Silica Gel Adsorption Research Articles

Determination of Selected Persistent Organochlorine Pollutants in Human Serum by Solid-phase Disk Extraction and Dual-Column Capillary Gas Chromatography with Electron Capture Detection

An improved clean-up method by solid-phase disk extraction was developed to isolate and concentrate trace levels of POPs (persistent organochlorine pollutants) in human serum prior to gas chromatography with electron capture detection on two different capillary columns, providing an improved selectivity. An EmporeTM C18 bonded silica extraction disk cartridge is used for the initial extraction and enrichment of the analytes. Subsequent clean-up is achieved by concentrated sulphuric acid and silica gel adsorption chromatography. Recoveries for selected POPs are ranging from 62 to 74% and a good reproducibility (RSD < 14%) is demonstrated. Human samples analysed under these conditions, show a similar relative concentration profile.

Experimental study of thermal phenomenon in PSA air dehumidification

A bench scale adiabatic PSA apparatus using a pair of vacuum-jacketed columns was constructed to study the non-isothermal effects involved in PSA air dehumidification with silica gel adsorbent. The thermal phenomenon coupled with the operating parameters, including half-cycle time, pressure ratio and velocity ratio of purge to feed, have been investigated systematically. A V-shaped temperature variation with time and a humped axial temperature distribution were observed for long cycle time due to thermal breakthrough. The temperature propagation speed and the local adsorption rate were obtained numerically from the experimental temperature distribution. The temperature propagation speed was greater than that for the mass because of the higher adsorption capacity of silica gel. The cycle time had no relation to the temperature propagation speed. A higher pressure ratio led to a faster temperature propagation, while the maximum temperature rise was almost the same. When the velocity ratio of purge to feed gas was less than 1.0, the desorption could not proceed completely and thus the mass transfer zone dispersed along the column. Recommendations for the design and operation of air drying PSA were also discussed on the basis of the above interpretation.

Thermodynamic investigation of trichloroethylene adsorption in water‐saturated microporous adsorbents

AbstractAdsorption of trichloroethylene (TCE) in adsorbents containing hydrophilic and hydrophobic micropores was investigated in order to determine the mechanisms responsible for TCE adsorption on mineral solids. A high‐pressure liquid chromatography method was used to measure TCE adsorption isotherms on three microporous adsorbents. Silica gel and zeolite type NaX were used as hydrophilic model adsorbents, and hexamethyldisilazane (HMDS)‐treated silica gel was used as a model hydrophobic adsorbent. Batch uptake and desorption isotherms were also measured on the hydrophilic silica gel. Uptake of TCE by all three adsorbents was linear over the concentration range investigated. However, the silica gel desorption isotherm was highly nonlinear, as indicated by its Freundlich isotherm exponent of 0.58. Capillary phase separation into hydrophobic micropores was postulated as being responsible for the isotherm hysteresis. Supporting this hypothesis was the conformance of the TCE adsorption isotherm to Dubinin–Radushkevitch volume filling of micropores theory. The enthalpies for TCE adsorption on all three solids were determined by van't Hoff analysis of distribution coefficients measured over a temperature range from 5 to 90°C. The TCE adsorption enthalpies on the silica gel and HMDS silica gel were exothermic, but on the zeolite adsorption was endothermic. High exothermic adsorption enthalpies on the silica gel adsorbents indicated that TCE adsorption was occurring in hydrophobic micropores, and that adsorption on surfaces with large radii of curvature contributed only minimally to the total uptake. This indicates that the predominant mechanism for TCE adsorption on these mineral solids is not partitioning into the vicinal water layer.

Adsorption equilibrium of solvent mixtures on silica gel and silica gel coated ceramics

This paper presents the sorption isotherms of the mixtures 2-propanol/water and n-heptane/methylcyclohexane as well as the isotherms of the pure solvents on the adsorbent silica gel and silica gel coated ceramics. The equilibrium data were obtained gravimetrically via an experimental set-up based on the through flowing method. The single component isotherms of all solvents are type IV of Brunauer classification, indicating strong interactions typically with a mesoporous solid. Due to small differences in molecule size, physical properties and interaction forces to silica gel, the mixtures of n-heptane and methylcyclohexane show a nonselective adsorption behavior over the complete composition range. An unusual phenomena has been observed for the adsorption equilibrium of the 2-propanol–water mixtures: at low relative pressures and low alcohol contents, 2-propanol is preferentially bounded so that the selectivity behavior according to the vapor liquid equilibrium of the fluid mixture is entirely inverted.

