Specific Rate Constants Research Articles

Combined Experimental and Theoretical Studies of the O(3P) + 1-Butene Reaction Dynamics: Primary Products, Branching Fractions, and Role of Intersystem Crossing.

Information on the detailed mechanism and dynamics (primary products, branching fractions (BFs), and channel specific rate constants as a function of temperature) for many important combustion reactions of O(3P) with unsaturated hydrocarbons is still lacking. We report synergistic experimental/theoretical studies on the mechanism and dynamics of the O(3P) + 1-C4H8 (1-butene) reaction by combining crossed molecular beam (CMB) experiments with soft electron ionization mass-spectrometric detection and time-of-flight analysis at 10.5 kcal/mol collision energy (Ec) to high-level ab initio electronic structure calculations of the underlying triplet and singlet potential energy surfaces (PESs) and statistical Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) computations of BFs including intersystem crossing (ISC). The reactive interaction of O(3P) with 1-butene is found to mainly break apart the 4-carbon atom chain, leading to the radical product channels ethyl + vinoxy (BF = 0.34 ± 0.11), methyl + C3H5O (BF = 0.28 ± 0.09), formyl + propyl (BF = 0.17 ± 0.05), ethyl + acetyl (BF = 0.014 ± 0.007), and butanal radical (ethylvinoxy) + H (BF = 0.013 ± 0.004), and molecular product channels formaldehyde + propenylidene/propene (BF = 0.15 ± 0.05) and butenone (ethyl ketene) + H2 (BF = 0.037 ± 0.015). As some of these products can only be formed via ISC from triplet to singlet PESs, from BFs an extent of ISC of 50% is inferred. This value is significantly larger than that recently observed for O(3P) + propene (22%) at similar Ec, underlying the question of how important it is to consider nonadiabatic effects for these and similar combustion reactions. Comparison of the derived BFs with those of statistical (RRKM/ME) simulations on the ab initio coupled triplet/singlet PESs shows good agreement, warranting the use of the RRKM/ME approach to provide information on the variation of BFs with temperature and to derive channel specific rate constants as a function of temperature (T) and pressure (p). Notably, ISC is predicted to decrease strongly with increasing temperature (from about 70% at 300 K to 46% at Ec = 10.5 kcal/mol, and about 1% at 2000 K). The present results lead to a detailed understanding of the complex reaction mechanism of O(3P) + 1-butene and, by providing channel specific rate constants as a function of T and p, should facilitate the improvement of current fossil-fuel (1-butene) as well as biofuel (1-butanol) combustion models.

Evaluation of assays for screening polycyclic aromatic hydrocarbon-degrading potential of bacteria

Within a 30-day incubation laboratory study, the polycyclic aromatic hydrocarbon (PAH) degradation profile of two bacteria, Planomicrobium sp. RNP01 and Rhodococcus sp. RNP05 were studied by three microtiter plate assays to reveal the combination of certain biological and biochemical characteristics which are reliable indicators in evaluation of bacterial biodegradation abilities. The three assays, which are hydrocarbon growth assay, 2,6-DCPIP assay and dehydrogenase activity assay revealed that Rhodococcus sp. RNP05 exhibited better potential for PAH degradation than Planomicrobium sp. RNP01. Differences between initial and final optical density and specific growth rate constants were significantly higher (r = 0.995, P < 0.05) in case of Rhodococcus sp. RNP05 on all tested substrates, as compared to Planomicrobium sp. RNP01. This was confirmed by GC-FID analyses. Dehydrogenase activity of Rhodococcus sp. RNP05 was higher (r = 0.9995, P < 0.05) than Planomicrobium sp. RNP01 and correlated positively with the hydrocarbon growth assay (r = 0.999, P < 0.05, for Rhodococcus sp. RNP05, r = 0.986, P < 0.05 for Planomicrobium sp. RNP01). This study has shown that the combination of these assays could be used for determining the bioremediation potential of PAHs in petroleum contaminated soil with the ability of screening a large number of bacterial strains.

