Main Rate-determining Step Research Articles

Waste-Based Ceramsite for the Efficient Removal of Ciprofloxacin in Aqueous Solutions

Ciprofloxacin (CIP), a compound with bioaccumulation toxicity and antibiotic resistance, is frequently detected in water at alarming concentrations, which is becoming an increasing concern. In this study, a low-cost ceramsite was developed from industrial solid wastes through sintering to remove CIP from wastewater. The effects of adsorbent dosage, initial pH, contact time, initial CIP concentration, and temperature were explored. More than 99% of CIP (20-60 mg/L) was removed at around pH 2-4 by the ceramsite. The kinetic data fitted well with the pseudo-second-order model, revealing that chemisorption was the main rate-determining step. The isotherm data was better described by the Freundlich model, suggesting that CIP was removed by the formation of multiple layers on the heterogeneous surface. Moreover, the removal efficiency was practically higher than 95% during five regeneration cycles, when different regeneration methods were used, including calcination, HCl, and NaOH washing, indicating that the ceramsite exhibited outstanding reusability in removing CIP. The primary mechanism of CIP removal by the ceramsite was found to be the synergism of adsorption and flocculation, both of which depended on the release of Ca2+ from the ceramsite. In addition, strong Ca-CIP complexes could be formed through surface complexation and metal cation bridging between Ca2+ and different functional groups in CIP.

Nitrogen-rich layered carbon for adsorption of typical volatile organic compounds and low-temperature thermal regeneration

Carbon-based adsorbents with a high adsorption capacity and low price have been widely used in the removal of volatile organic compounds (VOCs), but the poor gas selectivity and reusability limit their industrial applications. In this work, disc-like nitrogen-rich porous carbon materials (HAT-Xs) were synthesized to remove typical VOCs via adsorption. By controlling the synthesis temperature from 450 to 1000 °C, the C/N ratio of the HAT-Xs increased from 1.85 to 12.56. The HAT-650 synthesized at 650 °C with the high specific surface area of 305 m2 g−1 exhibits the highest adsorption capacity of 141 mg g−1 for ethyl acetate (which is 3.2 times for that of activated carbon), and 39.4 mg g−1 for n-hexane, 48.6 mg g−1 for toluene. Kinetic studies indicated that the adsorption is physical adsorption and that the interior surface diffusion is the main rate-determining step during the adsorption progress, the interior surface diffusion rate of ethyl acetate on HAT-650 is 1.455 mg g−1 min−0.5. At the same time, the desorption and reuse tests show that HAT-650 has excellent reusability with low desorption and regeneration temperature of 120 °C, and high desorption efficiency of 95.2% and that it could be a promising ethyl acetate adsorbent for industrial applications.

A strategy for the enhancement of trapping efficiency of gaseous benzene on activated carbon (AC) through modification of their surface functionalities.

Facile modification is a common, but effective, option to improve the uptake removal capacity of of activated carbon (AC) against diverse target volatile organic compounds (VOCs; e.g., benzene) in gaseous streams. To help design the routes for such modification, this research built strategies to generate three types of modified ACs by incorporating amine/sulfur/amino-silane groups under solvothermal or microwave (MW) thermal conditions. The adsorption performance has been tested using a total of six types of AC sorbents (three modified+three pristine forms) for the capture of 1Pa benzene (1atm and 298K). The obtained results are evaluated in relation to their textural properties and surface functionalities. Accordingly, the enhancement of AC surface basicity (e.g., point of zero charge (PZC)=10.25), attained via the silylation process, is accompanied by the reduced adsorption of benzene (a weak base). In contrast, ACs amended with amine/sulfur (electron-donating) groups using the MW technique are found to acquire high surface acidity (PZC of 5.99-6.05) to exhibit significantly improved benzene capturing capability (relative to all others). Their uplifted performance is demonstrated in terms of key performance metrics such as breakthrough volume (BTV10%: 163→443Lg-1), adsorption capacity (Q10%: 4.82→13.6mgg-1), and partition coefficient (PC10%: 0.516→1.67molkg-1 Pa-1). Based on the kinetic analysis, the overall adsorption process is found to be governed by pore diffusion as the main rate-determining step, along with surface interaction mechanisms. The results of this research clearly support the critical role of surface chemistry of AC adsorbents and their textural properties in upgrading air/gas purification systems.

