Membrane calcium-silicate

Faaland S, Einarsmd MA, Grande T. Reactions between calcium and strontium substituted lanthanum cobaltite ceramic membranes and calcium silicate sealing materials. Chem Mater. 2001 13 723. 

The exact location where these complex silicates are deposited depends on the operating conditions and the impurity concentrations in the various streams feeding the cell. Thus, Bissot s studies [117] showed calcium silicate of a different composition on the cathode side (Fig. 4.8.36), compared to the Ca2Si04 H2O and Ca(OH)2 formed on the anode side of the membrane and the absence of any precipitates on the cathode side in nine s studies [120]. It was suggested [119, 120] that this might be due to higher levels of calcium when it precipitated as calcium silicate closer to the anode side. 

Effect on Membrane Calcium deposits as hydroxide, silicate, aluminate, and other compounds in the cathode layer of the membrane, and causes physical disruption (holes). Current efficiency can decline to as low as 85%. 

Effect on Membrane. Silica in the membrane forms calcium silicate, which is less soluble than Ca(OH)2, and precipitates near the membrane cathode surface, thus reducing the current efficiency. 

Mechanism of Damage Silica enters the membrane as a neutral or cationic species and becomes a soluble anion as the pH increases in the region closer to the cathode. Silica can then trap calcium and precipitate as calcium silicate. It can also react with aluminum to precipitate as sodium aluminosilicate. These precipitates can form large crystals near the membrane cathode surface, causing physical damage to the normally smooth surface and reduce the OH rejection capacity. 

Diatoms are unicellular, photosynthetic microalgae that are abundant in the world s oceans and fresh waters. It is estimated that several tens of thousands of different species exist sizes typically range from ca 5 to 400 pm, and most contain an outer wall of amorphous hydrated silica. These outer walls (named frustules ) are intricately shaped and fenestrated in species-specific (genetically inherited) patterns5,6. The intricacy of these structures in many cases exceeds our present capability for nanoscale structural control. In this respect, the diatoms resemble another group of armored unicellular microalgae, the coccolithophorids, that produce intricately structured shells of calcium carbonate. The silica wall of each diatom is formed in sections by polycondensation of silicic acid or as-yet unidentified derivatives (see below) within a membrane-enclosed silica deposition vesicle 1,7,8. In this vesicle, the silica is coated with specific proteins that act like a coat of varnish to protect the silica from dissolution (see below). The silica is then extruded through the cell membrane and cell wall (lipid- and polysaccharide-based boundary layers, respectively) to the periphery of the cell. 

A membrane can be used in the so-called phase transfer catalysis as a separator between two immiscible liquids or a liquid and a gas. It serves as a well controlled contact surface. An interesting type of membrane reactor has been suggested in which a ceramic membrane is applied to regulate the contact between a gas and a liquid stream on the opposites of the membrane [De Vos, 1982 De Vos et al., 1982]. Hydrogenation of nitrobenzoic acid can be effectively performed with a porous calcium-aluminum silicate membrane reactor which essentially becomes a gas-liquid reactor. 

It is assumed that cement paste plays a role of semi-permeable membrane which is impermeable for the silicate anions formed as a result of reaction. Some authors postulate that this semi-permeable membrane is composed of sodium and potassium sUicates gel formed during reaction. A concentration gradient appears and linked with him the osmotic pressure. Recently, the difference in discussions of swelling mechanism, attributed to the sorption of water or to the osmotic pressure, became insignificant. There is a view that sodium and potassium ions can diffuse through the semi-permeable membrane more easily than the calcium ions. 

Silica is most dangerous in combination with other species that can form insoluble silicates. Any specification for silica presupposes that these other species (e.g calcium and aluminum) also are under control. Because of its mechanism of transport of silica, its concentration within the membrane actually goes through a maximum (Section 4.8.5). 

Sakai S., Ono T., Ijima H., Kawakami K. Aminopropyl-silicate membrane for microcapsule-shaped bioartificial organs control of molecular permeability. J. Membr. Sd. 2002 202 73-80 Sakai S., Ono T., Ijima H., Kawakami K. Permeability of alginate/sol-gel s)mthesized aminopropyl-silicate/alginate membrane templated by calcium-alginate gel. J. Membr. Sci. 2002 205 183-189... 

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