Soluble silica is not an environmental contaminant. It originates from well or surface water supplies and has concentrations of 1-120 mg/l as SiO2. Above 120 mg/l, colloidal or crystalline flakes of insoluble silica may be visible in neutral water (pH < 8.0). Higher concentrations of silica may be solubilized by having a carbonate or hydroxide alkalinity of at least 1.5 times the SiO2 concentration. Soluble silica in neutral water (pH 5-8) is present as a mixture of bisilicate, HSiO — as weak silicic acid, H2SiO3; and perhaps as SiO2—compounds analogous to HCO—, H2CO3, and CO2 in water (Camp 1963).
Removal of soluble silica from wastewater may be required for a few processes (State Water Pollution Control Board 1952) or in case of high pressure boiler feed, when a turbine is present (Crits 1968). Soluble silica can be removed
1. By warm or hot lime precipitation, with the addition of MgO (or naturally occurring magnesium, if enough is present) to reduce silica concentration by 80-95%— down to 1.0 mg/l (Applebaum 1968) in the effluent
2. From soft water using a desilicizer, a strongly basic anion resin in the hydroxide form (Applebaum 1968; Zunino 1962). Silica concentration in the effluent may be as low as 0.2 mg/l.
3. By strongly basic anion resins used in demineralization. Silica concentration in the effluent can be reduced to 0.002 mg/l if required.
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