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PubMed2000

Sorption of Cs(I) on Magnetite in the Presence of Silicates.

The sorption of H(4)SiO(4) on magnetite has been qualified and quantified using three different surface complexation models, CCM, DLM, and NEM. The three tested models can account for the sorption of silicates using the same stoichiometry, one neutral species binding on a neutral surface, and the same constant, error aside. Experiments have also been performed to demonstrate that the sorption of dissolved silicates has a nonnegligible effect on the behavior of the surface of magnetite. Then, the sorption of cesium is insignificant on the neat surface of magnetite and is increased up to 10-20% when silicates are present in solution. A theoretical model, where the rule of electrostatics is pointed out, has been developed to account for the experimental observations. This model allows the reproduction of the sorption of cesium in the presence of dissolved silicates for the following four cases: -concentration of silicates under solubility limit -concentration of silicates over solubility limit -binary mixtures of silica and magnetite -natural magnetite with silica as impurity. The reaction given in the model to account for the experimental observations proposes that silicates may act as a "bridge" between the surface of magnetite and cesium. Copyright 2000 Academic Press.

Journal Article↗

PubMed2004

Bilateral urinary calculi after treatment with a silicate-containing milk thickener.

UNLABELLED: Nephrocalcinosis and/or urinary calculi are rare in infants. Furosemide treatment during the neonatal period, vitamin D intoxication, hereditary diseases such as hyperoxaluria or distal tubular acidosis are among the most common aetiologies. We report the case of a 6-month-old boy with an extra-hepatic biliary duct atresia treated by the Kasai procedure and a gastro-oesophageal reflux treated with a silicate containing milk thickener (Gelopectose, 5.5% colloidal silicate) since the neonatal period. He did not present any other endogenous risk factor for urinary stone formation (normal urinary calcium/creatinine ratio; normal urinary magnesium excretion). The nephrolithiasis was discovered as the boy presented painful episodes of macroscopic haematuria. Ultrasound examination revealed bilateral nephrocalcinosis and multiple bilateral calculi without infection or urinary obstruction. Infrared spectroscopy revealed silicate as the major component suggesting silicate absorption to be responsible for the described symptoms. After replacement of the silicate-containing agent by a silicate-free milk thickener, the lesions were completely reversible as confirmed by repeated renal ultrasound examinations over a 2-month period. CONCLUSION: Silicate-containing milk thickeners can be responsible for urinary calculi and/or nephrocalcinosis.

Gastroesophageal Reflux↗

PubMed2005

Single-molecule studies of diffusion by oligomer-bound dyes in organically modified sol-gel-derived silicate films.

Single-molecule fluorescence spectroscopy is used to study dye diffusion within organically modified silicate (ORMOSIL) films. ORMOSIL films are prepared from sols containing tetraethoxysilane and isobutyltrimethoxysilane in 2:1 and 1:9 molar ratios. Nile red and a new silanized form of nile red that can be covalently attached to the silicate matrix are used as fluorescent probe molecules. The number and rate of single molecules diffusing through these films increases dramatically with increasing film organic content. Autocorrelation of the fluorescence images yields a quantitative measure of the relative populations of fixed and diffusing species. Surprisingly, both "free" and silicate-bound nile red exhibit relatively facile translational motions. Single-molecule/single-point fluorescence correlation spectroscopy (FCS) is used to measure the dye diffusion coefficients in submicrometer-scale film regions. The most common diffusion coefficients for "free" and silicate-bound nile red molecules in the 1:9 films are 3.9 x 10(-10) and 1.6 x 10(-10) cm(2)/s, respectively. The unexpectedly rapid diffusion of silicate-bound nile red is attributed to the presence of liquidlike silicate oligomers in the films. A lower bound for the molecular weight of the oligomers is estimated at 2900. Bulk solution-phase FCS experiments performed on "free" and silicate-bound nile red species extracted into chloroform solutions provide valuable support for these conclusions. Comparison of the results derived from experimental and simulated time transients indicates film heterogeneity occurs on sub-100-nm-length scales and likely results from the presence of inorganic- and organic-rich domains.

