Admittance measurements of kinetic and adsorption parameters of anodic mercury complex formation with macrobicyclic (222) ligand in propylenecarbonate and dimethylformamide

Admittance measurements were applied to investigation of the charge-transfer rate and mechanism of anodic complex formation between mercury and macrobicyclic ligand (222) as well as to the cathodic reduction of Hg²⁺-(222) complex formed in the bulk. From measurements in PC and DMF using adsorbable and non-adsorbable base electrolyte anions it was shown that the reactant adsorption effects are observable only if adsorption of ClO₄^? and (222) takes place at the same time. Corresponding charge-transfer rates were evaluated and potential dependence of the adsorption capacity for two ligand concentrations was given. At the half-wave potential apparent rate constants k_1/2 listed below were found (data from Fig. 9).     

Charge dynamics of a methane adsorption storage system: Intraparticle diffusional effects

The charge of natural gas adsorption storage systems is studied numerically, With emphasis given to the impact on its dynamics of intraparticle diffusional resistances to mass transport. Besides adsorption kinetics and thermal effects, the simulation model takes into account both mass transport inside the adsorbent and hydrodynamics of flow through the packed bed. Numerical results are presented for change with methane of a 50 liter cylindrical reservoir, filled with hypothetical adsorbents with diffusional time constants in the range 10^?3 s¹ D/R _p ² ≤ ∞. and with the adsorption equilibrium curve of a commercially available activated carbon with a good adsorptive storage capacity. An attempt is made to assemble the charge histories for different values ofD/R _p ² , in a single cure by using a modilied time scale.     

Studies on the adsorption of peptides of glycine/alanine on montmorillonite clay with or without co-ordinated divalent cations

The adsorption of simple peptides of glycine/alanine from their aqueous solutions onto montmorillonite, Ca²⁺ and Mg²⁺ exchanged montmorillonite clay studied UV spectrophotometrically at constant pH 7.02 and temperature 23 °C. The percent binding of Gly², Gly³, Gly⁴ and Gly-ala is calculated in terms of their optical density. The adsorption parameters, i.e. X_m and K_L have been calculated from Langmuir adsorption isotherm. Similar adsorption behaviour was observed with or without divalent cation exchanged adsorbent, but the percent binding and monolayer capacity appear to depend on the molecular weight, i.e. number of aliphatic carbon atoms of the adsorbates. The adsorption was significantly affected by the concentration of peptide, pH and temperature of the system. Equilibrium constant (K) and the free energies of adsorption (−ΔG) were determined from the isotherm measured under static conditions. Tetra glycine (Gly⁴) has positive −ΔG and K>1 showing greater adsorptibility, whereas for other peptides, −ΔG values were negative and K<1, thus showing very weak adsorption. A linear dependence of −ΔG on the number of aliphatic carbon atoms (n_c) from Gly² to Gly⁴ in adsorbate molecule was found. Thermodynamic data strongly support the quantitative data obtained from Langmuir adsorption isotherm. Ca²⁺ montmorillonite exhibited relatively better adsorption as compared to Mg²⁺ exchanged form or montmorillonite without Ca²⁺ or Mg²⁺. Results have shown that clay minerals might have played a significant role in prebiotic formation of proteins via adsorption of simple bio-oligomers on their surface.     

Decoration of MWCNTs with CoFe2O4 Nanoparticles for Methylene Blue Dye Adsorption

In the present work, a simple and viable method for producing multi-walled carbon nanotubes (MWCNTs) decorated with CoFe₂O₄ nanoparticles is presented. Chemical composition and crystal structure of the CoFe₂O₄/MWCNT composite was confirmed by X-ray diffraction measurements, while transmission electron microscopy was used to characterize the morphology and the distribution of nanocrystals in the composite. The obtained particles with relatively small diameter (about 14.9?nm) were found to be dispersed on the carbon nanotubes. The adsorption of methylene blue dye on CoFe₂O₄/MWCNT composites has been investigated. CoFe₂O₄/MWCNT composites show high adsorption capacity for methylene blue dye. Both Langmuir and Freundlich models describe the adsorption isotherms very well and the adsorption thermodynamic parameters (?G ⁰, ?H ⁰ and ?S ⁰) were calculated. The adsorption of methylene blue is generally spontaneous and thermodynamically favorable. The adsorption of methylene blue involves an endothermic process.     

