Adsorption behavior and kinetics of H2S on a potassium-promoted hydrotalcite

Adsorption of H₂S and the influence of steam on its adsorption capacity and kinetics were studied on a commercial potassium-promoted hydrotalcite. The sorbent shows a very high cyclic working capacity for H₂S compared to CO₂ and H₂O, even at lower partial pressures and at different operating temperatures ranging between 300 and 500 °C. The operating temperature does not significantly influence the cyclic working capacity for half-cycle times of 30 min. The adsorption mechanism, however, changes at higher temperatures. At lower temperatures (300 °C) a fast adsorption with a fast approach to steady state was observed. At higher operating temperatures, H₂S reacts with the hydrotalcite structure, forming strongly bonded sulfuric species on the sorbent. When using dry regeneration conditions, the first cycles in cyclic operation at higher temperatures show a significantly higher adsorption of H₂S (especially the first cycle), which cannot be desorbed during regeneration with N₂. After the first fast initial adsorption rate a continuous slow adsorption of H₂S occurs, probably caused by a surface reaction between H₂S and the hydrotalcite structure. This reaction is, however, reversible if steam is used.The adsorption mechanism for H₂S and H₂O was determined using multiple cyclic experiments comparable to previous studies performed for CO₂ and H₂O adsorption. It is evident that the adsorption mechanism developed for CO₂ on the same sorbents is also valid for H₂S, indicating that the developed mechanism is consistent for sour gas adsorption on this type of sorbents. The cyclic working capacity can be significantly increased if steam is used during the regeneration step of the sorbent. The mechanistic model developed for the adsorption of CO₂ and H₂O was successfully validated with more than 160 different TGA experiments. An operating temperature of 400 °C seems to be optimal to achieve a high cyclic working capacity for H₂S, because at higher temperatures the regeneration of the formed sulfuric species seems to be hindered resulting in a significant decrease in the cyclic working capacity.     

Polyacrylamide-gelatin polymeric networks: effect of pH and gelatin concentration on the swelling kinetics and mechanical properties

This work presents the synthesis of polyacrylamide-gelatin (PAM-G) semi-interpenetrating hydrogels, as well as the study of the swelling capacity of this material at different pH’s, and we report its Young modulus. The hydrogels were crosslinked with N,N′-methylenebisacrylamide and synthesized at different acrylamide/gelatin weight relationship. It was observed that the swelling capacity of the hydrogels increases when the gelatin concentration is increased; while the Young modulus (at the swelling equilibrium) decreases lightly. Therefore, the gelatin has a small effect in the Young modulus, unlike its influence in the swelling ability. The swelling experiments reveal that the PAM-G hydrogels increase its swelling capacity in alkaline mediums because the presence of the hydrophobic functional groups (mainly COO⁻) in the gelatin structure.     

Azacrown Ethers with Naphthyl Branches. Fluorescence Properties,Protonation and Metal Coordination

Starting from 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane, we have prepared two compounds by replacing the amine hydrogens with naphthyl or 3,5-bis(2′-oxymethylnaphthyl)benzyl units. The absorption and emission spectra of compounds 2 (N,N′-bis(2-naphthylmethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) and 3 (N,N′-bis[3,5-bis(2′-oxymethylnaphthyl)benzyl]-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) have been studied in CH₃CN:CH₂Cl₂ 1:1 (v/v) solution. For comparison purposes, the spectroscopic properties of N-methyl(2-methylnaphthalene)ethylamine (1) have also been investigated. For each compound, the absorption spectra are qualitatively very similar to that of naphthalene, with molar absorption coefficients as expected for the presence of one (1), two (2), and four (3) naphthyl chromophoric groups. The fluorescence spectra, however, are quite different from that of naphthalene. The naphthalene-type fluorescence (λ_max = 337 nm) is strongly quenched, particularly for compounds 1 and 2 which also exhibit a broad emission band in the visible region (λ_max ≈ 480 nm) assigned to a low lying charge-transfer excited state. In the case of compound 3, a quenched naphthalene-type band is accompanied by weak exciplex and excimer emissions. Upon titration with CF₃SO₃H, the charge transfer bands of 1 and 2 and the exciplex emission of 3 disappear and the naphthalene-type bands regain intensity. Titration plots show that in compounds 2 and 3 the protonation of the two nitrogens occurs stoichiometrically in two distinct steps. Titration with Zn²⁺ gives rise to 2.Zn²⁺ and 3.Zn²⁺ complexes. This article is dedicated to Professor Dedier Astruc.     