Determination of the rate-determining step of the oxygen reduction reaction of La0.8Sr0.2MnO3(LSM)- 8mol% yttria-stabilized zirconia(YSZ): Composition and microstructure

In this study, LSM-YSZ composite cathodes were analyzed by changing the firing temperature, composition, and operating temperature to determine the contribution of each step of the oxygen reduction reaction (ORR). The overall resistance of the cathode reaction was characterized by fitting the AC impedance spectra with an equivalent circuit model. It was found that initial reactions of ORR (dissociative adsorption) are the main rate-determining step (RDS) regardless of operating or sintering temperature, while reactions on LSM surface become the main RDS when the ratio of LSM catalysts has a relatively small proportion. The [LSM-YSZ]5:5 cathode fired at 1100 °C showed the best microstructure and lowest resistance in the ORR at an operating temperature of 700 °C (RHF: 0.18 Ωcm2, RMF: 0.20 Ω cm2, RLF: 0.25 Ωcm2, Rcathode: 1.14 Ωcm2). This demonstrates the potential use of LSM-YSZ cathodes for IT/LT-SOFC without the use of expensive materials, such as LSCF and BSCF.

In-Situ Formed Hydroxide Accelerating Water Dissociation Kinetics on Co3N for Hydrogen Production in Alkaline Solution

Sluggish water dissociation kinetics on nonprecious metal electrocatalysts limits the development of economical hydrogen production from water-alkali electrolyzers. Here, using Co3N electrocatalyst as a prototype, we find that during water splitting in alkaline electrolyte a cobalt-containing hydroxide formed on the surface of Co3N, which greatly decreased the activation energy of water dissociation (Volmer step, a main rate-determining step for water splitting in alkaline electrolytes). Combining the cobalt ion poisoning test and theoretical calculations, the efficient hydrogen production on Co3N electrocatalysts would benefit from favorable water dissociation on in-situ formed cobalt-containing hydroxide and low hydrogen production barrier on the nitrogen sites of Co3N. As a result, the Co3N catalyst exhibits a low water-splitting activation energy (26.57 kJ mol-1) that approaches the value of platinum electrodes (11.69 kJ mol-1). Our findings offer new insight into understanding the catalytic mechanism of nitride electrocatalysts, thus contributing to the development of economical hydrogen production in alkaline electrolytes.

Temperature Effect on Co-Based Catalysts in Oxygen Evolution Reaction.

Oxygen evolution reaction (OER), as the critical step in splitting water, is a thermodynamically "up-hill" process and requires highly efficient catalysts to run. Arrhenius' law suggests that the higher temperature, the faster the reaction rate, so that a larger OER current density can be achieved at a lower η. Herein, we report an abnormal temperature effect on the performance of Co-based catalysts, e.g., Co3O4, Li2CoSiO4, and Fe-doped Co(OH) x, in OER in alkaline electrolytes. The OER performance reached a maximum when the temperature increased to 65 °C, and the OER performance declined when the temperature became higher. The mechanism was investigated by using Co3O4 as a model sample, and we propose that at an optimal temperature (around 55-65 °C) the main rate-determining step changes from OH- adsorption dominant to a mixed mode and both the adsorption and the cleavage of the OH group can be rate-determining, which leads to the fastest kinetics.

