## 3 Results and Discussion
### Study on the Adsorption Mechanism  
SEM was performed to further explore the micro-morphology of the media and its changes in the operations of the bioretention systems (Figure 13a-d). Zeolite has a rough surface, folded, and a compact micropore structure. The micropore structure of 1-5 \(\upmu\)m is distributed all over the media surface. It can provide an ideal environment for the growth of the biofilm and can improve the bio-load and ion exchange performance. Figure 12 are the surfaces of post-experiment zeolite, sponge iron, and limestone, respectively. Compared to the original zeolite (Figure 13a), the zeolite used after the experiment exhibited a rough surface and more crystals. Polyhedral crystal aggregates and phosphate  
Figure 12: The assessment results of filtered runoff water quality in different bioretention systems. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sampling**} & \multicolumn{2}{c}{**Control**} & \multicolumn{2}{c}{**DS**} & \multicolumn{2}{c}{**SZ**} \\ \cline{2-9}  & **NH\({}_{4}\)\({}^{*}\)-N** & **TN** & **TP** & **NH\({}_{4}\)\({}^{*}\)-N** & **TN** & **TP** & **NH\({}_{4}\)\({}^{*}\)-N** & **TN** & **TP** \\ \hline
60 & 0.1468 & 0.5768 & 0.2764 & 0.1342 & 0.5836 & 0.2822 & 0.1214 & 0.6192 & 0.2594 \\
90 & 0.1927 & 0.5635 & 0.2438 & 0.1802 & 0.4812 & 0.3386 & 0.1672 & 0.5755 & 0.2573 \\
120 & 0.1867 & 0.5605 & 0.2528 & 0.1543 & 0.5551 & 0.2907 & 0.2152 & 0.4825 & 0.3024 \\
150 & 0.2129 & 0.5311 & 0.2561 & 0.1694 & 0.5295 & 0.3011 & 0.2124 & 0.5215 & 0.2660 \\
180 & 0.2398 & 0.5179 & 0.2423 & 0.2000 & 0.5243 & 0.2757 & 0.2237 & 0.5268 & 0.2495 \\
210 & 0.2179 & 0.5104 & 0.2717 & 0.2038 & 0.5129 & 0.2833 & 0.1929 & 0.5421 & 0.2650 \\
240 & 0.2453 & 0.4549 & 0.2998 & 0.2324 & 0.4831 & 0.2844 & 0.2162 & 0.5031 & 0.2807 \\
300 & 0.2611 & 0.4626 & 0.2764 & 0.2281 & 0.4957 & 0.2762 & 0.2108 & 0.5091 & 0.2801 \\
360 & 0.2699 & 0.4548 & 0.2753 & 0.2584 & 0.4666 & 0.2750 & 0.1844 & 0.5198 & 0.2957 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The weights of the pollution factors in water samples. precipitates were formed on the surface of the media due to the adsorption of phosphorus. Previous studies have also reported the formation of precipitates on the surface of similar materials [30; 47]. EDS results showed that the main elements on the surface of limestone and sponge iron were calcium, iron, aluminum, and silicon (Figure 12). The EDS peak intensity of Ca, Al, and Fe in the media decreased after the rainfall test (Figure 12). Combined with the results of SEM, the formation of phosphorus peaks in the media after rainfall indicates that phosphate precipitation has occurred on the surface of the media, and the possible reactions are as follows in Equations (10)-(12):  
\[2 Ca\equiv OH+H_{2}PO_{4}^{-}\rightarrow(Ca)_{2}HPO_{4}+H_{2}O+OH^{-} \tag{10}\]  
\[2 Al\equiv OH+H_{2}PO_{4}^{-}\rightarrow(Al)_{2}HPO_{4}+H_{2}O+OH^{-} \tag{11}\]  
\[2 Fe\equiv OH+H_{2}PO_{4}^{-}\rightarrow(Fe)_{2}HPO_{4}+H_{2}O+OH^{-} \tag{12}\]  
Figure 13: The figure shows the SEM images for (**a**) the zeolite before tests; (**b**) the zeolite after tests; (**c**) the post-experiment sponge iron; and, (**d**) the post-experiment limestone. The EDS spectrum is also illustrated for the following samples: (**e**) pre-experiment zeolite; (**f**) post-experiment zeolite; (**g**) post-experiment sponge iron; and, (**h**) post-experiment limestone. The XRD patterns, as shown in Figure 14, indicate that both zeolite and limestone have regular crystal structures, before and after the simulated rainfall. The characteristic diffraction peaks did not change significantly, which suggests that the crystal structure of the zeolite and the limestone did not transform during rainwater purification. Although the results of the SEM and EDS suggest that phosphate precipitates are present on the surface of zeolite and limestone, the characteristic peaks for phosphate were not found by XRD.