Journal Article↗

PubMed2003

In situ remediation of arsenic in simulated groundwater using zerovalent iron: laboratory column tests on combined effects of phosphate and silicate.

We performed three column tests to study the behavior of permeable reactive barrier (PRB) materials to remove arsenic under dynamic flow conditions in the absence as well as in the presence of added phosphate and silicate. The column consisted of a 10.3 cm depth of 50:50 (w:w, Peerless iron:sand) in the middle and a 10.3 cm depth of a sediment from Elizabeth City, NC, in both upper and lower portions of the 31-cm-long glass column (2.5 cm in diameter) with three side sampling ports. The flow velocity (upflow mode) was maintained at 4.3 m d(-1) during the 3-4-month experiments. As expected, dissolved As concentrations in different positions of the column generally followed the order: column influent > bottom port effluent > middle port effluent > top port effluent > column effluent. The steady-state As removal in the middle Peerless iron and sand mixture zone might be attributed to the continuous supply of corroded iron in the form of iron oxides and hydroxides that served as the sorbents for both As(V) and As(III). Consistent with previous batch study findings, dissolved phosphate (0.5 or 1 mg of P L(-1)) and silicate (10 or 20 mg of Si L(-1)) showed strong inhibition for As(V) and As(III) (1 mg of As(V) L(-1) + 1 mg of As(III) L(-1) in 7 mM NaCl + 0.86 mM CaSO4) removal by Peerless iron in the column tests. The presence of combined phosphate and silicate resulted in earlier breakthrough (C = 0.5C0) and earlier complete breakthrough of dissolved arsenic relative to absence of added phosphate and silicate in the bottom port effluent. Competition between As(V)/As(III) and phosphate/silicate forthe sorption sites on the corrosion products of Peerless iron seems to be the cause of the observations. This effect is especially important in the case of silicate for designing a PRB of zerovalent iron for field use because dissolved silicate is ubiquitous in terrestrial waters.

Arsenic↗

PubMed2005

Diatom carbon export enhanced by silicate upwelling in the northeast Atlantic.

Diatoms are unicellular or chain-forming phytoplankton that use silicon (Si) in cell wall construction. Their survival during periods of apparent nutrient exhaustion enhances carbon sequestration in frontal regions of the northern North Atlantic. These regions may therefore have a more important role in the 'biological pump' than they have previously been attributed, but how this is achieved is unknown. Diatom growth depends on silicate availability, in addition to nitrate and phosphate, but northern Atlantic waters are richer in nitrate than silicate. Following the spring stratification, diatoms are the first phytoplankton to bloom. Once silicate is exhausted, diatom blooms subside in a major export event. Here we show that, with nitrate still available for new production, the diatom bloom is prolonged where there is a periodic supply of new silicate: specifically, diatoms thrive by 'mining' deep-water silicate brought to the surface by an unstable ocean front. The mechanism we present here is not limited to silicate fertilization; similar mechanisms could support nitrate-, phosphate- or iron-limited frontal regions in oceans elsewhere.

Atlantic Ocean↗

PubMed2003

Silicate antibodies in Danish women with silicone breast implants.

OBJECTIVES: To use a new immunological assay to evaluate silicate antibody levels in women with and without silicone breast implants (SBIs). METHODS: Women (n=186) were identified through Danish population-based registers and categorized into six groups defined by prior breast surgery (breast implantation/breast reduction/no breast surgery) and by the presence or absence of prior hospital diagnoses of soft-tissue rheumatism (muscular rheumatism, ICD-8 codes 717.90 and 717.99). The women underwent blood tests, a silicate antibody assay and a clinical examination. Severity of rheumatic symptoms/signs was scored from 1 (none) to 5 (severe). RESULTS: The level of silicate antibodies was not significantly different between the three groups with prior soft-tissue rheumatism (P > 0.5), with the lowest value among women with SBIs. Among women who had no prior diagnosis of soft-tissue rheumatism, silicate antibody levels were highest in women with SBIs (P < 0.01). No significant correlations were observed between silicate antibody levels and symptom severity scores. CONCLUSIONS: Silicate antibodies were not consistently associated with SBIs and were not correlated with rheumatic symptoms. The clinical relevance of these antibodies remains questionable.

