International Journal of Chemical Kinetics最新文献

The complex effect of L-ascorbic acid on the oscillatory course of the Orbán–Epstein system, in which the Cu²⁺-catalyzed oxidation of thiocyanate ions with hydrogen peroxide occurs, was investigated. Our studies involved both the original and EDTA-perturbed system, analyzed in previous works. Potentiometric and spectrophotometric measurements were supported by numerical modeling of the proposed schemes of interaction of L-ascorbic acid with the components of the system. In this way the kinetic mechanism of the effect of EDTA on the Orbán–Epstein system, based on the idea of excess production of $�{\mathrm{HO}}_2^{•}$ radicals, enhancing the negative feedback loop in the oscillation cycle, was confirmed. The eventual involvement of other radicals, as, for example, OS(O)CN^• is possible, but kinetically indistinguishable from that of $�{\mathrm{HO}}_2^{•}$ species. The novel observation of the slowdown of oscillatory dynamics upon increasing concentrations of L-ascorbic was explained in terms of removal of HO₂Cu(OH)₂⁻ intermediate from reaction medium, presumably by the reduction of the central copper ion to elemental copper.

The kinetics of oxidation of pectin by acidic permanganate was studied at higher concentrations of pectin, previously. Consequently, the pseudo-first order plots of (ln (Abs. vs. time) were deviated from linearity. The results were interpreted in terms of two-stage reactions. Therefore, the results and postulated reaction mechanism were faced some debates which means that the redox reaction needs a further investigation. We expected that deviation may be attributed to the use of pectin of high concentration which leads to the creation of some complicated intermediates. These formed complexes may tend to retard the reaction rates and, hence, the noticed deviation from linearity. Therefore, in the present work pectin substrate of low concentrations was applied to check the linearity of pseudo first-order plots. Fortunately, all curves obtained from all plots were found to be fairly linear. This result was considered as evidence to support our expectation. Again, the experimental observations revealed second order rate constants in [H⁺] on contrary to that reported previously. Such diversity and contrariety observed may indicate the complexity and non-complementary nature of the present redox system. The activation parameters calculated by the least-squares method from the temperature dependency of the second-order rate constants were found to be ∆H^≠ = 52.67 kJ/mole, ∆S^≠ = –65.54 J/mole K, ∆G^≠ = 71.90 kJ/mole and E^≠ = 55.36 (±0.2–2.0 kJ/mole), respectively. However, the results obtained agreed with both inner and outer-sphere mechanism, a simultaneous two-electron change in one step of inner-sphere nature was suggested to be the more preferable pathway route for the electron-transfer. Speculated reaction mechanisms based on the experimental observations and the evaluated kinetic parameters were postulated and a pertinent discussion is presented.

Heavy metals adsorption by adsorbents prepared from natural materials is a low-cost, effective method for their removal from aqueous environments. This study aims to investigate the kinetic studies on the adsorption of hexavalent chromium from its aqueous solution on raw and activated cassava rhizome. The coarse and fine raw cassava rhizome powder was activated by thermal (drying), pyrolysis, 1% (v/v) acetic acid, and 1% (w/v) KOH solutions. The batch adsorption studies were carried out by varying time (0–60 min) with adsorption capacity (meq/g) as a dependent variable. Adsorption capacity increased with an increase in time up to 40 min and then remained constant for all adsorbents with variation in adsorption capacity from 1.5 to 2.1 meq/g in the order of fine pyrolysis > fine thermal > fine acid = coarse pyrolysis > fine alkali = coarse thermal > coarse acid > coarse alkali > raw fine > raw coarse. Then, the effect of time on adsorption capacity data was fit to mechanistic models of pseudo-first, pseudo-second, zeroth, and half-order kinetics. Adsorption of hexavalent chromium on raw coarse and fine cassava rhizome powder followed pseudo-first-order kinetics. Also, the data were fit to empirical models based on regression and neural network, and the goodness of model fit was evaluated using the correlation coefficient (R). Regression analysis indicates the optimal fit with an R value of 0.9948 when the effect of time on adsorption capacity was investigated. Neural network analysis reveals the best fit at an R value of 0.9985. Hence, a higher rate constant for fine particles compared to coarse particles indicates that fine particles have a greater effective surface area and enhanced diffusional properties, facilitating more efficient adsorption, which suggests that the finer particles interact more with the adsorbate, allowing for quicker transport and adsorption processes.

