Silica (silicon dioxide) is a compound of silicon and oxygen (Si02). It is a hard, glassy mineral substance which occurs in a variety of forms such as sand, quartz, sandstone, and granite. It is also found in the skeletal parts of various animals and plants. Silicon is the most abundant element on earth after oxygen which explains why most water supplies will contain some traces of silica. All natural water supplies contain some dissolved “silica” and most will also contain suspended or colloidal silica. In solution it can exist as silicic acid or silicate ion, depending upon the pH. Silica can also be found in certain foods including cucumbers, oats, brown rice, wheat, strawberries, onions, avocados, and root vegetables.
Silicon (Si) is a nonmetallic element that is abundant in supply as part of various compounds in the crust of the earth. Silicon is released during the weathering process and released under water during volcanic activity. It can be removed naturally through plankton fixation, sediment settling, or reactions of dissolved silicon with clay materials. Silicon is applied as an aid in steel where it is processed in high temperatures. Silica is only slightly soluble in water. The type and composition of the silica-containing minerals in contact with the water and the pH of the water are the primary factors controlling both the solubility and the form of silica in the resulting solution.
The solid crust of the earth contains 80% to 90% silicates or other compounds of silicon. Water passing through or over the earth dissolves silica from sands, rocks and minerals as one of the impurities it collects. The silica content in natural waters is commonly in the 5 to 25 mg/L range, although concentrations over 100mg/L occur in some areas.
Silicates are compounds which contain silicon and oxygen in combination with such metals as aluminum, calcium, magnesium, iron, potassium, sodium and others. Silicates are classed as salts. Silicates are widely distributed in such minerals as asbestos, mica, talc, lava, etc.
The silica content of water ranges from a few parts per million in surface supplies to well over 100 ppm in certain well waters.
In water treatment, we are concerned with silica because of its capacity to form scale deposits on surfaces it comes in contact with. In its colloidal form it consists of very fine particles in suspension.
Silica Removal Processes:
Strong base and ion exchange
Conventional precipitation technologies are messy and time consuming. When complete silica removal is required, various combinations of Reverse Osmosis and ion exchange processes are used. RO systems are currently unable to remove as much silica as ion exchange can but is far better at removing various forms of non-reactive silica and will remove many additional contaminants not addressed by ion exchange.