Values Of Kinetic Rate Constants Research Articles

Ag2O-adorned ZnO nanostructures: cooperative and sustainable nanomaterial system for effective reduction and mineralization of hazardous water pollutants

Organic water pollutants like nitroaromatics and synthetic dyes are causing serious threats to water. Ever-growing urban and industrial activities along with population explosion are rapidly contributing severe level of water contamination. Semiconducting nanomaterial-based photocatalysis has been proven to be an effective process for degradation of organic water pollutants. In the current study, visible light active Ag2O-adorned ZnO nanostructures were fabricated by a simple two-step hydrothermal method and the prepared nanostructures were utilized for the photocatalytic mineralization of rhodamine B (RhB) dye with visible light radiation. The catalytic potential of as-synthesized nanostructures was also investigated for the reduction of nitroaromatics (4-NP and 4-NA) and RhB dye in the presence of NaBH4. The Ag2O-adorned ZnO nanostructures prepared with 5% of silver nitrate denoted as ZnO/Ag2O (5%) demonstrated stupendous photomineralization activity against RhB dye as almost 100% degradation of RhB dye was achieved within 100min of reaction time at pH = 6. The kinetic study revealed that the degradation reaction followed the pseudo-first-order kinetics and the kinetic rate constant (k) of photodecolorization reaction for optimal catalyst was calculated to be 61.4 × 10-3min-1. The nanostructures revealed excellent recyclability and photostability as 95% activity of the catalyst was preserved even after the fifth cyclic run. The catalytic reduction of the 4-NP, 4-NA, and RhB dye was completed in 21, 12, and 40min, respectively, in the presence of ZnO/Ag2O (5%) and NaBH4 solution. The kinetic rate constant values for the reduction reactions were determined to be 229.6 × 10-3, 454.2 × 10-3, and 105.5 × 10-3min-1 for 4-NP, 4-NA, and RhB dye, respectively. Thus, the obtained results suggest that the components of the prepared nanosystem help in mutually strengthening the catalytic and photocatalytic abilities of each other, indicating the development of a cooperative and sustainable nanomaterial system in the current study.

Biogenic Pd-nanoparticles from Lantana trifolia seeds extract: Synthesis, characterization, and catalytic reduction of textile dyes

In the present work, the microwave-assisted biogenic synthesis of Palladium Nanoparticles (PdNPs) employing different concentrations of Lantana trifolia seeds extract as a reducing and stabilizing agent in accordance with the concept of environmentally friendly approach. The synthesized PdNPs were characterized using several analytical techniques to examine their morphology, microstructure, crystallinity, optical, and surface functional modification properties. The formation of PdNPs in the reaction solution was evidenced by a visible black color and broad continuous absorption spectra in the UV–Vis absorption spectra. The FTIR spectra were revealed that the phytochemicals were interacted with Pd metal in nanoparticle formation. The prepared PdNPs have a spherical shape and polydispersity nature with an average size of particles is 10 ± 2 nm. In addition, the synthesized PdNPs were well stabilized (zeta potential = −25.5 ± 2.8 mV) by the phytochemicals in seed extract and a face-centered cubic crystal (FCC) structure. The biosynthesized PdNPs were further investigated as a nanocatalyst in the catalytic reduction of Crystal Violet (CV) and Methyl Orange (MO) dyes in the presence of NaBH4 at room temperature. For CV and MO dyes reduction, the homogenous catalytic performance of PdNPs resulted in practically > 98 % of reduction with kinetic rate constant values of 0.393 min−1 (10 min) and 0.232 min−1 (18 min), respectively. Moreover, there is no significant loss in catalytic activity was observed after recovery and reusability of the biosynthesized PdNPs.

