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Environmental fate & pathways

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The vapour pressure of Si/FeSi silicate is extremely low and the substances and its constituents are in practice in-volatile substances (at NTP). However, Si/FeSi silicate may be emitted from anthropogenic sources into the atmosphere in small dust particles which eventually form aerosols in the atmosphere. Aerosols are then being washed out after a certain residence time.

FeSi silicate in particulate form is immobile in soil and sediment. Dissolution rate is generally low, but dependent also on environmental conditions. Adsorption/desorption behaviour of dissolved Si/FeSi silicate constituents are governed mainly by inorganic soil and sediment materials. Each dissolved constituent behave by a characteristic way and depending highly on local environmental conditions. The environmental fate of Si/FeSi silicate is for relevant parts connected to fate of its dissolution and transformation products. Dissolution, dissociation and speciation of dissociation products of Si/FeSi silicate is influenced by concentration of especially Si(OH) and Fe(II/III) -complexes together with smaller amounts of other elements (e. g. Ca, Al, Mg, S, Ni, Ba, Cu, Sr, Mn, Cd, and Ti).

Metals/elements can occur in different valences, associated with different anions or cations, and can be associated with adsorptive agents, such as Dissolved Organic Matter (DOM) in water, or bound to minerals in sediment or soil. Speciation highly depends on environmental conditions and chemistry and it makes the major differences in (bio) availability.

Silicon

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

Silicon present in FeSi alloys exists both in Si (0) and Si (IV) oxidation states/forms. The released form is expected to be in the Si(IV) oxidation state. If released in the environment from FeSi in elemental form Si (0), it is rapidly oxidized and hydrolysed to Si (IV) silica species. The rate of these transformations is highly dependent on particle size/surface area of released silicon particles and environmental conditions.

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 controlled). Dissolved silica may form precipitates with other elements like Al and Mg and may slowly form several types of clay minerals with these elements.

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 % (Weres et al. 1981).

 

Iron

Dissolved iron is present in the environment in two oxidation states Fe (II/III). Both states are non-volatile. Fe(II) “ferro” species are stable in anoxic and low oxygen conditions. Fe(II) is unstable in typical oxygenated water courses with a half-life of minutes – hours under favourable conditions, and oxidises easily to the Fe(III) state. The potential of the Fe(III) -Fe(II) couple, (0.77 V) is such that molecular oxygen can convert ferrous to ferric in acid solution or basic solution (Cotton et al., 1999).

Fe (III) “ferri” species are soluble and stable in water only at very low pH conditions. Normally Fe (III) reacts with water (hydrolysis) to form colloidal and insoluble ferric hydroxide Fe(OH)3which in typical aquatic environmental conditions slowly precipitates to sediments. Fe (OH)3is highly insoluble in water with Ksp= 1 x 10-36(CRC Handbook). Formation of ferric hydroxide at pH levels above 5.0 limits the presence of iron in aqueous systems. Under conditions of very low oxygen concentration, ferrous is freely soluble but ferric is not.

Heavy metals and organic matter may be strongly adsorbed to Fe precipitates. Fe (III) forms precipitates with phosphate. Iron ions, especially Fe(II) ions may be also adsorbed to dissolved organic material and some dissolved iron in natural waters may be present as soluble organic-complexes.

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 naturally found in elemental state in the environment. Calcium is also present in a relatively soluble form, 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.

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 soluble calcium hydrogen carbonate (Ca(HCO3)2“bicarbonate”.

CaCO3(s) + CO2+ H2OóCa(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 HCO3are 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.