Enthalpic characterization of activated carbons obtained from <Emphasis Type="Italic">Mucuna Mutisiana</Emphasis> with different burn-offs

Immersion enthalpies of activated carbon samples obtained by activation with steam at temperatures between 600 and 900 °C and activation times between 1 and 10 h were determined. The calorimetric liquids of immersion are CCl₄, water, NaOH, and HCl 2 M solutions, and the values of the immersion enthalpies are related to other properties of the activated carbons such as the surface area B.E.T., the micropore volume, the content of acid, and basic surface groups. The highest values for the immersion enthalpies take place for the polar solvent CCl₄ and for HCl solution, with values between 4.0 and 75.2 J g⁻¹ and 9.15 and 48.3 J g⁻¹, respectively.     

Calorimetric determination of activated carbons in aqueous solutions

The interactions among five samples of activated carbons, obtained from different lignocellulosic materials with different degrees of activation of approximately 20% and aqueous solutions of phenol and 4-nitro phenol are studied by means of the determination of immersion enthalpies. It is established that the obtained activated carbons are of a basic character and show values for the pH at the point of zero charge, pH_PZC, that range from 7.4 to 9.7 and, in all cases, higher total basicity contents than the values obtained for total acidity. The immersion heat of the activated carbons in CCl₄ and water is determined obtaining values which are higher for CCl₄ immersion and vary from 31.4 to 48.6 J g⁻¹. The hydrophobic factor, hf, it is calculated from the relation between of the immersion heat of the activated carbons in CCl₄ and the immersion heat in water, the obtained values were 2.98 and 6.75, which are greater than 1 due to the greater values obtained in CCl₄ when compared to the values obtained in water. Immersion enthalpies in phenol solution range from 7.6 to 13.9 J g⁻¹ and for the case of 4-nitro phenol such enthalpies range from 12.7 to 20.5 J g⁻¹; all the 5 samples studied showed a higher value for the heat of immersion in aqueous solutions of 4-nitro phenol.     

Enthalpic characterisation of activated carbon monoliths obtained from lignocellulosic materials

In this study, energetic interactions between activated carbon monoliths and various liquids were evaluated by determining immersion enthalpies in C₆H₆, H₂O and aqueous solutions of NaOH and HCl. Immersion enthalpies depend on both the surface chemistry and the interactions between specific groups, and were compared with results from volumetric titrations. Immersion enthalpies of activated carbon monoliths were between ?95.85 and ?176.5 J g^?1 for C₆H₆ and between ?11.19 and ?68.31 J g^?1 for H₂O; whereas immersion enthalpies in NaOH and HCl solutions were between ?20.36 and ?82.25 J g^?1 and ?18.81 and ?96.16 J g^?1, respectively. In support of these results, a high level of acidic groups was found on the surface of the activated carbon monoliths by Boehm volumetric titrations, with values between 719 and 1,290 g mol^?1, in agreement with the higher immersion enthalpies observed in NaOH. Correlations were established between immersion enthalpies in the liquids and the surface chemistry properties of the activated carbon monoliths determined by volumetric titrations, demonstrating that immersion enthalpy is a useful parameter for characterisation of these materials in specific liquids.     

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.     

Determination of partial immersion enthalpy in the interaction of water and activated carbon

A way to calculate the enthalpic contributions of each component of the mixture of activated carbon and water to the immersion enthalpy using the concepts of the solution enthalpies is presented. By determining the immersion enthalpies of a microporous activated carbon in water, with values that are between –18.97 and −27.21 Jg⁻¹, from these and the mass ratio of activated carbon and water, differential enthalpies for the activated carbon, ΔH_DIFacH_{{\rm DIF}_{\rm ac}} and water, ΔH_DIFwH_{{\rm DIF}_{\rm w}} are calculated, and values between –15.95 and –26.81 Jg⁻¹ and between –19.14 and –42.45 Jg⁻¹, respectively are obtained. For low ratios of the mixture, the components’ contributions to the immersion enthalpy of activated carbon and water differ by 3.20 Jg⁻¹.     

Solvent extraction behaviour of iodine and bromine in aqueous-organic systems,analogy with radioactive decay

The extraction of iodine and bromine under various conditions from their saturated aqueous solutions by CCl₄, C₆H₆ and o-xylene has been studied. The data obtained from the experiments carried out at various temperatures, for H₂O(I₂)−CCl₄ and H₂O(I₂)−C₆H₆ systems, exhibit an Arrhenius behaviour. The overall activation energy calculated for the extraction in the H₂O(I₂)−CCl₄ system, 650±50 cal·mol⁻¹ is lower than that of H₂O(I₂)−C₆H₆, 3600±300 cal·mol⁻¹. The use of the solubility parameter for the interpretation of the data in the extraction of iodine is investigated. The data obtained in multiple extractions are treated by using the analogy between extraction and radioactive decay. The half number of extraction for each system is determined. The complex curves obtained in the H₂O(I₂)−CCl₄ and H₂O(I₂) −Br₂)−CCl₄ systems are resolved into two components.     