Nitrogen compound distribution in a heating oil

A Number 1 fuel heating oil was studied for the organo‐nitrogen content. The organo‐nitrogen compounds were isolated by mild acid extraction followed by silica gel adsorption. Three extracts were studied ‐ a basic nitrogen extract in methylene chloride, a nonbasic nitrogen extract in methyl alcohol and a nonbasic nitrogen extract in methylene chloride. The major constituents of each extract were identified by GC/MS. There were seven major classes of nitrogen compounds detected in each extract. These compounds included alkyl‐substituted carbazoles, indoles, pyrazines, pyridines, pyrroles, quinolines and tetrahydroquinolines. The nitrogen content and distribution was compared to both tar‐sands and shale derived fuels.

Hydrogen purification from refinery fuel gas by pressure swing adsorption

AbstractHydrogen purification and recovery from various process streams constitutes the largest commercial use of pressure swing adsorption (PSA) technology. This study investigates the performance of a six‐bed, dual‐sorbent PSA operation for hydrogen purification from the refinery fuel gas. Major impurities are methane, ethane, propane and butane, and comprise 30–40% of the feed. The dual‐sorbent PSA bed consists of an initial layer of silica gel adsorbent for trapping heavier hydrocarbons and a subsequent layer of activated carbon for removing lighter hydrocarbons. A numerical simulation model of the H2‐PSA process developed with all the essential features of the actual operation shows that butane is more strongly adsorbed on activated carbon than silica gel, and, hence, is less easily desorbed from the former using simple pressure reduction in the PSA cycle. Therefore, the initial layer serves to prevent butane from degrading the adsorptive capacity provided by activated carbon for other lighter hydrocarbons. The simulation model agrees well with the experimental results from a laboratory unit as well as with available H2 ‐PSA plant data from a refinery. The results also indicate the importance of heat effects in this process. Extensive parametric studies, which show effects of feed velocity and cycle time on the variation of product recovery and purity obtainable from the industrial unit, provide a valuable guide for its proper operation.

Application of supercritical fluid technology to citrus oil processing

Two continuous processes for the fractionation of citrus oil by supercritical carbon dioxide (SCCO 2) have been developed to obtain the highly concentrated product of flavor; (1) countercurrent extraction process, (2) pressure swing adsorption process. In the countercurrent process, effects of pressure and solvent-to-feed ratio (S/F ratio) on the separation selectivity were investigated at a temperature of 333 K. The selectivity was improved with the increase in the S/F ratio. The HETS was calculated from the phase equilibria estimated by PR equation of state for a model mixture and the measured HETS decreased from 4 to 0.2 m with increase in the S/F ratio. In the pressure swing adsorption process, a cyclic adsorption and desorption process in SCCO 2 was developed with silica gel adsorbent at a temperature of 313 K. Highly concentrated fraction of flavor was continuously obtained at the desorption step and blowdown step.

Mechanisms of Beer Colloidal Stabilization

The effects of treatments that influence beer colloidal stability on foam-active protein and haze-active protein and polyphenols were studied. Unchillproofed beer was treated with bentonite, silica gel, and polyvinylpolypyrrolidone (PVPP). Bentonite adsorption removed both haze-active and foam-active protein. Silica gel adsorption removed haze-active protein but almost completely spared foam-active protein. This specificity results because silica gel binds to the proline residues in the protein; these are the same sites where polyphenols attach to create haze. As a result, only those proteins that can participate in haze formation are affected. PVPP removed approximately half of the haze-active polyphenol from beer but had little effect on foam-active or haze-active protein. Fining with gelatin greatly reduced the level of haze-active protein in the beer. Fining with tannic acid reduced the level of haze-active protein and, at higher addition rates, the concentration of haze-active polyphenol. Theoretical aspects of enzymatic chillproofing were considered. Since phenylalanine and tryptophan are present in hordein but not the foam-active barley lipid transfer protein 1 (LTP1), it is theoretically possible that an enzyme could be found that would attack haze-active but not foam-active protein.

Ambient air sampling for tritium--determination of breakthrough volumes and collection efficiencies for silica gel adsorbent.

