Chemical Reaction Steps Research Articles

Kinetics of heavy metal biosolubilization from electroplating sludge: effects of sulfur concentration

The removal of heavy metals from industrial sludge through biosolubilization using sulfur-oxidizing bacteria has been shown to be a promising technology, but the process with surplus concentration of sulfur causes re-acidification of the treated residues and creates environmental issues. Thus, the study for investigating the effect of sulfur concentration on the heavy metal biosolubilization system, with an emphasis on optimizing the sulfur concentration, is of immense importance. In this study, the experiments to investigate the effect of sulfur concentration on the performance of biosolubilization were carried out using 2–10 g/L elemental sulfur on heavy metal-laden electroplating sludge (50 g/L). The sludge-acclimatized, sulfur-grown Acidithiobacillus ferrooxidans isolate was used as sulfur-oxidizing bacteria. For the type of sludge used in this study, high pH reduction, short lag phase, and high heavy metal solubilization efficiencies were obtained in the treatment with 6 g/L sulfur. The kinetic study showed that the rate constant values of heavy metal solubilization were relatively high while using sulfur concentration of 6 g/L. The analysis using shrinking core model of fluid–particle reaction kinetics explicated that chemical reaction step controls the rate of heavy metal biosolubilization. The study provides an optimized strategy to design an efficient biosolubilization system for anticipated energy source.

Electron and hydrogen transfer in organic photochemical reactions

Electron and hydrogen transfers are basic steps in many chemical reactions. Photochemical or electronic excitation significantly influences the reactivity of chemical compounds and thus also these transfer processes. Furthermore, the formation of typical intermediates has a decisive impact on the outcome of the transformations. Based on these properties of photochemical processes, efficient homogeneous and heterogeneous photocatalytic transformations for application to organic synthesis have been developed. Efficient reactions without any chemical activation but induced by photochemical electron transfer can also be carried out. The photon acts as a traceless reagent. In the context of photochemical hydrogen transfer, two processes are frequently encountered. The two particles the electron and the proton are transferred either simultaneously or in two steps. In the latter case, the electron is transferred first and the proton follows. Both processes are applied to organic synthesis. Copyright © 2014 John Wiley & Sons, Ltd.

Kinetics of Forward Extraction of V(IV) in V(IV)-SO2- 4(H+, NA+)-Cyanex 302-Kerosene System Using Lewis Cell Technique

Kinetics of V(IV) extraction from acidic sulfate medium by Cyanex 302 (2,4,4-trimethylpentylmonothio phosphinic acid, [H2A2](o)) dissolved in kerosene were investigated using a Lewis cell operated at 3 Hz and flux (F) method of data treatment. The F (kmol/m2s) is inversely proportional to (1 + 0.01 [V(IV)]−1), (1 + 2000 [H+]), and (1 + 0.089 ) and directly proportional to (1 + 0.2 []). The activation energy (E a ) depends on the temperature region and is a function of reactants (f(R)). When log f(R) <−1.0, E a > 48 kJ/mol and when log f(R)>−0.3, E a < 20 kJ/mol. Within log f(R) values of −1.0 to −0.3, E a varies within 20–50 kJ/mol. The rate constant (k) at 293 K is 10−7.335 kmol/m2 s. The entropy changes in activation (ΔS ‡) were measured as negative value always. The complicated empirical rate equation was analyzed to provide extraction mechanisms in different parametric conditions, being supported by E a values. In most cases, the extraction process is either diffusion controlled or mixed controlled; a mixed-controlled process can be converted to a diffusion-controlled process at a lower temperature region and to a chemical-controlled process at a higher temperature region. For systems with log f(R) <−1.0, the process is completely chemical controlled. The rate-determining chemical reaction steps in order are: → ; and → at lower and higher concentration regions of . The negative ΔS ‡ values suggest that the rate-determining chemical reaction steps occur by SN2 mechanisms.

Recovery of gold from aqueous solutions by taurine modified cellulose: An adsorptive–reduction pathway

Abstract The aim of the study was to develop green techniques to recover precious gold by the taurine-modified cellulose (TDAC samples) from the aqueous solution. Adsorption, followed by reduction offered a method for the recovery of ionic and zerovalent gold. The spherical gold nanoparticles (5 nm average size) were investigated in the nanoparticle-embedded gel. Potential rate controlling steps of chemical reactions are modeled by the best-fit of thermodynamic (an endothermic process), kinetic curve (pseudo second order), and isotherm (Sips and 34.5 mg/g Langmuir adsorption capacity) parameters. The pH (2–6) and competing heavy metals (Cd, Co, Cr, Ni, and As) showed least interference, however, gold recovery altered at the high ionic strength (as NaCl). A complexing agent (acidic thiourea) desorbed 86% of ionic gold and pyro-crystallization technique yielded 90% metallic gold. The characterization including X-ray photoelectron spectroscopy detected metallic gold on the surface of TDAC, supporting the adsorptive–reduction mechanism. Performance characteristic of TDAC to concentrate gold in various forms can be viewed as a controllable technique in the water treatment applications.