Adult↗

PubMedDate not supplied

Mesoporous silicates as nanoreactors for carbon nanotube production in the absence of transition metal catalysts.

Carbon nanotubes were produced from either a template or the polymer-filled pore systems of mesoporous silicates of various structures and dimensions by heat treatment in the absence of air. Successful synthesis was done when the template molecules contained little or no oxygen. For SBA-15 material, where the structure-directing molecule used for synthesis of mesoporous silicate was polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer, no carbon nanostructures were formed. A peculiar carbon nanostructure was generated from the template for pore expanded MCM-41. To demonstrate carbon nanotube formation from polymer in the mesoporous silicates, the mesopores of MCM-41, MCM-48, and SBA-15 silicates were filled with divinyl-benzene polymer and then graphitized at 1300 K. The polymer was successfully transformed into carbon nanotubes for the MCM class silicate but not the SBA-15 silicate.

Catalysis↗

PubMed2005

Arginine kinase in the demosponge Suberites domuncula: regulation of its expression and catalytic activity by silicic acid.

In Demospongiae (phylum Porifera) the formation of the siliceous skeleton, composed of spicules, is an energetically expensive reaction. The present study demonstrates that primmorphs from the demosponge Suberites domuncula express the gene for arginine kinase after exposure to exogenous silicic acid. The deduced sponge arginine kinase sequence displays the two characteristic domains of the ATP:guanido phosphotransferases; it can be grouped to the 'usual' mono-domain 40 kDa guanidino kinases (arginine kinases). Phylogenetic studies indicate that the metazoan guanidino kinases evolved from this ancestral sponge enzyme; among them are also the 'unusual' two-domain 80 kDa guanidino kinases. The high expression level of the arginine kinase gene was already measurable 1 day after addition of silicic acid by northern blot, as well as by in situ hybridization analysis. Parallel determinations of enzyme activity confirmed that high levels of arginine kinase are present in primmorphs that had been exposed for 1-5 days to silicic acid. Finally, transmission electron-microscopical studies showed that primmorphs containing high levels of arginine kinase also produce siliceous spicules. These data highlight that silicic acid is an inorganic morphogenetic factor that induces the expression of the arginine kinase, which in turn probably catalyzes the reversible transfer of high-energy phosphoryl groups.

Amino Acid Sequence↗

PubMed1992

An aluminum silicate binding assay for quantitation of degradation of cholecystokinin octapeptide and other short peptides.

Most available techniques for the quantitation of enzymatic degradation of peptide hormones are time-consuming and require expensive equipment and/or novel reagents. Our aim here was to develop a rapid and sensitive assay for the measurement of degradation of cholecystokinin octapeptide (CCK-8) as well as other short, hydrophobic peptides. The proposed technique is based on our novel observation that intact CCK-8, but not its degradation product(s), binds to Lloyd reagent, a form of aluminum silicate. When radiolabeled CCK-8 was exposed to rat liver cytosol containing endogenous CCK-degrading activity, there was a time-dependent decrease in the binding of radiolabel to aluminum silicate [from 86 to 8% over 60 min at 37 degrees C]. The decrease in binding closely paralleled the extent of CCK-8 degradation over time as assessed by high-performance liquid chromatography and immunoprecipitation with specific polyclonal antibodies to CCK-8. While aluminum silicate did not efficiently bind to C-terminal and N-terminal CCK tetrapeptides, magnesium silicate bound to both tetrapeptides (> 82%), but not to their radiolabeled degradation products. Both aluminum and magnesium silicate also extensively bound (> 82%) to other peptide hormones including Met-enkephalin, somatostatin, and secretin, but did not bind their degradation products. These binding assays will be useful in studies of peptidases which degrade cholecystokinin or other small, hydrophobic peptides.

Aluminum Silicates↗

PubMed2001

Effects of Elevated Carbon Dioxide and Nitrogen Fertilization on Mycorrhizal Fine Roots and the Soil Microbial Community in Beech-Spruce Ecosystems on Siliceous and Calcareous Soil.