Cinética de la extracción sólido-líquido de antocianinas obtenidas a partir de Hibiscus rosa-sinensis

In this study the effect of temperature (25, 40 and 60 °C), solvent polarity using water (H2O)-ethanol (EtaOH) solvent with 100, 90:10 and 80:20 (H2O:EtaOH % v/v), and solvent-to-solid ratio (40/1, 60/1 and 80/1 mL/g) on the extraction yield of anthocyanins from Hibiscus rosa-sinensis was analyzed. The experimental data were fitted to Spiro and Siddique model with a good accuracy () and the specific extraction rate constant was perfectly related to the apparent diffusivity using the lumped parameter model. The major percentage of the anthocyanin extraction yield was 63.81±1.33, 68.95 ±1.53, and 83.09±3.14 when the used solvent was H2O, H2O-EtaOH (90:10), and H2O-EtaOH (80:20), respectively.  The diffusivity was affected mainly by temperature, tending to be more temperature sensitive as the solvent polarity decreased. Activation energy values (), in range from 19.64 to 29.28 kJ mol-1, allow to infer that the extraction of anthocyanins is favored when using an ethanolic aqueous solution as a solvent. Response surface methodology was successfully applied to the anthocyanin extraction yield, confirming that the process parameters (solvent polarity, solid-to-solvent ratio and temperature) affected the extraction yields.

Kinetics of carbon and nitrogen assimilation by heterotrophic microorganisms during wastewater treatment

The present study highlights microbial assimilation of carbon (glucose) and nitrogen (NH3-N) from wastewater using heterotrophic bioconversion process. Experiments were conducted to study the role of heterotrophic microbes towards removal of carbon and nitrogen at varying initial concentrations of carbon (COD, 533 to 1600mg/l) and nitrogen (NH3-N, 73 to 249mg/l) keeping the initial biomass of microorganisms constant. Removal of COD and ammonia from wastewater represented a first-order rate reaction, upon analysis of kinetics, indicating that the rate of reaction is dependent on the initial concentration of nutrients available. Rate equations were developed using the Monod model, and the maximum specific consumption rate (k4) and half saturation constant (Ks) values for NH3-N and COD were found to be 2.59mg/l/h and 64.13mg/l/h and 38.46mg/l and 2162.69mg/l, respectively. Assimilation of NH3-N followed the Freundlich isotherm model. The mass transfer coefficient for COD and NH3-N were found to be 0.13h-1 and 0.81h-1 respectively. The NH3-N is converted to N2O during nitrification, and observed values of N2O coincided with the empirically predicted values indicating the activity of heterotrophic nitrifiers. The regeneration/doubling time of heterotrophic microbial biomass varied from 26 to 121h. Statistical techniques, viz. analysis of variance, multi-linear regression analysis and principal component analysis, validated the results.

Improved CO2 Sorption Performance of Calcium Oxide (CaO) Sorbent with Nickel Oxide Additive

Currently, carbon capture and storage (CCS) technology appeared to be the most viable method to reduce carbon dioxide (CO2) emission from its existing sources. This paper reports the improved performance of calcium oxide (CaO)-based sorbents with the addition of nickel oxide (NiO) as an additive (denoted as CaO:NiO) in CO2 sorption process (CaO carbonation reaction). Pure CaO was used as benchmarked sorbent in this study. The samples were synthesized using the sol-gel method and CaO:NiO were evaluated for their performance in CaO carbonation at different sorption temperatures, from its CO2 sorption capacity, CO2 sorption rate, and kinetic performance. CaO:NiO has demonstrated an excellent CO2 sorption capacity of 0.79 mmol CO2/mmol CaO, meanwhile pure CaO exhibited lower CO2 sorption capacity of 0.61 mmol CO2/mmol CaO at 600 °C. During the chemical reaction controlled stage, the CO2 sorption rate of CaO:NiO was 0.78 mmol CO2/mmol CaO.min at 600 °C compared to pure CaO with sorption rate of 0.18 mmol CO2/mmol CaO.min. This study indicates that the addition of nickel oxide has successfully improved the CO2 sorption rate and capacity of CaO for CaO:NiO. Based on the kinetic study, it has been observed that the diffusion controlled stage has a greater resistance towards the CO2 sorption process (carbonation reaction) compared to rapid chemical reaction controlled stage. Experimental carbonation data for CaO:NiO fitted very well to the shrinking core model (SCM) with high correlation coefficient (R2 >0.936). The specific rate constants of rapid chemical reaction and diffusion-limited stages are directly proportional to the carbonation temperature. The activation energy required for CaO:NiO sorbent during the chemical reaction control stage was 31.81 kJ/mol.