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.     

Immersion enthalpy and the constants of Langmuir model in the 3-chloro phenol adsorption on activated carbon

The adsorption process of 3-chloro phenol from aqueous solution on a activated carbon prepared from African palm stone and which presents a specific surface area of 685 m² g⁻¹, a greater quantity of total acid groups and a pH_PZC of 6.8 is studied. The adsorption isotherms are determined at pH values of 3, 5, 7, 9 and 11. The adsorption isotherms are fitted to the Langmuir model and the values of the maximum quantity adsorbed that are between 96.2 and 46.4 mg g⁻¹ are obtained along with the constant K_L with values between 0.422 and 0.965 L mg⁻¹. The maximum quantity adsorbed diminishes with the pH and the maximum value for this is a pH of 5. The immersion enthalpies of the activated carbon in a 3-chloro phenol solution of constant concentration, of 100 mg L⁻¹, are determined for the different pH levels, with results between 37.6 and 21.2 J g⁻¹. Immersion enthalpies of the activated carbon in function of 3-chloro phenol solution concentration are determined to pH 5, of maximum adsorption, with values between 28.3 and 38.4 J g⁻¹, and by means of linearization, the maximum immersion enthalpy is calculated, with a value of 41.67 J g⁻¹. With the results of the immersion enthalpy, maximum quantity adsorbed and the constant K_L, establish relations that describe the adsorption process of 3-chloro phenol from aqueous solution on activated carbon.     

The extraction of Cr(III) from aqueous thiocyanate solutions and its separation from Co(II) and Ni(II)

For the system liquid anion-exchanger—Cr(III)−NCS⁻, an investigation has been made of the dependence of the percentage extraction of Cr(III) on parameters such as standing time of the Cr(III)−NCS⁻ solution, temperature, pH and type of exchanger. Quantitative extraction of e.g. 4·10⁻⁴ M Cr(III) by 0.1M Aliquat in CCl₄ is easily achieved at room temperature, using 4.75M KNCS−0.05N HCl as aqueous phase. At high Cr(III) concentrations, the complex anion present in the organic phase is Cr(NCS) ₆ ³⁻ ; when working with dilute metal ion solutions, the species extracted is Cr(NCS)₄ (H₂O) ₂ ⁻ . Separations of mixtures containing 10⁻²−10⁻⁴ M Co(II), Ni(II) and Cr(III) have successfully been accomplished.     

Platinum–tin complexes as catalysts for the anodic oxidation of borohydride

Platinum–tin complexes were prepared by the reduction of Pt(IV) with Sn(II) in HCl media and studied by light absorption spectrometry, X-ray photoelectron spectroscopy (XPS), and electron microscopy. The formation of three complexes, H₃[Pt(SnCl₃)₅], H₂[Pt(SnCl₃)₂Cl₂], and H₂[Pt₃(SnCl₃)₈], depending on HCl and SnCl₂ concentrations, has been shown. The glassy carbon (GC) electrode modified in the complexes solutions was found to be an electrocatalyst for borohydride oxidation in a 1.0-M NaOH solution. Comparison of BH₄ ⁻ electrooxidation on Pt and on GC modified with platinum–tin complexes has shown that catalytic hydrolysis of BH₄ ⁻ did not proceed in the latter case in contrast to its oxidation on the Pt electrode, and only direct BH₄ ⁻ oxidation has been observed in the positive potentials scan. The activity of Pt–Sn complexes for BH₄ ⁻ oxidation changes with time and eventually decreases due to Sn(II), bound in the complex with Pt(II), oxidation by atmospheric oxygen. The complexes may be renewed by addition of missing amounts of SnCl₂ and HCl.     

Crystallization and Phase Stability of CaSO<Subscript>4</Subscript> and CaSO<Subscript>4</Subscript> – Based Salts

