Observed Reaction Rate Constants Research Articles

Removal of Arsenic(III) from Groundwater by Nanoscale Zero-Valent Iron

Nanoscale zero-valent iron (NZVI) was synthesized and tested for the removal of As(III), which is a highly toxic, mobile, and predominant arsenic species in anoxic groundwater. We used SEM-EDX, AFM, and XRD to characterize particle size, surface morphology, and corrosion layers formed on pristine NZVI and As(III)-treated NZVI. AFM results showed that particle size ranged from 1 to 120 nm. XRD and SEM results revealed that NZVI gradually converted to magnetite/maghemite corrosion products mixed with lepidocrocite over 60 d. Arsenic(III) adsorption kinetics were rapid and occurred on a scale of minutes following a pseudo-first-order rate expression with observed reaction rate constants (K(obs)) of 0.07-1.3 min(-1) (at varied NZVI concentration). These values are about 1000x higher than K(obs) literature values for As(III) adsorption on micron size ZVI. Batch experiments were performed to determine the feasibility of NZVI as an adsorbent for As(III) treatment in groundwater as affected by initial As(III) concentration and pH (pH 3-12). The maximum As(III) adsorption capacity in batch experiments calculated by Freundlich adsorption isotherm was 3.5 mg of As(III)/g of NZVI. Laser light scattering (electrophoretic mobility measurement) confirmed NZVI-As(III) inner-sphere surface complexation. The effects of competing anions showed HCO3-, H4SiO4(0), and H2P04(2-) are potential interferences in the As(III) adsorption reaction. Our results suggest that NZVI is a suitable candidate for both in-situ and ex-situ groundwater treatment due to its high reactivity.

GAS PHASE REDUCTION OF CHLORINATED VOCs BY ZERO VALENT IRON

The removal of trichloroethylene (TCE) by reduction with elemental iron was examined in batch reactors representing three phase gaseous systems (Fe0 (solid), water vapor + TCE vapor + air mixture (gaseous), and condensed water on Fe0 surface (aqueous)) and two phase aqueous systems (Fe0 (solid), water (aqueous)) as the reaction medium. Two separate removal mechanisms were found for TCE removal in the gas phase reactors: [1] adsorption onto the Fe0 surface under low relative humidity (RH), and [2] reaction with Fe0 to form other organic compounds, such as ethylene, isomers of dichloroethylene, and vinyl chloride under 100% RH conditions in the gas phase. The adsorption of TCE vapor onto the Fe0 surface was fast initially and highest at low temperatures. The reduction reaction of TCE in the gas phase was first order with respect to the TCE concentration remaining in the gas phase under conditions of 100% RH at 15, 22, and 35°C, and the observed reaction rate constant (k obs) exhibited a maximum at elevated temperatures. The estimated k obs values for the degradation of dissolved TCE in aqueous systems were the same order of magnitude with the rate constants in the gas phase and exhibited an increasing rate at higher temperatures. The results suggest that Fe0 represents an economical choice for vapor phase barriers to limit the spread of contaminants in soil gas and reduce fugitive air emissions from contaminated sites.

Catalytic pathways in the ethanolysis of fenitrothion, an organophosphorothioate pesticide. A dichotomy in the behaviour of crown/cryptand cation complexing agents

The rates of displacement of 3-methyl-4-nitrophenoxide ion from the pesticide, fenitrothion, by alkali metal ethoxides in anhydrous ethanol were followed spectrophotometrically. Through product analysis experiments, which included 31P NMR and GC-MS, as well as spectrophotometric analysis, three reaction pathways were identified: nucleophilic attack at the phosphorus centre, attack at the aliphatic carbon, and a minor SNAr route ([Formula: see text]7%). Furthermore, a consecutive process was found to occur on the product of attack at the phosphorus centre. For purposes of kinetic treatment, the processes at the aliphatic and aromatic carbon were combined (i.e., the minor SNAr pathway was neglected), and the observed reaction rate constants were dissected into rate coefficients for nucleophilic attack at phosphorus and at aliphatic carbon. Attack at phosphorus was found to be catalyzed by the alkali metal ethoxides in the order KOEt > NaOEt > LiOEt. Catalysis arises from alkali metal ethoxide aggregates in the base solutions used (0–1.8 M); treatment of the system as a mixture of free ethoxide, ion-paired metal ethoxide, and metal ethoxide dimers resulted in a good fit with the kinetic data. An unexpected dichotomy in the kinetic behaviour of complexing agents (e.g., DC-18-crown-6, [2.2.2]cryptand) indicated that the dimers are more reactive than free ethoxide anions, which are in turn more reactive than ion-paired metal ethoxide. The observed relative order of reactivity is explained in the context of the Eisenman theory in which the free energy of association of the metal ion with the rate-determining transition state is largely determined by the solvent reorganization parameter. In contrast with displacement at the phosphorus centre, attack at the aliphatic carbon was not found to be catalyzed by alkali metals. In this case, the free ethoxide anion was more reactive than either the ion-paired metal ethoxide or the dimeric aggregate. The differing effects of alkali metals on the two pathways is ascribed largely to the leaving group pKa. For carbon attack, the pKa value estimated for demethyl fenitrothion, 2.15, is sufficiently low that metal ions are not required to stabilize the rate-determining transition state. In contrast, for phosphorus attack, 3-methyl-4-nitrophenoxide, with a pKa of 7.15, requires stabilization by metal ion interactions. Hence, alkali metal ions catalyze attack at phosphorus, but not attack at the carbon centres.Key words: organophosphorothioate, pesticide, fenitrothion, ethanolysis, alkali metal ethoxide, ion-pair reactivity, dimers, catalysis, competitive pathways.