KINETIC STUDY OF PALM OIL MILL EFFLUENT (POME) TREATMENT BY ACTIVATED SLUDGE

Kinetic parameter is a basis for design and optimization of activated sludge POME treatment. In fact, most of the kinetic parameter value used in design and optimization are default values taken from municipal wastewater. The kinetic parameters for POME treatment have not been thoroughlystudied and most of the system is using the Activated Sludge Models (ASM), either in modelling or design. Thus, the kinetic study of POME treatment by activated sludge system were carried out to obtain the kinetic parameters for the POME treatment plant design calculation.In this study, POME treatment was carriedout in batch studiesinto 14 L aeration tank with activated sludge for the biological oxidation process with optimum pH at 6.5 ± 0.1, MLVSS of 2000 ± 200 mg/Lfor HRT of 48 h and feeding with 650 ± 20 mg BOD3/L of anaerobic treated POME, while SRT was controlled at a range of 10 days to 20 days with interval of 2 days for the kinetic study experiment. From this kinetic study, the kinetic parameters for COD and BOD basis had been determined for maximum yields coefficient (Y), endogenous decay coefficient (kd), maximum specific substrate utilization rate (k) and half-velocity constant (Ks) at 0.2369 mg VSS/mg COD, 0.1060 day-1, 2.2717 day 1 and 758.7705 mg/L for COD basis whilst the kinetic parameters value for BOD basis were 0.6718 mg VSS/mg BOD3, 0.0658 day-1, 1.4136 day-1 and 556.1526 mg/L, respectively. Since the BOD is one of the discharge parameters in discharge regulatory, thus kinetic parameters for BOD basis will be used for the system design of POME treatment.

Mercury and methylmercury concentrations, sources and distribution in submarine canyon sediments (Capbreton, SW France): Implications for the net methylmercury production

Submarine canyons are important stocks of commercial interest fish, whose consumption is one of the main monomethymercury (MeHg) exposure to humans. Currently, biogeochemistry of mercury in those biologically productive system is unknown. In this work, inorganic mercury (Hg(II)) and organic mercury (MeHg) distributions were measured in sedimentary accumulative zones (slopes and terraces) against adjacent continental shelf sediments. Hg compound concentrations in these sediments show a huge range of concentrations (Hg(II) ranging from 18 to 973 ng g−1 and MeHg ranging from 0.07 to 2.03 ng g−1) exhibiting factors 50 and 20 fold, respectively. Higher values of mercury compounds were observed in canyon locations suggesting a high accumulation of mercury associated with higher values of clay fraction and organic matter content. The reactivity of mercury was investigated in sediment of three locations along Capbreton submarine canyon axis using slurry incubations experiments and isotopic tracers. Specific methylation and demethylation rate constants (kM and kD) were calculated. Results clearly showed that MeHg concentrations in these sediments are controlled by competing and simultaneous methylation and demethylation reactions mainly mediated by biotic process. Mercury reactivity was found higher in coastal stations compared to the offshore station due to more labile organic matter which may stimulate microbial activities. However, higher net MeHg production was estimated for the offshore station due to high Hg(II) concentrations suggesting a potential MeHg source for such marine environments.

A step-forward in the characterization of microalgal consortia: Microbiological and kinetic aspects

The characterization of a non-axenic microalgal culture was performed, including microbiological (massive sequencing of 18S and 16S rRNA) and kinetic aspects (maximum specific CO2 uptake rate, qmax, and half-saturation constant, KS). An innovative setup for the retrieval of kinetic parameters without CO2 limiting conditions was developed. The microbiological characterization showed that Acutodesmus deserticola was by far the predominant microalgae with a relative abundance of over 99.9%, while the following abundance order was observed for bacterial phyla: Proteobacteria> Bacteroidetes> Firmicutes > Chlorobi > Ignavibacteriae > Chloroflexi. The qmax and KS values were in the range of 0.027 – 0.038 gCO2 gVSS−1 h−1 and 4.0–18.2 gCO2 m-3, respectively, for irradiances from 45 to 140 μmol m−2 s−1. While increases in irradiance did not impact significantly the qmax values, a 3.1-fold increase in irradiance mediated a 4.5-fold decrease in KS. This suggested that the activation of the photosynthetic system driven by irradiance increases can be quantified by changes in KS.

