SPC cluster modeling of metal oxides: ways of determining the values of point charges in the embedded cluster model

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Equilibrium,kinetic and thermodynamic biosorption studies of Hg(II) on red algal biomass of Porphyridium cruentum

Among a variety of microbial materials employed for biosorption, algae have added advantages of non-toxic and autotrophic nature. In this study, biosorption of Hg(II) was studied with red algal biomass of Porphyridium cruentum. The parameters affecting biosorption such as dosage of biosorbent, pH, contact time, initial metal concentration, temperature and effect of foreign metal cations in binary system were evaluated. Kinetic data were described with the help of pseudo-first-order and pseudo-second-order kinetic models. Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherm models were applied to adsorption equilibrium data. According to the results, the maximum removal capacity (q_max) was 2.62?mg/g observed at pH 7 with 0.25?g/L of biosorbent dosage for Hg(II) solution containing 10?mg/L of metal ions. The Langmuir isotherm model fits best to the adsorption data while the kinetic data followed the pseudo-second-order model. Thermodynamics studies showed that the biosorption process of Hg(II) on P. cruentum was exothermic in nature.     

A model for the examination of phase behaviour in multicomponent systems

The Percus—Yevick equation for a multicomponent system of adhesive spheres is solved exactly. For an m-component system, the phase behaviour is determined by the number and nature of zeros of a system of $\text{m}\text{(}\text{m}\text{+ 1)}\text{2}$ quadratic equations.     

Thermodynamic and kinetic analysis of metal ion complexation by photochromic spiropyrans

This review focuses on the results of the studies of metal ion complexation by spiropyrans published mostly in recent years. Introduction of ionophore substituents in the molecules of photochromic spiropyrans induces a reasonably strong bonding in the complexes of these compounds with metal ions. The degree of complexation depends on the state (open or closed) of the photochromic spiropyran fragment. Directed synthesis of structures able to form complexes with metal ion requires knowledge of structure-property relationships based, among other, on the studies of the compounds synthesized earlier. This requires accurate determination of thermodynamic and kinetic parameters of complex formation. Difficulties of such quantitative analysis due to the presence of several interrelated equilibria in the system are discussed.     

Monosaccharides and the VO(IV) metal ion: Equilibrium,thermal studies and hypoglycemic effect

Protonation and complexation equilibriums of monosaccharides and the VO(IV) metal ion in aqueous solution were studied as well as their effect on the hyperglycemia of diabetic rats. The complexes formed were characterized by potentiometric titrations, paramagnetic resonance spectroscopy (EPR) and thermogravimetric–differential scanning calorimetry (TGA–DSC). The system involving d-gluconic acid (HGlu) and oxovanadium(IV) (VO²⁺) was chosen to study the serum glucose levels in alloxan-induced diabetic rats. A binuclear species was detected in small quantities, which was formed by coordination of two HGlu molecules and two VO²⁺ ions through a hydroxide bridge. The mononuclear species formed by HGlu and VO²⁺ were confirmed by EPR. The anisotropic spectra obtained from aqueous frozen solutions (77 K) are characteristic of mononuclear VO-hexoses. The cyclic sugars d-ribone-1,4-lactone (Riblac), d-galactone-1,4-lactone (Galac) and 2-deoxy-d-glucopyranose (dGlu) showed weak interactions with the metal ion and they are not able to hold the metal in solution above pH 4.6 resulting in hydrolysis of the metal ion. Also, the acute treatment with sugar complexes of HGlu–VO led to a significant hypoglycemic effect (23% and 18% by intraperitoneal or oral gavage treatment, respectively) in diabetic rats. These results show the potential effectiveness of VO–HGlu complexes as anti-hyperglycemic agents through intraperitoneal injection in alloxan-induced diabetic rats.     

Simultaneous kinetic determination of metal ion mixtures based on a ligand substitution reaction

The rates of the ligand substitution reactions between 2-(4-sulfophenylazo)-l,8-dihydroxy-3, 6-naphthalenedisulfonic acid (SPADNS) complexes of Co²⁺, Ni²⁺, Pb²⁺ and Cu²⁺ and EDTA (or CDTA) have been studied at 25.0°C, pH 8.0–9.3, by stopped-flow spectrophotometry. The reactions are first order in SPADNS complex and in incoming ligand. The absorbance change during the reaction progress can be kinetically separated for each component of a mixture, so that metal ions down to 10^-6 M can be determined. Components of very high reactivity, such as Cd²⁺, Mn²⁺ or Zn²⁺, can also be determined as part of a multicomponent mixture.     

Computer modeling of the structure of supramolecular systems: An example of ion exchangers

A method is proposed for the determination of the structure of supramolecular systems based on spectrum-structure correlations and quantum-chemical and molecular dynamic modeling. Application of the developed approach to the analysis of the structures carboxy- and sulfo-cation exchangers as alkali metal salts and an amino acid (glycine) suggests that the ion pairs in the studied systems are dissociated. This allows a conclusion that the retention of ions in the ion exchangers in chromatographic separation is due to not only the electrostatic interaction of the fixed and mobile ions, but also hydrogen bonding between the hydration shells of the counterions.     

