Sodium silicate
(endorsed 2005)
Sodium silicate, in the form of ‘activated silica,’ is used as a coagulant or a flocculation aid in the treatment of drinking water, in conjunction with a primary coagulant (e.g. alum). Soluble silicates (waterglass) can also be used to inhibit corrosion or sequester metals, and sodium silicate solution can be used to adjust pH in small water systems.
General description
Sodium silicate, Na₂O·xSiO₂, can be in the form of lumps of greenish glass (soluble in steam), white powders of varying degrees of solubility, or as cloudy or clear solutions of varying viscosity.
Soluble silicates can be differentiated by their ratio of silica to sodium oxide (SiO₂:Na₂O). This ratio, which ranges from 1.6 to 3.3 by weight, determines the physical and chemical properties of the product. Liquid silicates with a ratio of 1.6 have a pH of 13.2; whereas, at a ratio of 3.3 the pH is 11.0. The specific gravity of these solutions ranges between 1.4 and 1.6. The colloidal and polymeric properties of liquid silicates increase as the SiO₂:Na₂O ratio increases.
Appropriate materials for handling sodium silicate include cast iron, steel, fibreglass-reinforced plastic and polyethylene, and rubber linings.
Chemistry
Sodium silicate is produced by fusing high purity silica sand with sodium carbonate or potassium carbonate at 1000–1500°C. This results in an amorphous glass, which can be dissolved in water to form silicate solutions or ‘waterglass’.
In solution, silica is present in equilibrium between monomeric anionic species. The proportion of silica and alkali in a sodium silicate is usually expressed as the weight ratio of SiO₂ to Na₂O.
In drinking water treatment, solutions of activated or colloidal silica can be used for coagulation. Such solutions can be generated on site by partial or complete neutralisation of a dilute solution of sodium silicate by a mineral acid, an acid salt or chlorine. The activated silica solution obtained can be slightly alkaline or neutral, and is aged for a short time (1–2 hours) before use. The solution is then further diluted with 2–2.5 volumes of water. The activated silica solution has a shelf life of 1–2 days.
Typical use in Australian drinking water treatment
At one time, activated silica was commonly used as a coagulant aid (after a primary coagulant such as alum or ferric chloride), because it forms heavy, tough flocs that settle fast. However, polyacrylamide polymers have now largely replaced activated silica in most water treatment plants.
Soluble silicates are also used to protect metals from the corrosive effects of water by depositing a thin molecular film of silica (SiO₂) on metal surfaces. Silicate treatment is effective for corrosion control of concrete and a variety of metals: lead, copper, cast iron, ferrous metals, steel, galvanised steel, bronze, red and yellow brass, and nickel alloys. The pH and alkalinity of the water determine which silicate is suitable for this application.
Sodium silicate can also be used to sequester iron and manganese. Following metal oxidation, sodium silicate is added to hold oxidised metals in a colloidal suspension.
Concentrations of activated silica used in drinking water treatment can range from 1 to 10 mg/L (as SiO₂), and the concentration required varies with water quality, depending on factors such as pH, turbidity, colour, temperature and contaminant level.
The effectiveness of sodium silicate as a corrosion inhibitor depends on water quantities such as pH and bicarbonate concentration. The chemical is more effective under high-velocity flow conditions. Silicate is effective at high pH, and at a dosage over 15–20 mg/L (as SiO₂).
Silicate with a high ratio of Na₂O to SiO₂ will raise pH in weakly buffered waters. For corrosion control, relatively high concentrations (up to 24 mg/L) are required during the first 30–60 days of treatment, to form the initial protective coating. Thereafter, the silicate dosage is reduced incrementally in 30-day periods, until it reaches maintenance doses (4–8 mg/L).
As a metal sequestrant, sodium silicate (as SiO₂) should be added at up to 4–5 times the level of iron or manganese in the water.
Contaminants
The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. More than 20 elements are present as trace impurities in sodium silicate, including:
aluminium
cadmium
calcium
chloride
iron
magnesium
manganese
sulfate
Residual and by-product formation in drinking water
Sodium and silicate residues are present in finished water. When employed in drinking water treatment, sodium silicate should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.
Status
Sodium silicate was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.
References
Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association, Washington, DC.
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