Past industrial activities have left behind approximately 340 000 degraded sites across the EU, where contamination with Potentially Toxic Elements (PTE) prevails. Contaminated soils contain PTE in both mobile and bioavailable forms, which pose a risk for the environment and for human health in the case of exposure. Such degraded sites, also sometimes referred to as "old burdens", need to be rehabilitated in the most practical, economical, and environmentally acceptable way. Currently, there is no directive dealing exclusively with soil in the EU, so that the management of contaminated soil is affected by waste and landfill directives. Soil which is excavated at degraded sites is either a non-hazardous or a hazardous waste, which has to be managed according to the waste hierarchy, where landfilling is the least, and reuse or recycling are the most desirable options. However, contaminated soils can meet end-of-waste criteria through a R5 recovery operation. Heavily contaminated soils have to be treated by means of strict remediation measures that transform them into recycled material, which can be used beneficially in civil engineering. Immobilization is one of the most widely applied soil remediation approaches, and is recommended by the EU Commission as the Best Available Technique for the management of large volumes of soil.
The Old Zinc-Works, which are located in city of Celje, is one of most contaminated sites in Slovenia. The area of 17 ha is covered by approx. 1.5 M m3 of contaminated soil, classified as hazardous waste due to the high measured concentrations of different PTE (As, Zn, Pb, Cd, Cu, Mo), as well as SO42- and tar. A remediation method was developed for the treatment of this contaminated soil, which has been validated in the real case treatment which was performed at the old zinc-works area. An immobilization approach was used, where the bioavailable and mobile species of PTE are chemically and physically incorporated into a newly formed geotechnical composite. The latter is a mixture of crushed and homogenised contaminated soil and an immobilization additive. After establishing the optimum moisture content, the composite is compacted in layers by standard geotechnical methods, i.e. as engineering fill. Different recycled materials were experimentally tested as possible immobilization additives, e.g. two ashes from coal combustion, ash from the incineration of paper mill residues (PA), red mud, and two ladle slags from steelmaking. Their immobilization capabilities and technical characteristics were evaluated and compared in the contaminated soil/additive composites. There were substantial differences between the composites made using different immobilization additives, especially with regard to compressive strength and water permeability, and affinity for chemical immobilization of different PTE. Due to the fact that its characteristics were the most optimal, paper ash was utilized for the final application. This is because, at maximum degree of compaction of the soil/PA composite, pozzolanic-hydraulic activity of the ash was induced. Thus stable matrix was formed, in which contaminants are physically microencapsulated, or chemically adsorbed onto the surface and/or incorporated into the crystalline lattice of newly formed mineral phases. The hydration products are similar to those of Portland cement. The alkaline pH values of pore solution favour the formation of low soluble hydroxides of most of PTE. Hydraulic conductivity of composites, too, is very low, which means that very little water is percolated, and that the mass flow of the PTE is very slow.
