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Adsorption / desorption

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Reference
Endpoint:
adsorption / desorption: screening
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
other:
Justification for type of information:
A study on Adsorption/Desorption of disodium disilicate is scientifically not necessary, because silicates are naturally abundant in the environment. The anthropogenic input to the various compartments is a negligible contribution, compared to the concentrations contributed by the natural silica flux.
Monitoring data summarized in the OECD SIDS report (2004) the following: “Dissolved silica from commercial soluble silicates is indistinguishable from natural dissolved silica. Of the elemental composition of the earth’s crust, SiO2 makes up 59 % and similar percentages are present in many sediments and soils. Thus, silicon is the second most abundant element on earth. Compounds of silicon and oxygen are ubiquitous in the environment; they are present in inorganic matter, like minerals and soils as well as in organic matter, like plants, animals and man. By weathering of soil, rocks and sediments and by atmospheric deposition, silica is released into surface and ground waters from where it may be removed by precipitation and sedimentation or taken up by living organisms, especially diatoms. Dead sedimenting diatoms also contribute significantly to sediment silica (diatomaceous earth). Silica is found in all-natural waters with an average concentration of 10-20 mg SiO2/L.”
In addition, the adsorption/desorption of disodium disilicate is technically not feasible because it hydrolyses quickly to sodium disilicate (CAS No. 1344-09-8). The OECD SIDS report (2004) also reports a that pH and the concentration influences the polymerisation-depolymerisation equilibrium leading to a variety of mono-, oligo., and polymeric anions and amorphous silica. Therefore, calculations on the distribution in various environmental compartments are not feasible.
Moreover, the OECD SIDS (2004) report summarizes that: “Soluble silicates are insoluble in alcohols, like noctanol, making determination of a log Kow not feasible.”

Description of key information

A study on adsorption/desorption of disodium disilicate is not conducted due to several reasons:

a)     In accordance with column 2 of REACH Annex VIII, the determination of the adsorption/ desorption behaviour does not need to be conducted as disodium disilicate (delta-crystalline) is expected to have a low potential for adsorption (log Kow expected to be < 0 as the substance is not soluble in octanol).

b)    Silicates are naturally abundant in the environment. The anthropogenic input to the various compartments is a negligible contribution, compared to the concentrations contributed by the natural silica flux.

c)     A study on Adsorption/Desorption of disodium disilicateis technically not feasible because it hydrolyses quickly tosodium disilicate (CAS No. 1344-09-8).The OECD SIDS report (2004) also reports athat pH and the concentration influences the polymerisation-depolymerisation equilibrium leading to a variety of mono-, oligo., and polymeric anions and amorphous silica. Therefore, calculations on the distribution in various environmental compartments are not feasible.

Key value for chemical safety assessment

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