Application of heterogeneous vacancy solution theory to characterization of microporous solids

The vacancy solution theory of adsorption is re-formulated here through the mass-action law, and placed in a convenient framework permitting the development of thermodynamic ally consistent isotherms. It is shown that both the multisite Langmuir model and the classical vacancy solution theory expression are special cases of the more general approach when the Flory-Huggins activity coefficient model is used, with the former being the thermodynamically consistent result. The improved vacancy solution theory approach is further extended here to heterogeneous adsorbents by considering the pore-width dependent potential along with a pore size distribution. However, application of the model to numerous hydrocarbons as well as other adsorptives on microporous activated carbons shows that the multisite model has difficulty in the presence of a pore size distribution, because pores of different sizes can have different numbers of adsorbed layers and therefore different site occupancies. On the other hand, use of the classical vacancy solution theory expression for the local isotherm leads to good simultaneous fit of the data, while yielding a site diameter of about 0.257 nm, consistent with that expected for the potential well in aromatic rings on carbon pore surfaces. It is argued that the classical approach is successful because the Flory-Huggins term effectively represents adsorbate interactions in disguise. When used together with the ideal adsorbed solution theory the heterogeneous vacancy solution theory successfully predicts binary adsorption equilibria, and is found to perform better than the multisite Langmuir as well as the heterogeneous Langmuir model. (C) 2001 Elsevier Science Ltd. All rights reserved.

A generalised dynamic model for char particle gasification with structure evolution and peripheral fragmentation

A generalised model for the prediction of single char particle gasification dynamics, accounting for multi-component mass transfer with chemical reaction, heat transfer, as well as structure evolution and peripheral fragmentation is developed in this paper. Maxwell-Stefan analysis is uniquely applied to both micro and macropores within the framework of the dusty-gas model to account for the bidisperse nature of the char, which differs significantly from the conventional models that are based on a single pore type. The peripheral fragmentation and random-pore correlation incorporated into the model enable prediction of structure/reactivity relationships. The occurrence of chemical reaction within the boundary layer reported by Biggs and Agarwal (Chem. Eng. Sci. 52 (1997) 941) has been confirmed through an analysis of CO/CO2 product ratio obtained from model simulations. However, it is also quantitatively observed that the significance of boundary layer reaction reduces notably with the reduction of oxygen concentration in the flue gas, operational pressure and film thickness. Computations have also shown that in the presence of diffusional gradients peripheral fragmentation occurs in the early stages on the surface, after which conversion quickens significantly due to small particle size. Results of the early commencement of peripheral fragmentation at relatively low overall conversion obtained from a large number of simulations agree well with experimental observations reported by Feng and Bhatia (Energy & Fuels 14 (2000) 297). Comprehensive analysis of simulation results is carried out based on well accepted physical principles to rationalise model prediction. (C) 2001 Elsevier Science Ltd. AH rights reserved.

Solution techniques for transport problems involving steep concentration gradients: application to noncatalytic fluid solid reactions

Some efficient solution techniques for solving models of noncatalytic gas-solid and fluid-solid reactions are presented. These models include those with non-constant diffusivities for which the formulation reduces to that of a convection-diffusion problem. A singular perturbation problem results for such models in the presence of a large Thiele modulus, for which the classical numerical methods can present difficulties. For the convection-diffusion like case, the time-dependent partial differential equations are transformed by a semi-discrete Petrov-Galerkin finite element method into a system of ordinary differential equations of the initial-value type that can be readily solved. In the presence of a constant diffusivity, in slab geometry the convection-like terms are absent, and the combination of a fitted mesh finite difference method with a predictor-corrector method is used to solve the problem. Both the methods are found to converge, and general reaction rate forms can be treated. These methods are simple and highly efficient for arbitrary particle geometry and parameters, including a large Thiele modulus. (C) 2001 Elsevier Science Ltd. All rights reserved.

Human pigmentation genes: identification, structure and consequences of polymorphic variation

The synthesis of the visible pigment melanin by the melanocyte cell is the basis of the human pigmentary system, those genes directing the formation, transport and distribution of the specialised melanosome organelle in which melanin accumulates can legitimately be called pigmentation genes. The genes involved in this process have been identified through comparative genomic studies of mouse coat colour mutations and by the molecular characterisation of human hypopigmentary genetic diseases such as OCA1 and OCA2. The melanocyte responds to the peptide hormones a-MSH or ACTH through the MC1R G-protein coupled receptor to stimulate melanin production through induced maturation or switching of melanin type. The pheomelanosome, containing the key enzyme of the pathway tyrosinase, produces light red/yellowish melanin, whereas the eumelanosome produces darker melanins via induction of additional TYRP1, TYRP2, SILV enzymes, and the P-protein. Intramelanosomal pH governed by the P-protein may act as a critical determinant of tyrosinase enzyme activity to control the initial step in melanin synthesis or TYRP complex formation to facilitate melanogenesis and melanosomal maturation. The search for genetic variation in these candidate human pigmentation genes in various human populations has revealed high levels of polymorphism in the MC1R locus, with over 30 variant alleles so far identified. Functional correlation of MC1R alleles with skin and hair colour provides evidence that this receptor molecule is a principle component underlying normal human pigment variation. (C) 2001 Elsevier Science B.V. All rights reserved.