Ambient air samples for tritium (as HTO) can be collected using the solid adsorbent silica gel. The purpose of this study was to determine the maximum practical sampling volume and overall collection efficiency for water vapor collected on silica gel columns and to demonstrate the use of an impinger-based system to load water vapor onto silica gel columns. Breakthrough volumes (Vb) were measured and chromatographic efficiencies (expressed as the number of theoretical plates, N) were calculated for a 20 degrees C to 50 degrees C temperature range, with the relative humidity at approximately 30%. The tests yielded relative breakthrough volumes (air volume/adsorbent depth, m3 cm(-1)) of 0.36 for 20 degrees C, 0.20 for 30 degrees C, 0.15 for 40 degrees C, and 0.077 for 50 degrees C. For 18-cm columns, the average tritium tracer recoveries at 20 degrees C were 71% with no observed breakthrough for air volumes up to 5 m3, while at 40 degrees C mean tritium tracer recoveries dropped from 75% for volumes < or = 3.0 m3, to 0% for a volume of 5.0 m3. Frontal chromatographic profiles were measured for water vapor migrating through silica gel columns that were divided into 5 segments. The chromatographic efficiency of the silica gel columns was determined by graphical evaluation of the chromatography profiles. At a sampling rate of 0.25 L min(-1) and 30% relative humidity, the number of theoretical plates per adsorbent depth were 0.55 N cm(-1) at 20 degrees C, 0.68 N cm(-1) at 30 degrees C, 0.51 N cm(-1) at 40 degrees C, and 0.30 N cm(-1) at 50 degrees C. Chromatographic theory was used to estimate the overall collection efficiency of the silica gel columns as a function of the ratio of the sampling volume to breakthrough volume and the chromatographic efficiency. For a 9.5 m3 sample volume, 30% relative humidity, 0.25 L min(-1) sampling rate, and a 54-cm column, the overall collection efficiency was above 99.9% at 20 degrees C, above 95% at 30 degrees C, just below 80% at 40 degrees C, and <<80% at 50 degrees C.

Trapping and Measuring Radioiodine (Iodine-129) in Cartridge Filters

To understand the distribution of 129I trapped in AgS (silver-impregnated silica gel) adsorbent and to determine a calibration curve for measurement of this 129I, cartridges containing 10 in-line (arranged in series) filter elements, each packed with a 10-mm thickness of adsorbent, were fed from 20Bq (3.3 μg) to 4.6 MBq (768mg) of 129I at 150°C at a gas velocity of 22cm/s. The 129I in each filter element was measured after the adsorbent had been mixed homogeneously until the counting rates at the front and back surfaces of the filter element became equal. The amount of input 129I was allocated to each filter in proportion to the counting rates thus obtained.The first 7 of the 10 filter elements completely confined 4.6MBq of 129I. Therefore, the width of the adsorption zone was 7cm. Although each filter element in the cartridge could theoretically adsorb 2.92 MBq (485mg) of 129I, the first element captured only 1.42 MBq, which was 49% of its saturation value. Its DF had dropped to a level of 2.21 for its adsorption of only 17.4% of the saturated amount. As 129I continued to be deposited, the DF dropped to 1.45. The high gas velocity causes this phenomenon. Plots of counting rates vs. adsorption quantity provide a calibration curve covering a range of 20Bq to 2MBq of 129I with deviation of ± 13%. This curve is expressed by1n y = −0.57 + 0.961n x where y is the counting rate (counts/300s) and x the quantity of 129I in Bq.

The Chemical Characterization and Instability of Shale Derived Middle Distillate Fuels: ABSTRACT

Changes in fuel properties with time in storage have been a continuing problem in the use of shale-derived middle distillate fuels. The storage stability of middle distillate fuels is usually defined in terms of the formation of sediment, gum, color, and peroxides. A shale-derived fuel of marginal stability has been used as a source of nitrogen-rich polar extracts. These polar nitrogen heteroatonis were isolated by mild acid extraction followed by silica gel adsorption and were identified by combined capillary column GC/MS. Alkyl substituted pyridines were the prevalent class of compounds present. Other nitrogen functional groups present included the alkyl substituted: tetrahydroquinolines, quinolines, indoles, and carbazoles. This fuel was also analyzed for the metal ions content by ICP. The effect of adding these shale extract fractions as dopants to a stable shale diesel fuel were examined in terms of sediment formation, and peroxide number under accelerated storage stability test conditions. We have developed a 16 hour test method that predict accurately fuel behavior over a two year period. The activities of the extracts in inducing fuel instability were correlated with their chemical composition and will be reported.

