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Sediment toxicity

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Description of key information

Adsorption of Green liquor constituents to sediments is regarded low. Therefore the existing aquatic effect data can be used to substitute sediments toxicity effects data.

Key value for chemical safety assessment

Additional information

Substances which are potentially capable of depositing on or sorbing to sediments to a significant extent have to be assessed for toxicity to sediment-dwelling organisms. In addition, marine sediment effects assessment is necessary for substances that are known to be persistent in marine waters, and may accumulate in sediments over time. In general, substances with a Koc< 500 –1000 L/kg are not likely sorbed to sediment (SETAC, 1993)(TGD, 2003).

Green liquor discharge to surface waters is unlikely to affect specifically the sediment and sediment species. Adsorption of any of the GL constituents to sediment is not known to be strong. Anionic binding to sediment matrix is generally not regarded as an effective adsorption mechanism. Therefore binding of sulfate, sulfite and thiosulfate to sediments is not regarded effective.

Sulfide has strong affinity to divalent metals and could therefore be bound strongly to sediment minerals. The bioavailability and toxicity of metal sulfides are generally much lower than dissolved sulfides and therefore sediment toxicity is decreased if sediment pore water sulfides are precipitated. However, free dissolved sulfide or hydrogen sulfide from anthropogenic sources may be oxidized relatively rapidly in oxygen rich surface water and may therefore not enter the sediment surface layer. 

Instead, sulfides are highly relevant ions in the processes of natural sediments, since H2S is formed naturally in the bacterial decomposition of sulfur containing organic material and large quantities are detected in sediments.

In anaerobic sediments highly insoluble FeS is formed when released Fe2+reacts with H2S. The sulfate reduction cycle plays a major role in the mobilisation of phosphate and other nutrients from the sediment. Also, the removal of sulfide by the release of Fe2+results in an increase in the migration of other metals such as Cu, Zn and Pb from the sediment.

Sulfur reduction rates within the sediments are generally low in comparison to those within the overlying water. Sulfate concentrations varied little over time suggesting a constant turnover time for sulfate (Smith and Klug, 1981).

As a conclusion, since adsorption of GL constituents to sediments is regarded insignificant, the existing aquatic effect data can be used to substitute sediments toxicity effects data.