Seedling responses of three Australian tree species to toxic concentrations of zinc in solution culture

A frequently desired outcome when rehabilitating Zn toxic sites in Australia is to establish a self-sustaining native ecosystem. Hence, it is important to understand the tolerance of Australian native plants to high concentrations of Zn. Very little is known about the responses of Australian native plants, and trees in particular, to toxic concentrations of Zn. Acacia holosericea, Eucalyptus camaldulensis and Melaleuca leucadendra plants were grown in dilute solution culture for 10 weeks. The seedlings (42 days old) were exposed to six Zn treatments viz., 0.5, 5, 10, 25, 50 and 100 muM. The order of tolerance to toxic concentrations of Zn was E. camaldulensis > A. holosericea > M. leucadendra, the critical external concentrations being approximately 20, 12 and 1.5 muM, respectively. Tissue Zn concentrations increased as solution Zn increased for all species. Root tissue concentrations were higher than shoot tissue concentrations at all solution Zn concentrations. The critical tissue Zn concentrations were approximately 85 and 110 mug g(-1) DM for M. leucadendra, 115 and 155 mug g(-1) DM for A. holosericea and 415 and 370 mug g(-1) DM for E. camaldulensis for the youngest fully expanded leaf and total shoots, respectively. The results from this paper provide the first comprehensive combination of growth responses, critical external concentrations, critical tissue concentrations and plant toxicity symptoms for three important Australian genera, viz., Eucalyptus, Acacia and Melaleuca, for use in the rehabilitation of potentially Zn toxic sites.

Adsorption kinetics for the removal of chromium(VI) from aqueous solution by adsorbents derived from used tyres and sawdust

The batch removal of hexavalent chromium (Cr(Vl)) from wastewater under different experimental conditions using economic adsorbents was investigated in this study. These adsorbents were produced from the pyrolysis and activation of the waste tyres (TAC) and from the pyrolysis of sawdust (SPC). The performance of these adsorbents against commercial activated carbon F400 (CAC) has also been carried out. The removal was favoured at low pH, with maximum removal at pH = 2 for all types of carbon. The effects of concentration, temperature and particle size have been reported. All sorbents were found to efficiently remove Cr(VI) from solution. The batch sorption kinetics have been tested for a first-order reversible reaction, a first-order and second-order reaction. The rate constants of adsorption for all these kinetic models have been calculated. The applicability of the Langmuir isotherm for the present system has been tested at different temperatures. The thermodynamic parameters (AGO, K,) obtained indicate the endothermic nature of Cr(Vl) adsorption on TAC, SPC and CAC. (C) 2001 Elsevier Science B.V. All rights reserved.

Characterization of pore structure and coordination of titanium in TiO2 and SiO2-TiO2 sol-pillared clays

Titania sol-pillared clay (TiO2 PILC) and silica-titania sol-pillared clay (SiO2-TiO2 PILC) were synthesized by the sol-gel method. Supercritical drying (SCD) and treatment with quaternary ammonium surfactants were used to tailor the pore structure of the resulting clay. It was found that SCD approach increased the external surface area of the PILCs dramatically and that treatment with surfactants could be used to tailor pore size because the mesopore formation in the galleries between the clay layers follows the templating mechanism as observed in the synthesis of MCM-41 materials. Highly mesoporous solids were thus obtained. In calcined TiO2 PILC, ultrafine crystallites in anatase phase, which are active for photocatalytic oxidation of organics, were observed. In SiO2-TiO2 PILCs and their derivatives, titanium was highly dispersed in the matrix of silica and no crystal phase was observed. The highly dispersed titanium sites are good catalytic centers for selective oxidation of organic compounds. (C) 2001 Academic Press.

Characterisation of pore structure of carbon adsorbents using regularisation procedure

The constrained regularisation procedure was applied to compute the pore size distributions (PSDs, f(x)) for a variety of activated carbons using overall adsorption equation based on the combination of the Kelvin equation and the statistical adsorbed film thickness. The impact of the boundary values of relative nitrogen pressure p/p(0) was analysed on the basis of the corresponding alterations in the PSDs. Changes in microporosity and mesoporosity of activated carbons can be described adequately only when the range of p/p(0) is as wide as possible, as at a high initial p/p(0) value, the f(x) curves can be broadened with shifted maxima especially for micropores and narrow mesopores. Comparative analysis of the PSDs and the adsorption potential, adsorption energy and fractal dimension distributions gives useful information on the complete description of the adsorbent characteristics. (C) 2001 Elsevier Science B.V. All rights reserved.

