# Application of Iron-Modified Activated Carbon for Phosphate Removal in Aqueous Systems
## 3 Results and Discussion
### Effect of SMAC on the Sediment Layer  
Nutrient pollutants, which are the causes of algal blooms, require long-term management for improvement. The application of SMAC not only prevents the leaching of phosphates from the soil but also controls the phosphates present within the soil. This contribution is evidenced by the experiments shown in Figure 6. For the experiment, we filled a batch with P-Soil at concentration to a depth of 10 cm and added 1.5 L of distilled water to submerge the soil. Setting the surface at 0 cm, we collected samples of soil pore water at intervals of 2.5 cm to monitor phosphate concentrations. As a result, the phosphate concentrations at different depths reached equilibrium after 6 h. The measured concentra  
Figure 5: Phosphate elution rate and concentration in water by rpm during a 6 h experiment: (**a**) only P-Soil; (**b**) P-Soil with SMAC. In the experiment in (**a**), the stirring speed was set to 0, 40, and 80 rpm, and in (**b**), it was set to 0, 20, 40, 60, and 80 rpm. tion was approximate, which was established as the initial concentration. Subsequently, we covered the soil with 400 g of SMAC and added an additional 0.5 L of distilled water to initiate leaching immediately. Figure 6a,b represents conditions without and with hydraulic flow, respectively, where the results of (b) underwent stirring at the water surface at 100 rpm using a blade stirrer. Based on the results in Figure 6a,b, the reduction in phosphate through hydraulic stress showed an exponential decrease with soil depth. This phenomenon is known as hyperoleic exchange, demonstrated in the experimental study by [PERSON] et al. [41]. Hyporhei exchange refers to the circulation of water and solutes between surface water and sediment pore water. When surface water flows over the sediment bed, it creates pressure gradients and turbulence at the waterbed. These forces push water into the  
Figure 6: Phosphate elution rate at soil depth: (**a**) non-hydraulic condition; (**b**) hydraulic condition (100 rpm); (**c**) phosphate concentrations in water bodies (**a**,**b**). (**a**,**b**) were measured for two weeks, and (**c**) was measured within 24 h.  
sediment's pore spaces where the infiltrated water later returns to the water column, forming a continuous circulation loop known as hyporheic exchange. Its intensity and depth are largely governed by flow conditions and sediment characteristics. [PERSON] et al. demonstrated that stronger hydraulic flow increases the bed shear velocity, enhancing advective mixing and deepening the active exchange layer within the sediment. The results from the experiments also showed that the pore water concentration would decrease exponentially with the soil depth. Additionally, it was proven that sediments with larger particle diameters exhibited higher permeability, which facilitated faster and deeper solute transport. In our experiments, P-Soil had a \(D_{60}\) of 0.250 mm, exhibiting active hyperheic exchange under bed shear velocities ranging from approximately 0.01 to 0.04 m/s, which were well within the experimental parameters conducted in the research of [PERSON] et al., showing similar results. Additionally, since the SMAC particles applied to the surface had a much larger grain size (0.5-4 mm), the SMAC are unlikely to have affected the pore-scale water flow within the finer underlying soil. Thus, the presence of hydraulic flow appears to have played the dominant role in enhancing solute transport within the soil matrix. This provides experimental support for Figure 6, where the conditions of Figure 6b, with hydraulic flow, showed a significantly faster decrease in phosphate concentration than the results in Figure 6a. Figure 6c shows the phosphate concentration in the water body during the experiments conducted in Figure 6a,b. The results indicate a minimal amount of phosphate leaching from the P-Soil regardless of the hydraulic stress. This implies that the sorption kinetics of SMAC are sufficient in capturing phosphate before it escapes into the water column.