## 3 Results and Discussion
### Partitioning of Cd, Pb, and Zn in Uncontaminated, Contaminated, and Remediated Soils
In the remediated soils, the application of paper-ash substantially increased the soil's pH, which led to the precipitation of Cd, Pb, and Zn and their transfer from the easily soluble fractions to the sparingly soluble fraction associated with Fe/Mn oxides and hydroxides. In the early stage of remediation (Figures 3-5, soils that were not treated with sulfate), the Cd, Pb, and Zn were associated mainly with Fe/Mn oxides and hydroxides. After 28 days, Cd and Zn were redistributed to the residual fraction, while the Pb was between the residual and the fraction bound to Fe/Mn oxides and hydroxides. As the time elapsed, the CaO present in the paper-ash was hydrated, forming Ca(OH)\({}_{2}\) that, through the carbonation process, yielded CaCO\({}_{3}\). The newly formed carbonates, along with the hydroxides, bound the Cd, Pb, and Zn into insoluble compounds. The solubilities of the Cd, Pb, and Zn carbonates and hydroxides are presented in Table 2 (data from IUPAC-NIST Solubility Database [39]. After the stabilization in the mineral phases, until the end of the experiment, the Cd, Pb, and Zn remained associated mainly with the carbonate fraction and the fraction bound to the Fe/Mn oxides and hydroxides. The data from Figures 3-5 and Figures S3-5 further indicate that the addition of sulfate to soil samples had no effect on the remediation efficiency. The partitioning of Cd, Pb, and Zn in the remediated soil, which was not treated with sulfate (Figures 3-5), and in the sulfate-treated remediated soil (Figures S3-5), was almost the same. Due to the redistribution of Cd, Pb, and Zn from the water-soluble and the exchangeable fractions into the sparingly soluble fractions of soil composite (the formation of insoluble Cd, Pb, and Zn carbonates and hydroxides), the Cd, Pb, and Zn concentrations in the easily soluble fractions significantly decreased, indicating the effective remediation of the contaminated soil by paper-ash. The Cd concentration in the water-soluble fraction after the remediation was reduced from 0.8 to approximately 0.0005 mg/kg (Tables S4 and S7), which is 100 times lower than the limiting value for inert materials [37; 38]. In the exchangeable fraction, this decrease was from 1.5 to 0.0005 mg/kg (3000 times), demonstrating the effective immobilization of Cd also from the exchangeable fraction. The Pb concentration, which during the course of the experiment in the water-soluble fraction of contaminated soils, in general, ranged from 0.2 to 0.8 mg/kg, was reduced by about 250 times after the remediation (Tables S5 and S8). The concentrations of Pb in the water-soluble fraction were about 125 times lower than the limit value for inert materials [37; 38]. In the exchangeable fraction of soil that was not treated with sulfate, this decrease was from 8 to 0.1 mg/kg (80 times) after the remediation (Table S5), indicating the effective immobilization of Pb also in this easily soluble soil fraction. The effective immobilization of Pb from the exchangeable fraction was also observed in the sulfate-treated and remediated soil (Table S8). The Zn concentration in the water-soluble fraction was reduced from about 60 to 0.4 mg/kg (Table S6) after the remediation, which is 10 times lower than the limiting value for inert materials [37; 38]. In the case of Zn, similar concentrations were found in the sulfate-treated remediated soils (Table S9). In the exchangeable fraction, the Zn concentrations in soil that were not treated with sulfate decreased after the remediation by about 50 times, and in the sulfate-treated soil by about 100 times (Tables S6 and S9). This shows that the immobilization of Zn was also effective in the exchangeable soil fraction. It is important to stress that the Cd, Pb, and Zn were already effectively immobilized 28 days after the remediation and remained stabilized in the sparingly soluble soil composite fractions until the end of the experiment (1 year), demonstrating the remediation efficiency as well as the stability of the formed Cd, Pb, and Zn compounds.