Getting the adrenaline going: Crystal structure of the adrenaline-synthesizing enzyme PNMT

Background: Adrenaline is localized to specific regions of the central nervous system (CNS), but its role therein is unclear because of a lack of suitable pharmacologic agents. Ideally, a chemical is required that crosses the blood-brain barrier, potently inhibits the adrenaline-synthesizing enzyme PNMT, and does not affect other catecholamine processes. Currently available PNMT inhibitors do not meet these criteria. We aim to produce potent, selective, and CNS-active PNMT inhibitors by structure-based design methods. The first step is the structure determination of PNMT. Results: We have solved the crystal structure of human PNMT complexed with a cofactor product and a submicromolar inhibitor at a resolution of 2.4 Angstrom. The structure reveals a highly decorated methyltransferase fold, with an active site protected from solvent by an extensive cover formed from several discrete structural motifs. The structure of PNMT shows that the inhibitor interacts with the enzyme in a different mode from the (modeled) substrate noradrenaline. Specifically, the position and orientation of the amines is not equivalent. Conclusions: An unexpected finding is that the structure of PNMT provides independent evidence of both backward evolution and fold recruitment in the evolution of a complex enzyme from a simple fold. The proposed evolutionary pathway implies that adrenaline, the product of PNMT catalysis, is a relative newcomer in the catecholamine family. The PNMT structure reported here enables the design of potent and selective inhibitors with which to characterize the role of adrenaline in the CNS. Such chemical probes could potentially be useful as novel therapeutics.

Characterization of two HKT1 homologues from Eucalyptus camaldulensis that display intrinsic osmosensing capability

Plants have multiple potassium (K+) uptake and efflux mechanisms that are expressed throughout plant tissues to fulfill different physiological functions. Several different classes of K+ channels and carriers have been identified at the molecular level in plants. K+ transporters of the HKT1 superfamily have been cloned from wheat (Triticum aestivum), Arabidopsis, and Eucalyptus camaldulensis. The functional characteristics as well as the primary structure of these transporters are diverse with orthologues found in bacterial and fungal genomes. In this report, we provide a detailed characterization of the functional characteristics, as expressed in Xenopus laevis oocytes, of two cDNAs isolated from E. camaldulensis that encode proteins belonging to the HKT1 superfamily of K+/Na+ transporters. The transport of K+ in EcHKT-expressing oocytes is enhanced by Na+, but K+ was also transported in the absence of Na+. Na+ is transported in the absence of K+ as has been demonstrated for HKT1 and AtHKT1. Overall, the E. camaldulensis transporters show some similarities and differences in ionic selectivity to HKT1 and AtHKT1. One striking difference between HKT1 and EcHKT is the sensitivity to changes in the external osmolarity of the solution. Hypotonic solutions increased EcHKT induced currents in oocytes by 100% as compared with no increased current in HKT1 expressing or uninjected oocytes. These osmotically sensitive currents were not enhanced by voltage and may mediate water flux. The physiological function of these osmotically induced increases in currents may be related to the ecological niches that E. camaldulensis inhabits, which are periodically flooded. Therefore, the osmosensing function of EcHKT may provide this species with a competitive advantage in maintaining K+ homeostasis under certain conditions.

Mass balance characterisation of Al-7Si-Mg alloy microstructures as a function of solution treatment time

Mass balance calculations were performed to model the effect of solution treatment time on A356 and A357 alloy microstructures. Image analysis and electron probe microanalysis were used to characterise microstructures and confirm model predictions. In as-cast microstructures, up to 8 times more Mg is tied up in the pi-phase than in Mg2Si. The dissolution of pi is accompanied by a corresponding increase in the amount of beta-phase. This causes the rate of pi dissolution to be limited by the rate of beta formation. It is predicted that solution treatments of the order of tens of minutes at 540degreesC produce near-maximum T6 yield strengths, and that Mg contents in excess of 0.52 wt% have no advantage.

A semi-analytical solution for vibration of rectangular plates with abrupt thickness variation

A semi-analytical analysis of free vibration of plates with cross-sectional discontinuities due to abrupt changes in thickness is presented. A basic square element divided into suitable subdomains dependent upon the positions of these abrupt changes is used as the basic building element, Admissible functions that satisfy the essential or geometric boundary conditions are used to define the transverse deflection of each subdomain. Continuities in the displacement, slope, moment and higher derivatives between adjacent subdomains are enforced at the interconnecting edges. The resulting global energy functional from the proper assembly of the coupled strain and kinetic energy contributions of each subdomain is then minimized via the Ritz procedure to extract the frequencies and mode shapes. Contour plots of a range of new mode shapes are presented for the enhancement of understanding the dynamic behavior of this class of plates, (C) 2001 Elsevier Science Ltd, All rights reserved.