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Environmental fate & pathways

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Particulate/massive CaSi is immobile substance in soil, sediment and aquatic environments. CaSi is not volatile. Phototransformation and photolysis is not an important environmental fate process of this substance. CaSi is not biodegradable and its bioaccumulation potential is low.

The environmental fate of CaSi is for relevant parts connected to the fate of its dissolution/hydrolysis and transformation products. Dissolution, dissociation and speciation of the hydrolysis products of CaSi is influenced by the concentrations of especially Si (IV) and Ca (II) –complexes.

Silicon in the form of silicate minerals is the second most abundant element (after oxygen) in the earth’s crust. Silicon in its reduced elemental form is not naturally present in the environment. Si is always bound primarily with oxygen as silica/silicic acid in the environment.

Silicon present in CaSi alloys exists both in Si (0) and Si (IV) oxidation states/forms with amorphous SiO2 at the surface which dissolves into the Si (IV) oxidation state. If released in the environment from CaSi it is oxidized and hydrolysed to Si (IV) or precipitated silica (SiO2) species. The rate of these transformations is highly dependent on particle size/surface area of released CaSi particles and environmental conditions (KTH 2010).

Si (IV) in fresh water or seawater can occur in a number of chemical species, dissolved monomeric Si(OH)4, dimerized, trimerized, colloidal or in the form of aggregated colloids of different physical size or entirely as insoluble particulate matter. Saturated monomeric concentration range upper limits are ca. 60-140 mg/l (temperature and pH dependent). Dissolved silica may form precipitates with other elements like Al and Mg and may slowly form several types of clay minerals/silicates with these elements. (please provide here a reference).

 

In dilute solutions and most typical environmental pH values silica is present as monomeric silicic acid Si(OH)4. Since the dissociation constants of silicic acid is pKa19.9, pKa211.8, pKa3,412 & 12 (measured at 30 °C) only a high pH (> 9) changes the molecule into ionic form.

 

If the concentration of silica is close to the standard solubility (at pH = 7.0-9.2) the fraction of dimers, with respect to the silicic acid, is not more than 1.0 %, fraction of trimers ca. 0.1 %, tetramers and low-molecular cyclic polymers (up to 6 units SiO2) < 0.1 %.

 

Calciumis the fifth most abundant element (by mass) in the earth’s crust. Calcium is a component of several primary and secondary minerals in the soil, which are essentially insoluble. It occurs commonly in sedimentary rocks, e. g. dolomite, calcite, limestone, gypsum, and fluorite, and is not found in elemental state in the environment. Calcium in soluble form is present as a cation (Ca2+).

Depending highly on local environmental conditions, the dissolved Ca2+may stay in the solution, make soluble complexes, adsorb to surfaces or typically precipitate as calcium carbonate CaCO3.(please provide here a reference.).

 

Calcium carbonate (“calcite” in pure crystalline form) is poorly soluble in water but reacts with strong acids, releasing carbon dioxide. Calcium carbonate reacts with water saturated with carbon dioxide and forms solublecalcium hydrogen carbonate (Ca(HCO3)2“bicarbonate”.

CaCO3(s) + CO2+ H2O = Ca(HCO3)2

Calcium bicarbonate exists only inaqueoussolutions containing calcium (Ca2+), dissolved carbon dioxide (CO2), bicarbonate (HCO3–), and carbonate (CO32–) ions.

In soft-water lakes the calcium concentration is generally well below saturation level throughout the year. However, levels of calcium are generally so high that depletion by biota is hard to detect in normal analyses.

In Ca rich (hard-water) lakes a clear seasonal pattern can be observed regarding calcium concentrations. During active periods of photosynthesis calcium may be precipitated, as CaCO3following the equilibrium equation (reaction from right to left), since CO2and HCO3is consumed by algae. Calcium has an important role in the pH regulation and the whole carbon cycle of soils and surface waters. Because of its abundance and easy detection, the concentration of calcium ions is often used also as an indicator of water hardness.