WHAT ARE SILICATES AND WHY ARE THEY IN WATER?

Silicon is known to be present in all living organisms. This element occurs in the form of hydrated amorphous silica, referred to as opal, and is required for the production of structural materials in single-celled organisms through to higher plants and animals. For many life forms, silicon can even be considered to be an essential element. The assessment of the effects of soluble silicate emissions must be reviewed in this context.

The solid crust of the earth contains 80 to 90 percent silicates or silicates plus other compounds of silicon such as silica or silicon dioxide (SiO2). Silicates are those compounds which have a silicon-oxygen anion chemically combined with such metals as aluminum, calcium, magnesium, iron, potassium, sodium and others to form silicate salts. Most silicate salts, with the exception of sodium silicate, are only slightly soluble in water and are widely distributed in nature. Minerals such as asbestos, mica, talc, lava, etc., contain silicates.

To understand the effects of increased silicate concentrations, one must understand the biological function of silicate in water ecosystems, which is complex. In water systems, algae (and aquatic plants) are responsible for primary production: the transformation of inorganic nutrients and carbon dioxide into organic biomass. The algae biomass (density) is determined by several, potentially limiting, factors, such as temperature, light intensity and nutrient concentrations. All algae, such as green and bluegreen algae, require phosphorus and nitrogen as nutrients, but only diatoms (a type of algae) also require silicate for growth.

The ratio between Si and P, and the ratio between Si and N, determines which algae is (dominantly) present in the water. In natural waters, diatoms are often dominant in the spring (March-May). Their dominance is usually ended when all silicate is used and stored in the diatoms. When the diatom “bloom” collapses, it is often followed by dominance of other (non-diatom) algae. The effects of an increased silicate concentration can be: - Shifts in algae species composition. Additional silicate will increase the Si:P and Si:N ratio in the water, thus creating conditions more favorable for diatoms; - Increased algae biomass. As mentioned, the development of diatoms in the spring is usually ended when silicate is depleted.

Additional silicate will enable the algae to continue their (exponential) growth for a longer period of time; the biomass increase is directly related to the increase of silicate concentration; - Increased production in “trophic levels” of the food chain: increased primary production can result in an increased zooplankton biomass (grazing on the algae) and increased fish biomass (feeding on zooplankton).

Water moving over and through natural deposits will dissolve a small amount of various silicate minerals, making silicates a common contaminant of most waters. Natural physical and chemical weathering processes also produce many extremely small particles or colloids of silicate minerals. Thus, silicates are commonly found suspended in most water sources, usually at low concentrations. The silicates, or water-glass as they are often called, are actually used to make many products you might not normally think of. The use of sodium silicates is as a raw material for making silica gel. Also used in detergent; as a cement for glass, pottery, and stoneware; for fireproofing paper, wood, cement, and other substances; for fixing pigments in paintings and cloth printing; and for preserving eggs.

It is clear that increased silicate concentrations will affect the water ecosystem, but the effects and their intensity depend on the local situation, e.g., season, phosphorus, nitrogen and silicon concentration of the receiving water, and composition of the effluent (silicate or silicate in combination with phosphorus and nitrogen). Changes in ecosystems are generally regarded as undesirable. However, silicate will not only affect the ecosystem processes, but also the possibilities for the use of the water system by humans. Depending on the use functions of the water system, these effects can be positive as well as negative.