# Application of Iron-Modified Activated Carbon for Phosphate Removal in Aqueous Systems
## 3 Results and Discussion
### Prevention of Phosphate Release by Soil  
We evaluated the effectiveness of SMAC in preventing the leaching of phosphates from sediments saturated with phosphorous, and the results are shown in Figure 4. For this purpose, we compared three samples: the natural state of P-Soil (soil only) and the treatments where AC and SMAC were applied to the surface. This experiment confirmed the effectiveness of SMAC in preventing the leaching of phosphates through adsorption. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & 0.00 & 0.05 & 0.1 & 0.2 & 0.3 & 0.4 & 0.5 \\ \hline K & 0.1060 & 0.0496 & 0.0643 & 0.1096 & 0.1293 & 0.1365 & 0.1407 \\ n & 1.6598 & 1.7876 & 1.7425 & 1.9001 & 1.8488 & 1.8522 & 1.8517 \\ R\({}^{2}\) & 0.9053 & 0.9783 & 0.9565 & 0.7778 & 0.9194 & 0.9100 & 0.9121 \\ \hline \end{tabular}
\end{table} 
Table 2: Freundlich Constants. Figure 3: SEM-EDS elemental mapping images: (**a**) AC; (**b**) 0.2 M FeCl\({}_{3}\) of SMAC; (**c**) 0.5 M FeCl\({}_{3}\) of SMAC. Its analysis was performed at an accelerating voltage of 20 kV, a magnification of 5000\(\times\), and a working distance of 15 mm. The samples were coated with a thin platinum layer (\(\sim\)5 nm). Elemental mapping was conducted over a selected surface area of 20 \(\upmu\)m\({}^{2}\) to confirm the presence and distribution of iron on the SMAC surface. Iron was visualized as orange-colored dots overlaid on the SEM images. To prepare P-Soil for the experiment, the soil from the Chudong water area was immersed in a solution of concentration for 72 h until it reached equilibrium. We measured the concentration using absorbance every 24 h and found that equilibrium was reached around the 24 h mark. To ensure stability, we set the immersion duration to 48 h. As a result, the phosphate concentrations in the samples were 2100 mg/kg, 1933 mg/kg, and 2033 mg/kg for soil only, AC, and SMAC, in that order. The sediment measurement results from Site 1 of the sediment measurement network in the Chudong area of Daecheong Lake indicated phosphate concentrations ranging from 1000 to 1000-1500 mg/kg throughout the year, which is relatively high. In addition, the leaching of phosphates during the experiment was conducted over a period of 72 h.  
The results indicate minimal difference in phosphate leaching between the soil only and AC samples, with a small margin of 2.52%. Meanwhile, the SMAC sample exhibited a significantly reduced leaching amount, 4 to 5 times lower than the other samples. Phosphate leaching levels were 121 mg and 126 mg for soil only and AC, with leaching rates of 19.21% and 21.73%, respectively. In contrast, the SMAC sample recorded a leaching amount of 29 mg and a leaching rate of 4.75%. These results suggest that the SMAC layer effectively captures phosphate during its leaching process from soil to water, thereby reducing phosphate concentration in the water. This experiment demonstrates the significant effectiveness of SMAC in preventing internal phosphate loading from sediment in lake environments. Considering the typical phosphorus concentration in lake water, SMAC may also help prevent external phosphorus loading from accumulating in the sediment. To assess the effectiveness of SMAC across various soil environments, we conducted experiments simulating not only stagnant conditions but also environments with hydraulic flow. The results are shown in Figure 5. For this experiment, we used a blade stirrer to measure the leached phosphate over a 6 h period, preparing the P-Soil at a concentration of 2000 mgPO\({}_{4}\)/kg. Experiments in Figure 5a were conducted with P-Soil alone, whereas samples in Figure 5b were conducted with SMAC as a separating layer between P-Soil and the bulk water. Figure 4: Phosphate elution rate over time: “Soil Only”, “AC”, and “SMAC” indicate “P-Soil only”, “P-Soil + AC”, and “P-Soil + SMAC”, respectively. All samples contained 0.6 kg of P-Soil, and 30 g of AC or SMAC was applied to the AC and SMAC samples. They were conducted under static conditions (0 rpm) for 72 h to evaluate phosphate leaching behavior.