To study the responses of forests to global change, model ecosystems consisting of beech and spruce trees were maintained in open top chambers for 4 years under four conditions, namely with normal and elevated CO2 and with low and high nitrogen input, each replicated four times. Each open top chamber (height 3 m, diameter 3 m, soil depth 1.5 m) contained two separate soil compartments containing nutrient-poor siliceous and nutrient-rich calcareous soil. Here, we focus on the fine roots and the soil microbial community in these model ecosystems. At the time of planting, the fine roots were cut back according to forestry practice, and the newly formed roots were colonized by the indigenous soil microflora. After 4 years, the total biomass of fine roots, when averaged over all treatments, was almost the same in each of the two soil types; it was highest in the top 100 mm of soil (60%) and decreased sharply in deeper soil layers. Fungal biomass associated with the fine roots, consisting mainly of ectomycorrhizal fungi, was estimated using the ergosterol content as a marker. It was much higher in fine roots in the siliceous than in the calcareous soil, indicating considerably enhanced ectomycorrhiza formation in the nutrient-poor siliceous soil. Elevated atmospheric CO2 stimulated fine root production by ca. 85% and 43% in the top 100 mm of calcareous and siliceous soils respectively. Increased nitrogen input caused a slightly reduced production of fine root biomass in the calcareous soil but increased it by 33% in the siliceous soil. Marker substances for microorganisms were analyzed in the root-free soil. The amounts of carbon released by fumigation/extraction (a general marker for microbial biomass) and chitin (a marker for fungal biomass) were significantly higher in the top layer of the siliceous than of the calcareous soil, but they did not respond significantly to the treatments with elevated CO2 or the nitrogen fertilizer. The total concentration of the phospholipid fatty acids (PLFAs) and the number of the PLFAs did not differ between the two soil types. However, four of the eight most abundant PLFAs differed significantly between the two soil types. Principal component analysis revealed clearly separated clusters for the two soil types. Although analysis did not reveal any significant changes in response to the treatments, the concentration of the PLFA typical for ectomycorrhizal fungi was significantly higher under conditions of elevated CO2 in the nutrient-rich calcareous soil.

Journal Article↗

PubMed2005

Modification of fibrous silicates surfaces with organic derivatives: an infrared spectroscopic study.

This contribution explores the interaction of the fibrous silicates, palygorskite, sepiolite and chrysotile with a wide range of organic agents. Infrared spectroscopy (IR) methods are essential for the characterization of solid surfaces and for the investigation of the kind of bonds formed between the surface of these silicates and the organic moieties. Thus, when sepiolite or palygorskite are treated, e.g., with polyurethanes, alcohols, isocyanates, amines or pyridines, specific SiNHC or SiOC bonds are derived from the linkage of the differently located OH groups in these fibrous silicates with the organic moieties. On the other hand, more stable, covalent SiOSiC bondings are formed when the fibrous silicates, especially chrysotile, are reacted with heterofunctional silylating agents like chlorosilanes or ethoxysilanes carrying, alkyl, alkenyl or aryl groups. Such reactions may occur in the presence or absence of HCl. An absorption band at 960 cm(-1)--which we assigned to SiOH groups--is detected only in the presence of HCl. The evolution of this band is related to the degree of grafting of the organic radicals with the silanol groups of the silicates. HCl-generated silanol groups are the main bridges for the coupling of organosilyl groups on chrysotile and other silicates by covalent bonding, leading the way to the preparation of interesting new materials, including fibrous sheet polymers.

Journal Article↗

PubMed2003

Isomerization of cationic azobenzene derivatives in dispersions and films of layered silicates.

Photoisomerization reactions of cationic azobenzene dyes in solutions, dispersions, and films of layered silicates were studied by visible (Vis) spectroscopy. The dyes isomerized reversibly from thermodynamically more stable trans-isomers to cis-isomers when irradiated with ultraviolet (UV) light. Observed trends were compared with the optical changes of the dyes that occurred as a consequence of their adsorption at the silicate surface. Small fractions of the dyes are likely to have isomerized during the adsorption process, even without the UV-light irradiation. The aggregation of the dyes was another reaction taking place at the surface of the silicates. The extent of the UV-light-induced isomerization reactions was reduced for the adsorbed dye cations. The reaction proceeded readily for a dye with monovalent cations. However, the photoisomerization was practically negligible in both dispersions and films of layered silicates for a dye with bivalent cations, whereas the isomerization proceeded in solution. This phenomenon was interpreted in terms of the attractive electrostatic forces between the substrate and the dye cations, which hindered the isomerization reaction. The layer charge of silicates affected the orientation of the dye cations as observed by X-ray diffraction (XRD) measurements. However, the choice of silicate did not significantly affect the fundamental aspects and the described basic trends of the UV-light-induced isomerization reaction.