Summary.  Calcium sulfate occurs in nature in form of three different minerals distinguished by the degree of hydration: gypsum (CaSO₄·2H₂O), hemihydrate (CaSO₄·0.5H₂O) and anhydrite (CaSO₄). On the one hand the conversion of these phases into each other takes place in nature and on the other hand it represents the basis of gypsum-based building materials. The present paper reviews available phase diagram and crystallization kinetics information on the formation of calcium sulfate phases, including CaSO₄-based double salts and solid solutions. Uncertainties in the solubility diagram CaSO₄–H₂O due to slow crystallization kinetics particularly of anhydrite cause uncertainties in the stable branch of crystallization. Despite several attempts to fix the transition temperatures of gypsum–anhydrite and gypsum–hemihydrate by especially designed experiments or thermodynamic data analysis, they still vary within a range from 42–60°C and 80–110°C. Electrolyte solutions decrease the transition temperatures in dependence on water activity. Dry or wet dehydration of gypsum yields hemihydrates (α-, β-) with different thermal and re-hydration behaviour, the reason of which is still unclear. However, crystal morphology has a strong influence. Gypsum forms solid solutions by incorporating the ions HPO₄ ²⁻, HAsO₄ ²⁻, SeO₄ ²⁻, CrO₄ ²⁻, as well as ion combinations Na⁺(H₂PO₄)⁻ and Ln³⁺(PO₄)³⁻. The channel structure of calcium sulfate hemihydrate allows for more flexible ion substitutions. Its ion substituted phases and certain double salts of calcium sulfate seem to play an important role as intermediates in the conversion kinetics of gypsum into anhydrite or other anhydrous double salts in aqueous solutions. The same is true for the opposite process of anhydrite hydration to gypsum. Knowledge about stability ranges (temperature, composition) of double salts with alkaline and alkaline earth sulfates (esp. Na₂SO₄, K₂SO₄, MgSO₄, SrSO₄) under anhydrous and aqueous conditions is still very incomplete, despite some progress made for the systems Na₂SO₄–CaSO₄ and K₂SO₄–CaSO₄–H₂O. Corresponding author. E-mail: daniela.freyer@chemie.tu-freiberg.de Received December 17, 2002; accepted January 10, 2003 Published online April 3, 2003     

Basicity of 2-phenyl-5-R-1,3,4-oxadiazoles

We have studied the basicity of 2-phenyl-5-R-1,3,4-oxadiazoles (R = H, Me, CH₂Ph, t-Bu, CH₂Cl, CCl₃, CF₃) in aqueous sulfuric acid solutions. These compounds are weak organic bases (pK_BH ⁺ is −1.8 to −5.2). The values of pK_BH ⁺ determined on the H₀ and X acidity function scales agree well with each other. The substituent at the 5 position has a substantial effect on the basicity of the 1,3,4-oxadiazole ring. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 748–756, May, 2006.     

DSC analysis of human fat tissue in steroid induced osteonecrosis

Osteonecrosis (ON) of the femoral frequently occurs after steroid medication. One of the final pathways leading to steroid induced ON is thought to be pathologic fat metabolism. The pathobiological mechanism underlying the induction of fat metabolism outslides by steroids leading to ON has not been fully elucidated. The purpose of this study was to examine the intraoperative obtained gluteal fat tissue from ON patients with histology, gas chromatography (GC) and differential scanning calorimetry (DSC) and to compare them with otherwise healthy patient’s samples. The histological sections showed no significant differences compared with the control group. GC revealed that fraction of saturated fatty acids decreased in ON samples from mean values of controls of 24% to 21, the polyunsaturated fraction from 20 to 14%. The monounsaturated acids showed an increase from mean rate of 52% of the controls to 65% of steroid treated samples. DSC curves correlate with chromatographic analysis of the tissue fatty acids (Steroid treated, heating between 0–100°C: T _m=5.7°C, ΔH= −15.8J/g⁻¹; heating between −20–100°C: Tm= −9.96 and 5.85°C, ΔH= −59.17 and −16.2 J g⁻¹. Non-necrotic, heating between 0–100°C: two separable transition with Tm=5.7 and 9.9°C, total ΔH= −20.8 J g⁻¹; heating between −20–100°C: Tm= −10.9 and 4.95°C, total ΔH= −75.8 J g⁻¹.) Our preliminary findings are rather tendentious. Further investigations are needed with higher sample rate and under other anamnestic circumstances too.     

Conductivities of Several Ternary Electrolyte Solutions and Their Binary Subsystems at 293.15, 298.15, and 303.15 K

Conductivities were measured for the ternary systems NaNO₃–KNO₃–H₂O, NaCl–BaCl₂–H₂O, NaCl–LaCl₃–H₂O, and their binary subsystems NaNO₃–H₂O, KNO₃–H₂O, NaCl–H₂O, BaCl₂–H₂O, and LaCl₃–H₂O at (293.15, 298.15 and 303.15) K. The results were used to verify the generalized Young’s rule and the semi-ideal solution theory. Comparison of the results shows that the average relative differences between the predicted and measured conductivities are ≤4.2×10⁻³ for NaNO₃–KNO₃–H₂O, ≤4.6×10⁻³ for NaCl–BaCl₂–H₂O, and ≤8.9×10⁻³ for NaCl–LaCl₃–H₂O, indicating that the generalized Young’s rule and the semi-ideal solution theory can provide good predictions for the conductivity of mixed electrolyte solutions in terms of the data from their binary subsystems.     

