Silica, or silicon dioxide (\(\text{SiO}_2\)), is one of the most abundant compounds found on Earth, making up a significant portion of the planet’s crust in the form of sand and quartz. Its presence in rocks and water leads to questions about how it interacts with the environment. While bulk silica does not readily dissolve like table salt, it interacts with water through a slow chemical process. This process results in a biologically important, soluble form.
The Chemical Structure of Silica
The stability of silica is rooted in its atomic structure, consisting of a silicon atom covalently bonded to four oxygen atoms in a tetrahedral arrangement. These \(\text{SiO}_4\) tetrahedra link together to form a robust, three-dimensional network. Breaking this extensive network of strong silicon-oxygen covalent bonds requires a high amount of energy for the compound to dissolve.
Silica exists in two primary forms: crystalline and amorphous. Crystalline silica, such as quartz, possesses a highly ordered, repeating lattice structure, which maximizes bond strength and makes it exceptionally inert and insoluble. Amorphous silica, like silica gel, lacks this long-range atomic order. This structural disorder makes amorphous silica slightly more reactive and approximately three to four times more soluble than its crystalline counterpart.
Understanding Silica’s Interaction with Water
Silica undergoes a slow process known as hydrolysis rather than truly dissolving in water. The Si-O-Si bonds on the surface of the solid silica react with water molecules (\(\text{H}_2\text{O}\)), breaking the network structure. This reaction results in the formation of a simple, dissolved molecule called monomeric orthosilicic acid (\(\text{H}_4\text{SiO}_4\)). The rate of this dissolution is exceptionally slow under normal environmental conditions, but it increases significantly with higher temperatures and elevated pH levels.
The presence of this soluble acid is commonly referred to as dissolved silica. In natural water systems, the concentration of orthosilicic acid is typically limited to a few parts per million because the reaction reaches a low equilibrium. Dissolved silica exists as individual \(\text{H}_4\text{SiO}_4\) molecules, also known as reactive silica, that pass through filters. Colloidal silica, conversely, consists of ultra-fine, non-dissolved \(\text{SiO}_2\) particles suspended in the water. The formation of monomeric orthosilicic acid is the only way for silica to achieve a truly dissolved, bioavailable state.
Biological Absorption and Metabolism
Once \(\text{H}_4\text{SiO}_4\) is formed, it becomes the primary bioavailable form of silicon for humans and animals. This small, uncharged molecule is readily absorbed across the gastrointestinal tract wall and easily enters the bloodstream following ingestion. The body uses the absorbed silicon for various physiological functions, including the synthesis of collagen and the health of connective tissues.
The body does not store significant amounts of this element, rapidly processing it to maintain silicon homeostasis. A large portion, often more than 50%, of the absorbed orthosilicic acid is quickly excreted by the kidneys through the urine. At sites of active bone formation, \(\text{H}_4\text{SiO}_4\) is metabolized into the silicate ion (\(\text{SiO}_4^{4-}\)). This ion can substitute for phosphate ions in the bone mineral hydroxyapatite, playing a role in the mineralization process and bone structure.
Silica in Water Sources and Diet
Silica is a naturally occurring component in almost all freshwater sources, as water flows over rocks and sand. The concentration of dissolved silica in natural waters typically ranges between 5 and 25 milligrams per liter (mg/L), though levels can exceed 100 mg/L in certain groundwater supplies. The dissolved silica in drinking water is present as \(\text{H}_4\text{SiO}_4\) and is considered safe for human consumption.
Dietary intake provides the majority of silicon for most people, supplying an estimated 20 to 50 mg per day in a typical western diet. Common food sources rich in silicon include whole grains, cereals like oats and barley, and certain vegetables. Regulatory bodies, such as the U.S. Environmental Protection Agency (EPA), have not established a maximum contaminant level for dissolved silica in drinking water. This reflects its low toxicity and the body’s efficient mechanism for processing it.