with electron donor augmentation, has been shown to be one of the most cost‐effective alternatives. Numerous electron donors have been evaluated in the literature, but few studies have compared standard vs. slow‐release electron donors for sequential nitrate and perchlorate reduction. This study evaluated the efficacy and kinetics of biological reduction in soil microcosms augmented with emulsified oil (EO), glycerol, and mulch extract. Results indicated that EO and glycerol spiked at approximately 100 times stoichiometric excess (i.e., 100x) achieved similar overall reductions and degradation rates for nitrate and perchlorate, although nitrate appeared to exhibit zero order kinetics while perchlorate exhibited first order kinetics. The zero order rate constants for nitrate reduction were 3.32 mg/L d and 2.57 mg/L d for EO and glycerol, respectively. The first order rate constant for perchlorate reduction was 0.36 day−1 for both EO and glycerol. Stable chemical oxygen demand (COD) concentrations also highlighted the slow‐release properties of EO, which would reduce electron donor consumption in comparison to soluble substrates in soil remediation applications. The microcosms augmented with mulch extract failed to demonstrate any nitrate or perchlorate reduction due to the extract's lower COD concentration. Augmentation with compost or additional processing (i.e., concentration) would be necessary to make the extract a viable alternative.

Degradation of decabromodiphenyl ether (BDE-209) in microcosms mimicking sediment environment subjected to comparative bioremediation strategies

The aim of this study was to examine bioremediation strategies for BDE-209 contaminated sediments. Sediment microcosms were established to observe anaerobic debromination of BDE-209 under conditions representing three bioremediation strategies: biostimulation, bioaugmentation and natural attenuation. To simulate biostimulation, a defined mineral medium containing both a carbon source (sodium formate) and electron donor (ethanol) was added into sediments. Bioaugmentation was established by enrichment of the sediments using a culture of Dehalobium chlorocoercia strain DF-1, previously shown to dechlorinate polychlorinated biphenyls, to sediments. No amendments were made to the third set in order to represent natural attenuation. The biostimulation, bioaugmentation and natural attenuation strategies resulted in 55.3%, 40.2% and 30.9% reductions in BDE-209, respectively, after 180 days. Nona- through tri-BDEs were observed as products, with 17 PBDE congeners detected in 25 different proposed debromination pathways. At the end of the 180 day incubation period, the products for bioaugmentation, biostimulation and natural attenuation were tri-BDEs, tetra-BDEs and penta-BDEs, respectively. The proposed pathways revealed that meta- and ortho-Br removal were favored in sediments, and that debromination regiospecificity varied with each bioremediation strategy applied. Lastly, pseudo-first-order rate constants for BDE-209 reduction were calculated as 0.0049 d−1, 0.0028 d−1, 0.0025 d−1 for biostimulation, bioaugmentation and natural attenuation, respectively.

Synthesis of silver decorated silica nanoparticles with rough surfaces as adsorbent and catalyst for methylene blue removal

The discharge of dyes from industries to water reservoirs has attracted extensive attention worldwide, and it is of great importance to remove them efficiently. In this study, a series of silica nanoparticles with rough surfaces was prepared, and highly dispersed silver nanoparticles were loaded on its surface through a simple wet-impregnation method. The adsorption performance and catalytic activity of the as-prepared Ag/SiO2 for methylene blue (MB) removal was investigated. The results demonstrated that the Ag/SiO2 with smaller particle size and higher Ag loading amount shows high adsorption capacity and catalytic activity for MB. The maximum adsorption capacity was found to be ~55 mg/g, which is more than two times of pure silica nanoparticles. Besides, MB could be degraded by more than 99% of the initial concertation (40 mg/L) within 3 min and rate constant of the catalytic reduction achieved as high as 2.128 min−1. Moreover, the as-prepared Ag/SiO2 shows good stability in acidic environment and excellent reusability for at least eight successive cycles of adsorption and four cycles of catalytic reduction.

Mechanisms for Abiotic Dechlorination of Trichloroethene by Ferrous Minerals under Oxic and Anoxic Conditions in Natural Sediments.

Bench-scale experiments were performed on natural sediments to assess abiotic dechlorination of trichloroethene (TCE) under both aerobic and anaerobic conditions. In the absence of oxygen (<26 μM), TCE dechlorination proceeded via a reductive pathway generating acetylene and/or ethene. Reductive dechlorination rate constants up to 3.1 × 10-5 d-1 were measured, after scaling to in situ solid:water ratios. In the presence of oxygen greater than 120 μM, TCE dechlorination proceeded via an oxidative pathway generating formic/glyoxylic and glycolic/acetic acids, and oxidative dechlorination rate constants (again scaled to in situ conditions) up to 7.4 × 10-3 d-1 were measured. These rates correspond to half-lives of 60 and 0.25 years for abiotic TCE dechlorination under anaerobic and aerobic conditions, respectively, indicating the potentially large impact of aerobic TCE oxidation in the field. For both reductive and oxidative TCE dechlorination pathways, measured first-order rate constants increased with increasing ferrous iron content, suggesting the role of iron oxidation. Hydroxyl radical formation was measured and increased with increasing oxygen and ferrous iron content. Rate constants associated with TCE oxidation products increased with increasing hydroxyl radical generation rates, and were zero in the presence of a hydroxyl radical scavenger, suggesting that oxidative TCE dechlorination is a hydroxyl radical driven process.

