Silica hydrous

White, D.A., and Labayru, R., Synthesis of a manganese dioxide-silica hydrous composite and its properties as a sorption material for strontium, Ind. Eng. Chem. Res., 30(1), 207-210 (1991). 

Several types of mineralizations, massive-type stockwork mineralization and a sulfide-bearing sediment layer were described (Halbach et al., 1989). The outer portion of the massive sulfide sample (late-stage) is composed of barite, realgar, orpiment, amorphous silica and hydrous Fe-Mn oxides, small amounts of sphalerite, galena and pyrite. The central portion consists mainly of sphalerite, pyrite and galena with small... 

Apart from a few reports" on solid acid catalyzed esterification of model compounds, to our knowledge utilization of solid catalysts for biodiesel production from low quality real feedstocks have been explored only recently. 12-Tungstophosphoric acid (TPA) impregnated on hydrous zirconia was evaluated as a solid acid catalyst for biodiesel production from canola oil containing up to 20 wt % free fatty acids and was found to give ester yield of 90% at 200°C. Propylsulfonic acid-functionalized mesoporous silica catalyst for esterification of FFA in flotation beef tallow showed a superior initial catalytic activity (90% yield) relative to a... 

Silica used as a filler for rubbers is silicon dioxide, with particle sizes in the range of 10-40 nm. The silica has a chemically bound water content of 25% with an additional level of 4-6% of adsorbed water. The surface of silica is strongly polar in nature, centring around the hydroxyl groups bound to the surface of the silica particles. In a similar fashion, other chemical groups can be adsorbed onto the filler surface. This adsorption strongly influences silica s behaviour within rubber compounds. The groups found on the surface of silicas are principally siloxanes, silanol and reaction products of the latter with various hydrous oxides. It is possible to modify the surface of the silica to improve its compatibility with a variety of rubbers. 

Hydrous metal oxide powders, 25 100 Hydrous oxides, zirconium, 26 647 Hydrous silicates, in silica/silicate... 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

FA can interact with clay minerals and are known to form stable complexes with metal ions and hydrous oxides [59,61]. The operational technique for isolation of HA involves a pH-induced precipitation and it is likely that accessory minerals may be associated with the precipitation process. Complexes of HA and clay minerals are also formed, the increased ash content of HA suggesting that amorphous silica, iron hydroxides, and clay may aggregate with the HA fraction [58,60,61]. 

Silicate is another type of widely distributed substance that induces the release of histamine from platelets. It is a common constituent of plant tissues in the form of dissolved hydrous silica or silicic acid (24), which may contribute to the byssinotic syndrome. Cotton bracts contain 0.4-0.8% silica (24). For many of the byssinogenic substances, nanogram concentrations are sufficient to release histamine (48). 

A number of reagents containing oxide components are used in zeolite manufacture [19]. Silica is provided by addihon of sodium or other alkali silicate solutions, precipitated, colloidal, or fumed silica, or tetraalkylorthosihcate (alkyl = methyl, ethyl) and certain mineral silicates such as clays and kaolin. Alumina is provided as sodium aluminate, aluminum sulfate soluhon, hydrous aluminum oxides such as pseudo boehmite, aluminum nitrate, or aluminum alkoxides. Additional alkali is added as hydroxide or as halide salts, while organic amines and/or... 

Neither catalyst showed any evidence of crystalline components by X-ray diffraction. The permanganate on silica gel apparently remains as permanganate. The permanganate on activated carbon appears from the spectrum to have been reduced to Mn02 or to some other hydrous manganic oxide. 

Stepwise fluorination. Haimson and Knanth (1983) and Matheney and Knauth (1989) noted that the first fractions of oxygen were 0-depleted compared with oxygen recovered in later fractions, snggesting that the water-rich components of hydrous silica react preferentially in the early steps of fluorination. 

Silica makes up 12.6 mass-% of the Earth s crust as crystalline and amorphous forms. It was found that both modifications show similar main luminescence bands, namely a blue one centered at 450 nm ascribed to which substitutes for Si, red centered at 650 nm linked with non-bridge O, and dark-red at 700-730 nm linked with Fe. Time-resolved luminescence of hydrous volcanic glasses with different colors and different Fe, Mn, and H2O contents were measured and interpreted (Zotov et al. 2002). The blue band with a short decay time of 40 ns was connected with T2( D)- Ai ( S) and Ai C G)- Ai ( S) ligand field transitions of Fe " ", the green band with a decay time of approximately 250 ps with a Ti( G)- Ai( S) transition in tetrahedrally coordinated Mn ", while the red band with a much longer decay time of several ms with T1 (4G)- Ai( S) transitions in tetrahedrally coordinated Fe ". We detected Fe " " in the time-resolved luminescence spectrum of black obsidian glass (Fig. 4.43d). 