Exploring solution ICAR ATRP of methyl acrylate with AIBN as azo initiator: a kinetic modeling and simulation study

Supplementary techniques to atom transfer radical polymerization (ATRP) have been studied to reduce the concentration of catalytic complexes used, aiming at a less costly and more environmentally friendly process for synthesizing polymeric materials. Initiators for continuous activator regeneration ATRP (ICAR ATRP) is one technique in which a source of primary free radicals allows the reactivation of catalytic species used in the conventional ATRP. Due to the technological relevance of the topic, this work aims to expand the contribution to literature with kinetic modeling and simulation study for an ICAR ATRP system not yet explored. For this purpose, it was investigated solution ICAR ATRP of methyl acrylate with copper-based catalysts and azobisisobutyronitrile as ICAR initiator. The method of moments was employed to derive material balance equations. We find that the kinetic mechanism that best represented the trends of experimental data from literature was the one that considered specific side reactions of acrylates, such as backbiting and radical catalytic termination. In the model fitting procedure, a nonlinear optimization process had to be developed to estimate the values of kinetic rate constants not yet reported in the literature. The values obtained for the missing kinetic rate constants are consistent with similar experimental systems found in the literature. The deterministic model obtained was then employed to study the concentration profile maps. Simulations allowed verifying the slow release of primary free radicals from ICAR initiator to reduce the deactivators in activators species continuously. In addition, it was possible to confirm that the ICAR ATRP can be thermoregulated. Parametric analyses showed that variations in kinetic rate constants and reaction stoichiometry can shape monomer conversion and the molecular properties of the polymer (e.g., molecular weight, dispersity, and end functionality).

A numerical approach for detecting switch-like bistability in mass action chemical reaction networks with conservation laws

BackgroundTheoretical analysis of signaling pathways can provide a substantial amount of insight into their function. One particular area of research considers signaling pathways capable of assuming two or more stable states given the same amount of signaling ligand. This phenomenon of bistability can give rise to switch-like behavior, a mechanism that governs cellular decision making. Investigation of whether or not a signaling pathway can confer bistability and switch-like behavior, without knowledge of specific kinetic rate constant values, is a mathematically challenging problem. Recently a technique based on optimization has been introduced, which is capable of finding example parameter values that confer switch-like behavior for a given pathway. Although this approach has made it possible to analyze moderately sized pathways, it is limited to reaction networks that presume a uniterminal structure. It is this limited structure we address by developing a general technique that applies to any mass action reaction network with conservation laws.ResultsIn this paper we developed a generalized method for detecting switch-like bistable behavior in any mass action reaction network with conservation laws. The method involves (1) construction of a constrained optimization problem using the determinant of the Jacobian of the underlying rate equations, (2) minimization of the objective function to search for conditions resulting in a zero eigenvalue, (3) computation of a confidence level that describes if the global minimum has been found and (4) evaluation of optimization values, using either numerical continuation or directly simulating the ODE system, to verify that a bistability region exists. The generalized method has been tested on three motifs known to be capable of bistability.ConclusionsWe have developed a variation of an optimization-based method for the discovery of bistability, which is not limited to uniterminal chemical reaction networks. Successful completion of the method provides an S-shaped bifurcation diagram, which indicates that the network acts as a bistable switch for the given optimization parameters.

Modelling of the “surface explosion” of the $$\hbox {NO}+\hbox {H}_{2}$$ reaction over supported catalysts

We propose and numerically solve a phenomenological model for reduction of nitric oxide NO by hydrogen $$\hbox {H}_2$$ over supported catalysts. The model is based on the coupled system of PDEs with nonclassic conjugate conditions at the catalyst–support interface and includes the adsorption and desorption of particles of both reactants, surface diffusion of adsorbed molecules, and their surface reaction. We study the influence of the surface diffusivity, the particle jumping rate constants via the catalyst–support interface, and the concentration of reactants on the catalytic reactivity of the catalyst. We find that under specific values of kinetic rate constants, the turnover frequency of molecules of both reactants into $$\hbox {H}_2$$ O can possess one, two, or three peaks, whereas the turnover frequency of the reactant particles into $$\hbox {N}_2$$ , $$\hbox {N}_2$$ O, and $$\hbox {NH}_3$$ can have one or two peaks. We also give the other model based on the coupled system of ODEs. We show that turnover frequencies obtained by both models differ slightly only for large values of the surface diffusivity.