Supercritical Fluid Extraction of N-Nitrosamines in Hams Processed in Elastic Rubber Nettings

A method for analysing N-nitrosamines in hams processed in elastic rubber nettings by supercritical fluid extraction (SFE) is described. The study was carried out with the prototype of a commercial extractor with a silica gel adsorption cartridge integrally attached to the variable restrictor. The SFE method was compared with a solid-phase extraction procedure currently used for ham analysis. Both methods used the same gas chromatographic-chemiluminescence detection conditions. No significant difference (p < 0.05) was found between results obtained with the 2 methods. Repeatability standard deviation of the SFE method was 1.7 ppb, with a coefficient of variation (CV) of 2.7%, compared with 2.2 ppb, with a CV of 3.5%, for solid-phase extraction. SFE permits minimal use of solvent and more rapid analysis of nitrosamines.

NITROGEN COMPOUND DISTRIBUTION AND FUEL INCOMPATIBILITY REACTIONS IN A TAR SAND DERIVED MIDDLE DISTILLATE FUEL

ABSTRACT A high paraffin, high nitrogen, low sulfur jet fuel derived from Athabasca tar sands was studied. The organo-nitrogen compounds in the fuel were isolated by mild acid extraction followed by silica-gel adsorption. Three extracts were derived from this fuel: a basic nitrogen compounds extract, BNC, in methylene chloride, a non-basic nitrogen compounds extract, NBNC, in methyl alcohol and an NBNC extract in methylene chloride. The major constituents of each extract were identified and quantitated by combined capillary column GC/MS. Alkyl substituted pyrroles, pyridines, indoles, tetrahydro-quinolines, carbazoles, and quinolines were the prevalent organo-nitrogen compounds present. Incompatibility tests were conducted on this fuel using the oxygen overpressure method with both doped and undoped fuel systems. The dopant of choice was cumene hydroperoxide. Practically no sediment formation was observed for the undoped fuel (0.1 mg/100mL fuel). The doped fuel matrix induced incompatibility behavior in proportion to the quantity of dopant present.

Influence of matrix precursors on the microstructure and mechanical properties of C/C composites

SummaryThe microstructure and properties of uni‐directional pitch‐based carbon‐carbon composites are explained in terms of the chemical composition of pitch precursors. Pitches are characterized by standard procedures (elemental analysis, softening point and solubility tests), extrography which is a simple and rapid silica gel adsorption chromatographic technique, Fourier transform infra‐red and gas chromatography of the toluene‐soluble fraction. Pitch pyrolysis behaviour is monitored by hot‐stage microscopy. The main microstructural features of uni‐directional composites from pitches and commercial PAN‐based carbon fibres are determined by light microscopy and scanning electron microscopy.

NMR investigations of the possible cross reactions between cyanate and epoxy resins

AbstractThe possible cross reactions indicated by solid‐state NMR between cyanate functionalized resin and epoxy functionalized resin have been investigated by using both natural abundance and labeled monofunctional model compounds. These soluble products were isolated and purified by silica gel adsorption chromatography and gel permeation chromatography. They were fully characterized by high resolution 1H‐, 13C‐, 15N‐NMR spectroscopy and by mass spectrometry. The major cross‐reaction product is a racemic mixture of enantiomers, which contain an oxazolidinone ring formed by one cyanate molecule and two epoxy molecules. However, epoxy consumption lags cyanate consumption in the overall reaction as triazine formation from the cyanate is much faster than the two competing reactions, the cross reaction between cyanate and epoxy, and the self‐polymerization of epoxy, under the conditions investigated. The cross reaction between cyanate and epoxy is limited. Approximately 12% of cross reaction between cyanate and epoxy was found in the overall reaction. In addition to the cross reactions of epoxy and cyanate, the reactions of epoxy and the carbamate, which is the major side product for the curing reaction of cyanate resin in solution, have also been investigated, and the mechanism of these reactions discussed. From the reactions of epoxy and carbamate, several products related to cross reaction between epoxy and cyanate have been isolated and identified. It is suggested that the reaction of epoxy and carbamate is one of the pathways in the overall cross reaction between epoxy and cranate resins. Finally, the mechanism of the overall cross‐curing reaction between the diepoxy and dicyanate mixed resins is discussed. © 1994 John Wiley &amp; Sons, Inc.

