*3.3. Phosphorus Leaching*

Before application in bioretention facilities, the quantities of phosphorus that could leach from additive materials should be estimated to prevent potential eutrophication. PO4-P and TP-P were detected in the successive leaching solution and the results are shown in Table 2 and Figure 4.

**Table 2.** Phosphorus leaching quantities of PCB and HB in deionized water (DW) or artificial stormwater (AS).


<sup>1</sup> The negative value came from the original concentration of AS solution (3 mg/L PO4-P), which was subtracted in a calculation and indicated a decrease in AS.

**Figure 4.** Cumulative phosphorus compounds leached from PCB and HB in DW or AS. (**a**) PO4-P; (**b**) TP-P.

In general, PCB released more than HB in the first DW batching round: 1.47 μmol/g of PO4-P and 4.09 μmol/g of TP-P for PCB, while 0.19 μmol/g of PO4-P and 0.27 μmol/g were released for HB. After the first batch, the leaching quantities of phosphorus were in decline in each round and PCB released 2.68 μmol/g of PO4-P and 9.16 μmol/g of TP-P in total, while these values were 7.11 and 8.55 for HB, respectively. Compared to the compost (around 82 μmol/g of PO4-P in 6 rounds of leaching) [11] and poultry litter biochar (82.6–146.1 μmol/g of PO4-P in 10-days leaching) [15] in other studies, the phosphorus leaching quantities of PCB and HB were relatively low.

The inhibition effects of crosslinked polyurethane on the phosphorus leaching of HB can be observed from the different leaching tendencies of the two materials: The leaching quantities of HB increased as the number of rounds increased and it continued to release more and more PO4-P and TP-P at a nearly constant rate. The leaching tendency of HB was in agreement with former research on biochar leaching properties [44] and it can be predicted that additional phosphorus would be released with further batching. However, after being crosslinked and interpenetrated by the polyurethane, the release of phosphorus from the internal biochar was prevented, as previously reported [27]. PCB's first round of phosphorus leaching accounted for 44.65–54.85% of the total released quantities and the released quantities were only in the range of 2.67–3.16% for HB. The resilient and smooth network of polyurethane could resist scour caused by water and prevent itself

from weathering. Polymerization made the HB and polyurethane blend seamlessly, which avoided phosphorus on the surface of HB being washed away by waterpower. The reason for PCB releasing more TP-P than HB could be that the surface of PCB was brushed with organophosphorus flame retardants to meet the storage and transportation conditions [45].

Since the AS contained 3 mg/L of PO4-P, it was subtracted when calculating the cumulative phosphorus compounds leached from PCB and HB in AS, so the data present negative values. Our assumption from the negative values was that PCB and HB had a certain adsorption capacity to the 3 mg/L of PO4-P in AS. HB had a better treatment effect on phosphorus, whose adsorption capacity was unimpeded by batching rounds. It is likely that the alkalinity of HB (Table 1) brought this benefit, which could provide an alkaline condition to form hydroxyapatite precipitation with Ca2+ and PO4 3- in stormwater runoff [46]. Meanwhile, PCB had a relatively poor treatment performance on phosphorus due to its acidity in the water. It is believed that PCB also had a slight effect on phosphorus adsorption, reducing the leaching quantities of PO4-P and TP-P in AS. The mechanism of PCB phosphorus adsorption could be ion exchange or physical adsorption but this is inconclusive.
