# 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  
Figure 4 shows cumulative leaching percentages of heavy metals (Cr, Ni, Cu, Zn, As, Cd, and Pb) from S/S mixtures F-3 and F-5 over 90 days using deionized water as the leaching solution. Using phosphogypsum as a binding agent is to immobilize these metals and reduce their environmental mobility. However, the results show that the degree of effectiveness varies with the type of metal. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{8}{c}{**Chemical Composition [\%]**} \\ \cline{2-10}  & **Na\({}_{2}\)O** & **MgO** & **Al\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **P\({}_{2}\)O\({}_{5}\)** & **SO\({}_{3}\)** & **K\({}_{2}\)O** & **CaO** & **Other** \\ \hline Sediment & 0.36 & 1.85 & 13.11 & 43.89 & 0.41 & 0.53 & 1.82 & 1.17 & 36.92 \\ F-3 & 0.28 & 1.50 & 11.51 & 40.40 & 0.34 & 1.78 & 1.67 & 1.80 & 40.72 \\ F-5 & 0.31 & 1.65 & 12.56 & 41.06 & 0.37 & 2.39 & 1.76 & 2.51 & 37.39 \\ \hline \hline \multicolumn{10}{c}{**Heavy metals [\(\mu\)g/g]**} \\ \hline  & Cr & Ni & Cu & Zn & As & Cd & Pb \\ \hline Sediment & 348.6 & 53.2 & 322.4 & 1209 & 9.6 & 17.1 & 124.7 \\ F-3 & 310.9 & 46.3 & 290.9 & 1139 & 8.0 & 15.5 & 118.6 \\ F-5 & 319.3 & 47.0 & 293.7 & 1173 & 8.8 & 16.5 & 118.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Chemical compositions of samples. Figure 4: Cumulative leached metal fractions from S/S mixtures F-3 and F-5. Cadmium showed the highest cumulative leaching percentage, particularly in the 5% phosphogypsum mixture (F-5), with up to 9.01% leaching after 90 days. This indicates that cadmium remains relatively mobile even after treatment. Studies by [PERSON] et al. (2020) [43] support these findings, suggesting that phosphate amendments can sometimes increase cadmium leachability due to the formation of soluble cadmium-phosphate complexes, influenced by specific pH and redox conditions. On the other hand, lead elution was very low, up to 0.13% in the F-5 mixture, indicating effective immobilization, although the concentration was not high. This can be attributed to the formation of highly stable lead-phosphate compounds. [PERSON] et al. (2020) [44] demonstrated that lead forms insoluble complexes with phosphates, significantly reducing its leaching potential under various environmental conditions. Zinc showed significant leaching, particularly in the F-5 mixture, reaching 6.99% after 90 days, suggesting that zinc remains somewhat mobile even with higher phosphogypsum content. Zinc can form soluble complexes or remain adsorbed on the surface of particles, making it susceptible to leaching, especially under mildly acidic conditions [45]. Arsenic exhibited moderate leaching levels, with 2.07% for F-3 and 1.49% for F-5 after 90 days. The reduction in arsenic mobility is likely due to adsorption processes or the formation of arsenic-phosphate compounds. Nickel and chromium showed moderate leaching percentages, suggesting some stabilization but not complete immobilization. [PERSON] et al. (2022) [46] found that these metals can form relatively stable complexes within S/S matrices, although environmental conditions can still influence their mobility. Copper displayed moderate leaching levels, with a maximum of 0.69% for the F-5 mixture. While copper can form stable complexes with phosphates, some leaching is still observed. The mobility of copper in S/S systems can vary depending on the binding agent and environmental factors such as pH [47]. The ANS 16.1 leaching model utilizes [PERSON]'s diffusion theory to evaluate the diffusion rate, providing a basis for assessing the effectiveness of solidification/stabilization (S/S) treatments [26; 48]. This assessment is primarily achieved through the determination of diffusion coefficients (De) and the calculation of the leaching index (LX).