A potentiostatic study of hydrophobic ion transfer across lipid bilayer membranes: Part II. Transport of tetraphenylborate ions at high concentration in the aqueous solution

The transfer of tetraphenylborate (TPB^?) ions across black lipid membranes in aqueous solutions ranging from 10^?5M to 10^?3M, has been studied under potentiostatic conditions. A theoretical treatment of the problem accounts for the two time constants given and for the diffusion current for prolonged periods of time if one assumes that the adsorption-desorption transfer currents are large. This is tantamount to considering that these two processes are highly reversible and that the dipolar nature of the drop in potential between the closest approach and adsorption planes determines the adsorption. The resistances corresponding to the translocation and adsorption-desorption processes have thus also been evaluated. Using these results and those given by surface potential measurements on monolayers of glycerylmono-oleate, the number of adsorbed ions has been calculated. This has led to an explanation of the well known conductance maximum encountered when the concentration of the tetraphenylborate ions varies.     

Analysis and physical significancy of the kinetic parameters associated with albumin adsorption onto glassy carbon obtained by electrochemical impedance measurements

The kinetics of albumin adsorption onto a glassy carbon rotating disk electrode in a phosphate buffer, is reported from the time variations of the double-layer capacitance C_d, of the charge transfer resistance R_t and of the Tafel coefficient of the electrochemical reaction b; these three electrical quantities are determined by electrochemical impedance and faradaic current I measurements. The variations of C_d, 1/R_t, b  (R_tI)⁻¹ and I can be written under the form: α₀ + α₁ e^−t/τ1 + α₂ e^−t/τ2, where α₀, α₁, α₂, τ₁, τ₂ characterize a given electrical quantity. We demonstrate that this type of variation validates a two-step adsorption mechanism already proposed, starting from simply taking into account C_d(t). We analyze and discuss comprehensively various possibilities of interpreting the shifts between the time-constants associated with thevarious quantities as well as the correlations between these constants and those associated with physical adsorption phenomena.     

The effect of metal ions with different valences on the retardation of soil-bentonite barrier materials and its mechanism

In this paper, with K＋, Ca2＋ and Fe3＋ as the objects of study, retardation of soil-bentonite （SB） barrier materials for metal ions with different valences is investigated, and the adsorption mechanism, migration patterns and permeation behavior are explored so as to provide a theoretical basis for their application. The results show that the adsorption process for metal ions with different valences by SB barrier materials is fast, and the higher the valence, the greater the adsorption capacity. The fitting of the adsorption process conforms to pseudo-second-order adsorption kinetics and Langmuir-Freundlich adsorption equation, which explains that chemical adsorption is the dominating state and that the SB surface has certain heterogeneity. The permeability coefficient of K＋, Ca2＋ and Fe3＋ in SB each has a maximum and the higher the valence, the sooner the maximum appears. Also the higher the valence, the more obvious the effect on SB retardation performance; and the sooner the ion breaks through the barrier wall completely, that is, the wall＇s retardation performance for higher valent ions may decline.     

Adsorption and thermodynamics of biosurfactant,surfactin, monolayers at the air-buffered liquid interface

