The use of a four-stage sequential leaching procedure and the corresponding one-phase extractions for risk assessment of potential harmful substances in waste rock utilized in railway ballast

Abstract

The purpose of environmental legislation in the EU is prevention, minimization, and utilization of waste, respectively. When utilization is not possible, the purpose becomes the safe landfill disposal of waste, or disposal by other ecologically beneficial methods. In addition, material efficiency is an essential topic nowadays to promote the sustainable use of natural resources, waste materials, and industrial by-products, in agreement with the principle of sustainable development and LCA. To promote these goals, a four-stage sequential leaching procedure and determination of total concentrations was used in this research to determine the distribution of Cu, Pb, Zn, and V 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: (i) an acidic water-soluble fraction (H2O, pH = 4), (ii) an exchangeable fraction (CH3COOH), (iii) an easily reduced fraction (NH2OH-HCl), and (iv) oxidizable fraction (H2O2 + CH3COONH4). The results show that conditions and the size of ballast have a significant effect on the solubility of all heavy metals, and therefore on their mobility, bioavailability, and environmental risk. In addition, the total concentration of every element is much larger than its solubility in each four fractions (i)–(iv) or the sum of these concentrations – this sum can be called by the potential bioavailability – because the highest concentration is in the residual fraction. The leachability results determined here for waste rocks utilized as railway ballast show in good agreement with all earlier investigations determined for other waste or industrial by-products. Sequential leaching studies provide valuable information about the effect of conditions on the leachability/solubility, mobility, and bioavailability for risk assessment of harmful heavy metals. This information is necessary if we want to know the real environmental risk of metals in different conditions, possible in natural conditions now and in the future, i.e. not only in terms of the conditions pertaining to permission applications.