## 1 Introduction
[30] studied the reachability of Cd, Pb, Zn, and Cu from contaminated soils and soils amended with different additives. Three months after the soils' treatments, the BCR extraction procedure was applied to study the partitioning of the metals after the remediation. The results revealed that Cd and Pb mobility was effectively reduced after the addition of potassium humate, while Cu mobility was reduced after the addition of zeolite or fly ash. The addition of amendments did not influence the mobility of Zn. [PERSON] et al. [31] investigated the efficiency of the immobilization of Pb, Cd, and Zn in an artificially contaminated soil, using eggshell and banana-stem amendments. Three months after the soil amendments, [PERSON]'s sequential extraction procedure was applied. The results showed that in eggshell-treated soil, Pb, Cd, and Zn were transferred from the easily mobile to sparingly soluble soil fractions, while the banana-stem amendment reduced the exchangeable Cd content and increased its residual form in the soil. Useful information about the stabilization of metals in different mineral phases after treatments with immobilizing agents can be obtained with transmission electron microscopy (TEM), scanning electron microscope (SEM), and powder X-ray diffraction (XRD) [18; 32; 33]. XRD, SEM, and TEM, in combination with sequential extraction procedures, provide complementary data on the distribution of metals in different mineral phases of the contaminated soils, delivering valuable information about alternatives for the remediation of contaminated sites [34]. [PERSON] et al. [35] investigated the potential of biochars, red mud, and steel slag for the in situ stabilization of Cd, Cu, Pb, and Zn in soil. The results of the sequential extraction analysis demonstrated that the application of biochars increases the metal fractions associated with carbonate and organic matter. The application of red mud and steel slag substantially increased the soil pH, leading to metal precipitation and their transfer from the easily soluble forms to the sparingly soluble fractions associated with Fe/Mn oxides and alumo-silicates. The latter findings were supported by the XRD analyses. By the above-reported approaches using sequential extraction procedures and/or their combination with XRD or microscopic techniques, it was possible only to evaluate the short-term efficiency of remediation (after three months). Since only one experiment was performed after three months, it was not possible to follow the mechanisms of immobilization of contaminants during the remediation process, nor to evaluate the efficiency of remediation and stability of the formed mineral phases after a longer period of time. In Slovenia, the area around the abandoned zinc smelter \"Old Zinc-works\" site in the town Celje is heavily polluted with Cd, Pb, and Zn. Among the anions, sulfate is present as a contaminant. One of the promising methods for remediation is an in situ immobilization with paper-ash, which is locally available recycled material. The highly alkaline paper-ash (pH 12) is mixed with polluted soil, forming a geotechnical composite. The results reveal that the Cd, Pb, and Zn in a highly mobile water-soluble fraction are being significantly reduced after immobilization [18]. However, there is a lack of data on the stability of geotechnical composites and a lack of knowledge about which soil phases the Cd, Pb, and Zn were transferred to after the remediation. In the present work, an investigation based on modified Tessier's sequential extraction procedure in combination with XRD analyses was performed (i) to gain a deeper insight into the mechanisms of immobilization by determining with which mineral phases Cd, Pb, and Zn are associated in different time intervals in the soil after the remediation with paper-ash, (ii) to assess the stability of newly formed mineral phases of Cd, Pb and Zn after remediation, and (iii) to evaluate the environmental impacts of soil remediation.