Dissolution Of Silica Gel Research Articles

The Repetitive Detection of Toluene with Bioluminescence Bioreporter Pseudomonas putida TVA8 Encapsulated in Silica Hydrogel on an Optical Fiber.

Living cells of the lux-based bioluminescent bioreporter Pseudomonas putida TVA8 were encapsulated in a silica hydrogel attached to the distal wider end of a tapered quartz fiber. Bioluminescence of immobilized cells was induced with toluene at high (26.5 mg/L) and low (5.3 mg/L) concentrations. Initial bioluminescence maxima were achieved after >12 h. One week after immobilization, a biofilm-like layer of cells had formed on the surface of the silica gel. This resulted in shorter response times and more intensive bioluminescence maxima that appeared as rapidly as 2 h after toluene induction. Considerable second bioluminescence maxima were observed after inductions with 26.5 mg toluene/L. The second and third week after immobilization the biosensor repetitively and semiquantitatively detected toluene in buffered medium. Due to silica gel dissolution and biofilm detachment, the bioluminescent signal was decreasing 20–32 days after immobilization and completely extinguished after 32 days. The reproducible formation of a surface cell layer on the wider end of the tapered optical fiber can be translated to various whole cell bioluminescent biosensor devices and may serve as a platform for in-situ sensors.

Silica Gel as a Surrogate for Biogenic Silica in Batch Dissolution Experiments at pH 9.2: Further Testing of the Shrinking Object Model and a Novel Approach to the Dissolution of a Population of Particles

Recently, the increase in dissolved concentration in the batch dissolution of various salts or sucrose has been successfully modelled with three equations, one a cubic in time. However, from three separate earlier investigations with ocean sediments and phytoplankton frustules, there is residual suspicion that biogenic silica does not follow this behaviour. This paper shows that the Shrinking Object Model applies to the dissolution of sieved silica gel particles, as well as to a sample of unsieved, freeze-dried frustules of Odentella sp. Silica gel, being readily available in quantities that can be sieved, is a useful surrogate for biogenic silica in allowing problems of experimental design to be overcome. The dissolutions covered three possible analytic integrations that arise from the model: an exponential for approach to saturation with excess solid silica; the approach to near saturation with either a slight excess or deficiency of silica; dissolution at high under-saturation. Good agreement was found between experimental results and mathematical modelling. The paper provides template calculations by which future raw results can be parameterized. Nevertheless, the reasons for non-linear kinetics reported in earlier work have not been identified, and so controversy over non-linear dissolution kinetics is enhanced. Stirring regime was found to be important with silica gel dissolution, and so biogenic silica dissolution is therefore likely to be ‘transport limited’ at low stirring rate. Accordingly, all archived and future data should be scrutinized for stirring effects before being applied to the oceanic environment. A rigorous test for determining whether a substance’s dissolution deviates from the model is recommended as a preliminary to any future dissolutions, whether in batch or with the chemo-stat. A fixed amount of frustule sample is added to a series of buffered mixtures containing increasing background silicic acid concentrations. Absence of any problem is marked by a linear plot between the increase in silicic concentration accruing over a fixed reaction period and that of the background silicic acid. A novel mathematical proof is provided to justify the test’s use. The reasons for the earlier deviations from expected behaviour of, for example, oceanic sediments, are discussed. Lastly, the paper provides a novel approach to the dissolution of a population of particles of mixed sizes which will probably find ready future application in oceanography.

Interaction of Freshly Precipitated Silica Gel with Aqueous Silicic Acid Solutions under Ambient and Near Neutral pH-conditions: A Detailed Analysis of Linear Rate Law

