Sorption isotherm, glass transitions and state diagram for freeze-dried plum skin and pulp

Differential scanning calorimetry (DSC) was used to determine phase transitions of freeze-dried plums. Samples at low and intermediate moisture contents, were conditioned by adsorption at various water activities (0.11≤a w≤0.90) at 25°C, whereas in the high moisture content region (a w>0.90) samples were obtained by direct water addition, with the resulting sorption isotherm being well described by the Guggenheim-Anderson-deBoer (GAB) model. Freeze-dried samples of separated plum skin and pulp were also analysed. At a w≤0.75, two glass transitions were visible, with the glass transition temperature (T g) decreasing with increasing a w due to the water plasticising effect. The first T g was attributed to the matrix formed by sugars and water. The second one, less visible and less plasticised by water, was probably due to macromolecules of the fruit pulp. The Gordon-Taylor model represented satisfactorily the matrix glass transition curve for a w≤0.90. In the higher moisture content range T g remained practically constant around T g′ (-57.5°C). Analysis of the glass transition curve and the sorption isotherm indicated that stability at a temperature of 25°C, would be attained by freeze dried plum at a water activity of 0.04, corresponding to a moisture content of 12.9% (dry basis). © 2006 SAGE Publications.

Non-additivity of attractive potentials in modeling of N-2 and Ar adsorption isotherms on graphitized carbon black and porous carbon by means of density functional theory

We present a new approach accounting for the nonadditivity of attractive parts of solid-fluid and fluidfluid potentials to improve the quality of the description of nitrogen and argon adsorption isotherms on graphitized carbon black in the framework of non-local density functional theory. We show that the strong solid-fluid interaction in the first monolayer decreases the fluid-fluid interaction, which prevents the twodimensional phase transition to occur. This results in smoother isotherm, which agrees much better with experimental data. In the region of multi-layer coverage the conventional non-local density functional theory and grand canonical Monte Carlo simulations are known to over-predict the amount adsorbed against experimental isotherms. Accounting for the non-additivity factor decreases the solid-fluid interaction with the increase of intermolecular interactions in the dense adsorbed fluid, preventing the over-prediction of loading in the region of multi-layer adsorption. Such an improvement of the non-local density functional theory allows us to describe experimental nitrogen and argon isotherms on carbon black quite accurately with mean error of 2.5 to 5.8% instead of 17 to 26% in the conventional technique. With this approach, the local isotherms of model pores can be derived, and consequently a more reliab * le pore size distribution can be obtained. We illustrate this by applying our theory against nitrogen and argon isotherms on a number of activated carbons. The fitting between our model and the data is much better than the conventional NLDFT, suggesting the more reliable PSD obtained with our approach.

Modeling of adsorption in finite cylindrical pores by means of density functional theory

Adsorption of argon at its boiling point infinite cylindrical pores is considered by means of the non-local density functional theory (NLDFT) with a reference to MCM-41 silica. The NLDFT was adjusted to amorphous solids, which allowed us to quantitatively describe argon adsorption isotherm on nonporous reference silica in the entire bulk pressure range. In contrast to the conventional NLDFT technique, application of the model to cylindrical pores does not show any layering before the phase transition in conformity with experimental data. The finite pore is modeled as a cylindrical cavity bounded from its mouth by an infinite flat surface perpendicular to the pore axis. The adsorption of argon in pores of 4 and 5 nm diameters is analyzed in canonical and grand canonical ensembles using a two-dimensional version of NLDFT, which accounts for the radial and longitudinal fluid density distributions. The simulation results did not show any unusual features associated with accounting for the outer surface and support the conclusions obtained from the classical analysis of capillary condensation and evaporation. That is, the spontaneous condensation occurs at the vapor-like spinodal point, which is the upper limit of mechanical stability of the liquid-like film wetting the pore wall, while the evaporation occurs via a mechanism of receding of the semispherical meniscus from the pore mouth and the complete evaporation of the core occurs at the equilibrium transition pressure. Visualization of the pore filling and empting in the form of contour lines is presented.

