## 3 Results and Discussion  
### Soil Analysis  
The soil was exhaustively characterized, and its main characteristics and pseudototal metal contents are summarized in Table 2.  
The sandy-loam texture of the soil indicates a high sand content with relatively low amounts of silt and clay, which limits its capacity to retain heavy metals. Clay particles are key for metal retention due to their high surface area, cation exchange capacity, and the ability to form stable complexes with metal ions [40]. However, the low clay content in sandy-loam soils diminishes these effects, potentially increasing metal mobility and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Characteristic** & **Unit** & **Value** & **Element** & **Units** & **Concentration** \\ \hline pH\({}_{\text{(H2O)}}\) & - & 5.75 \(\pm\) 0.10 & Al & g kg\({}^{-1}\) & 57.71 \(\pm\) 8.69 \\ pH\({}_{\text{(H2C)}}\) & - & 4.85 \(\pm\) 0.10 & As & mg kg\({}^{-1}\) & 14.58 \(\pm\) 1.18 \\ Organic Matter & \% & 2.01 \(\pm\) 0.35 & Ca & g kg\({}^{-1}\) & 1.76 \(\pm\) 0.05 \\ N & \% & 0.13 \(\pm\) 0.01 & & & \\ P & mg kg\({}^{-1}\) & 28.60 \(\pm\) 1.04 & & & \\ Texture & - & sandy-loam & Cd & mg kg\({}^{-1}\) & nd \\ Ca\({}^{2+}\) & & 1.69 \(\pm\) 0.34 & Cr & mg kg\({}^{-1}\) & 68.75 \(\pm\) 2.18 \\ K\({}^{*}\) & & 18.11 \(\pm\) 4.84 & Cu & mg kg\({}^{-1}\) & 77.92 \(\pm\) 1.84 \\ Mg\({}^{2+}\) & cmol\({}_{(\tau)}\)kg\({}^{-1}\) & 0.50 \(\pm\) 0.10 & Fe & g kg\({}^{-1}\) & 48.81 \(\pm\) 3.08 \\ Na\({}^{+}\) & & 1.50 \(\pm\) 0.69 & K & g kg\({}^{-1}\) & 10.94 \(\pm\) 1.07 \\ Al\({}^{2+}\) & & 31.11 \(\pm\) 1.97 & Mg & g kg\({}^{-1}\) & 10.88 \(\pm\) 0.77 \\ ECEC & & 52.92 \(\pm\) 7.94 & Mn & g kg\({}^{-1}\) & 0.43 \(\pm\) 0.02 \\  & & & Na & mg kg\({}^{-1}\) & 135.83 \(\pm\) 3.54 \\ Fe oxides & & 35.08 \(\pm\) 4.17 & Ni & mg kg\({}^{-1}\) & 31.92 \(\pm\) 1.26 \\ Al oxides & g kg\({}^{-1}\) & 5.83 \(\pm\) 0.70 & Pb & mg kg\({}^{-1}\) & 9.50 \(\pm\) 1.32 \\ Mn oxides & & 0.15 \(\pm\) 0.02 & Zn & mg kg\({}^{-1}\) & 107.67 \(\pm\) 2.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Soil characteristic analysis. environmental risk, making it suitable for evaluating the effects of HANPs on Cd immobilization without interference from clay-mineral interactions. The soil showed a moderately acidic pH (5.75 in water and 4.85 in KCl), which is an essential factor in metal solubility and bioavailability. Acidic soils increase the solubility of heavy metals due to the abundance of H\({}^{+}\), which compete for sorption sites on soil particles [40]. The lower pH in KCl shows the exchangeable acidity, further highlighting the presence of H\({}^{+}\) and Al\({}^{3+}\) in the exchange complex. This acidic environment suggests higher Cd mobility and bioavailability as a result of the lack of HANP treatments. Organic matter plays an essential role in the biogeochemical cycling of heavy metals through mechanisms such as complexation, adsorption, and chelation [41]. However, the organic matter content in this soil was low (2%), limiting its capacity to retain heavy metals and interact with HANPs. The total N content was 0.13%, which is considered low for agricultural soils, showing limited N availability for plant growth, while the available phosphorus content was 28.60 mg kg\({}^{-1}\), which is within the medium range for agricultural soils [42]. The ECEC of the soil was 52.92 cmol(+)kg\({}^{-1}\), mainly due to the high concentration of exchangeable Al\({}^{3+}\). This is characteristic of acid soils, where Al\({}^{3+}\) dominates the exchange complex over basic cations such as Ca\({}^{2+}\), Mg\({}^{2+}\), and Na\({}^{+}\), which are present in much lower concentrations. The predominance of Al\({}^{3+}\) contributes to the high ECEC but also influences the soil buffering capacity, promoting Cd mobility by reducing the availability of exchange sites for Cd retention [43]. Iron and manganese oxides are key components that influence heavy metal retention. These oxides form stable complexes with metals, reducing their bioavailability [44].