The Use of a Five-stage Sequential Leaching Procedure for Risk Assessment of Heavy Metals in Waste Rock Utilized in Railway Ballast

Abstract

A five-stage sequential leaching procedure was used to determine the distribution of 10 metals in three combination samples taken from waste rock material, originating from a Finnish zinc mine, and used as railway ballast in Northern Finland. The leaching procedure consists of the following five sequential fractions: a water-soluble fraction (H₂O), an exchangeable fraction (CH₃COOH), an easily reduced fraction (NH₂OH-HCl), an oxidizable fraction (H₂O₂ + CH₃COONH₄), and a residual fraction (HF + HNO₃ + HCl). The results show, in accordance with all earlier corresponding studies, that a sequential extraction procedure is very suitable for evaluating the effects of external conditions on the solubility of harmful heavy metals, and that external conditions have a large effect on their leachability/solubility, and therefore on their mobility, bioavailability, and environmental risk. In addition, the total concentration of each element is much larger than its solubility in each of the first four fractions (1)–(4) – the potential bioavailability fractions – because the highest concentrations of all metals occurred in the residual fraction (5). The conditions of this phase – three strong acids and strong, long-lasting shaking – are never possible in nature, and therefore the residual fraction is called the inert phase. Thus, the total concentrations of the heavy metals are poor measures of real environmental risk and give no information about the effect of external conditions on their solubility. In addition, the dependency of different elements on the external conditions differs.

Acknowledgments

The authors wish to thank the technical staff of Suomen Ympäristöpalvelu Oy for the chemical analyses and Pöyry Environment Oy for sample materials. Thanks also to Licensed Translator Keith Kosola for revising the language.