Application of density functional theory to equilibrium adsorption of argon and nitrogen on amorphous silica surface

We present a new version of non-local density functional theory (NL-DFT) adapted to description of vapor adsorption isotherms on amorphous materials like non-porous silica. The novel feature of this approach is that it accounts for the roughness of adsorbent surface. The solid–fluid interaction is described in the same framework as in the case of fluid–fluid interactions, using the Weeks–Chandler–Andersen (WCA) scheme and the Carnahan–Starling (CS) equation for attractive and repulsive parts of the Helmholtz free energy, respectively. Application to nitrogen and argon adsorption isotherms on non-porous silica LiChrospher Si-1000 at their boiling points, recently published by Jaroniec and co-workers, has shown an excellent correlative ability of our approach over the complete range of pressures, which suggests that the surface roughness is mostly the reason for the observed behavior of adsorption isotherms. From the analysis of these data, we found that in the case of nitrogen adsorption short-range interactions between oxygen atoms on the silica surface and quadrupole of nitrogen molecules play an important role. The approach presented in this paper may be further used in quantitative analysis of adsorption and desorption isotherms in cylindrical pores such as MCM-41 and carbon nanotubes.

Role of enthalpy and entropy in moisture sorption behavior of pineapple pulp powder produced by different drying methods

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Study of the enthalpy-entropy mechanism from water sorption of orange seeds (C. sinensis cv. Brazilian) for the use of agro-industrial residues as a possible source of vegetable oil production

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

MOISTURE SORPTION CHARACTERISTICS OF PINEAPPLE PROCESSING WASTE: SHELL AND CENTRAL CYLINDER

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Alkaline hydrothermal treatment of titanate nanostructures

Since its initial proposal in 1998, alkaline hydrothermal processing has rapidly become an established technology for the production of titanate nanostructures. This simple, highly reproducible process has gained a strong research following since its conception. However, complete understanding and elucidation of nanostructure phase and formation have not yet been achieved. Without fully understanding phase, formation, and other important competing effects of the synthesis parameters on the final structure, the maximum potential of these nanostructures cannot be obtained. Therefore this study examined the influence of synthesis parameters on the formation of titanate nanostructures produced by alkaline hydrothermal treatment. The parameters included alkaline concentration, hydrothermal temperature, the precursor material‘s crystallite size and also the phase of the titanium dioxide precursor (TiO2, or titania). The nanostructure‘s phase and morphology was analysed using X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy. X-ray photoelectron spectroscopy (XPS), dynamic light scattering (non-invasive backscattering), nitrogen sorption, and Rietveld analysis were used to determine phase, for particle sizing, surface area determinations, and establishing phase concentrations, respectively. This project rigorously examined the effect of alkaline concentration and hydrothermal temperature on three commercially sourced and two self-prepared TiO2 powders. These precursors consisted of both pure- or mixed-phase anatase and rutile polymorphs, and were selected to cover a range of phase concentrations and crystallite sizes. Typically, these precursors were treated with 5–10 M sodium hydroxide (NaOH) solutions at temperatures between 100–220 °C. Both nanotube and nanoribbon morphologies could be produced depending on the combination of these hydrothermal conditions. Both titania and titanate phases are comprised of TiO6 units which are assembled in different combinations. The arrangement of these atoms affects the binding energy between the Ti–O bonds. Raman spectroscopy and XPS were therefore employed in a preliminary study of phase determination for these materials. The change in binding energy from a titania to a titanate binding energy was investigated in this study, and the transformation of titania precursor into nanotubes and titanate nanoribbons was directly observed by these methods. Evaluation of the Raman and XPS results indicated a strengthening in the binding energies of both the Ti (2p3/2) and O (1s) bands which correlated to an increase in strength and decrease in resolution of the characteristic nanotube doublet observed between 320 and 220 cm.1 in the Raman spectra of these products. The effect of phase and crystallite size on nanotube formation was examined over a series of temperatures (100.200 �‹C in 20 �‹C increments) at a set alkaline concentration (7.5 M NaOH). These parameters were investigated by employing both pure- and mixed- phase precursors of anatase and rutile. This study indicated that both the crystallite size and phase affect nanotube formation, with rutile requiring a greater driving force (essentially �\harsher. hydrothermal conditions) than anatase to form nanotubes, where larger crystallites forms of the precursor also appeared to impede nanotube formation slightly. These parameters were further examined in later studies. The influence of alkaline concentration and hydrothermal temperature were systematically examined for the transformation of Degussa P25 into nanotubes and nanoribbons, and exact conditions for nanostructure synthesis were determined. Correlation of these data sets resulted in the construction of a morphological phase diagram, which is an effective reference for nanostructure formation. This morphological phase diagram effectively provides a .recipe book�e for the formation of titanate nanostructures. Morphological phase diagrams were also constructed for larger, near phase-pure anatase and rutile precursors, to further investigate the influence of hydrothermal reaction parameters on the formation of titanate nanotubes and nanoribbons. The effects of alkaline concentration, hydrothermal temperature, crystallite phase and size are observed when the three morphological phase diagrams are compared. Through the analysis of these results it was determined that alkaline concentration and hydrothermal temperature affect nanotube and nanoribbon formation independently through a complex relationship, where nanotubes are primarily affected by temperature, whilst nanoribbons are strongly influenced by alkaline concentration. Crystallite size and phase also affected the nanostructure formation. Smaller precursor crystallites formed nanostructures at reduced hydrothermal temperature, and rutile displayed a slower rate of precursor consumption compared to anatase, with incomplete conversion observed for most hydrothermal conditions. The incomplete conversion of rutile into nanotubes was examined in detail in the final study. This study selectively examined the kinetics of precursor dissolution in order to understand why rutile incompletely converted. This was achieved by selecting a single hydrothermal condition (9 M NaOH, 160 °C) where nanotubes are known to form from both anatase and rutile, where the synthesis was quenched after 2, 4, 8, 16 and 32 hours. The influence of precursor phase on nanostructure formation was explicitly determined to be due to different dissolution kinetics; where anatase exhibited zero-order dissolution and rutile second-order. This difference in kinetic order cannot be simply explained by the variation in crystallite size, as the inherent surface areas of the two precursors were determined to have first-order relationships with time. Therefore, the crystallite size (and inherent surface area) does not affect the overall kinetic order of dissolution; rather, it determines the rate of reaction. Finally, nanostructure formation was found to be controlled by the availability of dissolved titanium (Ti4+) species in solution, which is mediated by the dissolution kinetics of the precursor.