Assuming a correlation between surface complexation and aqueous hydrolysis exists, the trend in strengths of surfaces complexes for An in different oxidation states onto a given mineral would be in the order An4+ > AnC>2+ > An3+ > AnOj. Several authors have provided evidence for linear relations between the first hydrolysis constant of metals and the intrinsic constant associated to the formation of surface species of metals as S-OMamorphous silica (Schindler Stumm 1987), hydrous ferric oxides (Dzombak Morel 1990), aluminum (hydr-)oxides and kaolinite (Del Nero et al. 1997, 1999a). 

Ratios of FeO (total iron expressed as FeO) to MgO versus silica in Mars rocks and soils. A diagonal line separates tholeiitic (TH) from calc-alkaline (CA) rocks tholeiitic magmas are anhydrous and calc-alkaline magmas are hydrous. All the Mars rocks plot in the tholeiitic field, as do GRS data (MgO was calculated from the Mg/Si ratio in Fig. 11.19). TES-derived data plot in the calc-alkaline field, but this is an artifact of alteration. After McSween et al. (2009). 

Silica occurs widely in Nature as quartz, often in large transparent crystals of characteristic shape but also in the translucent agglomerations of microscopic crystals known as chalcedony, which includes cherts and flint. Other natural crystalline varieties of Si02 include tridymite and cristobalite (opal is a semiprecious stone that consists of microcrystalline, hydrous cristobalite). All forms of silica involve three-dimensional networks of corner-linked Si04 tetrahedra. 

Adsorption of mercury. All chemicals used were Merck or Baker analytical quality reagents, unless stated otherwise. From the commercially available adsorbents the following were used silica gel 60 A porosity, 0.063-0.200 mm particle size (Merck) charcoal 0.3-0.5 mm particle size, gas-chromatographic quality (Merck) alumina R Woelm hydrous zirconium oxide HZ0-1, 100-200 mesh, (Bio-Rad). Except for zirconium phosphate, which was prepared according to Amphlett ( ), all other sorbents were prepared by coating (precipitation) on acid-purified silica gel, as described in (1 ). The Si0 -NH was prepared according to Leyden et al (11). L... 

Diatomite. also known as diatomaceous earth, or kieselgubr, consists mainly of accumulated shells or frustules of intricately structured amorphous hydrous silica secreted by diatoms, which are microscopic, onc-celled golden brown algae of the class Bacillariophyceae. Diatoms exist in many different environments and arc abundant in regions of oceanic upwelling. 

In Figure 10 the experimental ZPCs of hydrous silica-alumina coprecipitates (64) are compared with those of dried and ignited silica-alumina catalysts (49), some of the previously discussed aluminosilicate minerals, and the composition dependence derived from Equation 18 assuming ... 

Various methods are also available for the preparation of monodispersed hydrous metal oxide sols19 and silica sols20, 134. 

Aluminum occurs widely in nature in silicates such as micas and feldspars, complexed with sodium and fluorine as cryolite, and in bauxite rock, which is composed of hydrous aluminum oxides, aluminum hydroxides, and impurities such as free silica (Cotton and Wilkinson 1988). Because of its reactivity, aluminum is not found as a free metal in nature (Bodek et al. 1988). Aluminum exhibits only one oxidation state (+3) in its compounds and its behavior in the environment is strongly influenced by its coordination chemistry. Aluminum partitions between solid and liquid phases by reacting and complexing with water molecules and anions such as chloride, fluoride, sulfate, nitrate, phosphate, and negatively charged functional groups on humic materials and clay. 

The three-sheet or 2 1 layer lattice silicates consist of two silica tetrahedral sheets between which is an octahedral sheet. These three sheets form a layer approximately 10 A thick. The oxygens at the tips of the tetrahedra point towards the center octahedral sheet and substitute for two-thirds of the octahedrally coordinated hydroxyls. The 2 1 clay minerals include the mica and smectite groups which are by far the most abundant of the clay minerals. The pure end members of this type are talc, a hydrous magnesium silicate pyrophyllite, a hydrous aluminum silicate and minnesotaite, a hydrous iron silicate. 

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