Is the transformation/dissolution protocol suitable for ecotoxicity assessments of inorganic substances such as silica fume?

Performing ecotoxicity tests on poorly water soluble substances and in particular metals, metalloids, and metal oxides such as silica fume, can be problematic. Such substances may not be directly toxic to aquatic organisms but often have high concentrations of impurities present, due to production processes, which may result in ecotoxicological effects. This combined with possibly testing above the limit of solubility further exacerbates the interpretation of ecotoxicity test results. One approach to overcome this is to perform a transformation/dissolution (T/D) test to determine the quantities of elemental impurities which will consequently be in solution. These data can subsequently be compared to existing data to determine if there is likely to be an effect on aquatic organisms. This paper highlights research into determining the T/D potential of 2 different grades of amorphous silica fume (low and high grade purity) with complementary chronic ecotoxicity tests of the 2 substances to validate this approach. The low grade silica fume test substance was identified in the T/D assessments as being of concern for the potential to cause acute toxicity to aquatic organisms and had levels of impurities (e.g. Pb and Zn) in the solutions which exceeded the effect limits identified in the open literature. Consequently, silica fume would be hazard classified as acute 2 according to regulatory classification schemes. However, the results of the ecotoxicity hazard validation assessments in a Daphnia magna reproduction test and the sediment dwelling organism Chironomus riparius indicated that low and high grade silica fumes are not acutely or chronically toxic up to and including an initial loading concentration of 100 mg/L and 1000 mg/kg respectively. Hence, using the T/D test data alone may have resulted in a false hazard classification of silica fume (low grade).