*3.4. Leaching of Other Ions*

The main commercial processes for removing phosphorus from wastewater effluents are still chemical precipitation with metal ions [46]. The leaching pattern of metal ions could help to elucidate the mechanisms of the removal of phosphorus by PCB and HB. The leaching of low-valence metal ions (Na+, K+, Mg2+, Ca2+) as a function of batching rounds is shown in Figure 5 and summarized in Table 3. Generally, PCB leached fewer or nearly equal numbers of metal ions than HB in DW. AS prompted the metal ion-leaching quantities of HB but had no obvious impact on PCB.

**Table 3.** Cations leaching quantities of PCB and HB in DW or AS.


Na<sup>+</sup> was leached at a very low level in DW. From the energy dispersive spectroscopy results (Table 4) of PCB and HB, there was no Na+ on the analyzed surface of PCB but a small quantity on the surface of HB (0.62%). Hence, the low quantities of Na<sup>+</sup> in DW were reasonable. PCB and HB had similar cumulative release amounts but their release rates and patterns were not consistent: PCB released 3.13 μmol/g Na+ in the first round, accounting for 73.05% of total release quantities and released less during batching rounds. While HB only released 1.10 μmol/g Na+ in the first round, after that it leached at an almost constant rate. Because of the valence being the same, the pattern of K+ leaching was similar to Na+: the cumulative Na+/K+ leaching quantities increased logarithmically, which was consistent with previous research on biochar leaching [47]. AS motivates more metal ions to leach out from HB in the first round but keeps an approximate leaching rate afterwards, as in DW. The leaching quantities of PCB were significantly lower than HB, probably because the K<sup>+</sup> on the surface of HB had been released in the glycol–DW mixing process, most of which was cured in the inner structure of PCB during polymerization process.

**Figure 5.** Cumulative low valence metal cations leached from PCB and HB in DW or AS. (**a**) Na+; (**b**) K+; (**c**) Mg2+; (**d**) Ca2+


**Table 4.** Energy dispersive spectroscopy (EDS) contents of original and DW-leached materials.

The Mg2+ was released from PCB and HB linearly and a nearly equal amount of Mg2+ was released in each batching round. EDS results (Table 4) showed that there were 0.74% Mg elements on the surface of HB before leaching which could not be detected after leaching. This indicated that Mg2+ adhered to the surface of HB in the form of mineral ions and was washed away. The Mg2+ content on the surface of PCB dropped slightly after 8 rounds batching and this suggested that it mostly existed as compounds or was blocked in the polyurethane network. In AS, total Mg2+-releasing quantities and speed increased compared with those of DW: the first-round leaching quantity of PCB in AS was close to

that of DW and the release rate of Mg2+ in AS increased to 1.22–1.67 times that of in DW in the later leaching process. This increase indicated that the removal of phosphorus was not due to the magnesium–phosphorus precipitation.

The Ca2+ leaching rates of PCB and HB seems to be constants whether in DW or AS and are unaffected by the batching rounds. HB leached significantly more Ca2+ than PCB, since EDS results showed that HB had a higher Ca2+ content than PCB on the surface. Interestingly, HB leached the same Ca2+ quantities in AS as in DW, yet with a higher concentration in the first round. The equality of the leaching quantities proved the previous assumption that the reduction of PO4-P was caused by formation of hydroxyapatite precipitation with Ca2+ and PO4-P in stormwater runoff. On the contrary, the cumulative Ca2+-releasing quantities of PCB were negative and underwent a steady decline, consistent with the tendency of phosphorus leaching but unbalanced in their quantities. This made the mechanism of PCB adsorption to decrease phosphorus concentration uncertain. This could be partly attributed to the calcium–phosphorus precipitation when considering the negative value of Ca2+ releasing quantities but other phosphorus-removing approaches coexisted.

Overall, the metal cations of HB, existing as salts on its surface, were easy to washed away, especially for K+ and Ca2+ and this was proved by the SEM and EDS results. After the modification of crosslinked polyurethane, the leaching quantities of metal cations were significantly reduced. Through the correspondence of the leaching quantities between phosphorus and metal ions, it is clear that the mechanism of phosphorus removal by HB is calcium–phosphorus precipitation. PCB has several phosphorus removing approaches, including metal salts precipitation, which requires further research.
