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The WQA Glossary of Terms
This Glossary is published by the Water Quality Association (WQA) as a service to members and leaders in education, government, and industry.
© Copyright 2010 by Water Quality Association
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S
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Silica
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Friday,Jun 16, 2000
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As used in water chemistry, a collective term encompassing all reactive (dissolved) and inert (nonreactive and undissolved or colloidal) forms of the chemically resistant dioxide SiO2 of silicon, or silicates formed from silicon and oxygen in combination with one or more other minerals or metals. Silica, such as quartz for example, is present in almost all earth's minerals, rocks, soils, sands, and clays. It is found in fresh water in a range of 1 to 100 mg/L. In a laboratory water analysis report, silica is nearly always identified as silicon dioxide (SiO2), however when dissolved in water silica is actually hydrated to silicic acid (SiO2 nH2O) and associated anions. Below a pH of 9, silica is present mostly in the silicic acid form (SiO2 2H2O, also written as H4SiO4. At low pH, silicic acid can polymerize or condense to form uncharged colloid particles (known as colloidal silica) as small as 0.02 microns in size. As the pH of water exceeds 8, silicic acid (H4SiO4) increasingly dissociates into the bisilicate anions H3SiO4- and H2SiO42-. This will increase the solubility of the silica unless divalent and polyvalent cations are available in significant concentrations. These cations of calcium, magnesium, iron, and aluminum, for example, will likely cause precipitation of insoluble silicate salts, especially at higher pH levels. The precipitation of multivalent cations with the silicates tends to occur at a pH value of 1 to 2 points below the point at which the cation hydroxide solubilities would be exceeded. Silicates can interfere with the crystal growths of precipitating metal and mineral cations thereby keeping such silicate complexed particulates in the colloidal size ranges, e.g., silicates are a contributor to formations of colloidal iron and colloidal manganese.
Below pH 8, dissolved silica in the silicic acid form (H4SiO4) precipitates to SiO2 when the solubility of silicic acid is exceeded. Precipitated silica and silicates can be very difficult to redissolve. Silica concentrations above 100 mg/L in reverse osmosis waste streams (concentrate water) may cause silica deposition which will foul the membrane, however because silica is slow to crystalize some RO systems can operate safely with silicic acid concentrations as high as 140 mg/L in the RO concentrate without experiencing silica scale formation. The potential for silica precipitation should be evaluated whenever silica is present in an RO feedwater at a concentration above 20 mg/L. Silica is also objectionable in boiler feedwater because it may form a tenacious scale in the boiler itself, and because it volatilizes at high temperatures and redeposits on turbine blades. High concentrations of silica are likewise a concern in cooling tower makeup water because of the characteristic uncertainty about silica's solubility limits.
Inorganic silicate compounds, such as soluble sodium silicates (the simplest form of glass, known as water glass) have been employed for more than 70 years to treat water. These silicates are anodic corrosion inhibitors that will combine (react with) the free metal released at the anode site of corrosion activity in cipper, lead, cast iron and ferrous metals, steel, galvanized steel, bronze, red and yellow brass, and nickel alloys, to form an insoluble metal-silicate compound which seals off the corrosion site. Relatively high dosages, 25 to 50 mg/L, of silicate are required during the first 30 to 60 days of treatment. This should form the initial protective coating for corrosion control. Thereafter, the silicate dosage is reduced incrementally to maintenance feeds in the 4 to 10 mg/L range. There are reports of preferences of sodium silicate corrosion inhibitors for successful treatment of low alkalinity and
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