Modelling carbon monoxide conversion into methanol over supported catalysts

A deterministic mean field model for methanol, CH3OH, synthesis from CO and H2 over supported catalysts is proposed and solved numerically applying the finite difference method. The model is composed of a coupled system of PDEs subject to nonclassical conjugate conditions at the catalyst–support interface and includes the bulk diffusion of CO and H2 to and CH3OH from the surface of a bounded compartment. It also includes the adsorption and desorption of particles of both reactants and the surface diffusion of the adsorbed molecules and all intermediate reaction products. The influence of the initial concentration of H2, reactant adsorption and desorption rate constants, particle jumping rate constants via the catalyst–support interface, and reaction rate constants on the evolution of the catalytic reactivity of the supported catalyst is investigated. It is shown that under specific values of kinetic rate constants the turnover rate of CO and H2 into methanol can possesses one, two, or three peaks. The mechanism and conditions for arising of the second local maximum is discussed.

Immobilization of biosorbent in hydrogel as a new environmentally friendly fertilizer for micronutrients delivery

Abstract The aim of this work was to study the use of alginate-carboxymethyl cellulose-eggshell biocomposites as a material with controlled release of Cu2+ ions. The composition of biocomposites was optimized with Response Surface Methodology (5.0 wt% sodium alginate, 1.2 wt% carboxymethyl cellulose, 10.0 wt% eggshells). Physicochemical properties of biocomposites, kinetics and equilibrium of copper(II) ion sorption on structures were investigated. The General Rate Law model best described the kinetics data and the values of kinetic rate constants were 0.259, 0.358 min −1 for wet and dry capsules. The description of the sorption kinetics was also presented with the Weber-Morris diffusion model, which indicated a multi-stage process. The maximum sorption capacities for wet and dry biocomposites were 281 and 49 mg g−1. The composites presented a slow release of ions in the model soil solution (9 wt%/14 days). Studied biocomposites have a good potential as controlled release fertilizers fitting into the concept of sustainable development and precision agriculture. The new fertilizer formulations have the potential to by used as a new type of slow-release fertilizer that delivers micronutrient ions according to the plants uptake pattern with minimization of losses into the environment.

Hydrogel capsules with alfalfa as micronutrients carrier

The aim of this work was to study the properties of biocomposite based on sodium alginate and carboxymethyl cellulose with immobilized alfalfa as a material with controlled release of Cu2+ ions. Physicochemical (swelling, pHZPC) and sorption (kinetics, equilibrium) properties of the produced structures were investigated. Kinetic data were best described using a general rate law model, and the value of kinetic rate constant was 5.34 × 10−6. The sorption kinetics are also described using the Weber–Morris model, and the analysis revealed a three-step sorption. The maximum sorption capacity was determined using the Sips model and amounted to 9 mg g−1. Desorption of Cu2+ ions in soil solution (NaNO3 1 wt%) was carried out, and biocomposites released micronutrients in a slow way. The results showed that biocomposites with immobilized alfalfa are a very good material for potential applications in the agrochemical industry as a controlled released fertilizer.

In Vitro Ligand Binding Kinetics Explains the Pharmacokinetics of [18F]FE-PE2I in Dopamine Transporter PET Imaging.

Two of the most popular positron emission tomography (PET) tracers, [11C]PE2I and [18F]FE-PE2I, used to quantify dopamine transporters (DAT), display dissimilar kinetic behavior in in vivo assays. This difference can be explained by comparing values of kinetic rate constants, which characterize interaction of these tracers with DAT sites in vitro. At the same time, this kinetic analysis showed that the overall binding mechanism is similar for these two tracers and includes a fast step of complex formation followed by a slow isomerization step of this complex. Comparison with previous PE2I data revealed that isomerization of the DAT complex with PE2I occurs three times faster than in the case of FE-PE2I, which leads to the slower onset of peak specific binding of the former tracer in the DAT-rich regions. Therefore, ligands with slower isomerization on-rate, including [18F]FE-PE2I, seem to be better tracers in vivo, and their properties can be predicted in vitro.