Adsorption of surfactin, a powerful lipopeptide biosurfactant, at the air-liquid interface has been investigated in this article. The adsorption took place from buffered solutions containing relatively high concentrations of surfactin co- and counterions. Dynamic surface tension measurements were used to follow the self-assembly of surfactin at the interface until equilibrium surface pressure Π ^e is reached at a given surfactin concentration (C _s). Gibbs adsorption equation in conjunction with the Langmuir adsorption isotherm was used to predict surfactin surface excess as a function of the biosurfactant concentration up to the critical micelle concentration (CMC). The predicted surface excess at saturation (Γ _∞ ) is 1.05?±?0.05 μmol m^?2, corresponding to an area per molecule (A _∞ ) of 159?±?8 Å². The adsorption equilibrium constant (K?=?(1.5?±?0.6)?×?10⁶ M^‐?1) was also estimated from the nonlinear regression of Π ^e???C _s data in region B of the Π ^e???ln?C _s plot. The value of K suggests that surfactin has strong affinity for the interface, which is in line with its known high surface activity. Gibbs elasticity (E _G) of the interfacial surfactin monolayers, which is an important thermodynamic property, was also predicted at different surfactin concentrations. The limiting value (at the CMC) of E _G was found to be 183 mN m^?1, which is comparable to those reported in the literature for similar systems. The findings reported in this work reveal that the surface tension measurements coupled with appropriate theoretical analysis could provide useful information comparable to those obtained using highly sophisticated techniques.     

Adsorption of tert-butanol at the electrode from concentrated NaClO4 solutions

The parameters of the double layer for tert-butanol adsorption determined in the supporting electrolytes: 2 mol dm^?3, 3 mol dm^?3 and 4 mol dm^?3 NaClO₄, show an increase of tert-butanol adsorption on the mercury electrode together with the increase of NaClO₄ concentration. The adsorption of tert-butanol on the electrode takes place via the ?CH₃ group which is shown by the changes in the values of zero charge potentials, E _z. On the basis of the analysis of the changes of relative surface excess values, Γ′, and the parameters determining the maximum adsorption, the process of adsorption in the discussed systems can be recognized as a physical process. Besides, it can be said that a major drawback in the process of adsorption of the organic substance on the electrode is to remove water from the electrode surface.     

Medium effects: The adsorption of ions on a mercury electrode from mixed H2O-organic solvent systems

The problems met in the investigations of ionic adsorption at the mercury electrode from mixed solvent systems are discussed. An attempt is made to give such an interpretation of non-monotonic changes of the anion adsorption as function of solvent composition which could be equally well applied for interpreting the non-monotonic changes of the rate constants of electrode reactions. The practical difficulties of evaluating the surface excesses in the case of mixed solvents are discussed. Some experimental results for the adsorption of SCN^? ions from the system 0.2 M (NaSCN+NaF) in H₂O+acetone are presented. A suggestion for checking the consistency of the evaluated surface excesses is given.     

Efficiency of Acacia Gummifera powder as biosorbent for simultaneous decontamination of water polluted with metals

Biosorbent materials represent an interesting alternative to classic methods of metal removal from industrial effluents. Acacia biomass showed a higher absorption capacity for heavy metals than living biomass. This study aimed to evaluate the bioadsorption of Lead and Cadmium onto Acacia Gummifera gum, using batch experiment. The structural characterization of the biosorbent was carried out using FT-IR, SEM, BET, TGA and DSC analysis. The adsorption equilibrium was reached within 15 min. A maximum uptake of 18.3 mg.g⁻¹ Pb²⁺ and 9.57 mg.g⁻¹ Cd²⁺ was achieved at pH 6.5. The metal ions seemed to be removed exclusively by ion exchange, physical sorption and chelation. The biomass of A. Gummifera powder was found to be effective for lead and cadmium removal with respectively 97% and 86% sorption efficiency at a concentration of 100 mg/L, in aqueous media. Parameters affecting adsorption capacities such as biosorbent dosage, initial metal concentration, temperature, and pH are discussed in detail. Furthermore, adsorption thermodynamics, kinetics, and equilibrium were studied and fitted by different models. Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms were used to compare adsorption data at equilibrium. The adsorption kinetics data were found to be best fitted by the pseudo-second-order model, and the adsorption isotherm was well fitted with the Langmuir model. The calculated thermodynamic parameters (ΔG⁰, ΔS⁰ and ΔH⁰) indicated a spontaneous and exothermic biosorption of both metal ions onto Acacia Gummifera. Moreover, chromatograms obtained by size exclusion chromatography coupled with multi-angle laser light scattering detection system (SECMALLS) showed the formation of complexes between the arabinogalactan-peptide (AGP) and glycoprotein (GP) Acacia moieties and the two studied metal ions. The analysis of the FTIR spectra of dried Acacia and that of Acacia loaded with lead and cadmium in aqueous media suggests that the surface functional groups such as amides and carboxy groups might be involved in the metal removal process.The extent of adsorption for both metals increased along with an increase of the A. Gummifera biomass dosage. A. Gummifera biomass, which is safe, of low-cost, and highly selective, seems therefore to be a promising substrate for simultaneous trapping of Pd and Cd ions in aqueous solutions.     