Leaching kinetics of neodymium in sulfuric acid of rare earth elements (REE) slag concentrated by pyrometallurgy from magnetite ore

We studied the leaching kinetics of recovering neodymium in sulfuric acid from the rare earth elements (REE) slag concentrated by smelting reduction from a magnetite ore containing monazite. The leaching kinetics on neodymium was conducted at a reactant concentration of 1.5 g REE slag per L of 0.3 M H2SO4, agitation of 750 rpm and temperature ranging from 30 to 80 °C. Neodymium oxide included in the REE slag was completely converted into neodymium sulfate phase (Nd2(SO4)3) in H2SO4 after the leaching of 5 h, 80 °C. As a result, the leaching mechanism was determined in a two-stage model based on the shrinking core model with spherical particles. The first step was determined by chemical reaction, and the second step was determined by ash layer diffusion because the leaching of REEs by the first chemical reaction increases the formation of the ash layer affecting as a resistance against the leaching. By using the Arrhenius expression, the apparent activation energy of the first chemical reaction step was found to be 9 kJmol−1. After the first chemical reaction, leaching reaction rate was determined by the ash layer diffusion. The apparent activation energy of ash layer diffusion was found to be 32 kJmol−1.

Electrochemical Study of Cobalt Salen Compounds for Catalytic Biomass Degradation

ABSTRACT Heightened interest in sustainable energy sources have led to a multitude of forays into technologies dealing with biomass refining and energy conversion. While certain streams of biomass offer attractive possibilities as a virtually limitless feedstock, the inherent complexities associated with the constituent biomass polymers, especially the heterogeneous lignin polymer make efficient processing a difficult problem. To make efficient use of lignin as a potential feedstock for energy production or as a chemical precursor, high catalytic specificity and yield is desirable to deal with a broad range of chemical linkages in a manner that results in a single (or few) product(s). In this study, we investigate the use of organometallic Cobalt (salen) complexes and their application toward catalytic breakdown or modification of the lignin polymer. Various lignin model compounds have been used to study possible modes of converting lignin into a usable product in batch reactions with mixed results. In nearly all studies, regardless of yield, the catalyst undergoes deactivation, likely due to an irreversible conformational change or a change in oxidation state of the metal center.By employing non-aqueous cyclic voltammetry, we show that unfixed Co(salen) complexes form a highly reversible redox system. The largely conserved shape of the electrochemical response over successive scans suggests that these catalysts may be recoverable within an electrochemical redox system. Using lignin monomer analogs, we studied electro-catalytic reactions utilizing half-cell cyclic voltammetry and various other characterization techniques to study the electrochemical reversibility of the system and examine the reaction mechanism and oxidation products. Depending on the relative rates of the chemical reaction step and the characteristic response to CV scan rate, the catalytic process defined by the electrochemical oxidation of the catalyst and subsequent chemical reaction step between the ‘activated’ catalyst and the substrate should lead to a “steady state” response similar to that observed for a mass transport limited system. Chemical characterization techniques were also utilized to probe the catalyst oxidation state changes throughout the reaction in an attempt to resolve some of the issues of system deactivation. Keywords : lignin, cobalt salen, electrochemical oxidation, non-aqueous cyclic voltammetry ACKNOWLEDGEMENT We gratefully acknowledge the support of the University of Tennessee Office of the Vice President for Research for support of this work.

Stopped in its tracks: The RNA polymerase molecular motor as a robust sensor of DNA damage

DNA repair is often a complex, multi-component, multi-step process; this makes detailed kinetic analysis of the different steps of repair a challenging task using standard biochemical methods. At the same time, single-molecule methods are well-suited for extracting kinetic information despite time-averaging due to diffusion of biochemical components and stochasticity of chemical reaction steps. Here we discuss recent experiments using DNA nanomanipulation in a magnetic trap to study the initiation of transcription-coupled repair in a model bacterial system comprising the canonical Escherichia coli RNA polymerase and the Mfd translocase which specifically binds to it. These experiments provide kinetic insight into the reaction process, helping to explain how Mfd discriminates between transcribing RNAP and stalled RNAP. They also identify a reliably long-lived intermediate containing Mfd translocase and, potentially, RNA polymerase. This intermediate presumably serves as a platform for assembly of downstream repair components UvrAB(C).

