Silicic acid, lithium magnesium sodium salt

Endpoint summary

Additional information

This substance is an inorganic synthetic version of Hectorite clay, which commonly occurs in nature and is in the same family of clays as bentonite. The material is inorganic and therefore non-biodegradable, retaining its clay structure in the environment. At normal water course pH’s this material is stable and insoluble. It does not dissolve in water but disperses to form a clear sol and in the pH range 4 – 9 does not hydrolyse.

The majority of the applications for which this substance is used will not result in the material being released to water courses directly. Personal care product applications such as toothpaste, shower gels, shampoos, cosmetics etc and also household product applications such as dishwasher detergent and toilet cleaners will result in very low concentrations of this substance (<100 mg/l) being released into the domestic sewage system and finding its way into water treatment works. This substance has a high molecular weight, when hydrated forms a sol (or a gel at higher concentrations) and studies within the manufacturing effluent treatment facility suggest that the material sediments and is retained in the solid waste rather than going forward in the flume / purified water released to the river.

If low concentrations do get through the water treatment process and enter the water course, the fact that the material is inorganic and thus does not biodegrade, is not soluble, does not hydrolyse and is of a high molecular weight suggest that transfer across biological membranes is unlikely. In addition, short term aquatic toxicity tests carried out on fish and daphnia show this substance is not toxic to marine life. On this basis the need for long term aquatic testing is waived. 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.