Removal of benzidine based textile dye using different metal hydroxides generated in situ electrochemical treatment-A comparative study

Benzidine is used as an intermediate compound for the production of many types of dyes and it is found to be carcinogenic to human. The use of benzidine dyes are significantly withdrawn in most of the industries, but still it is used as colorant to dye the fabrics. The present study focuses on the treatment efficiency of electrochemical process in the removal of benzidine based triazo dye namely C.I. Direct Green 1 (DG1). Various key process parameters are employed to analyses the efficiency of ferrous and aluminum hydroxides on removal of colour is observed as 99.10%, chemical oxygen demand (COD) of dye solution observed as 75.71 and 89.65% for stainless steel (SS) and aluminum (Al) electrodes. The process efficiency is analyzed by observing the variation of pseudo first order kinetic rate constant values of 0.115 and 0.256 min−1, energy and electrode consumptions are found as 1.50 kWh/kg dye, 0.75 kWh/kg dye and 0.31 kg Fe/kg, 0.06 kg Al/kg dye for SS and Al electrodes respectively. The dye removal mechanism with metal hydroxides is observed as the partial degradation of azo bonds and followed by sweep coagulation with iron hydroxides/oxyhydroxides and charge neutralization with aluminum hydroxides/oxyhydroxides are supported by UV–visible, Fourier Transform Infrared (FTIR) and Scanning Electron Microscope and Energy Dispersive X-Ray (SEM-EDX) spectroscopy analysis of treated water and electrocoagulated flocs. Finally, the cost of electrocoagulation is evaluated for treatment of DG1 and it is identified to be 1.89$/kg dye for SS electrodes and 0.78$/kg dye for Al electrodes.

Photocatalytic degradation and photo-Fenton oxidation of Congo red dye pollutants in water using natural chromite\u2014response surface optimization

Refined natural Fe-chromite was characterized by XRD, FT-IR, reflected polarized microscope, XRF and UV spectrophotometer. Photocatalytic degradation and photo-Fenton oxidation of Congo red dye by Fe-chromite was investigated using 1 mL H2O2. The degradation of dye was studied as a function of illumination time, chromite mass, initial dye concentration, and pH. Fe-chromite acts as binary oxide system from chromium oxide and ferrous oxide. Thus, it exhibits photocatalytic properties under UV illumination and photo-Fenton oxidation after addition of H2O2. The degradation in the presence of H2O2 reached the equilibrium stage after 8 h (59.4%) but in the absence of H2O2 continued to 12 h (54.6%). Photocatalytic degradation results fitted well with zero, first order and second order kinetic model but it represented by second order rather than by the other models. While the photo-Fenton oxidation show medium fitting with the second order kinetic model only. The values of kinetic rate constants for the photo-Fenton oxidation were greater than those for the photocatalytic degradation. Thus, degradation of Congo red dye using chromite as catalyst is more efficient by photo-Fenton oxidation. Based on the response surface analysis, the predicted optimal conditions for maximum removal of Congo red dye by photocatalytic degradation (100%) were 12 mg/l, 0.14 g, 3, and 11 h for dye concentration, chromite mass, pH, and illumination time, respectively. Moreover, the optimum condition for photo-Fenton oxidation of dye (100%) is 13.5 mg/l, 0.10 g, 4, and 10 h, respectively.