Removal of toxic metals in a multi metal system using sorbents for potential application to urban stormwater treatment

In the context of increasing demand for potable water and the depletion of water resources, stormwater is a logical alternative. However, stormwater contains pollutants, among which metals are of particular interest due to their toxicity and persistence in the environment. Hence, it is imperative to remove toxic metals in stormwater to the levels prescribed by drinking water guidelines for potable use. Consequently, various techniques have been proposed, among which sorption using low cost sorbents is economically viable and environmentally benign in comparison to other techniques. However, sorbents show affinity towards certain toxic metals, which results in poor removal of other toxic metals. It was hypothesised in this study that a mixture of sorbents that have different metal affinity patterns can be used for the efficient removal of a range of toxic metals commonly found in stormwater. The performance of six sorbents in the sorption of Al, Cr, Cu, Pb, Ni, Zn and Cd, which are the toxic metals commonly found in urban stormwater, was investigated to select suitable sorbents for creating the mixtures. For this purpose, a multi criteria analytical protocol was developed using the decision making methods: PROMETHEE (Preference Ranking Organisation METHod for Enrichment Evaluations) and GAIA (Graphical Analysis for Interactive Assistance). Zeolite and seaweed were selected for the creation of trial mixtures based on their metal affinity pattern and the performance on predetermined selection criteria. The metal sorption mechanisms employed by seaweed and zeolite were defined using kinetics, isotherm and thermodynamics parameters, which were determined using the batch sorption experiments. Additionally, the kinetics rate-limiting steps were identified using an innovative approach using GAIA and Spearman correlation techniques developed as part of the study, to overcome the limitation in conventional graphical methods in predicting the degree of contribution of each kinetics step in limiting the overall metal removal rate. The sorption kinetics of zeolite was found to be primarily limited by intraparticle diffusion followed by the sorption reaction steps, which were governed mainly by the hydrated ionic diameter of metals. The isotherm study indicated that the metal sorption mechanism of zeolite was primarily of a physical nature. The thermodynamics study confirmed that the energetically favourable nature of sorption increased in the order of Zn < Cu < Cd < Ni < Pb < Cr < Al, which is in agreement with metal sorption affinity of zeolite. Hence, sorption thermodynamics has an influence on the metal sorption affinity of zeolite. On the other hand, the primary kinetics rate-limiting step of seaweed was the sorption reaction process followed by intraparticle diffusion. The boundary layer diffusion was also found to limit the metal sorption kinetics at low concentration. According to the sorption isotherm study, Cd, Pb, Cr and Al were sorbed by seaweed via ion exchange, whilst sorption of Ni occurred via physisorption. Furthermore, ionic bonding is responsible for the sorption of Zn. The thermodynamics study confirmed that sorption by seaweed was energetically favourable in the order of Zn < Cu < Cd < Cr . Al < Pb < Ni. However, this did not agree with the affinity series derived for seaweed suggesting a limited influence of sorption thermodynamics on metal affinity for seaweed. The investigation of zeolite-seaweed mixtures indicated that mixing sorbents have an effect on the kinetics rates and the sorption affinity. Additionally, the theoretical relationships were derived to predict the boundary layer diffusion rate, intraparticle diffusion rate, the sorption reaction rate and the enthalpy of mixtures based on that of individual sorbents. In general, low coefficient of determination (R2) for the relationships between theoretical and experimental data indicated that the relationships were not statistically significant. This was attributed to the heterogeneity of the properties of sorbents. Nevertheless, in relative terms, the intraparticle diffusion rate, sorption reaction rate and enthalpy of sorption had higher R2 values than the boundary layer diffusion rate suggesting that there was some relationship between the former set of parameters of mixtures and that of sorbents. The mixture, which contained 80% of zeolite and 20% of seaweed, showed similar affinity for the sorption of Cu, Ni, Cd, Cr and Al, which was attributed to approximately similar sorption enthalpy of the metal ions. Therefore, it was concluded that the seaweed-zeolite mixture can be used to obtain the same affinity for various metals present in a multi metal system provided the metal ions have similar enthalpy during sorption by the mixture.