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.     

Determination of the food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in human hair by solid-phase extraction and gas chromatography-mass spectrometry

Summary A method for the determination of the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in human hair has been developed and validated. Hair samples (200 mg) were dissolved in NaOH (1 M) and PhIP was isolated by successive solid-phase extraction on a polystyrene-divinylbenzene column and on a silica-based mixed-mode column with C₈ and-SO₃ ⁻ functional groups. Quantification was performed by gas chromatography-electron-impact ionization high-resolution mass spectrometry in selected-ion-monitoring mode. The method was validated for determination of PhIP in the concentration range 0.5–25 ng g⁻¹ hair with [²H₃]PhIP as internal standard. The limit of quantification was 0.26 ng g⁻¹ hair. Within-day and between-day precision were in the ranges 1–27% and 2–15% relative standard deviation, respectively. The hair sample used for method validation was found to contain 0.26 ng PhIP g⁻¹ hair.     

Direct coating of quantum dots with silica shell

This paper describes a method for direct coating of fluorescent semiconductor nanoparticles with silica shell. The fluorescent semiconductor nanoparticles used were CdSe _x Te_1–x nanoparticles coated with ZnS and succeedingly surface-modified with carboxyl groups, or quantum dots (Q-dots). The Q-dots were silica-coated by performing sol–gel reaction of tetraethyl orthosilicate (TEOS) using NaOH as a catalyst in the presence of the Q-dots. Quasi-perfect Q-dots/silica core-shell particles were formed at 5.0 M H₂O and 4.0 × 10⁻⁴ M NaOH. Under these concentrations of H₂O and NaOH, the particle size of Q-dots/silica particles could be varied from 20.1 to 38.1 nm as the TEOS concentration increased from 2.5 × 10⁻⁴ to 50 × 10⁻⁴ M. The Q-dots/silica particles showed fluorescence as well as the uncoated Q-dots.     

The role of Cherenkov and liquid scintilation counting in evaluating the anion-exchange separation of210Pb−210Bi−210Po

Using as eluent a sequence of 3M HCl, 12M HCl, and 8M HNO₃, a mixture of²¹⁰Pb,²¹⁰Bi, and²¹⁰Po may be clearly separated on a column of Dowex 1×2−100 anion exchange resin. A Cherenkov count in H₂O and the variation in count rate with time confirm that the nuclides emerge in the order²¹⁰Pb→²¹⁰Bi→²¹⁰Po. If 12M HCl is replaced by 1.5M H₂SO₄/2.3 M Na₂SO₄, a clean separation also results, but recovery of²¹⁰Po becomes considerably more difficult. All three nuclides are readily detectable by liquid scintillation counting, with the efficiency for²¹⁰Pb in the 60–70% range. The Cherenkov aqueous counting efficiency for²¹⁰Bi is ∼14–15%.     

Determination of heavy metals and iodide by stripping voltammetry in sodium chloride on mercury-graphite electrodes

The behavior of Cd(II), Pb(II), Cu(II), and I⁻ in the aqueous solutions of sodium chloride is studied by stripping voltammetry. A new version of using an indicator electrode from carbon glass ceramics modified with mercury for the consecutive stripping determination of Cd(II), Pb(II), Cu(II), and iodide is proposed. The mercury-graphite electrode was formed in the solution of a supporting electrolyte based on NH₄Cl, HCl, 0.05 M potassium tetraoxalate (KH₃C₄O₃ · 2H₂O), and 5 × 10⁻⁵ M mercury(II). At first, Cd(II), Pb(II), Cu(II), and then iodide were determined by anodic-cathodic stripping voltammetry after adding a sample solution (table salt, 10–100 mg/mL NaCl).     

Isopiestic Measurement of the Osmotic Coefficients of Aqueous {xH 2 SO 4 + (1− x)Fe 2 (SO 4 ) 3 } Solutions at 298.15 and 323.15 K

This study measures the osmotic coefficients of {xH₂SO₄ + (1−x)Fe₂(SO₄)₃}(aq) solutions at 298.15 and 323.15 K that have ionic strengths as great as 19.3 mol,kg⁻¹, using the isopiestic method. Experiments utilized both aqueous NaCl and H₂SO₄ as reference solutions. Equilibrium values of the osmotic coefficient obtained using the two different reference solutions were in satisfactory internal agreement. The solutions follow generally the Zdanovskii empirical linear relationship and yield values of a _w for the Fe₂(SO₄)₃–H₂O binary system at 298.15 K that are in good agreement with recent work and are consistent with other M₂(SO₄)₃–H₂O binary systems.