Kinetics of Nitric Oxide Absorption from Simulated Flue Gas by a Wet UV/Chlorine Advanced Oxidation Process

The mass transfer reaction kinetics of NO absorption by UV/chlorine advanced oxidation process were investigated in a lab-scale photochemical bubble reactor. Effects of several parameters on NO absorption rate were studied, including UV power, NO inlet concentration, SO2 concentration, active chlorine concentration of electrolyzed seawater, and reaction temperature. Results showed that NO absorption rate increased gradually with the increase of UV power, NO inlet concentration, and active chlorine concentration of electrolyzed seawater, but was almost independent of SO2 concentration and reaction temperature (below 313 K). The absorption process is a pseudo-0.2-order with respect to NO, as well as a pseudo-0.6-order with respect to active chlorine. The mass transfer process is the main rate-determining step for the NO absorption by UV/electrolyzed seawater process. The established NO absorption model is in good agreement with the experimental values.

ChemInform Abstract: Engineering Multimetallic Nanocrystals for Highly Efficient Oxygen Reduction Catalysts

AbstractReview: 88 refs.

The kinetic studies of the cobalt ion removal from aqueous solutions by dolomite-based sorbent

The kinetics of Co(II) ions adsorption on thermally activated dolomite was studied with respect to the calcination temperature of natural dolomite. The sorption of Co(II) onto all samples is reasonably fast: The first 30–35 min accounts for approximately 70–80 % of Co(II) removal from feed solutions. In order to select the main rate-determining step in the overall uptake mechanism, a series of experiments were performed and data obtained were interpreted in terms of film diffusion control, intraparticle diffusion, pseudo-first-order and pseudo-second-order models. From the modeling of kinetic data, it can be concluded that adsorption of Co(II) ions from aqueous solution by heat-treated dolomite is a complex phenomenon and occurs in a mixed diffusion mode—the kinetic data are well described by the pseudo-second-order equation. The possible multistage sorption mechanism involving film diffusion and intraparticle diffusion control steps as well as chemical interaction between Co(II) ions and calcined dolomite is proposed.

Decreasing the polarization resistance of LaSrCoO4 cathode by Fe substitution for Ba(Zr0.1Ce0.7Y0.2)O3 based protonic ceramic fuel cells

LaSrCo1−xFexO4−δ layered perovskites with K2NiF4 structure were synthesized successfully via the Pechini method and evaluated as cathode material for Ba(Zr0.1Ce0.7Y0.2)O3 (BCZY) electrolyte supported protonic ceramic fuel cells (PCFCs). X-ray diffraction results show that LaSrCo1−xFexO4−δ (0.0 ≤ x ≤ 0.5) were all single phase with K2NiF4 structure. Thermal expansion coefficient of LaSrCo0.5Fe0.5O4−δ is close to that of BCZY electrolyte. The conductivity of LaSrCo0.5Fe0.5O4−δ in air is 142 S cm−1 at 750 °C. At 750 °C, the Rp of LaSrCo0.5Fe0.5O4−δ is measured and the value is about 0.12 Ω cm2. The main rate-determining step in LaSrCo0.5Fe0.5O4−δ cathode is the dissociation of adsorbed oxygen molecules into atoms on the surface. Further, cathodic overpotential of 90 mV is obtained for LaSrCo0.5Fe0.5O4−δ cathode at current density of 0.2 A cm−2 at 750 °C. The substitution of Co by Fe causes an enhancement of the better electrochemical properties implying that LaSrCo0.5Fe0.5O4−δ could be applied as possible cathode for PCFCs.

Kinetic studies of manganese removal from aqueous solution by adsorption on natural zeolite

<p> The kinetics of manganese adsorption onto natural zeolite (clinoptilolite) were studied with respect to initial metal ion concentration and adsorbent mass. In order to select the main rate-determining step in the overall uptake mechanism, a series of experiments were performed under batch conditions from single ion solutions. Data obtained from the kinetic experiments are interpreted in terms of Pseudo-second order kinetic model, Weber and Morris model and model proposed by Furusawa and Smith.</p><p> The adsorption kinetics is reasonably fast. It means that in the first 20 min approximately 75% of Mn<sup>2+</sup> is adsorbed from solutions. From the kinetic data can be concluded that adsorption of manganese ions from solution by natural zeolite is more efficiency at higher adsorbent mass and at lower manganese concentration in solution. Amount of Mn adsorbed on zeolite increase at higher manganese concentration in solution.</p><p> From the kinetic models, can be concluded that intraparticular diffusion is more likely to be the rate determining step, but at higher concentrations of Mn ions in solution, film diffusion may be take a part in a rate determining step.</p>

