Calorimetric study of the immersion heats of lead(II) and chromium(VI) from aqueous solutions of Colombian coffee husk

Summary This work shows the result of the study of the Pb(II) and Cr(VI) ions adsorption by means of a Calvet type calorimeter of heat conduction that can operate at 150şC. The calorimeter was electrically calibrated to establish its sensitivity and reproducibility, obtaining K=12.95±0.05 W V^-1 and chemically it was examined with tris-(hydroxymethyl)-aminomethane (THAM)-HCl system, obtaining ΔH= -30.91±0.03 kJ mol^-1. The activated carbon sample obtained from coffee husk and the calorimetric results obtained were related to other techniques used to perform this type of studies.     

Effect of solvent media and condensed benzene rings on thermochemical behaviour of dibenzo-18-crown-6 in solution

The specific and non-specific interactions of twelve activated carbon cloth samples prepared from commercial cotton fabric, and that present different activation degrees are studied through the determination of immersion enthalpies in CCl₄ and H₂O, and in aqueous solutions of NaOH and HCl. The immersion enthalpies found for the solvents CCl₄ and H₂O are in a range of 5.49–45.84 and 1.77–7.76 J g⁻¹, respectively. The enthalpic values for the materials in aqueous solutions of NaOH and HCl, allow characterizing the chemical surface of these materials, which are in a range of 6.63 and 21.49 J g⁻¹, finding through them important relations in company with other characterizing techniques used in the study of these materials.     

On the mechanisms of phenol adsorption by carbons

The removal of phenol and related compounds from dilute aqueous solutions by activated carbons corresponds to the coating of the micropore walls and of the external surface by a monolayer. This process is described by an analog of the Dubinin—Radushkevich—Kaganer equation. On the other hand, as suggested by immersion calorimetry at 293 K, in the case of concentrated solutions, the mechanism corresponds to the volume filling of the micropores, as observed for the adsorption of phenol from the vapor phase. The equilibrium is described by the Dubinin—Astakhov equation. It follows that the removal of phenol from mixtures with water depends on the relative concentrations, and the limiting factor for adsorption is either the effective surface area of the carbon, or the micropore volume.     

Energetics of methane adsorption on microporous activated carbons

The influence of microporous carbon surface oxidation on energetics of methane adsorption at 308 K is discussed. Obtained adsorption heats and integral molar entropies of the adsorbate show that microporous carbon surface oxidation changes the methane adsorption process. This is probably resulted by the existence of an endothermic effect during adsorption in oxidized carbon micropores.     

Thermodynamics of Dipeptides in Water. Calorimetric determination of enthalpy change values related to proton transfer processes of a series of dipeptides in water

Calorimetric measurements on a series of dipeptides having methionine as first common term enabled us to determine the thermodynamic quantities related to the proton transfer processes at the free α-amino and α-carboxyl groups of the dipeptides using methionyl-methionine (MetMet) as reference compound. The drawing forces were found to be the solvation processes of zwitterions, ions and undissociated molecules of dipeptides, hydrophobic effects of side-chains playing an important role. The results were interpreted in terms of the reciprocal influence of the dipeptides components. Relative and absolute scales related to the enthalpic contribution to the acidity and basicity of the dipeptides studied are supplied. This revised version was published online in July 2006 with corrections to the Cover Date.     

Removal of copper from aqueous solutions by adsorption on activated carbons

The adsorption isotherms of Cu(II) ions from aqueous solutions in the concentration range 40–1000 mg l⁻¹ on two samples of granulated and two samples of activated carbon fibres containing varying amounts of associated oxygen have been reported. The adsorption isotherms are type I of BET classification showing initially a rapid adsorption tending to be asymptotic at higher concentrations. The amounts of oxygen associated with the carbon surface has been enhanced by oxidation with nitric acid and ammonium persulphate in the solution phase and with oxygen gas at 350°C and decreased by degassing of the oxidized carbon samples at 400, 650 and 950°C. The adsorption of Cu(II) ions increases on oxidation and decreases on degassing. The increase in adsorption on oxidation depends on the nature of the oxidative treatment while the decrease in adsorption on degassing depends on the temperature of degassing. This has been attributed to the increase in the carbon–oxygen acidic surface groups on oxidation and their decrease on degassing. Suitable mechanisms consistent with the results have been proposed.     

Calorimetric study of liquid gadolinium-based alloys

The enthalpies of mixing of liquid Gd-Si (1770±5 K) and Al-Gd (1760±5 K) alloys have been measured by high-temperature isoperibolic calorimetry. The calorimetric study of the gadolinium-based liquid alloys demonstrates the great negative enthalpies of mixing, which is associated with the contribution of GdSi and GdAl₂ intermetallides into the liquid-state thermodynamics. The comparison of obtained results with literature data has been performed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Calorimetric investigation of the Cu-Sn-Bi lead-free solder system

The thermodynamic properties play a crucial role in the development of new solder materials. In this work a calorimetric investigation of the ternary Cu-Sn-Bi system was carried out by using a Calvet-type calorimeter in order to obtain the molar limiting partial enthalpy of Cu in liquid Sn-Bi alloys with different compositions. The molar limiting partial enthalpy of Cu at 820 K was determined in the Sn-75 at% Bi, Sn-43 at% Bi and Sn-26 at% Bi liquid bath showing an endothermic behaviour. The results are compared with the literature data available for Cu in the pure liquid Bi and Sn and then discussed.     

