To test the hypothesis that silicification occurs under acid conditions in the silicon deposition vesicle (SDV), the acidity of the SDV of the pennate diatoms Navicula pelliculosa (Brebisson et Kutzing) Hilse, N. salinarum (Grunow) Hustedt, and Nitzschia sigma (Kutzing) Smith was determined during development of new frustule valves. Cells were incubated with the weak base 3-(2,4-dinitroanilino)-3'-amino-N-methylpropylamine (DAMP) followed by immunocytochemical localization in whole cells and on ultrathin sections. After resupplying silicate to cells synchronized by silicon depletion, the uptake of this nutrient from the medium was the same with or without DAMP; new valves developed without morphological aberrations that could conceivably have been caused by the probe. DAMP was found in cellular compartments known to be acidic, such as vacuoles active as lysosomes, the lumen of thylakoids, and microbodies. In the nucleus and mitochondria, which are circumneutral and basic compartments, the probe did not appear. Besides its presence in acidic compartments, DAMP was specifically accumulated within the SDV during formation of new valves; during the process of valve maturation, the SDV seemed to become increasingly acidic. In control experiments using the ionophores chloroquine, valinomycin, and nigericin, the compartmental location of DAMP was clearly disturbed, resulting in a random intracellular distribution. Accumulation of the fluorescent probe rhodamine 123, which can be translocated over membranes by a reducing potential, confirmed that the SDV can translocate weak bases. The results with DAMP suggest that the pH of the SDV is important in the silicification of diatoms: It facilitates a fast nucleation and aggregation of silica particles, thus increasing the rate of formation of the mature frustules. In addition, the acidic environment might protect the newly formed valves against dissolution before completion and coverage by the organic casing prior to their secretion.

Silicon deposition in diatoms: control by the pH inside the silicon deposition vesicle

Kinetics of reaction between silicic acid and amorphous silica surfaces in NaCl solutions

http://faculty.eas.ualberta.ca/konhauser//Reprints/ChemicalGeology-NY%282003%29.pdf

The effect of cyanobacteria on silica precipitation at neutral pH: implications for bacterial silicification in geothermal hot springs

Currently there are several models discussed to describe the formation of monodispersed silica particles by the controlled hydrolysis and condensation of tetraethylorthosilicate (TEOS) in mixtures of water/ammonia and ethanol. The present study will show results on an extended range of experimental conditions. Particle growth followed a first-order reaction kinetic with respect to TEOS concentration. Reaction orders for the ammonia and water content were 0·97 and 1·18, respectively. Only monomeric (≈ 70–90%) and dimeric (≈ 10–25%) silicate units were detected during the growth reaction and an activation energy of 27 kJ mol−1 (6·5 kcal mol−1) was indicated. Particles revealed an ultramicroporous internal structure. A narrower size distribution and a more spherical shape were observed at higher water concentrations. The results suggested an aggregation mechanism of nanometer-sized primary nuclei during the first stage of the reaction followed by a monomer addition growth mechanism thereafter.

http://www.ipfdd.de/fileadmin/user_upload/mbz/lectures/Microsystems/Giesche1994.pdf

Synthesis of monodispersed silica powders I. Particle properties and reaction kinetics

http://homepages.see.leeds.ac.uk/%7Eearlgb/Publications/Tobler%20et%20al%20GCA_09.pdf

Quantification of initial steps of nucleation and growth of silica nanoparticles: An in-situ SAXS and DLS study

The polymerization of silicic acid in dilute aqueous solutions was studied experimentally and theoretically. Two basic processes were studied: the formation of colloidal particles by homogeneous nucleation, and their further growth by deposition of dissolved silica upon them. It was found that the state of ionization of the silica surface controls the rate of polymerization. The rate of deposition of dissolved silica on the surface of amorphous silica is proportional to the surface density of ionized silanol groups. The extent of surface ionization also determines the value of the surface tension, and thus also the rate of homogeneous nucleation. Added salts accelerate both molecular deposition and homogeneous nucleation by increasing the extent of surface ionization and decreasing the solubility of silica. Except for fluoride, salt ions were found not to have any specific catalytic effect. Fluoride was confirmed to be a powerful catalyst. Aluminum and boron were found to inhibit the reaction at pH 8. A successful quantitative theory of the homogeneous nucleation of colloidal silica particles was developed. The Lothe-Pound factor was shown to be about 3.34 × 1025 (kg H2O)−1. The nucleation model and results of our data analysis are incorporated in a computer code which models the homogeneous nucleation and growth of colloidal silica particles. This code is able to reproduce much of our data to within experimental error, and may be used to generate useful predictions for conditions that are typical of geothermal brines. The concentration of “adsorbed silica” on the surface of amorphous silica was determined, and was found to increase with the concentration of dissolved silica in the ambient solution. It is probably a reaction intermediate between dissolved silica and solid amorphous silica.

/pdf/kinetics-of-silica-polymerization-282wyph0j7.pdf

Kinetics of silica polymerization

Nickel sulfate catalyzes the reaction of hydrogen sulfide with oxygen in aqueous solution. This reaction was studied, and an empirical rate expression and a reaction mechanism were deduced. The rate of oxidation is independent of oxygen concentration and pH over the range investigated. The reaction rate is one half order in nickel, and it changes from second to first order in sulfide with increasing concentration. The oxidation reaction is an autocatalytic, free radical chain reaction. Nickel catalyzes the chain initiation step, and polysulfido radical ions propagate the chains. Colloidal sulfur is a major, frequently undesirable reaction product. Sodium sulfite suppresses formation of colloidal sulfur by converting it to thiosulfate. Cobalt is an equally potent catalyst, but a colloidal dispersion of cobalt oxysulfide is produced. Iron compounds are much weaker catalysts; iron citrate and iron HEDTA (N-hydroxyethylenediaminetriacetic acid) were the best among those tested.

Oleh Weres

Papers

Kinetics of silica polymerization

Catalytic Oxidation of Aqueous Hydrogen Sulfide in the Presence of Sulfite