Vaporization kinetic study of lavender and sage essential oils

The thermal behavior of lavender and sage essential oils (EOs) and also that of their main components (camphor and 1,8-cineole) were studied by thermogravimetry (TG) at different scan rates under inert gas atmosphere up to about 140 °C. All the samples investigated undergo a single-step evaporation starting from ambient temperature without evidence of side decomposition processes. On the basis of the onset temperatures (T on) extrapolated from the TG curves, the following increasing stability trend is assessed: $$1{,}8{\text{-cineole}} < {\text{sage}} < {\text{lavender}} < {\text{camphor}}.$$ However, the difference of T on values among the first three samples was found to be slight. To the aim of assessing a thermal stability scale among the samples, a kinetic analysis of evaporation was performed. A first approach was based on a simple model-fitting kinetic method, widely used in the past for studying evaporation of EOs. The results were compared with those obtained with a more reliable approach based on the integral isoconversional method by Kissinger–Akahira–Sunose in which the activation energy (E a) versus reaction extent (α) plot of each sample was obtained. Because of the compensation effect, neither activation energy nor the pre-exponential factor, alone, could be considered to compare the stabilities. Lastly, the Arrhenius kinetic rate constant values at given temperatures within the experimental range were calculated, and camphor was found to be the more stable component, while lavender seems to vaporize faster than any other sample tested.

Bioaccumulation—Depuration kinetics and effects of phenanthrene on Mediterranean mussel (Mytilus galloprovincialis)

In this study, Mediterranean mussel species, Mytilus galloprovincialis, were exposed to phenanthrene (PHE) due to its ubiquitousness and bioavailability in the aquatic environment. Kinetic parameters of the PHE bioaccumulation and depuration were calculated for an 11-day uptake and subsequent 11-day depuration periods. Those values of kinetic rate constants for uptake and depuration of PHE were calculated as varying between 93–177 and 0.18–0.25, respectively. Negative correlation was observed between the PHE concentrations and kinetic rate constants. Bioaccumulation Factors (BAFs) for the mussels exposed to different PHE concentrations were calculated from both kinetic rate constants and from the experimental data and found between 509–701 and 441–658, respectively. Experimental quantitative structure activity relationship (QSAR) values for PHE were found comparable to the calculated QSARs reported in literature. Two sub-lethal biomarker methods (filtration rate and lysosomal stability) were applied to observe the responses of the mussels throughout the experimental period. Results of both biomarkers show that PHE negatively affects mussels during the uptake of PHE, and mussels were able to recover physiologically at the end of depuration period according to the filtration rate results.

Isomeric C5H11Si+ ions from the trimethylsilylation of acetylene: An experimental and theoretical study

The gas phase offers a unique medium to conduct the electrophilic addition reaction of (CH3)3Si+ (trimethylsilylium ion) to acetylene. However, this deceptively simple reaction displays a remarkable dependence on the gas phase pressure, revealing the interplay of competitive pathways. In FT-ICR mass spectrometry at ca 10−8mbar, the nascent (CH3)3Si+–acetylene complex undergoes a rearrangement process yielding the CH2C(CH3)Si(CH3)2+ ion. This structure has been assigned on the basis of the ion-molecule reactivity displayed by the sampled C5H11Si+ adducts, matching the one of the model ion obtained from 2-(trimethylsilyl)propene. Whereas the absolute values of kinetic rate constants could not discriminate between isomeric species, the branching ratios for competitive addition–elimination channels in the reaction with i-C3H7OH and t-C4H9OCH3 were found to be diagnostic of different structures. The pathways leading from the (CH3)3Si+–acetylene complex primarily formed to the candidate C5H11Si+ isomers have been investigated by ab initio quantum chemical calculations at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311G(2d,p) level. The energy profiles show that the path to the CH2C(CH3)Si(CH3)2+ isomer is associated to the lowest activation energy barrier, below the reactants energy level. The energy released in the (CH3)3Si+–acetylene association process, remaining stored in the complex formed at low pressure, thus allows the isomerization to a species holding the positive charge on electropositive silicon. Interestingly, the most stable of the conceivable isomers, (E)-(CH3)CHCHSi(CH3)2+, is not accessed because of an activation energy barrier protruding above the reactants energy level. The combined information of ion-molecule reactivity and ab initio calculations of potential isomers and rearrangement pathways has thus afforded a comprehensive view of the (CH3)3Si+ addition reaction to acetylene under various pressure regimes.