Sorption of Cu(II) Ions from Aqueous Solution with Synthesized B-Doped Heptazine-Based g-C/N/H Polymers

In this study, boron doped heptazine-based g-C/N/H polymers (B-doped heptazine-based g-C/N/H polymers) were synthesized by the pyrolysis process of the mixture of low cost H3BO3 and melamine. X-ray power diffractometer (XRD) and Fourier transform infrared spectroscopy (FT-IR) were applied to characterize the successful doping of B atoms into the nanostructure of the graphic-C3N4 (heptazine-based g-C/N/H polymers). The synthesized material was conducted to investigate the sorption capacity for Cu(II) ions from aqueous solution and the sorption conditions including the doped amount, the solution pH, the contact time, and initial concentration were optimized. The results indicated that the sorption process was dependent on solution pH and the optimum pH was from 3.0 to 6.0. The kinetics was well evaluated by pseudo-second-order which indicated that the main rate-determining step was controlled by the chemical sorption process. The maximum sorption capacity calculated from Langmuir model amounted to 149.3 mg g−1 with the initial concentration range from 80 ∼ 800 mg L−1. So, B-doped heptazine-based g-C/N/H polymers have the potential application as a low cost adsorbent for high concentration Cu(II) ions in aqueous solution.

Adsorption of Lignosulfonate Compound from Aqueous Solution onto Chitosan-Silica Beads

The main objective of this study is to investigate the possibility of crosslinked chitosan-tetraethoxy orthosilane (TEOS) (chitosan-silica) beads to be used as an adsorbent material to adsorb the lignosulfonate compound in solution. Different parameters affecting the adsorption capacity such as contact time, adsorbent dosage, initial concentration, pH, ionic strength, and temperature have been investigated. Adsorption isotherms of lignosulfonates onto chitosan-silica beads were also studied. Batch adsorption experiments were carried out and the optimum lignosulfonate adsorption onto chitosan-silica beads occurred at contact time of 30 minutes, the adsorbent dosage of 40 g/L, initial concentration of 50 mg/L, pH 5, and a temperature of 45°C. Adsorption isotherms studied through the use of graphical methods revealed that the adsorption of lignosulfonates onto chitosan-silica beads follows the Langmuir model, with the maximum adsorption capacity being 238.3 mg/g at pH 7. Adsorption is dependent on the ionic strength. The adsorption of lignosulfonate on chitosan-silica beads was best described with the pseudo-second-order kinetic model with a rate constant of 0.32 g · mg−1 · min−1, while intra-particle-diffusion was the main rate-determining step in the lignosulfonate adsorption process. The chitosan-silica beads investigated in this study were thus exhibited as a high potential adsorbent for the removal of lignosulfonate from solution.

Interionic Hydration Structures of NaCl in Aqueous Solution: A Combined Study of Quantum Mechanical Cluster Calculations and QM/EFP-MD Simulations

The association process of NaCl in aqueous solution was studied by a combination of quantum mechanical calculations on NaCl(H(2)O)(n) (n = 1-6) clusters and quantum mechanical/effective fragment potential-molecular dynamics (QM/EFP-MD) simulations for NaCl in 292 EFP waters. The interionic hydration structures (IHSs) were topologically classified as "ring" (R), "half-bridge" (H), and "full-bridge" (F) types on the basis of the quantum mechanical calculations. Subsequent IHS analysis on QM/EFP-MD simulations revealed that the NaCl contact ion pair (CIP) mainly involved R type hydration structures while the solvent-separated ion pair (SSIP) was composed of two different groups of F-type hydration structures. Our IHS analysis also discovered H type hydration even at large separation interionic distances (∼7 Å), which is denoted as a dissociating ion pair (DIP). The analysis was able to reveal the most complete interionic structures and their reorganizations of the association process. A strong correlation between the IHSs and interionic distance suggests that not only the solvent reorganization but also the local IHS changes are equally important. Mechanistically, it is suggested that the conversion between ring-type and full-bridge hydration structures is the main rate-determining step of ion-pair association.

