## 3 Results and Discussion
### Assessment of Cd Distribution in Different Soil Fractions
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Soil + HANPs** & **Cd** & **F1** & **F2** & **F3** & **F1 + F2 + F3** \\ \hline - & 0 & nd & nd & nd & nd \\
1\% & 0 & nd & nd & nd & nd \\ \hline - & 5 & nd & 1.04 \(\pm\) 0.3 e & 0.79 \(\pm\) 0.34 d & 1.83 \(\pm\) 0.64 d (36.60\%) \\
1\% & 5 & nd & 1.41 \(\pm\) 0.26 d & 0.50 \(\pm\) 0.24 d & 1.91 \(\pm\) 0.50 d (38.20\%) \\ \hline - & 10 & 0.11 \(\pm\) 0.03 c & 3.49 \(\pm\) 1.28 c & 2.45 \(\pm\) 0.85 c & 6.05 \(\pm\) 2.16 c (60.50\%) \\
1\% & 10 & 0.04 \(\pm\) 0.02 d & 3.73 \(\pm\) 0.02 c & 2.13 \(\pm\) 0.49 c & 3.77 \(\pm\) 0.53 c (37.70\%) \\ \hline - & 25 & 0.28 \(\pm\) 0.07 b & 8.72 \(\pm\) 3.2 b & 6.11 \(\pm\) 2.13 b & 15.11 \(\pm\) 5.40 b (60.44\%) \\
1\% & 25 & 0.11 \(\pm\) 0.04 c & 9.31 \(\pm\) 1.23 ab & 5.33 \(\pm\) 0.39 b & 14.75 \(\pm\) 1.66 b (59.00\%) \\ \hline - & 50 & 0.73 \(\pm\) 0.54 a & 13.56 \(\pm\) 3.84 a & 11.3 \(\pm\) 1.65 a & 25.59 \(\pm\) 6.03 a (51.18\%) \\
1\% & 50 & 0.61 \(\pm\) 0.11 a & 14.77 \(\pm\) 2.8 a & 11.03 \(\pm\) 0.84 a & 26.41 \(\pm\) 3.75 a (51.82\%) \\ \hline
**Soil + HANPs** & **Cd** & **F4** & **F5** & **F6** & **\% Recovery** \\ \hline - & **0** & nd & nd & nd & - \\ \hline 1\% & 0 & nd & nd & nd & - \\ \hline - & 5 & 1.12 \(\pm\) 0.18 d & 0.13 \(\pm\) 0.03 d & 0.66 \(\pm\) 0.08 d & 74.80 \\ \hline 1\% & 5 & 0.82 \(\pm\) 0.25 d & nd & 1.14 \(\pm\) 0.21 d & 77.30 \\ \hline - & 10 & 1.87 \(\pm\) 0.63 c & 0.79 \(\pm\) 0.32 c & 0.57 \(\pm\) 0.13 e & 92.80 \\ \hline 1\% & 10 & 1.36 \(\pm\) 0.16 c & nd & 2.44 \(\pm\) 0.62 c & 75.65 \\ \hline - & 25 & 4.68 \(\pm\) 1.57 a & 1.97 \(\pm\) 0.81 b & 1.43 \(\pm\) 0.43 d & 92.76 \\ \hline 1\% & 25 & 3.40 \(\pm\) 0.26 b & nd & 3.42 \(\pm\) 0.76 c & 86.28 \\ \hline - & 50 & 7.80 \(\pm\) 1.33 a & 4.38 \(\pm\) 0.78 a & 5.86 \(\pm\) 1.05 b & 87.25 \\ \hline 1\% & 50 & 5.58 \(\pm\) 0.55 a & 0.08 \(\pm\) 0.03 d & 8.96 \(\pm\) 0.79 a & 82.05 \\ \hline \end{tabular} \(\!\!\)nd: not detected. F1 + F2 + F3 in parentheses: percentage (%) of total Cd associated with these fractions over total Cd. Different letters next to the values indicate significant differences among treatments for each fraction at a significance level of \(p<\) 0.05.  
\end{table} 
Table 3: Fractions F1 to F6 of sequential Cd extraction (mg kg\({}^{-1}\)). results of phytotoxicity tests and pot experiments, where reductions in germination rates and root growth were observed at Cd concentrations \(\geq\) 10 mg kg\({}^{-1}\). This indicates the limited effectiveness of HANPs in immobilizing Cd in these highly available fractions at the highest Cd contamination levels. In the less bioavailable fractions (F4-F6), the influence of HANPs is more significant. Specifically, the Cd content complexed with organic matter (F5) decreased significantly in soils treated with HANPs compared to untreated soils. This indicates that HANPs may modify the Cd distribution from organic matter-bound forms to more stable and less bioavailable forms. In this line, an increase in the formation of stable metal-phosphate complexes in soils amended with HANPs was observed in a previous study [47]. Additionally, a significant increase in irreversibly adsorbed Cd (F6) was also observed in soils treated with HANPs, particularly at higher Cd concentrations (10, 25 and 50 mg kg\({}^{-1}\)). This suggests that HANPs promote Cd immobilization through mechanisms such as chemisorption or precipitation, leading to the formation of highly stable Cd-phosphate complexes. In fact, a previous study demonstrated that hydroxyapatite application reduced bioavailable Cd concentrations by transforming exchangeable and soluble fractions into more stable forms [22]. Similarly, it was found that combining HA with humic acids decreased Cd availability in maize while enhancing crop performance, highlighting the potential synergistic benefits of using multiple amendments [48]. These findings support that HANPs alone may effectively transform Cd into less bioavailable forms (F6), thereby reducing its translocation to plant tissues, as observed in the pot experiments with quinoa.