Comparison of MCNPX-C90 and TRIPOLI-4-D for fuel depletion calculations of a Gas-cooled Fast Reactor

The Gas-cooled Fast Reactor is one of the reactor concepts selected by the Generation IV International Forum for the next generation of innovative nuclear energy systems. Several fuel design concepts are being investigated. Burnup depletion of mixed fuel of uranium and plutonium, cooled with gas in a fast neutron energy spectrum must be simulated. Various codes are being developed and/or adapted to improve the quality of the results, and also to reduce the computing time required for the simulations.     

Growth/no growth interfaces of Bacillus cereus, Staphylococcus aureus and Salmonella enteritidis in model systems based on water activity, pH, temperature and ethanol concentration

This study focuses its attention on the boundary between the growth and no growth of three strains ofSalmonella enteritidis , Bacillus cereus and Staphylococcus aureus in the presence of growth controlling factors such as temperature, pH, water activity (A_w) and ethanol concentration. Preliminarly, the minimal values of pH, A_wand temperature, and the maximum ethanol concentrations allowing the growth of the considered micro-organisms were determined. The calculation of these values enabled the use of logistic model to evaluate the growth/no growth boundary for the bacteria in relation to the considered independent variables. The location of the growth/no-growth boundaries for S. enteritidis and Staph. aureus were strongly affected, at the same ethanol concentration, by temperature, pH and A_w. Among the considered species, Staph. aureus was endowed with the highest sensitivity to low pH values whileB. cereus's growth/no growth interface, was quite unaffected by the combination of the stresses, when the physico–chemical conditions were above the minimum for growth. The effects of temperature, A_wand ethanol on the limitation of growth of the considered species were not merely additive. It was possible to identify the combinations of such factors preventing the growth of Salmonella enteritidis, Staph. aureus and B. cereus.     

Regulation of insulin-like growth factors I and II and their binding proteins in human bone marrow stromal cells by dexamethasone

Müller cells are highly permeable to potassium ions and play a major role in maintaining potassium homeostasis in the vertebrate retina during light-evoked neuronal activity. Potassium fluxes across the Müller cell's membrane are believed to underlie the light-evoked responses of these cells. We studied the potassium currents of turtle Müller cells in the retinal slice and in dissociated cell preparations and their role in the genesis of the light-evoked responses of these cells. In either preparation, the I-V curve, measured under voltage-clamp conditions, consisted of inward and outward currents. A mixture of cesium ions, TEA, and 4-AP blocked the inward current but had no effect on the outward current. Extracellular cesium ions alone blocked the inward current but exerted no effect on the photoresponses. Extracellular barium ions blocked both inward and outward currents, induced substantial depolarization, and augmented the light-evoked responses, especially the OFF component. Exposing isolated Müller cells to a high potassium concentration did not cause any current or voltage responses when barium ions were present. In contrast, application of glutamate in the presence of barium ions induced a small inward current that was associated with a substantially augmented depolarizing wave relative to that observed under control conditions. This observation suggests a role for an electrogenic glutamate transporter in generating the OFF component of the turtle Müller cell photoresponse.     

Integrated ambient ozone evaluation by passive samplers and clover biomonitoring mini-stations

An ozone monitoring network was set up using passive samplers and biological mini-stations of two clones of white clover (Trifolium repens L. cv. Regal), NC-R (O(3)-resistant) and NC-S (O(3)-sensitive). This paper reports on a pilot study performed in the period June-October 1999 in the Rome municipal area by using five biomonitoring mini-stations and ozone passive samplers with a new nitrite based design. This combined methodology can be used to obtain information on the biological implications of the injury due to tropospheric ozone. The two techniques can integrate data for the short-medium period and can be placed in different urban and rural sites, proving to be a very useful tool for ozone concentration mapping.