User Guide to Thresholds and Classification - Environmental ...

348User Guide for Thresholds and ClassificationsUnderlying this is the assumption that, although speciation can occur, the species will remain available underenvironmentally relevant conditions. This may not always be the case, as described above, and anyevidence available that would suggest changes to the bioavailability over the course of 28 days, should becarefully examined. The bioaccumulation of metals and inorganic metal compounds is a complex processand bioaccumulation data should be used with care. The application of bioaccumulation criteria will need tobe considered on a case-by-case basis taking due account of all the available data.A further assumption that can be made, which represents a cautious approach, is that, in the absence of anysolubility data for a particular metal compound, either measured or calculated, the substance will besufficiently soluble to cause toxicity at the level of the L(E)C50, and thus may be classified in the same wayas other soluble salts. Again, this is clearly not always the case, and it may be wise to generate appropriatesolubility data.This section deals with metals and metal compounds. Within the context of this guidance document, metalsand metal compounds are characterised as follows, and therefore, organo-metals are outside the scope ofthis section.Metals, M0, in their elemental state are not soluble in water but may transform to yield the available form.This means that a metal in the elemental state may react with water or a dilute aqueous electrolyte toform soluble cationic or anionic products, and in the process the metal will oxidise, or transform, from theneutral or zero oxidation state to a higher one.In a simple metal compound, such as an oxide or sulphide, the metal already exists in the oxidised state,so that further metal oxidation is unlikely to occur when the compound is introduced into an aqueousmedium. However, while oxidisation may not change, interaction with the media may yield more solubleforms. A sparingly soluble metal compound can be considered as one for which a solubility product canbe calculated, and which will yield a small amount of the available form by dissolution. However, itshould be recognised that the final solution concentration may be influenced by a number of factors,including the solubility product of some metal compounds precipitated during thetransformation/dissolution test, for example, aluminium hydroxide.Application of aquatic toxicity data and solubility data for classification of metals and metalcompoundsInterpretation of aquatic toxicity dataAquatic toxicity studies carried out according to a recognised protocol should normally be acceptable as validfor the purposes of classification. Section A9.3 should also be consulted for generic issues that are commonto assessing any aquatic toxicity data point for the purposes of classification.Metal complexation and speciationThe toxicity of a particular metal in solution, appears to depend primarily on (but is not strictly limited to) thelevel of dissolved free metal ions. Abiotic factors including alkalinity, ionic strength and pH can influence thetoxicity of metals in two ways. By influencing the:January 2012 EPA0109