The present study investigates the adsorption performance of SnOFe₂O₃TiO₂ nanocomposite for the removal of Basic Fuchsin (BF) dye from aqueous solution. The nanocomposite was synthesized via the coprecipitation method and characterized using various techniques such as TEM, SAED, and EDAX. TEM images confirmed the nanocomposite's crystalline structure with nanoscale particle sizes, while SAED patterns revealed its polycrystalline nature. EDAX analysis revealed the presence of Ti (54.9%), O (39.5%), Fe (4.8%), and Sn (0.7%), indicating TiO₂ as the dominant phase, with Fe and Sn contributing additional functionalities. Adsorption experiments were conducted to examine the effect of various operational parameters, such as pH, contact time, adsorbent dose, and initial dye concentration. The adsorption capacity increased with pH, reaching a maximum at pH 8.5. The adsorption process followed pseudo-second-order kinetics with a high correlation coefficient (R² = 0.99), indicating chemisorption as the primary mechanism. The Langmuir isotherm model best described the equilibrium data, with a maximum adsorption capacity (Q_max) of 344.42 mg/g at 25°C. Thermodynamic analysis showed that the adsorption process was spontaneous and exothermic, with a slightly negative enthalpy change (ΔH°) of −0.008314 kJ/mol and a negative Gibbs free energy (ΔG°) of −3.150 kJ/mol at 298 K. The SnOFe₂O₃TiO₂ nanocomposite exhibited excellent reusability, retaining over 90.71% of its adsorption capacity after five consecutive cycles of regeneration using a simple desorption process. These results demonstrate the potential of SnOFe₂O₃TiO₂ nanocomposite as a highly efficient and sustainable adsorbent for the removal of BF dye from wastewater.

AMPD (2-Amino-2-methyl-1,3-propanediol) is a candidate hindered amine for CO₂ separation from biogas. In this study, the performance of water-lean mixtures of AMPD with the organic solvents ethylene glycol (EG) and propanol (PrOH) was explored. Hindered amines load more CO₂, while organic solvents desorb CO₂ at low temperature. As a result, the proposed mixtures will be useful for improved CO₂ removal from biogas. Thus, CO₂ solubility in the blends AMPD/H₂O (AMPD = 1 M) and AMPD/EG/PrOH/H₂O (AMPD = 1 M, EG:PrOH = 1:1, H₂O = 15 wt.%) was measured at 303, 313, and 323 K. The equilibrium CO₂ partial pressure varied in the 0.05–240 kPa range. Upon replacing water with the chosen organic solvents, the rate of CO₂ desorption in AMPD doubled, and low-temperature regeneration (T = 363 K) was possible. Besides, the heat of CO₂ absorption was lowered too, as evident from the Gibbs–Helmholtz equation. Since these outcomes were encouraging, absorption kinetics were also studied in the fast pseudo-first order reaction regime using the stirred cell technique and rate constants at 308 K were reported. Such water-lean mixtures were hitherto scarcely studied, and we filled this gap in this work.

The hydrolysis of carboxylate ester, that is, p-nitrophenyl acetate (PNPA) with α-nucleophiles, such as acetohydroxamic acid (AHA) and benzohydroxamic acid (BHA), has been studied in mixed surfactant systems (CTAB + C₁₆-10-C₁₆, 2Br⁻; CTPB + C₁₆-10-C₁₆, 2Br⁻; CTAB + C₁₆-12-C₁₆, 2Br⁻; and CTPB + C₁₆-12-C₁₆, 2Br⁻) at pH 8.0 and 27°C. Each reaction proceeded with pseudo-first-order kinetics. The rate of reactions with a system of mixed gemini surfactants has been shown to accelerate noticeably. The gemini mixed system had a higher rate constant than the single surfactant system. Gemini surfactants' spacer length has been tested in single systems, and their rate data are contrasted with those of monomeric surfactants, such as cetytrimethylammonium bromide (CTAB) and cetytriphenylphosphonium bromide (CTPB).

The kinetics of highly efficient oxidation of cadaverine (CAD) by Ce(IV) in both HClO₄ and H₂SO₄ solutions were examined using UV–Vis absorption spectra at numerous temperatures. The last products of CAD oxidation were recognized as ammonia and 5-aminopentanal, that is, an essential organic compound in both biological and chemical settings. From the obtained data, it's clear that the reactions’ kinetics demonstrated a first-order dependence in [Ce^IV], where they exhibited lower than unit orders with regard to [CAD] over the studied concentration range. In HClO₄ and H₂SO₄, the oxidation reactions showed positive and negative incomplete unit orders in [H⁺], correspondingly. Based on the obtained results, the mechanistic reactions’ pathways were suggested. The reliable rate laws were derived, and the reactions’ rate constants were estimated. In addition, the activation and thermodynamic parameters were calculated and discussed. This study illuminated the role of the oxidant, medium, temperature, and other conditions on the oxidation kinetics and mechanisms of these redox systems.