Kinetic Study of the Acid Hydrolysis of Parthenium hysterophorus L. for xylose yield in the Production of Lignocellulosic ethanol

Parthenium hysterophorus L commonly known as Parthenium weed is one of the seven obnoxious weed in the world. Being a lignocellulosic biomass, with a composition of Lignin: 13.9%; Hemicellulose: 21.01% and Cellulose: 27.8%, it may be considered as the potential source of fuel ethanol production though pretreatment, saccharification and fermentation route. In the present work, efforts have been made to increase the yield of xylose during the hydrolysis process. Hydrolysis of Parthenium hysterophorus was carried out with 1%, 3% & 5% H2SO4 solutions. Each set of biomass was allowed to soak for 1 h & 2 h for each acid concentration. Finally the substrates were treated for up to 10 min in the temperature range of 190 230 C. The xylose yield has been studied for every set of parameters. The maximum value of xylose yield obtained is 46.41 mg/g of dry Parthenium, when hydrolysed with 5% H2SO4 at a temperature of 230 C in 2 h soaking time and 10 min treatment time. The values of kinetic rate constants show the adherence to Arrhenius relationship with significant changes in the concentration of acid and soaking period.

Reaction kinetics and validity of BOD test for domestic wastewater released in marine ecosystems

With urbanization of coastal cities, marine pollution is becoming a severe problem. The rates of biodegradation, decomposition, and ratification of pollutants get slowed down due to salinity. The higher temperatures prevalent in tropical regions significantly affect reaction rates. Multiple factors influence the rate of biodegradation, making the process complex. Hence, prediction and evaluation of the assimilative capacity of the marine environment due to wastewater discharges is becoming a difficult task. Biochemical oxygen demand (BOD) is a wet oxidation process, which follows first-order kinetics. The values of kinetic rate constants are expected to differ with varying salinities and temperatures. Research is carried out using glucose-glutamic acid and domestic wastewater to evaluate the impact of salinity on biodegradation of carbonaceous waste at 20°C and 27°C. The findings confirm the hypothesis of slow biodegradation of carbonaceous organic matter in marine waters. An inverse relationship between rate of biodegradation and salinity was observed. BOD exertion at 20°C (5days) and 27°C (3days) for the marine environment is comparable at selected salinities thereby confirming the validity of BOD test of shorter duration at elevated temperature.

Computation of restoration of ligand response in the random kinetics of a prostate cancer cell signaling pathway

Mutation and/or dysfunction of signaling proteins in the mitogen activated protein kinase (MAPK) signal transduction pathway are frequently observed in various kinds of human cancer. Consistent with this fact, in the present study, we experimentally observe that the epidermal growth factor (EGF) induced activation profile of MAP kinase signaling is not straightforward dose-dependent in the PC3 prostate cancer cells. To find out what parameters and reactions in the pathway are involved in this departure from the normal dose-dependency, a model-based pathway analysis is performed. The pathway is mathematically modeled with 28 rate equations yielding those many ordinary differential equations (ODE) with kinetic rate constants that have been reported to take random values in the existing literature. This has led to us treating the ODE model of the pathways kinetics as a random differential equations (RDE) system in which the parameters are random variables. We show that our RDE model captures the uncertainty in the kinetic rate constants as seen in the behavior of the experimental data and more importantly, upon simulation, exhibits the abnormal EGF dose-dependency of the activation profile of MAP kinase signaling in PC3 prostate cancer cells. The most likely set of values of the kinetic rate constants obtained from fitting the RDE model into the experimental data is then used in a direct transcription based dynamic optimization method for computing the changes needed in these kinetic rate constant values for the restoration of the normal EGF dose response. The last computation identifies the parameters, i.e., the kinetic rate constants in the RDE model, that are the most sensitive to the change in the EGF dose response behavior in the PC3 prostate cancer cells. The reactions in which these most sensitive parameters participate emerge as candidate drug targets on the signaling pathway.