The admittance of an electrochemical cell with adsorption of electroactive species at the DME

Equations for the electrode admittance in, the case of adsorption of electroactive species have been derived, considering both a.c. and d.c. aspects of adsorption. It was shown that due to the d.c. aspects the function also contains a drop-time dependent term f(t_m): =c_OD_O^1/2+c_RD_R^1/2 +f(t_m)?s Under normal polarographic conditions the value of is constant, so c_O and c_R are drop-time dependent and consequently also the value of the Warburg coefficient σ, as well as the adsorption parameters C_LF and C_HF.     

Cadmium removal from aqueous solution by biochar obtained by co-pyrolysis of sewage sludge with tea waste

Resource utilization is a critical pathway for sustainable solid waste treatment. Biochar was prepared from the co-pyrolysis of sewage sludge and tea waste. Brunauer–Emmett–Teller measurement, scanning electron microscopy and Fourier transform infrared analysis were employed to characterize the biochar. Then, the interface behavior between biochar and Cd from aqueous solution was investigated. The effect of adsorbent dose and pH on Cd adsorption was evaluated. Adsorption kinetics and the adsorption isotherm were studied, and the adsorption mechanism was explored. The results showed that the suitable adsorbent dose was 4 g L^?1 and the optimal pH of the Cd solution remained at 6.0. Cadmium sorption on the biochar could be well described by the pseudo-second order kinetic model (R ² > 0.98). The adsorption process was described using the Langmuir (R ² > 0.86), Freundlich (R ² > 0.86), Temkin (R ² > 0.84) and Dubinin–Radushkevich (R ² > 0.86) isotherm models. The proportion of organic constituents in biochar was 69.2–72.4%. Minerals that originated in biochar played an important role during the Cd adsorption process, and the contribution of minerals accounted for 27.6–30.8% of the total adsorption. The main mechanism of the Cd adsorption process by biochar involved ion exchange, surface complexation, electrostatic interaction, surface co-precipitation, and other mechanisms. Therefore, biochar created by the co-pyrolysis of sewage sludge and tea waste could be used as an adsorbent for the removal of metal ions from contaminated water.     

Removal of uranium(VI) from aqueous solutions using nanoporous ZnO prepared with microwave-assisted combustion synthesis

The adsorption of the uranyl ions from aqueous solutions on the nanoporous ZnO powders has been investigated under different experimental conditions. The adsorption of uranyl on nanoporous ZnO powders were examined as a function of the contact times, pH of the solution, concentration of uranium(VI) and temperature. The ability of this material to remove U(VI) from aqueous solution was followed by a series of Langmuir and Freunlinch adsorption isotherms. The adsorption percent and distribution coefficient for nanoporous ZnO powders were 98.65 % ± 1.05 and 7,304 mL g^?1, respectively. The optimum conditions were found as at pH 5.0, contact time 1 h, at 1/5 Zn²⁺/urea ratio, 50 ppm U(VI) concentration and 303 K. The monomolecular adsorption capacity of nanoporous ZnO powders for U(VI) was found to be 1,111 mg g^?1 at 303 K. Using the thermodynamic equilibrium constants obtained at different temperatures, various thermodynamic parameters, such as ΔG°, ΔH° and ΔS°, have been calculated. Thermodynamic parameters (ΔH° = 28.1 kJ mol ^?1, ΔS° = 160.30 J mol^?1 K^?1, ΔG° = ?48.54 kJ mol^?1) showed the endothermic and spontaneous of the process. The results suggested that nanoporous ZnO powders was suitable as sorbent material for recovery and adsorption of U(VI) ions from aqueous solutions.     

