# 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
This range indicates that the S/S treated waste containing cadmium is less immobilized compared to other metals but is still within the threshold that allows for safe disposal in sanitary landfills. The use of phosphogypsum as an immobilization agent in the S/S process has proven effective in reducing the leachability of the investigated metals. Phosphogypsum's role in forming stable complexes with metal ions likely contributes to the observed high LX values, as it enhances the structural integrity of the S/S matrix and its resistance to leaching. Figure 5: Average leaching indices (LX) in S/S mixtures F-3 and F-5. The effectiveness of phosphogypsum in immobilizing heavy metals suggests that it can be a viable agent in S/S treatment formulations, especially for applications requiring strict leachate control. The ability to achieve high LX values not only demonstrates the feasibility of using such materials in environmentally sensitive applications but also provides a framework for the development of guidelines and regulations governing the use of S/S treated waste. When comparing the slope values from the graph showing the relationship between the cumulative fraction of leached metal (mg m\({}^{-2}\)) and time (Figure 6), it is noted that surface wash-off appeared to be the main controlling leaching mechanism for chromium, copper and lead for both mixtures, since the values are below 0.4. This suggests that these metals are predominantly released from the surface of the S/S material, rather than diffusing through the matrix or dissolving, implying that the surface of the S/S matrix retains some solubility or that physical processes at the surface facilitate metal release. The relatively low slope values for these metals highlight the effectiveness of the S/S process in limiting their mobility and subsequent environmental impact. In the F-5 mixture, arsenic also displays slope values below 0.4, similarly suggesting surface wash-off as the controlling mechanism. Conversely, for both S/S mixtures, nickel shows slope values ranging between 0.4 and 0.6, indicating that diffusion is the dominant leaching mechanism for this metal. In this range, the leaching process is characterized by the gradual migration of nickel ions from the interior of the matrix to the surface. Similarly, arsenic in the F-3 mixture falls into this category, suggesting that diffusion plays a more significant role in its leaching compared to the F-5 mixture. For zinc and cadmium in both mixtures, slope values exceeding 0.6 indicate that dissolution is the primary leaching mechanism, suggesting these metals have higher solubility within the S/S matrix. This could result in increased mobility and potential for environmental release. While these findings highlight some limitations of the current S/S treatment for zinc and cadmium, the gradual release of these metals may still be manageable with proper monitoring to ensure it remains within regulatory limits. Figure 6: Leaching mechanism determined based on slope values from graphs of cumulative leached fraction of metals versus time for S/S mixtures.