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

The gas-phase reaction between calcium monocation and fluoromethane: Ca(+)+CH(3)F-->CaF(+)+CH(3) was theoretically analyzed. The potential energy hypersurface was explored by using density functional theory methodology with different functionals and Pople's, Dunning's, Ahlrichs', and Stuttgart-Dresden basis sets. Kinetics calculations (energy and total angular momentum resolved microcanonical variational/conventional theory) were accomplished. The theoretically predicted range for the global kinetic rate constant values at 295 K (7.2x10(-11)-5.9x10(-10) cm(3) molecule(-1) s(-1)) agrees reasonably well with the experimental value at the same temperature [(2.6+/-0.8)x10(-10) cm(3) molecule(-1) s(-1)]. Explicit consideration of a two transition state model, where the formation of a weakly bounded prereactive complex is preceded by an outer transition state (entrance channel) and followed by an inner transition state connecting with a second intermediate that finally leads to products, is mandatory. Experimental observations on the correlation, or lack of correlation, between reaction rate constants and second ionization energies of the metal might well be rationalized in terms of this two transition state model.

Desorption kinetics of hydrophobic organic chemicals from sediment to water: A review of data and models

Resuspension of contaminated sediment can lead to the release of toxic compounds to surface waters where they are more bioavailable and mobile. Because the timeframe of particle resettling during such events is shorter than that needed to reach equilibrium, a kinetic approach is required for modeling the release process. Due to the current inability of common theoretical approaches to predict site-specific release rates, empirical algorithms incorporating the phenomenological assumption of biphasic, or fast and slow, release dominate the descriptions of nonpolar organic chemical release in the literature. Two first-order rate constants and one fraction are sufficient to characterize practically all of the data sets studied. These rate constants were compared to theoretical model parameters and functionalities, including chemical properties of the contaminants and physical properties of the sorbents, to determine if the trends incorporated into the hindered diffusion model are consistent with the parameters used in curve fitting. The results did not correspond to the parameter dependence of the hindered diffusion model. No trend in desorption rate constants, for either fast or slow release, was observed to be dependent on K(OC) or aqueous solubility for six and seven orders of magnitude, respectively. The same was observed for aqueous diffusivity and sediment fraction organic carbon. The distribution of kinetic rate constant values was approximately log-normal, ranging from 0.1 to 50 d(-1) for the fast release (average approximately 5 d(-1)) and 0.0001 to 0.1 d(-1) for the slow release (average approximately 0.03 d(-1)). The implications of these findings with regard to laboratory studies, theoretical desorption process mechanisms, and water quality modeling needs are presented and discussed.

G protein threshold behavior in the human neutrophil oxidant response: measurement of G proteins available for signaling in responding and nonresponding subpopulations

Threshold behavior is an important aspect of signal transduction pathways that allows for responses to be turned on or off. Human neutrophil responses to N-formyl peptides, including oxidant production and release, exhibit threshold behavior with respect to the number of G proteins available for signaling; progressive treatment of neutrophils with pertussis toxin causes the conversion of responding cells to nonresponding cells. To quantify the threshold level of G proteins required for signaling of N-formyl peptide stimulated oxidant production in a neutrophil population, we used a plasma membrane associated G protein quantification assay in conjunction with a sorting flow cytometer and measured differences in the average number of G proteins available for signaling per cell in both the responding and the nonresponding subpopulations after pertussis toxin treatment. Although there appeared to be a threshold separating responding cells and nonresponding cells for a given sample, no discrete threshold was measured across multiple treatment conditions. A mathematical model of the early steps in signaling suggests that cell-to-cell variability in signal parameters, such as numbers of signal components and values of kinetic rate constants, obscures the measurement of a discrete threshold and leads to an apparent decrease in the threshold level of G proteins available for signaling as the total G proteins are decreased.