Kinetic Analysis of Hepatic Metabolism Using Hyperpolarized Dihydroxyacetone.

Hyperpolarized carbon-13 magnetic resonance (HP-MR) is a new metabolic imaging method the does not use ionizing radiation. Due to the inherent chemical specificity of MR, not only tracer uptake but also downstream metabolism of the agent is detected in a straightforward manner. HP [2-13C] dihydroxyacetone (DHA) is a promising new agent that directly interrogates hepatic glucose metabolism. DHA has three metabolic fates in the liver: glucose production, glycerol production and potential inclusion into triglycerides, and oxidation in the tricarboxylic acid cycle. Each pathway is regulated by flux through multiple enzymes. Using Duhamel's formula, the kinetics of DHA metabolism is modeled, resulting in estimates of specific reaction rate constants. The multiple enzymatic steps that control DHA metabolism make more simplified methods for extracting kinetic data less than satisfactory. The described modeling paradigm effectively identifies changes in metabolism between gluconeogenic and glycogenolytic models of hepatic function.

Dynamics studies of O<sup>+</sup> + H<sub>2</sub>→ OH<sup>+</sup> + H reaction

In the present work, the long-range interaction potential part of potential energy surface (PES) of OH<sub>2</sub><sup>+</sup> system is revised and the new resulting PES apparently is more reasonable than the old one in the long-range part. Based on the new PES, the dynamics calculations of O<sup>+</sup> +H<sub>2</sub>→ OH<sup>+</sup> + H reaction are carried out at a state-to-state level of theory by using time-dependent quantum wave packet method with second order split operator in a collision energy range from 0.01 to 1.0 eV. The dynamic properties such as reaction probability, ro-vibrational resolved statereaction probability, integral cross section, differential cross section, and state specific rate constant are calculated and compared with available theoretical and experimental results. The results of ro-vibrational resolved state reaction probability reflect some dynamic properties such as resonances which is attributed to the deep well located on the reaction path. The vibrational resolved state reaction probability indicates that the excitation efficiency of the OH<sup>+</sup> product is relatively low. The results of integral cross sections indicate that the present results are in better agreement with the experimental values than with previous theoretical calculations, especially in the low collision energy region. However, the state specific rate constant results underestimate the experimental values. The comparison betweenour calculations and the experimental results indicates that the contribution of the rotational excitation of H<sub>2</sub> molecule should be included in the calculations. However, only the initial state <i>v</i> = 0, <i>j</i> = 0 is calculated in the present work. We suppose that the deviation of the present results from the experimental data is due to the fact that the rotational excitation of reactant isnot included in the present calculation. The differential cross section signals indicate that the complex-forming reaction mechanism isdominated in the case of low collision energy, but it transforms into abstract reaction mechanism as the collision energy further increases.

Effect of energy content of the nitraminic plastic bonded explosives on their performance and sensitivity characteristics

Abstract Information about the forty nine nitraminic plastic bonded explosives (PBXs) and different nitramines were collected. Fillers of these PBXs are nitramines 1,3,5-trinitro-1,3,5-triazinane (RDX) and β-1,3,5,7-tetranitro-1,3,5-tetrazocane (β-HMX), cis-1,3,4,6-tetranitro-octahydroimidazo-[4,5-d]imidazole (bicyclo-HMX, BCHMX) and e-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (e-HNIW, CL-20) which are bonded by polyfluoro-elastomers, polydimethyl-siloxane, poly-glycidyl azide, polyisobutylene, polystyrene-butadiene, poly-acrylonitrile-butadiene and hydroxyl-terminated polybutadiene in addition to a melt cast compositions based on 2,4,6-trinitrotoluene. For thirty two of these PBXs the relationships are specified and analyzed between heats of their combustion and relative explosive strengths; by means of these relationships it might be possible to estimate, which groupings in the macromolecule of binder could be liable to their primary fission in the PBXs initiation. Similarly, for forty two of these explosives, the relationships are described and analyzed between their enthalpies of formation and impact sensitivities; here is especially attention paid to PBXs filled by BCHMX. Specific rate constants from Vacuum Stability Test (VST) of four nitramines and twenty PBXs are introduced into relationships with their enthalpies of formation. Regarding to all the mentioned cases, increasing of energy content of the studied explosives leads to increase of the relative explosive strength or initiation reactivity, respectively. Exception with the opposite trend, the outputs of VST are for BCHMX, where in PBXs are matrices with the esteric plasticizers or the energetic poly-glycidyl azide. Admixture of RDX or HMX, respectively, into the BCHX PBXs gives ternary PBXs whose thermal stability, in the sense of applied VST, is higher comparing to the original binary explosives.

