The challenges of hazard identification and classification of insoluble metals and metal substances for the aquatic environment

The OECD is currently harmonizing procedures for aquatic hazard identification of substances. Such a system already exists in Europe where it is recognized that special consideration must be given to sparingly soluble metals and metal compounds (SSMMCs) because standard hazard testing procedures designed for organic chemicals do not accommodate the characteristics of SSMMCs. Current aquatic hazard identification procedures are based on persistence, bioaccumulation, and toxicity (PBT) measurements. Persistence measurements typically used for organic substances (biodegradation) do not apply to metals. Alternative measurements such as complexation and precipitation are more appropriate. Metal bioaccumulation is important in terms of nutritional sufficiency and potential food chain transfer and toxicity. Unlike organic substances, metal bioaccumulation potential cannot be estimated using log octanol-water partition coefficients. Further, bioaccumulation and bioconcentration factors are often inversely related to exposure concentration for most metals and organisms, and hence are not reliable predictors of chronic toxicity or food chain accumulation. Metal toxicity is due predominately to the free metal ion in solution. In order to assess the toxicity of SSMMCs, the rate and extent of transformation to a soluble form must be measured.