# Utilization of Phosphogypsum and Sediment in Subgrade Material for Pavement Construction
## 3 Results
### Physicochemical and Thermal Characterization of Sediment Samples Treated with Phosphogypsum
#### 3.1.6 ANS 16.1 Test
The average values of diffusion coefficients (De) and LX values are presented in Table 7 and Figure 5. In this context, the average diffusion coefficients (De) for the F-5 mixture demonstrated a range of values: from 1.22 \(\times\) 10\({}^{-8}\) cm\({}^{2}\)s\({}^{-1}\) for cadmium, indicating moderate mobility, to 9.45 \(\times\) 10\({}^{-12}\) cm\({}^{2}\)s\({}^{-1}\) for lead, suggesting limited mobility. According to [PERSON] and [PERSON] (1980) [31], the diffusion coefficients of metals from S/S mixtures can span a wide range, from values indicative of very mobile metals (approximately 1 \(\times\) 10\({}^{-5}\) cm\({}^{2}\)s\({}^{-1}\)) to those that represent practically immobilized metals (around 1 \(\times\) 10\({}^{-15}\) cm\({}^{2}\)s\({}^{-1}\)). The diffusion coefficients observed for the F-3 and F-5 mixtures, which lie between 10\({}^{-8}\) and 10\({}^{-12}\) cm\({}^{2}\)s\({}^{-1}\), suggest that the metals within these mixtures exhibit moderate mobility. \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline \multirow{2}{*}{**Mixtures**} & \multicolumn{3}{c}{\(\overline{D}\)**\(\mathbf{e}\) (cm\({}^{2}\)/s)**} & \multicolumn{3}{c}{} \\ \cline{2-9}  & **Cr** & **Ni** & **Cu** & **Zn** & **As** & **Cd** & **Pb** \\ \hline F-3 & \(1.98\times 10^{-12}\) & \(2.06\times 10^{-10}\) & \(4.93\times 10^{-11}\) & \(2.43\times 10^{-9}\) & \(1.59\times 10^{-10}\) & \(5.62\times 10^{-9}\) & \(1.24\times 10^{-12}\) \\ F-5 & \(1.73\times 10^{-11}\) & \(3.13\times 10^{-10}\) & \(1.56\times 10^{-10}\) & \(8.10\times 10^{-9}\) & \(2.70\times 10^{-10}\) & \(1.22\times 10^{-8}\) & \(9.45\times 10^{-12}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Mean diffusion coefficients De (cm\({}^{2}\)s\({}^{-1}\)) in S/S mixtures F-3 and F-5. These findings have significant implications for the environmental performance of S/S treatments. Metals with diffusion coefficients in the range of 10\({}^{-8}\) to 10\({}^{-12}\) cm\({}^{2}\)s\({}^{-1}\) are not as readily leachable as highly mobile species, yet they are not fully immobilized. The moderate mobility of these metals implies that, over time, there may be a gradual release of contaminants into the surrounding environment. This controlled release may be beneficial in certain scenarios, as it can mitigate acute toxicity risks by preventing the sudden leaching of high concentrations of metals. However, it also necessitates continuous monitoring and potential long-term management to ensure that the release rates do not exceed acceptable environmental thresholds. According to the Canadian Environmental Protection Agency [32], the leaching index (LX) values serve as a crucial criterion for assessing the suitability of solidification/stabilization (S/S)-treated waste for various use and disposal scenarios. An LX value above 9 signifies effective treatment, making the S/S-treated waste suitable for \"controlled use\" applications, such as quarry rehabilitation, lagoon closure, or use as a road base. LX values ranging from 8 to 9 indicate that the treated waste is appropriate for disposal in sanitary landfills. Conversely, LX values below 8 denote that the S/S treated waste is unsuitable for disposal, indicating a higher potential for leaching and environmental contamination [32; 48; 49]. Figure 5 illustrates the average leaching index (LX values) for the tested mixtures, demonstrating that the S/S treatment effectively immobilizes the metals. Both S/S mixtures exhibit LX values greater than 9 for the majority of metals, suggesting high immobilization efficiency. The F-3 mixture shows LX values above 9 across all metals, confirming its suitability for \"controlled use\" applications. This high level of immobilization indicates that the metals are securely bound within the matrix, reducing the likelihood of their release into the environment under standard conditions of use. For the F-5 mixture, while most metals also show LX values above 9, cadmium is an exception, with its LX value ranging between 8 and 9.