Quantum mechanical study of the 16O + 18O18O → 16O18O + 18O exchange reaction: Integral cross sections and rate constants.

The isotopic exchange reaction, 16O + 18O18O → 16O18O + 18O, involving excited ozone, , as intermediate complex, was investigated by means of a time independent quantum mechanical approach using the TKTHS potential energy surface (PES) [V. G. Tyuterev et al., J. Chem. Phys. 139, 134307 (2013)] of ozone. State-to-state integral cross sections were calculated for collision energies lower than 0.4 eV. Then specific and thermal rate constants were computed between 10 K and 350 K using these cross sections. The full quantum thermal rate constant is found to be in better agreement with the most recent experimental data than with previous studies where other O3 PESs were employed, confirming therefore the higher accuracy of the TKTHS PES. However, the present theoretical thermal rate constant still remains below the measured rate, maybe due to the neglect of non-adiabtic couplings.

Impact of Phosphate on Ferrate Oxidation of Organic Compounds: An Underestimated Oxidant.

Ferrate (K2FeO4) is a powerful oxidant and up to 3 mol of electrons could be captured by 1 mol of ferrate in the theoretical conversion of Fe(VI)-Fe(V)-Fe(IV)-Fe(III). However, it is reported that the utilization efficiency of the ferrate oxidation capacity is quite low because of the rapid autodecomposition of intermediate iron species, which negatively influences the potential of ferrate on organic pollutants control. We accidentally found that for the ferrate oxidation of carbamazepine (CBZ), bisphenol S (BPS), diclofenac (DCF), and ciprofloxacin (CIP), the determined reaction rate constants were 1.7-2.4 times lower in phosphate buffer than those in borate buffer at pH 8.0. For the reaction of ferrate with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) at pH 7.0, the determined reaction stoichiometries were 1:1.04 in 100 mM phosphate buffer, 1:1.18 in 10 mM phosphate buffer, and 1:1.93 in 10 mM borate buffer, respectively. The oxidation ability of ferrate seems depressed in phosphate buffer. A kinetic model involving the oxidation of ABTS by Fe(VI), Fe(V) and Fe(IV) species was developed and fitted the ABTS•+ formation kinetics well under different buffer conditions. The results showed that phosphate exhibited little influence on the oxidation ability of Fe(VI) and Fe(IV) species, but decreased the specific rate constants of ABTS with Fe(V) species by 1-2 orders of magnitude, resulting in the outcompeting of Fe(V) autodecomposition pathway. The complexation between phosphate anions and Fe(V) species may account for the inhibition effect of phosphate buffer. Considering that many studies regarding ferrate oxidation were carried out in phosphate buffer, the actual oxidation ability of ferrate may be underestimated.

Direct measurements of channel specific rate constants in OH + C3H8 illuminates prompt dissociations of propyl radicals

