Silicic acid, lithium magnesium sodium salt

Toxicity to aquatic algae and cyanobacteria

Key value for chemical safety assessment

EC50 for freshwater algae:

100 mg/L

EC10 or NOEC for freshwater algae:

6.1 mg/L

Additional information

The test report prepared by Chemex correctly notes that there was inhibition of algae apparent through changes in growth rates over 72 hours and reports that the IyC50 could not be determined. The headline result of IyC50 43 mg/l is misleading and implies that there is a toxic effect.

However, Laponite is known to have ion exchange properties and it is likely that the substance would impact on the growth media required for algal growth. There are two indicators that there is an effect; the first is that recovery of the substance measured by analysing for lithium shows poor response and secondly, the shape of the dose –effect curve. 

The report correctly adjusts the headline figures to account for ‘measured’ concentration, but since metal ions cannot degrade, it is suspected that ion exchange processes will be responsible, and the substance can therefore be designated as a ‘difficult substance’ requiring special considerations. For this reason, the nominal concentrations are considered a more reliable indicator.

Secondly, the dose response curve shows an effect at low concentrations (demonstrated by the low IC10 ) and at the top dose level, inhibition is still slight; these curves are normally concave, with increasing inhibition at higher levels, but the curves with Laponite are at best described as convex, with flattening response with concentration.

These curves, together with the known chemical properties of Laponite and the recognition that substances with ion exchange and sequestering properties are special cases, suggest that Laponite would not be inhibitory to algal growth under natural conditions.