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Environmental fate & pathways

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Particulate/massive CaSi is immobile substance in soil, sediment and aquatic environments. The environmental fate of CaSi is for relevant parts connected to the fate of its dissolution and transformation products. Dissolution, dissociation and speciation of the dissociation products of CaSi are influenced by the concentrations of especially Si (IV) and Ca (II) –complexes.

Silicon is not known to be naturally present in the environment in its reduced elemental form. Normally Si is always bound primarily with oxygen as silica/silicic acid in the environment.

Silicon in CaSi alloys exists both in Si (0) and Si (IV) oxidation states/forms. The released form is expected to be at the Si (IV) oxidation state. If released in the environment from CaSi in elemental form Si (0), it is rapidly oxidized and hydrolysed to Si (IV) silica (SiO2) 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. 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 pKa1 9.9, pKa211.8, pKa3,4 12 & 12 (measured at 30 °C) only a high pH (> 9) changes the molecule into ionic form.

 

Dissolved silica is not volatile and partitions primarily in the aquatic phase. Saturated monomeric concentration range upper limits are ca. 60-160 mg/l (temperature and pH controlled). Dissolved silica may form precipitates with other elements (e.g. Al and Mg) and may slowly form several types of clay minerals with these elements.

 

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 %.

 

Adsorption of dissolved silica to soil and sediments is not strong and it is relatively mobile in soil. Adsorption takes place primarily to inorganic materials and in lesser extent to organic material. A dynamic adsorption/desorption equilibrium between soil/sediment particles and water keeps the surface water aquatic concentrations of dissolved silica relatively constant (ca. 2-15 mg/l). Higher concentrations can be found in ground waters.

 

Calcium  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+ H2O <-> Ca(HCO3)2

 

Calcium bicarbonate exists only in aqueous solutions 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 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.

In soil, the most prevailing and common adsorption mechanism of Ca is its retention by weak electrostatic forces. Calcium adsorption/desorption and consequently its mobility and bioavailability is much related to the soil cation exchange capacity (CEC), since solid phases of soil particles often carry a negative surface charge and electrostatic cationic retention (outer sphere/cation exchange) dominates.

Some anions form insoluble precipitates with calcium e.g. PO43-, MoO-(and CO32-). These processes may lead in to true ”fixation” of Ca into soil matrix and the magnitude of Ca retarded by these mechanisms is dependent of conditions and of the availability of these counter anions.