*2.5. Column Experiments*

In order to simulate the performance of the filter additives under an actual working situation, column experiments were carried out in the laboratory. The river sand (washed by DW and dried) and filter additives (unwashed) were mixed evenly according to a mass ratio of 10/1. In 3 PVC columns (30 cm in height, 6 cm in diameter and 3 mm holes of sieve tray at the bottom), filter papers were placed at the bottom and evenly mixed geomedia filled the columns, denoted as PCB-Column, HB-Column and Sand-Column. Sand-Column was filled only with river sand as a control group. For each 2 cm of mixed filling, a wooden hammer was employed to drop 10 times from 5 cm above the filling until it was filled to 24 cm. Washed and dried gravels were placed on top of the mixture filling to prevent current scour. After filling, peristaltic pumps (BT01-100) were used at the top of the columns to pump DW for 3 h at a speed of 15 mL/min. The inflow velocity was calculated according to the rainfall intensity formula in China. In this study, rainfall occurred once a year and lasted for 3 h, the catchment ratio was 15 and the infiltration flow per minute was calculated as 15 mL. In total, 50 mL of effluent was collected every 20 min by the effluent tubing at the bottom in order to detect the contents of PO4-P. The detection methods were the same as above. AS was pumped at the same rate for 3 h after the DW was pumped in for 3 h. AS included 3 mg/L PO4-P. Effluent was collected and measured as above.

## **3. Results and Discussion**

#### *3.1. Polymerization Process and Microstructure*

The Polyurethane-biochar crosslinked material (PCB) was obtained via a one-shoot method. The reaction of PCB and the interaction with the addition of HB are shown in Figure 1. Glycol contained a hydroxyl group that could reacted with an isocyanate group from MDI to obtain a urethane linkage and a monomeric unit was formed by the two constitutional units. Furthermore, the polymer chain was linked through urethane linkages between monomeric units. HB was crosslinked between two monomeric units [22].

**Figure 1.** The reaction of polyurethane-biochar crosslinked material (PCB) via the one–shoot method with the addition of HB.

The properties of PCB depend on various factors (chain rigidity, cross-linking degree, intermolecular bonds, etc.) and can be changed in a wide range by the proper selection of raw materials [35]. Considering the application of PCB in bioretention facilities, durability, resilience, porousness and hydrophilia were demanded by the multi-field (water-soil-air) coupling effects. Glycol was chosen as the main polyol source that would reduce the length of carbon chain and improve the hard segment content. The hard segment can improve the initial modulus and tensile strengths of polyurethane materials [36] and crosslinked polymerization would make up for the brittleness of HB to make it resilient against weathering. HB, as an inner material, was blocked in the network of polyurethane by the interaction between carbonyl groups and HB. The blocking way was referred from the FTIR analysis (Figure 2): The interaction between the HB and polyurethane was observed by the shifted wavenumber of carbonyl groups at around 1600 cm−1. As a carbon-rich material, HB possessed abundant hydroxyl groups [37] and was more likely to have this interaction [38].

**Figure 2.** Fourier transform infrared (FTIR) spectra of PCB and hardwood biochar (HB).

The polyol source also influenced the microstructure of polyurethane composites [39]. Small molecular weight polyol has a better smoothness and porousness. As shown in Figure 3, the concave-convex surfaces and throats of PCB can be clearly seen, yet HB has a flat shape and few holes and bumps. This difference in structure may account for the improvements in the hydraulic properties of PCB and HB.

(**a**) (**b**)

**Figure 3.** The scanning electron microscopy (SEM) images of (**a**) PCB and (**b**) HB.