Synthesis and characterisation of titanium sol-gels in varied gravity environments

Sol-gel synthesis in varied gravity is only a relatively new topic in the literature and further investigation is required to explore its full potential as a method to synthesise novel materials. Although trialled for systems such as silica, the specific application of varied gravity synthesis to other sol-gel systems such as titanium has not previously been undertaken. Current literature methods for the synthesis of sol-gel material in reduced gravity could not be applied to titanium sol-gel processing, thus a new strategy had to be developed in this study. To successfully conduct experiments in varied gravity a refined titanium sol-gel chemical precursor had to be developed which allowed the single solution precursor to remain un-reactive at temperatures up to 50oC and only begin to react when exposed to a pressure decrease from a vacuum. Due to the new nature of this precursor, a thorough characterisation of the reaction precursors was subsequently undertaken with the use of techniques such as Nuclear Magnetic Resonance, Infra-red and UV-Vis spectroscopy in order to achieve sufficient understanding of precursor chemistry and kinetic stability. This understanding was then used to propose gelation reaction mechanisms under varied gravity conditions. Two unique reactor systems were designed and built with the specific purpose to allow the effects of varied gravity (high, normal, reduced) during synthesis of titanium sol-gels to be studied. The first system was a centrifuge capable of providing high gravity environments of up to 70 g’s for extended periods, whilst applying a 100 mbar vacuum and a temperature of 40-50oC to the reaction chambers. The second system to be used in the QUT Microgravity Drop Tower Facility was also required to provide the same thermal and vacuum conditions used in the centrifuge, but had to operate autonomously during free fall. Through the use of post synthesis characterisation techniques such as Raman Spectroscopy, X-Ray diffraction (XRD) and N2 adsorption, it was found that increased gravity levels during synthesis, had the greatest effect on the final products. Samples produced in reduced and normal gravity appeared to form amorphous gels containing very small particles with moderate surface areas. Whereas crystalline anatase (TiO2), was found to form in samples synthesised above 5 g with significant increases in crystallinity, particle size and surface area observed when samples were produced at gravity levels up to 70 g. It is proposed that for samples produced in higher gravity, an increased concentration gradient of water is forms at the bottom of the reacting film due to forced convection. The particles formed in higher gravity diffuse downward towards this excess of water, which favours the condensation reaction of remaining sol gel precursors with the particles promoting increased particle growth. Due to the removal of downward convection in reduced gravity, particle growth due to condensation reaction processes are physically hindered hydrolysis reactions favoured instead. Another significant finding from this work was that anatase could be produced at relatively low temperatures of 40-50oC instead of the conventional method of calcination above 450oC solely through sol-gel synthesis at higher gravity levels. It is hoped that the outcomes of this research will lead to an increased understanding of the effects of gravity on chemical synthesis of titanium sol-gel, potentially leading to the development of improved products suitable for diverse applications such as semiconductor or catalyst materials as well as significantly reducing production and energy costs through manufacturing these materials at significantly lower temperatures.

