Effect of the Concentration of Sodium Chloride on the Polymerization of Silicic Acid

Silica deposition from the high salinity geothermal water of the Fushime area in Kagoshima prefecture occurs within a relatively short time. Therefore, the effect of sodium chloride concentration on the polymerization of silicic acid and the precipitation of silica was studied by spectrophotometry and gel chromatography at 25 and 90°C. The concentration range of sodium chloride was 0.05-2.5 mol dm^-3, and the pH range 6 - 9. At a given pH, the rate of decrease in the monosilicic acid concentration increased by increasing the sodium chloride concentration. It results from an increase in the supersaturated concentration of monosilicic acld due to a decrease in the solubility of amorphous silica. The precipitation rate of silica also increased by increasing the sodium chloride concentration, because the reaction between polysilicic acids was accelerated due to the decrease in electrostatic repulsion among polysilicic acids.