Journal Article↗

PubMed2000

Kinetics of silicate exchange in alkaline aluminosilicate solutions.

In strongly alkaline aqueous KOH solutions containing SiIV in large excess over AlIII, the kinetics of exchange of monomeric silicate with small acyclic aluminosilicate solute species is much more rapid than with either cyclic aluminosilicates or any all-silicate anions. Selective inversion recovery 29Si NMR studies of homogeneous solutions of stoichiometric composition 3.0 mol kg-1 of SiO2, 0.1 mol kg-1 of Al2O3, and 8.0 mol kg-1 of K2O in 60-75% D2O gave rate constants of 2.0 +/- 0.2 kg mol-1 s-1 and 17 +/- 4 s-1 for the forward and reverse reactions of monomeric silicate with (HO)3AlOSiOn(OH)(3-n)(n+1)- (n = 2 or 3) at 0 degree C. These rate constants are more than 10(4)-fold faster than those extrapolated from 60 to 90 degrees C for comparable reactions of silicate anions. The greater lability of acyclic aluminate centers relative to silicate is ascribed partly to the availability of HO- groups for condensation reactions on Al and mainly to the ease of expansion of the coordination number of AlIII beyond 4. The latter attribute is diminished when AlIII is constrained to be tetrahedral in cyclic structures. With respect to the mechanism of formation of zeolites from alkaline aqueous media, it is suggested that small, labile AlOSi units add rapidly to growing zeolitic structures "on demand", whereas the more kinetically inert cage or ring structures cannot. This would explain why a silicate or aluminosilicate structure that is dominant among solute species at equilibrium in the presence of a particular cation may bear little or no geometric relation to the zeolitic framework promoted kinetically by that same cation.

Journal Article↗

PubMed2006

Interaction energy and surface reconstruction between sheets of layered silicates.

Interactions between two layered silicate sheets, as found in various nanoscale materials, are investigated as a function of sheet separation using molecular dynamics simulation. The model systems are periodic in the xy plane, open in the z direction, and subjected to stepwise separation of the two silicate sheets starting at equilibrium. Computed cleavage energies are 383 mJ /m(2) for K-mica, 133 mJ /m(2) for K-montmorillonite (cation exchange capacity=91), 45 mJ /m(2) for octadecylammonium (C(18))-mica, and 40 mJ /m(2) for C(18)-montmorillonite. These values are in quantitative agreement with experimental data and aid in the molecular-level interpretation. When alkali ions are present at the interface between the silicate sheets, partitioning of the cations between the surfaces is observed at 0.25 nm separation (mica) and 0.30 nm separation (montmorillonite). Originally strong electrostatic attraction between the two silicate sheets is then reduced to 5% (mica) and 15% (montmorillonite). Weaker van der Waals interactions decay within 1.0 nm separation. The total interaction energy between sheets of alkali clay is less than 1 mJ /m(2) after 1.5 nm separation. When C(18) surfactants are present on the surfaces, the organic layer (>0.8 nm) acts as a spacer between the silicate sheets so that positively charged ammonium head groups remain essentially in the same position on the surfaces of the two sheets at any separation. As a result, electrostatic interactions are efficiently shielded and dispersive interactions account for the interfacial energy. The flexibility of the hydrocarbon chains leads to stretching, disorder, and occasional rearrangements of ammonium head groups to neighbor cavities on the silicate surface at medium separation (1.0-2.0 nm). The total interaction energy amounts to less than 1 mJ /m(2) after 3 nm separation.