OH + molecules are an important class of reactions in combustion and atmospheric chemistry. Consequently, numerous studies have measured rate constants for these processes over an extended temperature range. A large majority of these experimental studies have utilized the decay of [OH] profiles (monitored either by absorption or laser-induced fluorescence) to obtain total rate constants. However, there are limited direct measurements of channel specific rate constants in this important class of reactions, particularly at combustion relevant temperatures. In the present experiments, we have directly measured site-specific rate constants for abstraction of the secondary CH bond in OH + C3H8 at high temperatures. Atomic resonance absorption spectrometry (ARAS) was used to monitor the formation of H-atoms from shock-heated mixtures of tert-butylhydroperoxide and C3H8 at high temperatures. Simulations for the experimental H-atom profiles are sensitive only to abstraction of the secondary CH bond leading to unambiguous measurements of the rate constants for this reaction. Over the T-range, 921 K < T < 1146 K, rate constants from the present experiments for OH + C3H8 → H2O + i-C3H7 can be represented by the Arrhenius expression,k=(3.935±1.387)×10−11exp(−1681±362K/T)cm3molecule−1s−1Simulations of the lower temperature data (T < 1000 K) indicate that the H-atom profiles are also influenced to a minor extent by the thermal dissociation of iso-propyl, i-C3H7 → H + C3H6, at short time-scales. Direct dynamics calculations were performed to examine in greater detail the potential role of prompt dissociations of i-C3H7 and n-C3H7 (formed from the title reaction) in interpreting the lower temperature (< 1000 K) data from the present work. These simulations suggest that prompt dissociation of propyl radicals does not influence the present experimental observations but has a minor influence on higher temperature combustion simulations.

Degradation of trinitrotoluene by transgenic nitroreductase in Arabidopsis plants

The explosive 2,4,6-trinitrotoluene (TNT) is a highly toxic and persistent environmental pollutant. TNT is toxic to many organisms, it is known to be a potential human carcinogen, and is persistent in the environment. This study presents a system of phytoremediation by Arabidopsis plants developed on the basis of overexpression of NAD(P)H-flavin nitroreductase (NFSB) from the Sulfurimonas denitrificans DSM1251. The resulting transgenic Arabidopsis plants demonstrated significantly enhanced TNT tolerance and a strikingly higher capacity to remove TNT from their media. The highest specific rate constant of TNT disappearance rate was 1.219 and 2.297 mL/g fresh weight/h for wild type and transgenic plants, respectively. Meanwhile, the nitroreductase activity in transgenic plant was higher than wild type plant. All this indicates that transgenic plants show significantly enhanced tolerances to TNT; transgenic plants also exhibit strikingly higher capabilities of removing TNT from their media and high efficiencies of transformation.

Mathematical modelling of flat plate biofilm photobioreactors with circular and rectangular configurations

Flat plate photobioreactors (FPPBRs) using bacterial biofilm have gained much recent attention due to operational ease, improved light conversion efficiency and reduction of process cost, particularly in hydrogen production. In this study, two comprehensive mathematical models, one explaining the dynamics of a batch type FPPBR used for the development of biofilm and the other a deterministic model (both temporal and spatial) to predict the performance of a continuous FPPBR using Rhodopseudomonas sp. have been developed for both circular and rectangular configurations. The system equations have been solved using MATLAB 2013. From batch studies, the maximum specific growth rate and half saturation constant for the microorganism have been determined to be 0.07 h−1 and 1.946 g l−1 respectively. An “Instantaneous attachment and proliferation” mechanism has been proposed to explain the behaviour of biofilm right from the early stage of attachment to the reversal from attached to planktonic state. The flow patterns of substrate medium through the biofilm have been generated using COMSOL Multiphysics software. From the perspective of the hydrogen yield, the models predict that the FPPBR geometry plays a crucial role by demonstrating the superior performance of the circular reactor in comparison to the rectangular counterpart. It is expected that the mathematical models developed here will help in the design, scale-up and control of FPPBRs to be used particularly for hydrogen production using suitable microorganisms.

Thermo-analytical study of a melt cast composition based on cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5 d]imidazole (BCHMX)/trinitrotoluene (TNT) compared with traditional compositions

The thermal behavior and decomposition kinetics of a melt cast composition based on mixture of bicyclic nitramine, cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole (BCHMX) and insensitive explosive, 2,4,6-trinitrotoluene (TNT) (60/40 by wt %) have been investigated. This behavior was studied by non-isothermal TG and DSC techniques and the decomposition kinetics were determined by the isoconversional methods named Kissinger and modified Kissinger-Akahira-Sunose (KAS). For comparison, the traditional TNT casted compositions based on 1,3,5-trinitro-1,3,5-triazinane (RDX) named composition B and β-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (β-HMX) were studied. Also Vacuum Stability Test (VST) was used isothermally at 120 °C for six hours to determine the specific rate constants. Despite of the volatility of TNT, the obtained results showed that the reactivity of the studied cyclic nitramines in mixtures was increased in the order HMX < BCHMX < RDX, because of their incorporation into the melted TNT (acidic solvent). The kinetic compensation effect and outputs of the Vacuum Stability Test clearly exemplify this reactivity. Nevertheless, taking into consideration the recently published facts about this composition and its application in practice, the mixture is a promising composition which has high relative explosive strength compared with the other studied compositions.