Kinetics of reductive denitrification by nanoscale zero-valent iron

Zero-valent iron powder (Fe 0) has been determined to be potentially useful for the removal of nitrate in the water environment. This research is aimed at subjecting the kinetics of denitrification by nanoscale Fe 0 to an analysis of factors affecting the chemical denitrification of nitrate. Nanoscale iron particles with a diameter in the range of 1–100 nm, which are characterized by the large BET specific surface area to mass ratio (31.4 m 2/g), removed mostly 50, 100, 200, and 400 mg/l of nitrate within a period of 30 min with little intermediates. Compared with microscale (75–150 μm) Fe 0, endproduct is not ammonia but N 2 gas. Kinetics analysis from batch studies revealed that the denitrification reaction with nanoscale Fe 0 appeared to be a pseudo first-order with respect to substrate and the observed reaction rate constant ( k obs) varied with iron content at a relatively low degree of application. The effects of mixing intensity (rpm) on the denitrification rate suggest that the denitrification appears to be coupled with oxidative dissolution of iron through a largely mass transport-limited surface reaction (<40 rpm).

Zinc ionization by interaction with argon plasma

A low-temperature zinc plasma was generated by secondary ionization using an electron cyclotron resonance argon plasma. The ionization efficiency of zinc samples of the metal, the bromide, and the iodide were examined at different sample crucible temperatures. It was found that the ionization rate of zinc metal is the highest among the tested samples. The observed reaction rate constants , defined as = nZn+/nZnnAr+ in the present work, are 1.8 × 10-18 m-3, 1.7 × 10-19 m-3 and 3.7 × 10-19 m-3 for zinc metal, zinc bromide, and zinc iodide, respectively. The large reaction constant of zinc metal may be due to the small dissociation energy into atom vapour, compared with the large dissociation energies of the zinc halides.

Relation between Third-Phase Properties and Rate of Dehydrohalogenation of 2-Bromooctane in Phase Transfer Catalytic System.

The purpose of this paper is to investigate the relationship between the properties and catalytic activity of the third phase in a phase transfer catalytic system. The conditions of formation of the third phase are investigated by changing the kind of phase transfer catalyst (PTC), organic solvents, and concentration of potassium hydroxide in the aqueous phase. When tetrabutylammonium bromide, (Bu) 4 NBr, is used as the phase transfer catalyst, the third phase is formed with both dodecane and toluene as organic solvents. On the other hand, when tetrahexylammonium bromide, (Hex) 4 NBr, is used as the phase transfer catalyst, the third phase forms with dodecane but not with toluene. The volume of the third phase, the base strength, the concentration of catalyst, water, and organic solvent in the third phase were measured. Solidification phenomena are observed for some cases of higher KOH concentrations in the aqueous phase. Dehydrohalogenation of 2-bromooctane in the organic phase was carried out in a batch reactor with the potassium hydroxide solution in the aqueous phase. The observed reaction rate constant (k obs ), which is determined from the first order kinetics, increases remarkably with the increase in concentration of KOH in the aqueous phase (ξ KOH ) for the condition of third phase formation. However, for the solidification condition, the reaction rate drops sharply because only the external surface of the solid is effective for the reaction. The distribution coefficient of reactant between the organic phase and the third phase, (K A ) and the reaction rate constant in the third phase (k third ) can be related to the concentration of water in the third phase (C H2 O) independent of the catalysts and organic solvents. Since PTC exists in the third phase, it can be reused without any loss of catalytic activity.

