## 3 Results and Discussion
### Investigation of the Mineralogical Composition of the Remediated Soils  
To identify the mineral phases that were formed after the remediation, XRD analyses were performed in uncontaminated, contaminated, and remediated soils during each time interval as a sequential extraction procedure. In the case of contaminated soil without the addition of gypsum, there is no difference in XRD spectra in comparison to the uncontaminated soil, neither from the scope of analysis of crystalline phases nor from the scope of analyses for the potential presence of amorphous phases. In addition, there was no difference between the contaminated soil with the addition of gypsum in comparison to the uncontaminated soil with the addition of gypsum. The only difference between the contaminated soil and contaminated soil with added gypsum was that the characteristic peaks of gypsum were detected in the XRD spectra of the latter, as can be seen in Figure S6.  
The potential formation of secondary crystalline and amorphous phases of Cd, Pb, and Zn minerals was somehow expected after artificial contamination of soil. Therefore, special care was taken during the refinement of the XRD spectra to identify the potential presence of commonly occurring Cd, Pb, and Zn carbonate and sulfate minerals. However, no such phases were identified.  
Since the data from the partitioning study revealed that the sulfate treatment does not influence the remediation efficiency, only the soils that were not treated with sulfate are presented. Sections of the XRD patterns of the contaminated and remediated soils, along with the XRD patterns of the uncontaminated soil, are presented in Figure 6.  
Figure 6: XRD patterns of (**A**) remediated soil and (**B**) corresponding section of the XRD patterns from 8.9\({}^{\circ}\)–11.8\({}^{\circ}\) 20 with time elapsed after the remediation.  
The results of the XRD analyses showed that a new mineral phase was formed due to the addition of paper-ash to the soil. A mono-sulpho-aluminate hydration product from the group of calcium aluminate hydrate minerals (CAH) was identified. Its relative quantity was not changed significantly during the experiment. The hydration product CAH, which was formed in the remediated soils, and the carbonation process, which was proven in the case of paper-ash, contribute to the effective immobilization of the Cd, Pb, and Zn. These XRD data supported the results obtained by the sequential extraction procedure.  
In the reported literature, the BCR extraction scheme [30], the [PERSON]'s sequential extraction procedure [31], and the use of sequential extraction procedures in combination with XRD analysis [34; 35] were reported for the investigation of the immobilization of heavy metals in the soil after the remediation with different soil amendments. Since in these articles sequential extraction procedures were applied for only one-time interval after the remediation, by the reported approaches [30; 31; 34; 35], it was only possible to estimate the efficiency of remediation by evaluating the redistribution of metals from the easily soluble to more sparingly soluble soil mineral phases. In the present investigation, the use of modified Tessier's sequential extraction procedure in combination with the complementary XRD analysis performed in time intervals of 7, 28, 56, 90, 120, and 365 days after the remediation, enabled us to follow the mechanisms of the remediation of Cd, Pb, and Zn with paper-ash, the study of the efficiency of the remediation and the stability of the newly formed insoluble mineral phases of Cd, Pb, and Zn. To the best of our knowledge, such a comprehensive methodological approach has not yet been reported.