Calorimetric Studies of Hydration of Cementitious Materials Varying in Basicity

This paper presents results of thermochemical and thermokinetic studies of the hydration of portland cement, alkaline cements with mineral additives and slag alkaline binder. The influence of the modulus of basicity of the binders on the thermochemical and thermokinetic characteristics of hydration was estimated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of the concentration of polybasic aminoanilides and aminobenzimidazoles on the acidity functionH 0 of aqueous solutions of HCl

The effect of the concentrations of the reactants and reaction products on the protonating ability of aqueous HCl is investigated in the example of the cyclization of polybasic aminoanilides to aminobenzimidazoles at 25°C and 50°C. It is established that the acidity functionH ₀ ^s of HCl solutions in organic-aqueous solvents of constant composition is equal toH ₀, the acidity function for an aqueous solution with the same concentration of HCl on the mole-ratio scale. This can be utilized for the quantitative prediction ofH ₀ ^s. The decrease in the basicity of indicators in mixed solvents is a consequence of the decrease in the activity coefficient of the unionized form of the base standardized to water _B. It is observed that not onlyH ₀ ^s but also the basicity constant of the reactant may vary during the reaction. The latter depends on the concentration of the reactant, as well as on the effect of the product concentration on the value of _B for the reactant.Deceased April 8, 1991.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 488–491, March, 1993.     

Immersion enthalpy of carbonaceous samples in aqueous solutions of monohydroxilated phenols

The immersion enthalpies of modified activated carbons were determined, with commercial Carbochem^TM–PS230 (CAG) as the initial activated carbon, which was modified by: chemical treatment with HNO₃ 7 mol L⁻¹ (CAO) and thermal treatment under flow of H₂ (CAR) in function of the adsorbed quantity of monohydroxilated phenols, catechol, resorcinol and hydroquinone at a pH of 7 in aqueous dissolutions in order to characterize the solid–solution interaction and evaluate the influence of the chemical characteristics of the activated carbon in the phenol adsorption. The results show a variation in the immersion enthalpy in function of the adsorbed quantity of phenol and the initial dissolution concentration; which shows that the intensity of the interaction changes in function of the composition of the liquid phase. The immersion enthalpies present the following arrangement: catechol > resorcinol > hydroquinone, with a −ΔH_inm of 35.7; 30.8 and 24.6 Jg⁻¹, respectively, at a pH of 7 for a 100 mg L⁻¹ phenol monohydroxilated solution.     

量热研究铝与钠蒙脱土交换反应

Exchange enthalpies of aluminum with Na-montmorillonite suspension are determined at 3O3.15K and PH=3, and 4, using titration calorimetry and the exchange reaction mechanism are further studied by combining calorimetric results and exchange isotherm determined by static method. The adsorption of aluminum on Na-montmorillonite is endothermic, and therefore exchange reaction is a process driven by entropy. X-ray diffraction patterns of montmorillonite saturated by Na+ and Al3+ are determined at wet and dry state respectively. The results show Al3+ adsorbed on the clay can dramatically reduce the expansion of montmorillonite because Al3+prevents osmotic swelling.     

Calorimetric Study of the Acidity of a New Family of Mesoporous Catalysts

Thermally stable mesoporous aluminophosphates (AIPO) and silicoaluminophosphates (SAPO) were prepared at room temperature in the presence of a cationic surfactant and an organic base. These materials possess high surface areas and regular mesopores of approximately 35 Å diameter. By contrast to microporous crystalline aluminophosphate molecular sieves, mesoporous compounds are amorphous and characterized by Al/P ratios greater than 1. These particularities are responsible for a strong Lewis acidity, as evidenced by ammonia adsorption microcalorimetry. Mesoporous materials are more acidic than the microporous analogues and the amount of strong acid sites increases with the silicon content. This revised version was published online in July 2006 with corrections to the Cover Date.     

Relation between immersion enthalpy and the acidity of clay pillared minerals

Total acidity for a series of modified clays obtained from a natural vermiculite is determined through temperature programmed desorption (TPD) using ammonia as probe molecule. Results obtained for the acidity range from 15.1 to 68.5 meq/100 g. Immersion enthalpies of the clays in benzene, water and aqueous solutions of NH₃ 0.058 M and NaHCO₃ 0.053 M are determined. The results obtained show that immersion enthalpies in benzene and water are between −6.26 and −25.6 J g⁻¹ and −2.10 and 5.55 J g⁻¹, respectively and are smaller than the values obtained for the immersion enthalpies in the solutions. Immersion enthalpy values in NH₃ solution are greater than the obtained using NaHCO₃. Linear relations between the total acidity of the clays and the immersion enthalpies in the basic solutions are determined. An interaction factor using ammonia is calculated since the relation between the immersion enthalpy in ammonia solution and in water and it may be deduced that the relation with the total acidity is of second order tendency between them.     