Thermochemistry of n-alkylamines interaction with ETS-10 titaniunsilicate

According to the data from calorimetric adsorption of methylamine, under the best experimental conditions, 10 mg ETS-10 was saturated with 20 μl portions of an aqueous solution of methylamine at 0.05 mol dm⁻³ concentration corresponding to 0.490 mmol of methylamine per gram of solid. From calorimetric titration data, the thermodynamic values of ΔH, ΔG and ΔS were calculated, showing favourable n-alkylamine-ETS-10 interactions, from which the most favourable occurs for n-butylamine. The adsorption of these bases did not affect the structure of the titaniunsilicate as the XRD profile of ETS-10 is not altered. The FTIR spectra showed that with the increase in the amine mass the bands corresponding to the symmetric and asymmetric deformations at 1390 and 1467 cm⁻¹ increase in intensity along with the NH₂ deformation at 1530 cm⁻¹ as expected. The thermogravimetry shows two mass losses for ETS-10 that correspond to physisorbed water and water in smaller channels and cavities. After methylamine adsorption, the same behaviour was observed, however for butyl- and pentylamine three other mass losses were observed corresponding to decomposition and/or release of n-alkylamines.     

Estimating of surface activity of Cl− and Br− ions in the (In-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4] and (In-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] systems by different methods

Differential capacitance curves in the (In-Ga)/[N-methylformamide + mc KCl + (1 ? m)c KClO₄] and (In-Ga)/[N-methylformamide + mc KBr + (1 ? m)c KClO₄] systems are measured using an ac bridge for the following molar portions m of the surface-active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The Cl^? and Br^? anions specific adsorption in the systems can be described quantitatively by the Frumkin isotherm. The principal parameters of Cl^? and Br^? anions adsorption at the (In-Ga)/N-methylformamide interface are determined by different methods. Unlike Ga/N-methylformamide interface, where the adsorption energy increased in the sequence I^? ≈ Br^? < Cl^?, at the (In-Ga)/N-methylformamide interface it increased in the reverse sequence: Cl^? < Br^? < I^?. The adsorption parameters at the charge density q = 0, obtained by three different methods, are close to each other. However, the parameters α₁ and α₂, which characterize the charge effect on the adsorption energy, when determined by the analyzing of dependences of adsorption potential drop E _ads on ln(mc), differ from those determined by two other methods. The error may be caused by the assuming that the adsorption potential drop is proportional to the coverage of dense layer with the specifically adsorbed ions.     

Adsorption properties of CdS-CdTe system semiconductors

The adsorption of carbon(II) oxide and ammonia on nanofilms of solid solutions and binary compounds of the CdS-CdTe system is studied by means of piezoquartz microweighing, FTIR IR, and measuring electroconductivity. Allowing for the conditions and composition of semiconductor systems, we determine the mechanisms and principles of adsorption processes by analyzing the α _p = f(T), α _T = f(p), and α _T = f(t) experimental dependences; IR spectra; the thermodynamic and kinetic characteristics of adsorption; the acid-base, electrophysical, and other characteristics of adsorbents; the electron nature of adsorbate molecules; and the obtained acid-base characteristics: the composition and adsorption characteristics and composition state diagrams. Previous statements on the nature and retention of local active centers responsible for adsorption and catalytic processes upon changes in their habitus and composition (as components of systems of the A^IIIB^V-A^IIB^VI and A^IIB^VI-A^IIB^VI types) on the surface of diamond-like semiconductors are confirmed. Specific features of the behavior of (CdS) _x (CdTe)_{1 ? x} solid solutions are identified in addition to general features with binary compounds (CdS, CdTe), as is demonstrated by the presence of critical points on acid-base characteristics-composition and adsorption characteristics-composition diagrams. On the basis of these diagrams, the most active adsorbents (with respect to CO and NH₃) used in designing highly sensitive and selective sensors are identified.     