Experimental study of the ammonia adsorption characteristics on the composite sorbent of CaCl2 and multi-walled carbon nanotubes

A new composite sorbent made of CaCl2 and multi-walled carbon nanotube (MWCNT) was prepared for solid–gas ammonia adsorption refrigeration systems in the paper. The composite sorbent was fabricated by impregnating MWCNTs matrix with CaCl2 to improve the adsorption performance of the chemical sorbent. The ammonia adsorption capacity of the composite sorbent and the MWCNTs matrix were measured using a Rubotherm magnetic suspension balance. Experimental results showed that the maximum adsorption amount of ammonia by the MWCNTs matrix and the composite sorbent are 90.05 mg gCNT−1 and 859.85 mg gsorbent−1 under the experimental conditions. Moreover, the chemical reaction plateaus and steps of the reaction between CaCl2 and NH3 were analyzed and the adsorption capacities of the composite sorbent at different working conditions were evaluated. The results indicated that although the adsorption capacity of ammonia on the pure MWCNTs is relatively low, the addition of MWCNTs into the chemical sorbent can avoid the agglomeration and prevent the adsorption performance attenuation of reactive salt.

Understanding a Substrate’s Product Regioselectivity in a Family of Enzymes: A Case Study of Acetaminophen Binding in Cytochrome P450s

Product regioselectivity as influenced by molecular recognition is a key aspect of enzyme catalysis. We applied large-scale two-dimensional (2D) umbrella sampling (USP) simulations to characterize acetaminophen (APAP) binding in the active sites of the family of Cytochrome P450 (CYP) enzymes as a case study to show the different regioselectivity exhibited by a single substrate in comparative enzymes. Our results successfully explain the experimentally observed product regioselectivity for all five human CYPs included in this study, demonstrating that binding events play an important role in determining regioselectivity. In CYP2C9 and CYP3A4, weak interactions in an overall large active site cavity result in a fairly small binding free energy difference between APAP reactive binding states, consistent with experimental results that show little preference for resulting metabolites. In contrast, in CYP1A2 and CYP2E1, APAP is strongly restrained by a compact binding pocket, leading to a preferred binding conformation. The calculated binding equilibrium of APAP within the compact active site of CYP2A6 is able to predict the experimentally documented product ratios and is also applied to explain APAP regioselectivity in CYP1A2 and CYP2C9. APAP regioselectivity seems to be related to the selectivity for one binding conformation over another binding conformation as dictated by the size and shape of the active site. Additionally, unlike docking and molecular dynamics (MD), our free energy calculations successfully reproduced a unique APAP pose in CYP3A4 that had been reported experimentally, suggesting this approach is well suited to find the realistic binding pose and the lowest-energy starting structure for studying the chemical reaction step in the future.

Mathematical Modelling of Pool Fire Burning Rates in a Full- Scale Ventilated Tunnel

A computational fluid dynamic model with full coupling between gaseous and liquid phases is developed to predict buring rates of liquid pool fires in ventilated full-scale tunnel. Rates of fuel release are calculated using predictions of flame feedback to the surface of the pool. A pool fire in tunnel is modelled as an unsteady process, from the time of ignition until convergence to a quasi-steady burning rate. This feedback supports sustained flame above the pool surface and controls the burning rate of the fuel. The numerical model solves three dimensional, time-dependent Navier-Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. Turbulent combustion process is modelled by an Eddy Dissipation Concept (EDC) by using two chemical reaction steps to CO prediction. The numerical model is shown to possess the ability to predict the effect of ventilation on burning rate and the initial growth period in a fullscale tunnel fire. The current study indicates that CO generation is relatively independent of position in the overfire region, and correlated solely as a function of mixture fraction. While no correlation of soot concentrations in terms of the mixture fraction is found. Abundant CO and soot are formed around the fire base, which is later deflected near the tunnel ceiling, and the backflow brings about the toxic products with a noticeable smoke stratification as the airflow velocity is below a critical value.