Low Temperature H2S Removal with Metal-Doped Nanostructure ZnO Sorbents: Study of the Origin of Enhanced Reactivity in Cu-Containing Materials

Sulfidation of pure and metal-doped ZnO nanostructure sorbents (M0.03Zn0.97O, M = Fe, Co, Ni, Cu) was studied in order to clarify the effect of metal on the transformation kinetics at 200−350 °C. The solids were prepared by coprecipitation from metal nitrate solution followed by calcination at 400 °C. Reaction with H2S was studied by thermal gravimetric analysis (TGA) using a gas mixture containing 0.2 vol % H2S in equimolar H2−N2. It was found that at 350 °C the TGA sulfidation profiles of all studied samples are similar, with the interface reaction being the main rate-determining step. After lowering the temperature to 250 °C the transformation of Cu0.03Zn0.97O continues to be controlled by the interface reaction with only a slightly decreased rate. In contrast, for all other samples the diffusion resistance appears, provoking a significant drop of their transformation rates. This finding shows that during sulfidation of Cu-doped ZnO the diffusion is faster than for all other sorbents. The same effect w...

Dynamics Study of TiO<sub>2</sub> Semiconductor Photoelectrode by EIS in the Ammonia Degradation

The photoelectrocatalytic activation and semiconductor character parameter have been investigated by the electrochemical impedance spectroscopy (EIS) and Mott-Schottky test on the Ti/TiO2 photoanode in ammonia system. Based on the evolution of electrical parameters of the equivalent circuit, the most probable dynamics has been stated that the redox reaction in the Ti/TiO2 film photoanode interface is the main rate-determining step. According to the products analysis, the reaction mechanism of ammonia degradation by TiO2 has been presented.

Comparison on the leaching kinetics of chalcocite and pyrite with or without bacteria

The acid leaching, ferric leaching, and bioleaching of chalcocite and pyrite minerals were conducted in two sets of 3L stirred reactors. The dissolution rates of copper and iron were correlated with leaching conditions. In the acid leaching process, the dissolution rate of chalcocite was around 40wt.% while that of pyrite was less than 4%. In the ferric leaching process with high ferric concentration, only 10 wt.% of iron in pyrite was leached out at the same retention time though the copper recovery over 60 wt.% in chalcocite. For the bioleaching process, the chalcocite leaching rate was highly increased, nearly 90 wt.% of copper was leached out, and the iron dissolution of pyrite exceeded 70 wt.%. For the two minerals, the bioleaching shows the highest leaching rate compared with the acid leaching or ferric leaching. In uncontrolled bioleaching process, pyrite could be dissolved effectively. The experimental data were fitted to the shrinking core and particle model. The results show that in all the leaching tests, the chalcocite leaching was mainly controlled by diffusion, while for the pyrite leaching, chemical reaction is the main rate-determining step.