Predictions of permeability, surface area and average dissolution rate during reactive transport in multi-mineral rocks

The interactions between minerals and reactive fluids have great impact on the pore structure and hydrologic properties of rocks. A 3D pore-scale reactive transport model is applied to predict multi-mineral reactions in porous rocks. Eight mineral phases are distinguished in 2D slices of a reservoir sandstone by using Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN) imaging technique. The 2D image is then registered into the corresponding cross-section of the 3D rock image obtained from micro-computed tomography (CT) scanning. Reactive transport with chemical dissolution is simulated directly on the 3D image of the sandstone. In the simulations, each mineral is assigned with a specific intrinsic reaction rate constant. The dissolution-induced changes in the pore structure are measured. The pore size distribution of the rock is analysed to investigate the evolution of the pore space. The changes of surface area and permeability are compared with single mineral simulations to illustrate the importance of mineralogical heterogeneity on reactive transport. The results show that the 3D pore-scale mineralogical heterogeneity causes overestimation of the permeability variations during reactive transport. The average effective dissolution rates for the rock are also measured. The simulations with single mineral assumptions present an overall reaction rate which is 10 times higher than the predictions with multi-mineral dissolution at the start of reactions. Relationships between average reaction rate and time are proposed to characterise the evolution of dissolution rate. A correction is also suggested to reduce the errors in predictions of reaction rates with single mineral assumptions.

Transcription factor target site search and gene regulation in a background of unspecific binding sites

Response time and transcription level are vital parameters of gene regulation. They depend on how fast transcription factors (TFs) find and how efficient they occupy their specific target sites. It is well known that target site search is accelerated by TF binding to and sliding along unspecific DNA and that unspecific associations alter the occupation frequency of a gene. However, whether target site search time and occupation frequency can be optimized simultaneously is mostly unclear. We developed a transparent and intuitively accessible state-based formalism to calculate search times to target sites on and occupation frequencies of promoters of arbitrary state structure. Our formalism is based on dissociation rate constants experimentally accessible in live cell experiments. To demonstrate our approach, we consider promoters activated by a single TF, by two coactivators or in the presence of a competitive inhibitor. We find that target site search time and promoter occupancy differentially vary with the unspecific dissociation rate constant. Both parameters can be harmonized by adjusting the specific dissociation rate constant of the TF. However, while measured DNA residence times of various eukaryotic TFs correspond to a fast search time, the occupation frequencies of target sites are generally low. Cells might tolerate low target site occupancies as they enable timely gene regulation in response to a changing environment.

Methanogenic degradation of tetramethylammonium hydroxide by Methanomethylovorans and Methanosarcina

This study evaluated the methanogens responsible for methanogenic degradation of tetramethylammonium hydroxide (TMAH) in a continuous flow bioreactor. The enriched methanogens attained an estimated maximum specific TMAH degradation rate and half-saturation constant of 39.5 mg TMAH/gVSS/h and 820 mg/L, following the Monod-type kinetic expression for methanogenic TMAH degradation. Presence of sulfide more than 20 mg/L significantly extended lag period and slowed down specific TMAH degradation rates. The results of terminal restriction fragment length polymorphism (T-RFLP), cloning/sequencing, and quantitative real-time PCR analyses targeting on the methyl coenzyme M reductase alpha subunit (mcrA) genes retrieved from the bioreactor and batch experiments indicated that Methanomethylovorans species were the dominant methanogens responsible for methanogenic degradation of TMAH. The isolated TMAH-degrading methanogen from the bioreactor, however, was identified closely related to Methanosarcina mazei. It is likely that a very low TMAH environment in the bioreactor favored the growth of Methanomethylovorans hollandica, while the much higher TMAH in the isolation growth medium proliferated Methanosarcina mazei.