Evolution of tryptophan biosynthetic pathway in microbial genomes: a comparative genetic study

Biosynthetic pathway evolution needs to consider the evolution of a group of genes that code for enzymes catalysing the multiple chemical reaction steps leading to the final end product. Tryptophan biosynthetic pathway has five chemical reaction steps that are highly conserved in diverse microbial genomes, though the genes of the pathway enzymes show considerable variations in arrangements, operon structure (gene fusion and splitting) and regulation. We use a combined bioinformatic and statistical analyses approach to address the question if the pathway genes from different microbial genomes, belonging to a wide range of groups, show similar evolutionary relationships within and between them. Our analyses involved detailed study of gene organization (fusion/splitting events), base composition, relative synonymous codon usage pattern of the genes, gene expressivity, amino acid usage, etc. to assess inter- and intra-genic variations, between and within the pathway genes, in diverse group of microorganisms. We describe these genetic and genomic variations in the tryptophan pathway genes in different microorganisms to show the similarities across organisms, and compare the same genes across different organisms to find the possible variability arising possibly due to horizontal gene transfers. Such studies form the basis for moving from single gene evolution to pathway evolutionary studies that are important steps towards understanding the systems biology of intracellular pathways.

Kinetics of Direct Leaching of Natural Alunite in KOH

Alunite is a potential resource for production of alumina and potassium sulfate. Hydrometallurgy is the conventional process employed for this purpose and direct leaching in KOH is suitable as one of the process steps because it does not require prior calcination. In this paper dissolution kinetics of pure natural alunite in KOH is described. Kinetics of dissolution of alunite in potassium hydroxide follows the shrinking core model. The rate of reaction is controlled by surface chemical reaction step and the order of reaction with respect to KOH is 1.327. The activation energy of this reaction was estimated as 94.18 kJ/mol and a kinetic model for alunite dissolution in potassium hydroxide was obtained.

Theoretical Study of Gas Hydrate Decomposition Kinetics—Model Development

In order to provide an estimate of the order of magnitude of intrinsic gas hydrate dissolution and dissociation kinetics, the "Consecutive Desorption and Melting Model" (CDM) is developed by applying only theoretical considerations. The process of gas hydrate decomposition is assumed to comprise two consecutive and repetitive quasi chemical reaction steps. These are desorption of the guest molecule followed by local solid body melting. The individual kinetic steps are modeled according to the "Statistical Rate Theory of Interfacial Transport" and the Wilson-Frenkel approach. All missing required model parameters are directly linked to geometric considerations and a thermodynamic gas hydrate equilibrium model.

Multiphoton reaction of DTTCI observed by femtosecond pump–probe and two-pulse correlation measurements

To understand the elementary steps of chemical reactions, unimolecular reactions are important and considered to be model reactions. We carried out pump–probe and two-pulse correlation (2PC) measurements of the multiphoton-induced reaction of DTTCI (3,3′-diethyl-2,2′-thiatricarbocyanine iodide), a kind of cyanine dyes, under the red-tail excitation condition and found that both photoisomerization and photodegradation were caused by a two-photon process and the reaction time for photoisomerization was approximately 0.5ns. Excitation dynamics was examined by 2PC measurements and the temporal character of the intermediate state was analyzed.

Tracing the steps of photoinduced chemical reactions in organic molecules by coherent two-dimensional electronic spectroscopy using triggered exchange.

We establish coherent triggered-exchange two-dimensional (TE2D) electronic spectroscopy as an expansion of pump-repump-probe transient absorption spectroscopy and uniquely elucidate the role of higher-lying electronic states in ultrafast photochemistry. As an example, this is demonstrated for a molecular switch present in two ring-open conformations. The formation of a new species-the radical cation-is observed and its precursor state is identified via TE2D.

Forced bursting and transition mechanism in CO oxidation with three time scales

The mathematical model of CO oxidation with three time scales on platinum group metals is investigated, in which order gaps between the time scales related to external perturbation and the rates associated with different chemical reaction steps exist. Forced bursters, such as point—point type forced bursting and point—cycle type forced bursting, are presented. The bifurcation mechanism of forced bursting is novel, and the phenomenon where two different kinds of spiking states coexist in point—cycle type forced bursting has not been reported in previous work. A double-parameter bifurcation set of the fast subsystem is explored to reveal the transition mechanisms of different forced bursters with parameter variation.

QM/MM modelling of ketosteroid isomerase reactivity indicates that active site closure is integral to catalysis

Ketosteroid isomerase (Δ⁵-3-keto steroid isomerase or steroid Δ-isomerase) is a highly efficient enzyme at the centre of current debates on enzyme catalysis. We have modelled the reaction mechanism of the isomerization of 3-oxo-Δ⁵-steroids into their Δ⁴-conjugated isomers using high-level combined quantum mechanics/molecular mechanics (QM/MM) methods, and semi-empirical QM/MM molecular dynamics simulations. Energy profiles were obtained at various levels of QM theory (AM1, B3LYP and SCS-MP2). The high-level QM/MM profile is consistent with experimental data. QM/MM dynamics simulations indicate that active site closure and desolvation of the catalytic Asp38 occur before or during formation of dienolate intermediates. These changes have a significant effect on the reaction barrier. A low barrier to reaction is found only when the active site is closed, poising it for catalysis. This conformational change is thus integral to the whole process. The effects on the barrier are apparently largely due to changes in solvation. The combination of high-level QM/MM energy profiles and QM/MM dynamics simulation shows that the reaction involves active site closure, desolvation of the catalytic base, efficient isomerization and re-opening of the active site. These changes highlight the transition between the ligand binding/releasing form and the catalytic form of the enzyme. The results demonstrate that electrostatic interactions (as a consequence of pre-organization of the active site) are crucial for stabilization during the chemical reaction step, but closure of the active site is essential for efficient catalysis to occur.

