WHAT IS A TYPICAL CONCENTRATION FOR SILICA (SiO₂) or SILICATES IN FRESH SURFACE AND GROUND WATER SUPPLIES ON EARTH?

Soluble silicates are economical, effective, and environmentally responsible chemicals which have been used for more than 70 years to protect metals from the corrosive effects of water. They are classified as corrosion inhibitors because they can deposit protective films onto various metal surfaces, isolating the metal from any further corrosive attack, and because they raise water pH which can make it less corrosive to metals. Silicates do not contribute zinc or phosphorous to treated water. Laboratory and field experience has shown that silicate corrosion inhibitors are effective in many different types of water. Protection is provided in both acidic and alkaline water. In harder water slightly more silicate is needed to achieve the same degree of corrosion inhibition, since some of the injected silica may react with hardness ions before it has a chance to bond onto metal surfaces.

The term “silica” refers to the compound silicon dioxide (SiO₂) Water is typically analyzed for silica as part of normal water quality analysis procedures. Silica levels are usually reported as mg SiO₂/L. “Silicate” is a generic term for compounds that contain silicon, oxygen, and one or more metals. They can be naturally occurring or synthetic. As an example, PQ’s synthetic sodium silicate compounds can be represented by the generalized formula Na₂O xSiO₂, where “x” varies from 1.60 to 3.22 for commercial products. Naturally occurring silicate minerals make up nearly 90% of the earth’s crust. Dissolved silica is a minor but ubiquitous constituent of the hydrosphere. Commercial soluble silicates have a higher degree of polymerized silica species than natural dissolved silica because of higher concentrations; however, when diluted they depolymerize to molecular species that are indistinguishable from natural dissolved silica.

Sodium silicate adds silicate anions, together with sodium and hydroxyl ions, to water. Silica is found to some extent in all natural waters and is believed to be ecologically harmless. The charged, polymeric nature of the silica found in synthetic silicate solutions is responsible for its reaction with metals and corrosion inhibition properties. The sodium content of water will increase slightly with sodium silicate addition. This issue has been raised as a concern in some instances.

Silica (SiO₂) is an oxide of silicon, and is present in almost all minerals: It is found in surface and well water in the range of 1 - 100 mg/i. Silica is considered to be colloidal in nature because of the way it reacts with adsorbents. A colloid is a gelatinous substance made up of non­diffusible particles that remain suspended in a fluid medium. Silica is objectionable in cooling tower makeup and boiler feed water. Silica evaporates in a boiler at high temperatures and then redeposits on the turbine blades. These deposits must be periodically removed or damage to the turbine will occur. Silica is not listed in the Primary or the Secondary Drinking Water Standards issued by the US EPA.