Interaction of freshly precipitated silica gel with aqueous solutions was studied at laboratory batch experiments under ambient and near neutral pH-conditions. The overall process showed excellent reversibility: gel growth could be considered as an opposite process to dissolution and a linear rate law could be applied to experimental data. Depending on the used rate law form, the resulting rate constants were sensitive to errors in parameters/variables such as gel surface area, equilibrium constants, Si-fluxes, and reaction quotients. The application of an Integrated Exponential Model appeared to be the best approach for dissolution data evaluation. It yielded the rate constants k dissol ∼ (4.50 ± 0.68) × 10−12 and k growth ∼ (2.58 ± 0.39) × 10−9 mol m−2 s−1 for zero ionic strength. In contrast, a Differential Model gave best results for growth data modeling. It yielded the rate constants k dissol ∼ (1.14 ± 0.44) × 10−11 and k growth ∼ (6.08 ± 2.37) × 10−9 mol m−2 s−1 for higher ionic strength (I ∼ 0.04 to 0.11 mol L−1). The found silica gel solubility at zero ionic strength was somewhat lower than the generally accepted value. Based on the $${}^{298}K_{\rm SS}^{(I=0)} = 10^{-2.754\pm 0.017}$$ and $$^{298}K_{\rm DBP}^{(I=0)} = 10^{-2.728\pm 0.003},$$ standard Gibbs free energy of silica gel formation was calculated as $$\Updelta{G}_{\rm f}^0(298\,{\rm K}) \sim -\hbox{850,463} \pm 98\hbox{ J mol}^{-1}$$ and −850,318 ± 20 J mol−1, respectively. Activation energies for silica gel dissolution and growth were determined as $$E_{\rm A}^{\rm dissol} \sim 62.0\pm 3.2\,\hbox{kJ mol}^{-1}$$ and $$E_{\rm A}^{\rm growth} \sim 48.8\pm 4.4\,\hbox{ kJ mol}^{-1},$$ respectively. An universal value for growth of any silica polymorph, $$E_{\rm A}^{\rm growth} \sim 37.4 \pm 9.4\,\hbox{kJ mol}^{-1},$$ is not consistent with the value for silica gel growth, which questions the hypothesis about one unique activated complex controlling the silica polymorph growth.

A new method for dissolution of silica gel in a pentafluoropropionyl fluoride—tertiary amine system

A new procedure for the preparation of amino complexes of silicon tetrafluoride was proposed. For this purpose, silica gel is dissolved in a fluoroanhydride—methanol system at room temperature followed by the precipitation of SiF4 solvate that was formed with tertiary amine (pyridine, 1,10-phenanthroline, 2,2′-bipyridyl). The yields of the formed complexes are 69–83%.

Speciation of dissolved silicates in natural waters containing alkaline and alkaline-earth ions. A case study--waters from arid lands (North West China).

The concentration of silica in water samples from the desert area of Xinjiang, N. W. China, has been measured by colorimetry with ammonium molybdate. The observed pattern of dependence of the concentration of silica on the concentration of sodium ion (Na(+)) in the water samples is consistent with the pattern obtained by experiments on in-vitro dissolution of silica gel in sodium chloride (NaCl) solution. This indicates that the dissolution of silica in the hydrologic system in this area depends on the concentration of Na(+). Calcium ion (Ca(2+)), which is known to play an important role on the dissolution of silica on the basis of in-vitro experiments, was observed to take little part in the dissolution of silica in actual natural water samples. This implies that the Ca(2+) is bound to the hydrogen carbonate anion or that the Ca(2+) content of natural water containing salts is very low, owing to precipitation. In these samples silicate-Na(+) was identified as the dissolution species of silica; it was also ascertained that Ca(2+) did not form complexes with silicate species. These observations resulted from direct identification of dissolved chemical species by use of FAB-MS (fast atom bombardment mass spectrometry). The research indicates that in water samples in this critically arid region the concentration of "dissolved" silica is basically determined by the concentration of Na(+), indicative of pure inorganic conditions in the desert area of Xinjiang, N.W. China.

Influence of the addition of alkanolamines and tetramethylammonium hydroxide on the shape and size of zeolite-A particles

Influences of the addition of alkanolamines and alkylammonium hydroxide into the starting suspensions on the shape and the size of zeolite-A (NaAlSiO4·xH2O) particles were investigated by optical microscopy and liquid-state NMR measurements. Zeolite-A was prepared from a mixture of NaAlO2, Na2SiO3·9H2O, alkanolamine or alkylammonium hydroxide, and H2O by hydrothermal treatment. The addition of triethanolamine (TEA) into the starting suspension led to the enlargement of the particle size of the prepared zeolite-A, whereas the addition of tetramethylammonium hydroxide (TMAOH) decreased it. TEA was confirmed to coordinate to Al to form [N(C2H4O)3Al(OH)]− complex, decreasing the degree of supersaturation of free [Al(OH)4]− and inhibiting the excess nucleation of zeolite. On the other hand, the addition of TMAOH facilitated the dissolution of silica gel in the starting suspension, very probably due to its higher basic property, and increased the degree of supersaturation of Si species, which resulted in the decreases in the particle size.