The trace analysis of musk xylene in biological samples: Problems associated with its ubiquitous occurrence

Musk xylene (1-tert-butyl-3,5-dimethyl-2,4,6-trinitrobenzene) (MX), a synthetic musk often used in fragrances has previously been described to occur in human tissue. In this article a specific and sensitive method for the determination of MX in blood is described. It includes a simple clean-up by silica gel adsorption chromatography followed by GC/MS detection with negative chemical ionisation (NCI) and multiple ion detection of the molecular ion and M-30. The absolute detection limit in the MID-mode was 50 fg of MX. 11 human blood samples of 3 individuals were analysed to elucidate the suitability of this method. The MX concentrations ranged from 66 to 270 pg/g plasma or 12 to 49 ng/g blood lipids, respectively. In the course of the method evaluation the hazard of sample contamination during the clean-up procedure was investigated and MX was found to be present in several materials in the laboratory. Some of these contamination sources could be eliminated.

Characterization of adsorbents by high-performance liquid chromatography using aromatic hydrocarbons Porous graphite and its comparison with silica gel, alumina, octadecylsilica and phenylsilica

The retention characteristics of 52 aromatic hydrocarbons, mostly alkylbenzenes, were measured on porous graphite using methanol as the mobile phase. There is a linear dependence of log k′ on the number of carbon atoms, (i) for polymethylbenzenes whose methyl groups are all in ortho positions and (ii) for n-alkylbenzenes above butylbenzene. As with alumina and silica gel, the ortho effect for graphite was also observed, but graphite shows much higher α values (the increase in log k′ for addition of a carbon atom) than any of the other packing materials studied. The superior selectivity of graphite is in line with its crystalline structure, consisting of a flat, crystalline array of hexagonally arranged carbon atoms. In terms of optimum eluent and retention graphite behaves like a reversed-phase material, but in terms of selectivity towards isomeric compounds it behaves like the oxide adsorbents silica gel and alumina.

Separation of actinoid(III) and actinoid(IV) by solvent extraction and adsorption

The extraction of americium(III) and thorium(IV) by thenoyltrifluoroacetone (TTA) and dibenzoylmethane (DBM) and the adsorption of the ions on silica gel were studied so as to develop a method for the separation of plutonium(III) and plutonium(IV) in natural waters. When the aqueous phase was free from strong complexing anions, the separation of americium(III) and thorium(IV) was possible by the TTA and DBM extraction methods in the pH ranges 1–2.5 and 3–6 respectively and they were roughly separated by the silica gel adsorption method. However, 0.03 M sulphate solution interfered with the separation by the adsorption method and shifted the suitable pH range for the separation by solvent extraction. Fluoride ion may also affect the extraction and adsorption, judging from its complex formation constant with actinoid(IV). The concentrations of complexing anions should be considered for the separation of actinoid(III) and actinoid(IV) in natural waters.

Influence of silica gel surface purity on the coverage density of chemically bonded phases

The surface chemistry of eight silica-gels with different siliceous matrix structures have been described. The influence of surface impurities on the chemical modification process (coverage density) with monooctadecylsilane has been studied. The surfaces of the adsorbents, before chemical modification, were cleaned by extraction with 20% HCl. Bare and modified materials were characterized using different physico-chemical methods e.g.: SIMS, CP/MAS NMR, porosimetry, pH measurement, elemental analysis and chromatography (GC & HPLC). The presence of trace amounts of different metallic elements creating new adsorption centers on the surfaces of the packings, have been demonstrated. Moreover, it has been shown that washing of silica gel adsorbents increases the coverage density of alkylsilyl ligands and improves the chromatographic properties of prepared packings.

The Characterization of Two New Low Molecular Weight Proteins (LMPs) from Streptococcus pyogenes

Two novel extracellular mitogenic substances were isolated from Streptococcus pyogenes strain NY-5 and characterized. The purification steps involved an initial enrichment of the proteins from culture supernatant by silica gel adsorption, followed by ion exchange chromatography and gel filtration. The purified materials were homogeneous in SDS-PAGE, showed estimated molecular weights of 12 kD and isoelectric points of 4.7 and 4.3, respectively. Both proteins (LMP-12k-4.3pI and LMP-12k-4.7pI) demonstrated lymphocyte transformation activity at a concentration of 0.1 microgram/ml. The LMP-12k-4.7pI showed a 69.2% homology of the amino acid sequence with that of a phosphocarrier protein of Staphylococcus aureus and with a total identity in the active centre. The same protein was also isolated from streptococcal group C strain H46A with an N-terminal amino acid sequence being identical. The LMP-12k-4.7pI demonstrated biochemical properties identical with those of the earlier described streptococcal pyrogenic exotoxin type D. The LMP-12k-4.3pI did not show such a clear relation to other functional proteins.