Determination of the Rate Constant for the Reaction of Hydroxyl and Oxide Radicals with Cysteine in Aqueous Solution

The techniques of pulse radiolysis and absorption spectroscopy have been used to determine absolute rate constants for the reaction of hydroxyl and oxide radicals with cysteine (CysS) species in aqueous solution. Using measured equilibrium constants for radical disulfide anion formation, over the pH range 6.0-13.0, observed reaction rate constants were resolved into their constituent reactions .OH + CysSH-->H2O + CysS. and .O(-) + CysS-( + H2O)-->CysS. + 2OH- with calculated limiting rate constants of (5.35 +/- 0.82) x 10(9) and (3.28 +/- 0.49) x 10(8) dm3 mol-1 s-1, respectively. The rate constant for hydroxyl radical reaction with CysS- was found to be the same as that for CysSH.

Decomposition of 2,6-dichlorophenolindophenol in strongly acidic solutions: a potential kinetic method for the determination of pH

The problem of pH measurements at extreme pH values is well known. Higher pH values are obtained at pH values of below 1 because of the so-called ‘acid error’. The acid error depends on various parameters and it is not always reproducible. In order to determine accurate pH values at high acidities, a kinetic study of the decomposition of 2,6-dichlorophenolindophenol (DCPl) in strongly acidic solutions was investigated by applying the stopped-flow technique. The DCPl is unstable at lower pH values (below 2) and the reaction rate constant for its decomposition in the pH range 0–2 is dependent on the pH. A good correlation between pH and the observed reaction rate constant of the decomposition of DCPl (kob) was found for low pH values. The reaction rate constant, k1, was calculated by three different approaches for the evaluation of the experimental data. The weighted mean value of k1 was found to be (77 ± 1)× 10–3 l mol–1 S–1, with confidence limits of 95%. A knowledge of k1 is also important in analysis as DCPl is the main reagent for the determination of assorbic acid in acidic solutions.

Automation of the Batch Method for Reaction Kinetic Studies Using Flow Injection Analysis. Kinetic Study of Hydrolysis Of N4-Acetylsulfanilamide, Acetylsalicylic Acid and Phenyl Phosphate

Abstract The automation of the discontinuous (batch) method for kinetic stucfies using flow-injection analysis (FIA) is described. Aliquots of the reaction mixture are automatically injected in an appropriate manifold and the kinetic profile of the reaction is obtained as a series of absorbance peaks. Observed reaction rate constants are calculated using the Guggenheim and non-linear fitting approaches. The new method is evaluated in the kinetic study of the acid hydrolysis of N4-acetylsulfanilamide by colorimetric monitoring of sulfanilamide, the alkaline hydrolysis of acetylsalicylic acid (aspirin) by colorimetric monitoring of salicylate, and the enzymic hydrolysis of phenyl phosphate with alkaline phosphatase by colorimetric monitoring of phosphate. The automated flow-injection batch method can be used in kinetic studies of reactions with t1/2 greater than 200 s.

Competitive reaction and quenching of vibrationally excited O+2 ions with SO2, CH4, and H2O

Vibrationally excited O+2 ions injected into a He buffered flow tube react rapidly with SO2 and H2O by charge transfer and with CH4 to produce CH3O+2 , CH+3 , and CH+4 . It is found that the rapidly reacting states at thermal energy are O+2 (v≥2) for SO2 and CH4 and O+2 (v≥3) for H2O, while the lower vibrationally excited states are rapidly quenched. When the reactions of SO2 and CH4 are studied in Ar buffer as a function of kinetic energy it is found that the vibrational temperature of O+2 established through collisional excitation by the Ar buffer is perturbed by quenching collisions with the reactant molecules. This leads to observed reaction rate constants that change with reactant gas concentration. For the reaction of O+2 with CH4 the influence of kinetic and vibrational energy on the branching ratio of the reaction channels has been investigated. The present vibrational relaxation data for O+2 (v) by CH4, in conjunction with other recent measurements, allows a rather detailed picture of the mechanism to be drawn for this complicated reaction that involves the making and breaking of four chemical bonds.

A kinetic investigation of the thionitrite from (±)-2-acetylamino2-carboxy-1,1-dimethylethanethiol as a possible nitrosating agent

In strongly aqueous acid solution (±)-2-acetylamina-2-carboxy-l,1-dimethylethyl thionitrite (RSNO) undergoes denitrosation to give the corresponding thiol. The reaction is reversible and normally lies well over on the side of the thionitrite, but can proceed in the denitrosation direction in the presence of traps for free nitrous acid such as added sodium azide or sulphamic acid. At sufficiently high trap concentration to ensure complete irreversibility, the reaction was found to be first-order in RSNO and acid-catalysed; the observed reaction rate constant is approximately proportional to hO. Catalysis was observed by Cl–, Br–, SCN–, and SC(NH2)2 and the second-order rate constants were 4.6 × 10–3, 13.0 × 10–3, 34 × 10–3, and 27 × 10–3 l mol–1 s–1, respectively. The order of reactivity is as expected with the exception of that of thiourea. N-Methyl-4-nitroaniline was converted quantitatively to the corresponding nitrosamine by RSNO, but in the presence of added sodium azide, no nitrosamine was detected, this shows that RSNO like nitrosamines generally and alkyl nitrites, acts as a nitrosating agent towards amines etc., under these experimental conditions, by prior hydrolysis or via nitrosyl halide etc. formation. A kinetic analysis has enabled the reactivity of the nitrous acid traps hydrazoic acid and sulphamic acid to he established quantitatively.

