**4. Conclusions**

Polyurethane-biochar crosslinked material (PCB) has been successfully manufactured using polyurethane and hardwood biochar (HB) in order to improve the hydraulic performance of bioretention facilities. This improvement was confirmed through a characteristic analysis of PCB using FTIR spectra, SEM images and hydraulic parameter tests. Biochar was crosslinked through urethane linkages in the polyurethane network. Saturated water content was doubled due to the hydrophilia and porousness of polyurethane. The internal throats, confirmed by the SEM images, increased the permeability coefficient of the filter additive by two orders of magnitude.

From the perspective of phosphorus release and adsorption, PCB is a feasible filter additive in bioretention facilities for stormwater treatment. The network of polyurethane

restrained the release of phosphorus from interpenetrated HB with a decreasing cumulative rate of phosphorus leaching and reduced the metal cation leaching quantities compared with HB. The superiority of the adsorption capacity of PCB should be emphasized: for the typical phosphate concentration of stormwater runoff, the equilibrium adsorption quantity of PCB is 93–206 mg/kg for phosphate, which is a result of various factors, including its suitable pH, cation supply and porousness. PCB has a high (93.84%) and stable phosphate removal rate in column experiments, owing to the hydrophilia and porousness of polyurethane.

Overall, the present study offers a feasible filter additive with modified hydraulic properties and environmentally friendly advantages for bioretention facilities to use in stormwater treatment. Changes in the properties via the adjustment of the formula and ratio in PCB preparation lead to a variation in pore size and the functional group should be examined in further research. Meanwhile, the effect of PCB on the removal of other pollutants in stormwater should be investigated in the future.

**Author Contributions:** Methodology, Y.M.; formal analysis, Y.M.; investigation, Y.M.; resources, Y.W.; data curation, Y.M.; writing—original draft preparation, Y.M.; writing—review and editing, Y.W. and C.W.; visualization, Y.M.; supervision, Y.W. and C.W.; project administration, Y.W. and C.W.; funding acquisition, Y.W. and C.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China, grant number U1765204 and 41772340.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in this article.

**Acknowledgments:** The authors would like to acknowledge the instrumentation and technical support of the State Key Laboratory of Hydrology Water Resources and Hydraulic, Hohai University.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