The Impact of River Discharge and Water Temperature on Manganese Release from the Riverbed during Riverbank Filtration: A Case Study from Dresden, Germany

The climate-related variables, river discharge, and water temperature, are the main factors controlling the quality of the bank filtrate by affecting infiltration rates, travel times, and redox conditions. The impact of temperature and discharge on manganese release from a riverbed were assessed by water quality data from a monitoring transect at a riverbank filtration site in Dresden-Tolkewitz. Column experiments with riverbed material were used to assess the Mn release for four temperature and three discharge conditions, represented by varying infiltration rates. The observed Mn release was modeled as kinetic reactions via Monod-type rate formulations in PHREEQC. The temperature had a bigger impact than the infiltration rates on the Mn release. Infiltration rates of &lt;0.3 m3/(m2·d) required temperatures &gt;20 °C to trigger the Mn release. With increasing temperatures, the infiltration rates became less important. The modeled consumption rates of dissolved oxygen are in agreement with results from other bank filtration sites and are potentially suited for the further application of the given conditions. The determined Mn reduction rate constants were appropriate to simulate Mn release from the riverbed sediments but seemed not to be suited for simulations in which Mn reduction is likely to occur within the aquifer. Sequential extractions revealed a decrease of easily reducible Mn up to 25%, which was found to reflect the natural stratification within the riverbed, rather than a depletion of the Mn reservoir.

Generation and reduction of bulk nanobubbles by ultrasonic irradiation

To develop a compact generator of bulk nanobubbles (ultrafine bubbles), ultrasound was irradiated to ultrapure water. Effects of ultrasonic power and frequency on nanobubble concentration and diameter were investigated. Nanobubbles with a diameter of 90–100 nm were generated. Number concentration of nanobubbles increased with time and approached asymptotically to an equilibrium value which was about 1.5 × 109 mL−1. Nanobubble concentration increased with increasing ultrasonic power and decreasing frequency. For comparison, ultrasound was also irradiated to high concentration nanobubble water prepared by pressurized dissolution method. Reduction of nanobubbles was measured and nanobubble concentration decreased to the equilibrium value. Nanobubble concentration decreased with increasing ultrasonic power and frequency.Generation and reduction of nanobubbles by ultrasound was modeled to analyze experimental data. The generation rate coefficient of nanobubbles increased with decreasing frequency because cavitation collapse became stronger. The reduction rate constant of nanobubbles at 488 kHz and 1 MHz in high concentration nanobubble water became much higher than those in ultrapure water due to aggregation and floatation of nanobubbles. The equilibrium number concentration increased with decreasing frequency. The results calculated by our model were in good agreement with the experimental data. The nanobubble generator using ultrasound is compact, simple in operation and produce nanobubbles in a short time. This method is contamination-free because the liquid pump is unnecessary.

N, S doped carbon dots—Plasmonic Au nanocomposites for visible-light photocatalytic reduction of nitroaromatics

Abstract

Reduction–Oxidation of Chromium Ions in Aqueous Solution by Treatment with Atmospheric-Pressure Direct-Current Discharge in Argon

Kinetic features of the oxidation–reduction processes of chromium ions in a potassium dichromate aqueous solution subjected to treatment in argon atmospheric-pressure dc discharge have been investigated in the discharge current range of 20–80 mA and the dichromate concentration range of 0.1–0.5 mmol/L. The solution served as the discharge cathode. It has been revealed that the discharge treatment stimulates the reduction reactions of Cr6+ to Cr3+ ions and the reverse reactions of oxidation of Cr3+ to Cr6+ ions; i.e., the oxidation–reduction reactions are reversible. The limiting degree of reduction of Cr6+ ions has been found to depend on the discharge current and the initial concentration of the solution. The apparent rate constants of oxidation and reduction have been determined, and the energy efficiency of the process has been evaluated. The data are compared with the results obtained earlier for a discharge in air.

Synthesis and Evaluation as a Blood-Brain Barrier-Permeable Probe of 7-N-(PROXYL-3-yl-methyl)theophylline.

