A critical surface area concept for acute hazard classification of relatively insoluble metal-containing powders in aquatic environments

A method is proposed for determining the hazard identification, based on acute aquatic toxicity, and subsequent classification if necessary, of metals and sparingly soluble inorganic metal compounds. The method is based on establishing a relationship between the measured reaction kinetics of the substance with an aqueous medium and the measured surface area of the substance loaded to the medium. The total dissolved metal concentration at a given time expressed as a function of measured surface area is then compared with an appropriate acute toxicity value, the median lethal effective concentration (L(E)C50), as measured in standard toxicity tests with the corresponding soluble metal salt to yield a critical surface area independent of particle size that will deliver the L(E)C50 to the medium. The critical surface area can then be converted to the conventional 100, 10, and 1 mg/L cutoff points to calculate a hazard identification line that can be used straightforwardly thereafter to establish the hazard classification of any form of the substance once its specific surface area is measured. Further transformation testing of the forms that the substance may be produced in is not required. The application of the critical surface area method to determine the hazard classification according to specific surface area is illustrated by worked examples for nickel metal and zinc metal. Selection of appropriate L(E)C50 values is critical, because different values can substantially change the final classification.