Changes in the Surface Area of Silica-supported 12-Molybdophosphoric Acid in Relation to the Degree of Loading and the Calcination Temperature

A technical silica gel (Grace-113) was impregnated with 12-molybdophosphoric solution to attain loadings of 0.72–13.4 wt.%. The air-dried solids were then calcined for 3 h at 400, 600 and 800°C, respectively, followed by determination of their N2 adsorption isotherms. The silica gel was thermally stable and suffered about 15% loss in area at 800°C. The impregnated silicas showed considerable decrease in surface area as a function of their heteropoly acid content. Two processes are suggested as taking part in the modification of pore structure: (1) diffusion of the aqueous heteropoly acid solution and its subsequent deposition; and (2) dissolution of silica gel in the acidic aqueous solution followed by interaction with HPMo to form either molybdosilicate or P–Si exchange products. Correlation with the previously studied catalytic conversion of isopropanol indicated that the activity was independent of the surface area. Catalytic dehydration on the 600°C and 800°C products seems to be associated with silicate, molybdate or MoO3 surface species rather than being due to surface acidity or the extent of the surface area.

Mechanism of Silica Gel Dissolution in Water in the Presence of Pyrocatechol

Abstract The rates of dissolution of silica gel in aqueous alkaline solutions containing pyrocatechol were measured. At pH 10, all of the silica gel was dissolved in the presence of pyrocatechol at 25 °C for 48 h (SiO2 0.3 g and pyrocatechol 0.04 mol in 15 ml of water). The rates of silica-gel dissolution increased with increasing pyrocatechol concentration. Four moles of pyrocatechol were needed to dissolve one mole of SiO2. The rate of dissolution was first order in solid SiO2 concentration. Langmuir-type adsorption of pyrocatechol to the silica-gel surface was assumed to explain the rate dependence on the pyrocatechol concentration.

Surface Chemistry and Durability of Borosilicate Glass

ABSTRACTPreliminary results show that glass durability is dependent on reactions occurring at the glass-solution interface. CSG glass (18.2 wt. % Na2O,5.97 wt. % CaO, 11.68 wt. % Al2O3, 8.43 wt. % B2O3, and 55.73 wt. % SiO2) dissolution and net surface H+ and OH- adsorption are minimal at near neutral pH. In the acid and alkaline pH regions, CSG glass dissolution rates are proportional to 2 to , respectively. In contrast, silica gel dissolution and net H+ and OH" adsorption are minimal and independent of pH in acid to neutral solutions. In the alkaline pH region, silica gel dissolution is proportional 0.9 to . Although Na adsorption is significant for CSG glass and silica gel in the alkaline pH regions, it is not clear if it enhances dissolution, or is an artifact of depolymerization of the framework bonds.

The structure directing effect of cations in aqueous silicate solutions. A 29Si-NMR study

Studies of the dissolution of amorphous silica gel in aqueous hydroxides by 29Si-MAS-NMR, give information on the oligomerisation of dissolved monomeric silicic acid and on the interaction between cations and the silica gel surface. The dissolution rate increases in the order (LIOH ≈ CsOH) < (RbOH ≈ NaOH) < KOH, indicating different promoting effects of the corresponding cations in the dissolution reaction. We attribute these differences to hydration properties of adsorbed alkali metal cations. Formation of silicate oligomers at high pH-values proceeds via cyclic trimeric silicate anions which, at high pH-values are more stabilised than the linear silicate anions. Further, the structure of highly polymerised silicate species depends on the alkali metal cation. Here we show for the first time that tetramethylammonium (TMA) hydroxide initially inhibits the dissolution of amorphous silica gel, which is caused by strong adsorption of rather hydrophobic TMA-cations on the silica gel surface. The effect of sodium/TMA-ratios on the dissolution rate was studied, showing that in the case where sodium is present, the rate of silica gel dissolution substantially increases. Studies on the reaction mechanism for the formation of cubic octameric silicate anions stabilised by TMA-cations are also reported.

Silica gel dissolution in aqueous alkali metal hydroxides studied by 29SiNMR

The rate of silica gel dissolution in aqueous alkaline media was investigated using 29Si nuclear magnetic resonance spectroscopy. A profound difference of alkali metal hydroxides on the dissolution rate of amorphous silica gel was observed. The dissolution rate increases in order LiOH ≈ CsaOH) < (RbOH ≈ NaOH) < KOH, as was confirmed by the β = silicomolybdate complexation method. Silica gel dissolution involved formation of monomeric silicic acid, Q 0. The monomeric anions oligomerise into dimer species, which in turn form cyclic and lineartrimer species. The structure of highly polymerised silicate species depends on the alkali metal cation, i.e. low pH silicate solutions have structurally different silicate species as a function of alkali metal hydroxide, as is shown by 29Si-NMR spectroscopy. Potassium cations gave rise to more polymerised silica species compared with other alkali metal cations studied. On the contrary, when lithium hydroxide and silica gel are mixed in a molar SiO 2/Li 2O = 1:1, a microcrystalline phase is formed which consists of lithium silicate crystals. Crystallisation of the lithium silicate proceeds via monomeric silica being in solution.