Photocatalytic degradation of gaseous toluene by using immobilized titania/silica on aluminum sheets

The aim of this study was to prepare a highly active immobilized titania/silica photocatalyst and to test its performance in situ toward degradation of toluene as one of the major toxic indoor contaminants. In this work, two different titania layers immobilized on Al sheets were synthesized via low temperature sol-gel method employing presynthesized highly active titania powders (Degussa P25 and Millennium PC500, mass ratio 1:1): (a) with a silica/titania binder and a protective layer and (b) without the binder. The photocatalysts were characterized by X-ray diffraction, nitrogen sorption measurements, scanning electron microscopy (SEM), infrared spectroscopy, and UV-vis diffuse reflectance spectroscopy (DRS). The in situ photocatalytic degradation of gaseous toluene was selected as a probe reaction to test photocatalytic activity and to verify the potential application of these materials for air remediation. Results show that nontransparent highly photocatalytically active coatings based on the silica/titania binder and homogeneously dispersed TiO2 powders were obtained on the Al sheets. The crystalline structure of titania was not altered upon addition of the binder, which also prevented inhomogeneous agglomeration of particles on the photocatalyst surface. The photoactivity results indicate that the adsorption properties and photocatalytic activity of immobilized photocatalysts with the silica/titania binder and an underlying protective layer were very effective and additionally, they exhibited considerably improved adhesion and uniformity. We present a new highly photocatalytically active immobilized catalyst on a convenient metallic support, which has a potential application in an air cleaning device.

Metal Retention Experiments for the Design of Soil-Mix Technology Permeable Reactive Barriers

Soil-mix technology is effective for the construction of permeable reactive barriers (PRBs) for in situ groundwater treatment. The objective of this study was to perform initial experiments for the design of soil-mix technology PRBs according to (i) sorption isotherm, (ii) reaction kinetics and (iii) mass balance of the contaminants. The four tested reactive systems were: (i) a granular zeolite (clinoptilolite-GZ), (ii) a granular organoclay (GO), (iii) a 1:1-mixture GZ and model sandy clayey soil and (iv) a 1:1:1-mixture of GZ, GO and model soil. The laboratory experiments consisted of batch tests (volume 900mL and sorbent mass 18g) with a multimetal solution of Pb, Cu, Zn, Cd and Ni. For the adsorption experiment, the initial concentrations ranged from 0.01 to 0.5mM (2.5 to 30mg/L). The maximum metal retention was measured in a batch test (300mg/L for each metal, volume 900mL, sorbent mass 90-4.5g). The reactive material efficiency order was found to be GZ>GZ-soil mix>GZ-soil-GO mix>GO. Langmuir isotherms modelled the adsorption, even in presence of a mixed cations solution. Adsorption was energetically favourable and spontaneous in all cases. Metals were removed according to the second order reaction kinetics; GZ and the 1:1-mix were very similar. The maximum retention capacity was 0.1-0.2mmol/g for Pb in the presence of clinoptilolite; for Cu, Zn, Cd and Ni, it was below 0.05mmol/g for the four reactive systems. Mixing granular zeolite, organoclay and model soil increased the chemisorption. Providing that GZ is reactive enough for the specific conditions, GZ can be mixed to obtain the required sorption. Granular clinoptilolite addition to soil is recommended for PRBs for metal contaminated groundwater. The laboratory experiments consisted of batch tests with a multimetal solution of Pb, Cu, Zn, Cd and Ni. The four reactive materials chosen were granular zeolite, clinoptilolite and model sandy clayey soil, granular organoclay and a mix of clinoptilolite, model soil and organoclay. The reactive material efficiency order was found to be granular clinoptilolite>clinoptilolite-soil mix>clinoptilolite-soil-organoclay mix>granular organoclay. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Hydrothermal stability and catalytic activity of aluminum-containing mesoporous ethane-silicas

Aluminum was incorporated into the mesoporous framework of ethane-silica by one-pot condensation of Al(OiPr)(3) with 1,2-bis(trimethoxysilyl)ethane using octadecyltrimethylammonium chloride as surfactant. Powder X-ray diffraction patterns, nitrogen sorption analysis, and TEM results reveal the formation of an ordered mesoporous material with uniform porosity. Al-27 MAS NMR confirms the incorporation of aluminum in the framework. The synthesized materials exhibit extremely high hydrothermal stability in boiling water (no obvious change of mesostructure and textural properties was observed even after refluxing in water for 100 h), which could be mainly contributed to the ethane-bridged mesoporous framework. The aluminum-containing mesoporous ethane-silicas are efficient catalysts for the alkylation of 2,4-di-tert-butylphenol by cinnamyl alcohol to yield a flavan.