Adsorption of mercury from single and multicomponent metal systems on activated carbon developed from cherry stones

The adsorption of mercury from a single/multi-solute aqueous solution by activated carbon (AC) prepared from cherry stones (CS) by chemical activation with H₃PO₄, ZnCl₂ or KOH is studied. Three series of AC (i.e., P, H₃PO₄; Z, ZnCl₂; K, KOH) were prepared by controlling the impregnation ratio and carbonization temperature. The textural characterization of AC was carried out by gas adsorption, mercury porosimetry and density measurements. The surface chemistry was analyzed by the pH of the point of zero charge (pH_zpc), FT-IR spectroscopy and Boehm’s method. Experiments of mercury adsorption were conducted by the batch method, using aqueous solutions of mercury and of mercury, cadmium and zinc without pH adjustment. The ACs possess a wide range of pore volumes and sizes. Their microporosity is usually well developed. The meso- and macropore volumes are higher for the P carbons and K carbons, respectively. BET surface areas as a rule range between 1000 and 2000 m²?g^?1. The pH_zpc is much lower for the P carbons. The content of acidic oxygen surface groups is lower for the K carbons, whereas the content of basic groups is higher for these carbons. The kinetics of the adsorption process of mercury is faster for ACs with high volumes of large size pores. However, the surface groups have a marked unfavorable influence on the kinetics. The pseudo-second order rate constant (k₂×10^?3, g/mol?h) is higher by the order Z-4-800 (67.69)>K-3-800 (43.45)>P-3.44-400 (36.98). The incorporation of zinc and cadmium to the mercury solution usually decelerates the adsorption process for the P carbons and Z carbons and accelerates it for the K carbons. The amount adsorbed of mercury is much larger for the K carbons than for the other ACs. For the Z carbons, competition effects of zinc and cadmium on the adsorption of mercury are negligible, which indicates that mercury adsorbs specifically on surface active sites of these adsorbents.     

Trace determination of metal ion inhibitors of the urea-urease system by aph-stat kinetic method

Summary A p_H-stat, kinetic method, based on the decreased rate of the ureasecatalyzed hydrolysis of urea, was developed for the trace determination of eight metal ion inhibitors. The kinetic parameters,v _M andK _m for the urease system and the apparent inhibition constants,K ₀, for the metal ion interactions were measured. The sensitivities of the metal ions in decreasing order ofK ₀ are: Ag(I) > Hg(II) > Cu(II) > Cd(II)> > Mn(II) Co(II) > Pb(II) > Ni(II).Ag(I) was determined in the range of 2–10 · 10^–8 M with a maximum error of 11.6% relative which corresponds to an absolute error of 0.5 ng. The precision of individual determinations was ±2% relative. Results clearly indicate the adaptability of enzymatic inhibition to trace metal ion analyses.

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Zusammenfassung Zur Spurenbestimmung von acht Metallionen, die als Hemmstoffe der Ureasewirkung auf die Harnstoffhydrolyse wirken, wurde eine kinetische Methode mit Hilfe eines p_H-Staten ausgearbeitet. Die kinetischen Parameterv _M undK _m für das Ureasesystem und die HemmungskonstantenK ₀ für die Einwirkung der Metallionen wurden gemessen. Nach absteigendenK ₀-Werten ergab sich folgende Empfindlichkeitsreihe: Ag(I) > Hg(II) > Cu(II) > > Cd(II) > Mn(II) Co(II) > Pb(II) > Ni(II).Ag(I) wurde in der Größenordnung 2–10 · 10^–8 Mol mit einem maximalen Fehler von rel. 11,6% bestimmt, was einem absoluten Fehler von 0,5 ng entspricht. Die Genauigkeit der Einzelbestimmungen betrug ±2% rel. Die Ergebnisse zeigen eindeutig die Anwendbarkeit der Fermenthemmung für die Analyse von Metallspuren.

     

A kinetic model for the growth of znte by metal organic chemical vapour deposition

A kinetic model for the metal organic chemical vapour deposition (MOCVD) growth of ZnTe is presented, taking into account the competitive adsorption of organometallic precursors. By assuming that diethylzinc (DEZn) and diethyltellurium (DETe) or di-isopropyltellurium (DIpTe) are adsorbed onto the surface by two sites, the model yields the growth rate as a function of the gas-phase concentrations of the constitutents and is corroborated by experimental results obtained by the MOCVD growth (at 400°C with DETe or 350°C with DIpTe), which shows asymmetric behaviour: for a given DETe or DIpTe pressure (10^?4 atm), the growth rate as a function of DEZn partial pressure passes through a maximum, whereas, at the same constant DEZn pressure, the growth rate increases monotonically when the DETe or DIpTe partial pressure increases     