Modeling of a continuous photocatalytic reactor for isovaleraldehyde oxidation: Effect of different operating parameters and chemical degradation pathway

An investigation of isovaleraldehyde (ISOV) photocatalytic oxidation was conducted at initial concentrations ranging from 25 to 150mg/m3 and different relative humidities (5–90% RH) in order to characterize the process performances close to indoor air conditions. Experiments were carried out in two different reactors: cylinder and flat-plate photoreactor (planar reactor) at different air gap (20–60mm) and gas residence times (0.67–5.0s). A plug flow reactor system was developed in order to perform kinetic studies of (i) isovaleraldehyde removal, (ii) selectivity of CO2, (iii) by-products formation and removal. It appears that ISOV removal efficiencies increased with lower inlet concentrations, lower air gap and higher gas residence times.In small amounts, the presence of water vapor has a promoting effect on the degradation due to the formation of OH radicals.Evaluating different kinetic models by least squares analysis, it was shown that the Langmuir-Hinshelwood (L-H) model could give a good correlation with the experimental results. Thus, the effect of ISOV gas-phase concentration, air gap and light intensity on chemical conversion rates is discussed.A kinetic model based on Langmuir-Hinshelwood (L-H) approach and taking into account the mass transfer step was developed. This allows us to determine L-H constants regardless of the transfer aspect. This last is estimated by semi-empirical correlation. The separation between the mass transfer and the chemical reaction steps is obtained. The effect of UV light intensity is also considered.

Alteration of Bentonite From Ughelli By Nitric Acid Activation: Kinetics And Physicochemical Properties

The physicochemical properties of Ughelli bentonite were modified by nitric acid activation to improve its performance. The bentonite mineral was reacted with different concentrations of nitric acid ranging from 2 to 16 mol.L -1 . The reactions were performed at different times and temperatures to study the process kinetics. The chemical composition and physical characteristics of the natural and treated samples were analyzed. To investigate the adsorptive performance of the modified samples, they were used to bleach soya bean oil. The surface area and adsorption capacity increased from 117.6 m 2 .g -1 to 364 m 2 .g -1 and 29 to 95 %, respectively, after activating with 14 mol.L -1 of HNO 3 . Kinetics studies showed that the activation process were controlled by the chemical reaction step and can be given by: 1 - (1 - X) 1/3 = k t ; where X is the fraction of the cations removed at time t . This study has proved that bentonite from Ughelli can be modified by nitric acid activation to increase its performance as adsorbent in the vegetable oil purification industry.

Quantum Chemical Study of the Initial Step of Ozone Addition to the Double Bond of Ethylene

The mechanisms of the initial step in chemical reaction between ozone and ethylene were studied by multireference perturbation theory methods (MRMP2, CASPT2, NEVPT2, and CIPT2) and density functional theory (OPW91, OPBE, and OTPSS functionals). Two possible reaction channels were considered: concerted addition through the symmetric transition state (Criegee mechanism) and stepwise addition by the biradical mechanism (DeMore mechanism). Predicted structures of intermediates and transition states, the energies of elementary steps, and activation barriers were reported. For the rate-determining steps of both mechanisms, the full geometry optimization of stationary points was performed at the CASPT2/cc-pVDZ theory level, and the potential energy surface profiles were constructed at the MRMP2/cc-pVTZ, NEVPT2/cc-pVDZ, and CIPT2/cc-pVDZ theory levels. The rate constants and their ratio for reaction channels calculated for both mechanisms demonstrate that the Criegee mechanism is predominant for this reaction. These results are also in agreement with the experimental data and previous computational results. The structure of DeMore prereactive complex is reported here for the first time at the CCSD(T)/cc-pVTZ and CASPT2/cc-pVDZ levels. Relative stability of the complexes and activation energies were refined by single-point energy calculations at the CCSD(T)-F12/VTZ-F12 level. The IR shifts of ozone bands due to formation of complexes are presented and discussed.