Silica removal using ion-exchange resins"a*M. Ben Sik Ali a, B. Hamrounl , S. Bouguecha b, M. Dhahbi b°Laboratoire Eau et Technologies Membranaires, FacultO des Sciences de Tunis,

In water treatment, silica is a problem because of its tendency to form deposits. It is often necessary to include reducing silica concentration to permit increased cycles of concentration without scale. The behavior of aqueous silica and the various water treatment processes used for its removal were reviewed. Silica that exists in water in equilibrium with ion silicate can be removed by strong base anion-exchange resins operated in the hydroxide cycle. With the aim of silica removal by ion exchange, some thermodynamic and kinetic aspects were studied at different temperatures. Selectivity coefficients were determined and activity coefficients were calculated from an appropriate model. Silica concentrations were obtained by the measure of the absorbance of yellow or blue-colored silicomolybdic acid. Anion species concentrations were determined by ionic chromatography.

A strong anion-exchange resin is often useful in water treatment, especially for silica removal. Some thermodynamic and kinetic aspects were developed in this work. Thermodynamic equilibrium constants and selectivity selections determined by a graphical method have the same value. They show a better selectivity for silica when the resin is in the hydroxide form. With silica, anion-exchange reactions were rapid and reversible. They were in a third-order reaction at 20°C and 30°C. Rate constants were determined at these temperatures. Applied to geothermal water, silica removal by a strong ion-exchange resin suffers from the competition of other anions present.