Efficient uranium(VI) biosorption on grapefruit peel: kinetic study and thermodynamic parameters

The uranium(VI) biosorption by grapefruit peel was studied from aqueous solutions. Batch experiments was conducted to evaluate the effect of contact time, initial uranium(VI) concentration, initial pH, adsorbent dose, salt concentration and temperature. The equilibrium process was well described by the Langmuir, Redlich–Peterson and Koble–Corrigan isotherm models, with maximum sorption capacity of 140.79 mg g⁻¹ at 298 K. The pseudo second order model and Elovish model adequately describe the kinetic data in comparison to the pseudo first order model and the process involving rate-controlling step is much complex involving both boundary layer and intra-particle diffusion processes. The effective diffusion parameter D _i and D _f values were estimated at different initial concentration and the average values were determined to be 1.167 × 10⁻⁷ and 4.078 × 10⁻⁸ cm² s⁻¹. Thermodynamic parameters showed that the biosorption of uranium(VI) onto grapefruit peel biomass was feasible, spontaneous and endothermic under studied conditions. The physical and chemical properties of the adsorbent were determined by SEM, TG-DSC, XRD and elemental analysis and the nature of biomass–uranium (VI) interactions was evaluated by FTIR analysis, which showed the participation of COOH, OH and NH₂ groups in the biosorption process. Adsorbents could be regenerated using 0.05 mol L⁻¹ HCl solution at least three cycles, with up to 80% recovery. Thus, the biomass used in this work proved to be effective materials for the treatment of uranium (VI) bearing aqueous solutions.     

Equilibrium and kinetic studies on the reactions of alkylcobalamins with cyanide

Ligand substitution equilibria of different alkylcobalamins (RCbl, R = Me, CH(2)Br, CH(2)CF(3), CHF(2), CF(3)) with cyanide have been studied. It was found that CN(-) first substitutes the 5,6-dimethylbenzimidazole (Bzm) moiety in the alpha-position, followed by substitution of the alkyl group in the beta-position trans to Bzm. The formation constants K(CN) for the 1:1 cyanide adducts (R(CN)Cbl) were found to be 0.38 +/- 0.03, 0.43 +/- 0.03, and 123 +/- 9 M(-1) for R = Me, CH(2)Br, and CF(3), respectively. In the case of R = CH(2)CF(3), the 1:1 adduct decomposes in the dark with CN(-) to give (CN)(2)Cbl. The unfavorable formation constants for R = Me and CH(2)Br indicate the requirement of very high cyanide concentrations to produce the 1:1 complex, which cause the kinetics of the displacement of Bzm to be too fast to follow kinetically. The kinetics of the displacement of Bzm by CN(-) could be followed for R = CH(2)CF(3) and CF(3) to form CF(3)CH(2)(CN)Cbl and CF(3)(CN)Cbl, respectively, in the rate-determining step. Both reactions show saturation kinetics at high cyanide concentration, and the limiting rate constants are characterized by the activation parameters: R = CH(2)CF(3), DeltaH = 71 +/- 1 kJ mol(-1), DeltaS = -25 +/- 4 J K(-1) mol(-1), and DeltaV = +8.9 +/- 1.0 cm(3) mol(-1); R = CF(3), DeltaH = 77 +/- 3 kJ mol(-1), DeltaS = +44 +/- 11 J K(-1) mol(-1), and DeltaV = +14.8 +/- 0.8 cm(3) mol(-1), respectively. These parameters are interpreted in terms of an I(d) and D mechanism for R = CH(2)CF(3) and CF(3), respectively. The results of the study enable the formulation of a general mechanism that can account for the substitution behavior of all investigated alkylcobalamins including coenzyme B(12).     

Study of a heavy metal biosorption onto raw and chemically modified Sargassum sp. via spectroscopic and modeling analysis

In this study, raw and formaldehyde-modified Sargassum sp. are used for heavy metal removal. A series of experiments shows that the chemical modification by formaldehyde improves biosorption capacity by approximately 20%. Solution pH plays an important role in the metal uptake. According to X-ray photoelectron spectroscopic and Fourier transform infrared spectroscopic analysis, the possible organic functional groups in the metal binding include carboxyl, ether, alcoholic, hydroxyl, and amino functional groups. A new model that includes a series of coordination reactions among a generalized functional group, alkaline earth metal ions and heavy metal ions, is developed for simulation of biosorption process. The model well describes the single- and multiple-species metal biosorption process under different conditions such as pH. The biosorption of heavy metals is due to the ion exchange between the heavy metals and alkaline earth metals and their adsorption onto the free sites of the seaweeds. Slightly more than half of the metal uptake is due to ion exchange. The metal affinity for the functional groups follows a descending order of lead > copper > alkaline earth metal.     

The structure of the ion: An ion kinetic energy spectrometry study

The kinetic energy release (T) values for the loss of CO from ions from five compounds have been obtained and are consistent with at least two different structures for the ion. This result is supported by the T value obtained for the decomposition of the molecular ions.