Mechanism of Ion Permeation in a Model Channel: Free Energy Surface and Dynamics of K+ Ion Transport in an Anion-Doped Carbon Nanotube

The mechanism of the ion permeation is investigated for an anion-doped carbon nanotube, as a model of the K+ channel, by analyzing the free energy surface and the dynamics of the ion permeation through the model channel. It is found that the main rate-determining step is how an ion enters the channel. The entrance of the ion is mostly blocked by a water molecule located at this entrance. Only about 10% of K+ ions which reach the mouth of the channel can really enter the channel. The rejection rate sensitively depends on the location of this water molecule, which is easily controlled by the charge of the carbon nanotube; for example, the maximum permeation is obtained when the anion charge is at a certain value, -5.4e in the present model. At this charge, the facile translocation of the ion inside the channel is also induced due to the number of fluctuations of the ions inside the channel. Therefore, the so-called "Newton's balls", a toy model, combined with a simple ion diffusion model for explaining the fast ion permeation should be modified. The present analysis thus suggests that there exists an optimum combination of the length and the charge of the carbon nanotube for the most efficient ion permeation.

Modulation of Na,K-ATPase by phospholipids and cholesterol. II. Steady-state and presteady-state kinetics.

The effects of phospholipid acyl chain length (n(c)) and cholesterol on several partial reactions of Na,K-ATPase reconstituted into liposomes of defined lipid composition are described. This regards the E(1)/E(2) equilibrium, the phosphoenzyme level, and the K(+)-deocclusion reaction. In addition, the lipid effects on some steady-state properties were investigated. Finally, the effects of cholesterol on the temperature sensitivity of the phosphorylation and spontaneous dephosphorylation reactions were investigated. The fatty acid and cholesterol composition of the native Na,K-ATPase membrane preparation showed a remarkable similarity to the lipid composition known to support maximum hydrolytic capacity as determined from in vitro experiments. The main rate-determining step of the Na,K-ATPase reaction, the E(2) --> E(1) reaction, as well as several other partial reactions were accelerated by cholesterol. This regards the phosphorylation by ATP as well as the E(1) - P --> E(2)-P reaction. Moreover, cholesterol shifted the E(1)/E(2) equilibrium toward the E(1) conformation and increased the K(+)-deocclusion rate. Finally, cholesterol significantly affected the temperature sensitivity of the spontaneous dephosphorylation reaction and the phosphorylation by ATP. The effects of cholesterol were not completely equivalent to those induced by increasing the phospholipid acyl chain length, indicating that the cholesterol effects are not entirely caused by increasing the hydrophobic bilayer thickness, which indicates an additional mechanism of action on the Na,K-ATPase.

Tyr-143 facilitates interdomain electron transfer in flavocytochrome b2.

The role of Tyr-143 in the catalytic cycle of flavocytochrome b2 (L-lactate:cytochrome c oxidoreductase) has been examined by replacement of this residue with phenylalanine. The electron-transfer steps in wild-type and mutant flavocytochromes b2 have been investigated by using steady-state and stopped-flow kinetic methods. The most significant effect of the Tyr-143----Phe mutation is a change in the rate-determining step in the reduction of the enzyme. For wild-type enzyme the main rate-determining step is proton abstraction at the C-2 position of lactate, as shown by the 2H kinetic-isotope effect. However, for the mutant enzyme it is clear that the slowest step is interdomain electron transfer between the FMN and haem prosthetic groups. In fact, the rate of haem reduction by lactate, as determined by the stopped-flow method, is decreased by more than 20-fold, from 445 +/- 50 s-1 (25 degrees C, pH 7.5) in the wild-type enzyme to 21 +/- 2 s-1 in the mutant enzyme. Decreases in kinetic-isotope effects seen with [2-2H]lactate for mutant enzyme compared with wild-type, both for flavin reduction (from 8.1 +/- 1.4 to 4.3 +/- 0.8) and for haem reduction (from 6.3 +/- 1.2 to 1.6 +/- 0.5) also provide support for a change in the nature of the rate-determining step. Other kinetic parameters determined by stopped-flow methods and with two external electron acceptors (cytochrome c and ferricyanide) under steady-state conditions are all consistent with this mutation having a dramatic effect on interdomain electron transfer. We conclude that Tyr-143, an active-site residue which lies between the flavodehydrogenase and cytochrome domains of flavocytochrome b2, plays a key role in facilitating electron transfer between FMN and haem groups.