Kinetic analysis of myoglobin autoxidation by isoelectric-focusing electrophoresis.

The autoxidation of horse myoglobin was studied in the presence or absence of catalase (EC 1.11.1.6) and/or superoxide dismutase (EC 1.15.1.1) at various pH values (6.6-7.8). Changes in the percentages of oxymyoglobin and metmyoglobin during the reaction were analysed by means of isoelectric focusing on Ampholine gel plates. Oxymyoglobin was decreased in a first-order manner, with an accompanying increase in metmyoglobin, under the various conditions studied. The observed reaction rate constants obtained under these conditions were pH-dependent; however, they were also greatly affected by the presence of the enzymes. The pH-dependence of the overall reaction was explained by the acid-base three-state model of myoglobin proposed by Shikama & Sugawara [(1978) Eur. J. Biochem. 91, 407-413]. The reaction process of myoglobin autoxidation was explained by the model suggested by Winterbourn, McGrath & Carrell [(1976) Biochem. J. 155, 493-502], indicating that superoxide anion and hydrogen peroxide are involved in the reaction mechanism.

On statistical and thermodynamic approaches to ion polar molecule collisions

Capture collision rate constants for ion polar molecule systems obtained from an average free energy function are shown to be equivalent to those obtained from a canonical activated complex theory formalism. Results provide a satisfactory least upper bound for observed reaction rate constants and compare well with Chesnavich, Su, and Bowers’ results obtained using microcanonical variational transition state theory. The average free energy approach is extended to the ion–quadrupole interaction, and capture collision rate constants are obtained providing a satisfactory least upper bound on experimental rate constants. A thermodynamic model for ion polar neutral collisions is developed and the difference between capture collision rate constants and momentum transfer collision rate constants rationalized in terms of a repulsive entropic force which prevents capture but produces isotropic scattering. The success of the results of Barker and Ridge in accounting for momentum transfer rates is thus explained and connected to the success of the average free energy result in accounting for reaction rate constants. It is argued that the agreement between theory and experiment indicates that rotational inelasticity plays a significant role in a gas phase transport property, i.e., the momentum transfer collision rate of ions in polar gases.

Studies on the formulation and admixture of parenteral preparations. I. Degradation of ascorbic acid and cyanocobalamin by sodium bisulfite added to ascorbic acid injection (author's transl)

Sodium bisulfite, which is frequently added as an antioxidant to ascorbic acid injections, was found to react with ascorbic acid and to effect the reduction of the vitamin concentration under anaerobic conditions. The rate of disappearance of ascorbic acid by sodium bisulfite in solution was pseudo first-order. In the pH range of 4 to 8 the dominating degradation process may be a reaction between monohydrogen ascorbate ion and bisulfite ion. The accompanying degradation of sodium bisulfite also followed the pseudo first-order. From the observed reaction rate constants of sodium bisulfite, the following Arrhenius equation was obtained. [numerical formula] k'obs at 25° was calculated as 2.30×10-4 hr-1. Assuming the average ambient temperature to be 25°, sodium bisulfite added to ascorbic acid injection will remain for as long a period as 300 to 400 days during its shelf-life. It was also found that cyanocobalamin was degraded by sodium bisulfite at considerably high velocity ; i.e., the half life of cyanocobalamin in aqueous solution of pH 5.3 at 25° was shortened to 180 and 38 minutes by the addition of 10 and 100 times moles of sodium bisulfite respectively. Therefore, cyanocobalamin injection is incompatible with commercially available ascorbic acid injections which generally contain sodium bisulfite. In conclusion, utilizing such an intra- and inter-formula reactive substance as sodium bisulfite seems not to be reasonable in the formulation of parenteral preparations.

Carbon-isotope effects in enzyme systems II. Studies with formic andd dehydrogenase

It has been shown that formic acid 12C is utilized more rapidly by the formic acid dehydrogenase enzyme system than is formic acid 14C. Regardless of the 14C/ 12C concentration ratios of the material, the observed reaction rate constants were recorded as k 12 C = 1.51 · 10 −3 and k 14 C = 1.19 · 10 −3 at 37°C at pH 7 in phosphate buffer.