*3.5. Phosphate Adsorption*

Isothermal adsorption experiments can help to estimate the adsorption capacity and properties of additive materials in a bioretention system. The phosphate adsorption results of PCB-DW and HB-DW in DW and AS are shown in Figure 6. PCB-DW had a compelling advantage in phosphate adsorption to contrast to HB-DW: PCB-DW had a stronger equilibrium adsorption capacity, which was 1.32–1.58 times of that of HB-DW. The adsorption rates of PCB-DW were 70–98% under at different concentrations, while HB-DW could adsorb 44–74% of phosphate. PCB-DW and HB-DW adsorbed more in DW than in AS over the tested concentration range but with minimal growth. At a typical phosphate concentration range of 2–5 mg/L in stormwater runoff, the equilibrium adsorption was 93–206 mg/kg for PCB and 60–142 mg/kg for HB.

**Figure 6.** Adsorption isotherms of PCB-DW and HB-DW in DW or AS.

As confirmed by previous research, phosphate was bound to the biochar not only by electrostatic adsorption but also by covalent bonds, forming highly valent cationicphosphate crystals, including magnesium [48], iron, alum or calcium [46]. There are many factors and complex evolvement courses for PO4-P adsorption by polyurethane: the main mechanism of phosphate removal is adsorption, which occurs as a result of electrostatic attraction between two oppositely charged ions, where pH plays an important role, preferring to remain around 7 [29]. This explained why PCB (pH = 6.62) showed a poorer adsorption capacity in the leaching experiments but PCB-DW (pH = 6.98) did better in the isothermal adsorption experiments. Adsorption in AS was inferior to that in DW, which indicated that additional Ca2+ in AS could not promote the progress of precipitation, instead weakening it and the bivalent and multivalent cations leaching from themselves were adequate for removing phosphorus. In this study, the superiority of PCB-DW was the multiple factors, including the weak acidic conditions with a pH around 7, an abundant supply of bivalent and multivalent cations and a relatively high BET (Table 1). A thorough, quantitative analysis of the factors of phosphate adsorption is still required, however.

The results of the isothermal adsorption of PCB-DW and HB-DW were also fitted to two adsorption models, as shown in Table 5. The Freundlich model fitted the PO4- P adsorption data of the PCB-DW better, with *R*<sup>2</sup> > 0.99, while the Langmuir model was better for HB-DW. Ahmed also found that the Freundlich model had the best fit for the adsorption of nutrients onto polyurethane materials [23]. With the inverse of the characteristic constants (*1/n*) < 1 in Freundlich models and the Langmuir model coefficient *RL* being between 0 and 1, we can draw the conclusion that the adsorption of PO4-P occurred easily for both PCB-DW and HB-DW. The adsorption of PO4-P by PCB-DW and HB-DW was nonlinear. With the increase in the PO4-P concentration in the solution, its adsorption capacity gradually becomes saturated, which was also confirmed by the bending of the fitting curves in Figure 6. *KF* (the Freundlich model's volumetric-affinity parameter), to some extent, proved that, compared to HB-DW, PCB-DW had a better adsorption affinity for phosphate. The *qmax* in the Langmuir model reflected the potential maximum adsorption capacity of the materials and PCB-DW had higher *qmax* than HB-DW no matter in DW or AS. Therefore, PCB-DW can be used as an additive with high adsorption performance in stormwater treatment.


**Table 5.** Parameters for Freundlich and Langmuir isotherms of phosphate adsorption on PCB-DW and HB-DW.