Calorimetric investigations on molecular interactions in a mixed layer adsorbed on a homogeneous surface

The adsorption and differential heats of adsorption of mixtures of methanol and tetrahydrofuran (mole ratio 11), methanol and cyclopentane (11 and 14) and tetrahydrofuran and cyclopentane (11 and 14) on a graphitized carbon black (Sterling MT) surface were determined.The dependence of the intermolecular interactions on the composition of the adsorbed layer was established. From an analysis of the experimental results, the mechanism of adsorption of the equimolar methanol-tetrahydrofuran mixture was described, in which both homomolecular. and heteromolecular association were taken into consideration.

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Zusammenfassung Es wurde die Adsorption und die differentiellen Adsorptionswärmen äqnimolarer Gemische aus Methanol mit Tetrahydrofuran bzw. aus Methanol und Tetra-hydrofuran mit Cyclopentan (Molverhältnis 11 bzw. 14) an mit Graphit überzogenen Ruß Sterling MT Oberflächen bestimmt.Dabei wurde die Abhängigkeit der intermolekularen Wechselwirkungen von der Zusammensetzung der adsorbierten Schicht aufgedeckt. Ausgehend von der Analyse der experimentellen Ergebnisse wurde der Mechanismus der Adsorption eines Methanol-Tetrahydrofurangemisches beschrieben, wobei sowohl homoals auch heteromolekulare Assoziation berücksichtigt wurde.

     

Bioregeneration of activated carbons by bacterial degraders after adsorption of surfactants from aqueous solutions

The feasibility of activated carbon (AC) bioregeneration after adsorption of biologically resistant surface-active substances (SAS) by a community of strains of SAS-degrading bacteria of the genus Pseudomonas are analyzed. It was shown that the degree of AC equilibrium saturated with nonionic SAS under static conditions was 23–35%. The contribution of bacterial destruction of SAS to the restored capacity was 20–23% and, for all practical purposes, did not depend on the AC porosity.

The efficiency of regeneration of the microporous AC after adsorption by it of the nonionic and anionic SAS under dynamic conditions was 22 and 95% of the respective capacities of fresh samples. The extent of regeneration of the mesoporous AC after dynamic sorption of nonionic SAS was 85%. Results indicate that regeneration is based on desorptive and migrational movements of the adsorbate toward the biocenosis of SAS-degrading bacteria located on the external surface of the AC grain. The adsorptive properties of the AC–SAS system and the sorbents’ porous structure determine the probability of desorption of molecules and the rate of their diffusion into the peripheral zone of the grain. The AC regeneration efficiency depends on the nature of the adsorbate distribution in its porous structure.     

A Calorimetric Study of Alloy Formation in Au-Cu and Au-In Systems

A calorimetric study of alloy formation in Au-Cu and Au-In systems was carried out. The heats of mixing, ΔH _mix, were measured at 1380,1484 and 1604 K for Au-Cu and at 1387, 1449 and 1505 K for Au-In in the full concentration range. The results of experiments were described by approximating equations. Observed values of ΔH _mix were discussed using the phase diagrams of the systems. It was shown that the systems become closer to the ideal state with increasing temperature. From the experimental data, the dependencies of the enthalpies of the systems on the composition and temperature were estimated. The simultaneous effects of composition and temperature on enthalpy are represented in 3D diagrams. The specific heats of the alloys, in the temperature range of experiments, were derived from the slopes of enthalpy planes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Simultaneous determination of enthalpies of dilution and dissociation of protocatechuic acid in aqueous salt solutions by calorimetric measurements

Protocatechuic acid, a major metabolite of antioxidant polyphenols found in green tea, has been shown to prevent carcinogenesis or antitumor growth in vitro and in vivo studies. The dilution enthalpies Δ_dilH_m, dissociation enthalpies Δ_disH_m and thermodynamic dissociation constants K_a for protocatechuic acid in aqueous NaCl or KCl solutions were simultaneously determined by mixing-flow microcalorimetry at T = 298.15 K. In order to verify the reliability of the fitted dissociation parameters, the values of dissociation enthalpies and thermodynamic dissociation constants were also determined by isothermal titration calorimetry and electrical conductivity methods. The K_a values obtained through the proposed method agree well with those reported in literatures and obtained by other techniques. Enthalpic interaction coefficients (h₂, h₃ and h₄) were computed according to the McMillan–Mayer model. The trends of h₂ and Δ_disH_m for protocatechuic acid with increasing salt molality in both the salt solutions were obtained. The different influence of KCl and NaCl on the values of h₂ and Δ_disH_m were discussed in terms of (solute + solute) and (solute + solvent) interactions. The results showed that it is possible to perform in a single calorimetric experiment the simultaneous determination of dilution enthalpies, dissociation enthalpies and constants in a given solvent.