Journal Article↗

PubMed1993

Cytotoxic and genotoxic effects of calcium silicates on human lymphocytes in vitro.

Calcium silicate has been considered as a possible replacement for asbestos due to its heat and fire resistance. We describe the cytotoxic and genotoxic potential of calcium silicate using peripheral human blood lymphocytes. Calcium silicates at concentrations of 10 and 100 micrograms/ml significantly increased the frequencies of chromosomal aberrations (CAs) and sister-chromatid exchanges (SCEs). The increases in CAs and SCEs were dose-dependent, though not linearly. A significant decrease in the proliferation rate index was observed with increased dose of calcium silicates. The induction of chromatid-type aberrations indicates that the clastogenic activity of calcium silicate is S-phase-dependent.

Adult↗

PubMed1986

Effects of the inhalation of dusts from calcium silicate insulation materials in laboratory rats.

The effects of respirable dust from three commercially produced calcium silicate insulation materials were examined in laboratory rats by long-term inhalation and injection techniques. These calcium silicate products have been used as replacements for asbestos in the insulation of the engine rooms of ships, and the particle size distribution of the dust clouds generated for the experimental study closely matched those found in ships during the installation of this type of material. One year of exposure to a dust cloud of 10 mg/m3 of respirable dust had no discernible effect on the length of survival of treated animals compared to controls. No pulmonary lesions were found that appeared associated with the inhalation of calcium silicate per se, but one sample did contain significant amounts of quartz and this did produce a few small pulmonary nodules. While two small pulmonary neoplasms, one malignant and one benign, were found in dusted animals, neither was the cause of death, and the incidence was not significantly different from the control group where no tumors were found. One peritoneal mesothelioma was found in an animal from one of the inhalation groups, but this was considered to be a spontaneous tumor as none of over 100 animals injected intraperitoneally with 25 mg of calcium silicate developed these tumors. While the white blood cell count of dusted animals, compared to controls, was significantly raised in all treated groups at the end of the dusting period, these figures were within the published normal ranges for laboratory rats. It was concluded that the three tested calcium silicate products were harmless to the rats of this species at the doses tested.

Animals↗

PubMed2004

Silicate complexes of sugars in aqueous solution.

Certain sugars react readily with basic silicic acid to form soluble 2/1 (sugar/silicic acid) silicate complexes. Failure of monohydroxy compounds to give soluble products under these conditions indicates that the sugar silicates are chelates: five-membered diolato rings. Only furanose forms react. Pyranose sugars are stable under these conditions. Because all glycosides fail to react with silicic acid under these conditions, reaction appears to involve the anomeric position (C1 in aldoses, C2 in ketoses), which has a more acidic hydroxy group. Reaction is completed only when the anomeric hydroxy group is cis to an adjacent hydroxy group. The appropriate furanose form must have sufficient natural abundance and solubility to provide an observable product, as measured by (29)Si and (13)C NMR spectroscopy. These structural and practical constraints rationalize the successful reaction of the monosaccharides ribose, xylose, lyxose, talose, psicose, fructose, sorbose, and tagatose and the disaccharides lactulose, maltulose, and palatinose. Glucose, mannose, galactose, and sucrose, among others, failed to form complexes. This high selectivity for formation of sugar silicates may have ramifications in prebiotic chemistry.

Carbohydrate Conformation↗

PubMed1959

The metabolism of silicon in the rat and its relation to the formation of artificial siliceous calculi.

The urinary excretion of silicon in the rat was found to be enhanced beyond normal levels by the administration of various chemical forms of silicon. The excretion was enhanced to a much greater degree by the administration of ethyl silicate than by magnesium trisilicate, sodium metasilicate, or water glass. The tolerance level of rats to sustained daily doses of ethyl silicate fed via stomach tube was approximately 15 to 30 mg. of silicon per rat per day. Urinary silicon excretion was found to be a straight line function of the concentration of ethyl silicate administered, via stomach tube, with approximately 18 per cent of the administered silicon appearing in the urine at all levels tested. Using sustained dietary additions of ethyl silicate as a means of enhancing urine silicon levels, artificial siliceous urinary calculi were consistently produced on zinc pellets implanted in the bladders of rats.

Animals↗