Reversible and irreversible adsorption of particles on homogeneous surfaces

The kinetics of localized reversible and irreversible adsorption of interacting particles on homogeneous surfaces was analysed. Asymptotic analytical equations were derived for the surface blocking parameter B(0), and for adsorption kinetics and adsorption isotherms in the limit of low and high surface concentrations. It was found that the geometrical blocking effect was much more pronounced than the Langmuir model predicts, especially for high surface concentrations and low ionic strengths of suspensions.The new adsorption isotherm formulated indicates that for a large adsorption constant, K_a, the equilibrium surface concentration becomes proportional to K^−1/3_a, whereas in the Langmuir model this quantity is approached as K⁻¹_a (for K_a ≫I). In the case of irreversible adsorption the theoretical predictions were experimentally tested by applying the direct microscope observation method. Monodisperse suspensions of negatively charged latex particles were used in these experiments with silanized mica sheets as the adsorbing surface. Our theoretical predictions were quantitatively confirmed, indicating that the Langmuir model is not appropriate for describing localized adsorption of particles on homogeneous surfaces.     

Adsorption of organics on MSC5A in supercritical CO2, chromatographic measurements & stop & go simulation

Chromatographic measurements were made for the adsorption of benzene, toluene and m-xylene on molecular sieving carbon (MSC) in supercritical fluid CO₂ mixed with organics. Supercritical chromatograph packed with MSC was used to detect pulse responses of organics. Adsorption equilibria and adsorption dynamics parameters for organics were obtained by moment analysis of the response peaks. Dependences of adsorption equilibrium constants, K ^?, and micropore diffusivity,?D, on the amount adsorbed were examined. The dependencies of adsorption equilibrium constants, K ^?, and micropore diffusivity, D, of benzene, toluene and m-xylene, on molarity of benzene with each parameters of temperature or pressure were obtained. It was found that the values of K ^? and D for an organic substance depended on the amount adsorbed of other organics strongly. And stop & go method was used as simulation method of perturbation chromatography for investigating adsorption equilibrium and rate. Numerical solution for multicomponent chromatogram in time domain could be obtained by appropriate model equations with experimental conditions. This simulated chromatogram can be compared with experimental chromatogram to determine the adsorption equilibrium and rate parameters. In addition, molecular simulation of multicomponent adsorption equilibria was performed, and potential parameters were determined by comparing the simulation with experimental results. Simulation soft ware is Cerius2 (Version?4.2) made by MSI. The purpose of performing simulation is to elucidate an adsorption mechanism on the molecule level.     

Adsorption of UO2 2+ on poly(N,N-diethylacrylamide-co-acrylic acid): effects of pH, ionic strength, initial uranyl concentration, and temperature

We prepared poly(N,N-diethylacrylamide-co-acrylic acid) (P(DEA-co-AA)) microgels which could efficiently remove UO₂ ²⁺ from aqueous solutions. In this study, the effect of adsorption parameters such as pH value, adsorbent dose, shaking time, and temperature has investigated. It is found that the pseudo-second-order model is more suitable for our experiment. The adsorption kinetic data indicated that the chemical adsorption was the swiftness processes, the adsorption equilibrium could be achieved within 30 min. And there are very good correlation coefficients of linearized equations for Langmuir isotherm model, which indicated that the sorption isotherm of the hydrogel for UO₂ ²⁺ can be fitted to the Langmuir isotherm model. The adsorption process was spontaneous (?G ⁰ < 0) and exothermic (?H ⁰ < 0). The adsorbed UO₂ ²⁺ can be desorbed effectively by 0.1 M HNO₃ and the adsorption capacity is not significantly reduced after five cycles. Present study suggests that this P(DEA-co-AA) can be used as a potential adsorbent for sorption UO₂ ²⁺ and also provide a simple, fast separation method for removal of UO₂ ²⁺ ions from aqueous solution.