The drug-nitroxide radical hybrid-compound 7-N-((2,2,5,5-tetramethylpyrrolidine-1-yloxy(PROXYL))-3-yl-methyl)theophylline (3) was synthesized by coupling 7-N-tosyltheophylline with 3-hydroxymethyl-PROXYL, HMP). The stability of 3 relative to that of HMP was examined in the presence of the anti-oxidant, ascorbic acid (AsA). The initial reduction rate constants of 3 and HMP were 11.9±5.3 and 6.1±5.2 M-1 min-1, respectively. In the presence of glutathione (GSH), these constants increased slightly to 22.3±6.8 and 9.1±2.4 M-1 min-1, respectively. Two-dimensional cranial electron paramagnetic resonance imaging of mice intravenously injected with 3 via the tail vein revealed that probe 3 enters the mouse brain by passing through the blood-brain barrier (BBB).

Study on Carbothermal Reduction of Titania in Molten Iron

Abstract In order to improve the reduction rate of titania in molten iron, various iron powders containing C, Si, Mn and S were melted. The experiments were carried out on the reduction of titania through a high-temperature tube furnace at 1,723–1,823 K. The quantitative effects of C, Si, Mn, S and temperature on the reduction of titania were investigated in the current study. The results demonstrated that when the carbon content, the manganese content and the temperature increased by 1 %, 0.1 % and 100 K, the reduction rate increased by 0.008 %/h, 0.001125 %/h and 0.0235 %/h, respectively; when the sulfur content increased by 0.01 %, the reduction rate decreased by 0.004875 %/h; the reduction rate was irregular with the change of silicon content in molten iron. The phase at the reaction interface after the experiment was confirmed to be the Fe2Ti3O9 which was considered to be the combination product between iron oxide and titania; the lower titanium oxides were unstable and hard to be observed. The reduction was affected by the concentration of various elements in molten iron and the activity interaction coefficients between various elements. The rate constants for reduction were calculated at 1,723 K, 1,773 K and 1,823 K; the apparent activation energy was calculated as 209 kJ/mol through the rate constants and temperatures according to the Arrhenius equation.

Enhanced catalytic reduction of nitrophenols by sodium borohydride over highly recyclable Au@graphitic carbon nitride nanocomposites

In this study, a photochemical green synthesis using thermal exfoliation process is developed to fabricate Au@graphitic carbon nitride (g-C3N4) nanocomposite, highly recyclable and reusable, for the catalytic reduction of nitrophenols by NaBH4. Au nanoparticles (Au NPs) in the diameter of 5–15 nm are deposited onto the surface of g-C3N4 in 3–6 layers of structure. The synthesized Au@g-C3N4 nanocomposites exhibit excellent catalytic activity and stability in the reduction of nitrophenols including 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2,4-nitrophenol, and 2,4,6-nitriophenol. The catalytic performance of Au@g-C3N4 is highly dependent on the initial nitrophenol concentration, Au loading, inorganic anions, and pH. The rate constant of 4-nitrophenol reduction over Au@g-C3N4 (2 wt%) is 26.4 times that of pure Au NP in the presence of 7 mM of NaBH4 at pH 5. Moreover, Au@g-C3N4 can be reused for at least 10 consecutive cycles without considerable loss of catalytic activity. The presence of anions (0.1 M) such as H2PO4−, SO42-, HCO3−, and NO3− decreases the rate of 4-nitrophenol reduction by a factor of 1.2–8.8; whereas there is a 1.2-time increase in rate constant upon the addition of Cl− ion. The detection of H radical adducts indicates that Au NPs adsorbs BH4− ions and forms Au-H species. The porous and conductive g-C3N4 provides large surface area for nitroarene adsorption and subsequent electron transfer from the Au-H species to 4-nitrophenol, which results in accelerating the reduction of 4-nitrophenol. Results clearly demonstrate that Au@g-C3N4 is a promising green catalyst of enormous potential for nitroaromatic reduction, which provides a new venue for tailoring Au-based nanomaterials in elucidation of a wide variety of heterogeneous catalytic reactions.

Dynamics of Photocatalytic Hydrogen Production in Aqueous Dispersions of Monolayer-Rich Tungsten Disulfide

Two-dimensional tungsten disulfide (WS2) is an emerging semiconducting photocatalyst featuring high optical absorption, carrier mobility, and catalytic activity toward hydrogen evolution. While characterization of its optical and electrocatalytic properties has advanced, less is known about its ultrafast carrier dynamics and intrinsic photocatalytic activity in aqueous systems producing hydrogen. This work removed extraneous variables often found in photoelectrochemical systems, thereby allowing the intrinsic proton reduction rate for monolayer-rich WS2 nanosheets to be estimated via transient absorption lifetimes and a developed kinetic scheme. Addition of a hole scavenger, ascorbic acid (AA), resulted in a 3-fold increase in carrier lifetimes following photoexcitation. Longer electron lifetimes with AA yielded a 14-fold increase in hydrogen production. An intrinsic proton reduction rate constant was extracted that may be extended to any photoelectrochemical or electrochemical hydrogen evolution scheme i...