*3.6. Stormwater Infiltration Experiments*

In order to explore the actual operation effects after PCB and HB are added into bioretention facilities as additives, column tests were conducted, simulating the rainwater infiltration process and evaluating the suitability and feasibility of the materials. Two simulated rainfall events were carried out, each of which lasted for 3 h. DW was pumped into three columns at a rate of 15 mL/min in the first event. The PO4-P concentrations of outflows were detected every 20 min and the variations in the outflow concentrations are shown in Figure 7. The outflows from the PCB-Column had higher PO4-P concentrations compared with the HB-Column. With the increase in DW inflow volume, the effluent concentration decreases linearly, nearly to 0. Traces of PO4-P were detected in the outflow of the HB-Column, while the content rose slightly during the infiltration process. The

tendency of the concentration to grow in the HB-Column outflow was in accordance with the leaching patterns in the batching experiments.

**Figure 7.** Concentration of PO4 <sup>3</sup><sup>−</sup> in outflows from the three columns in the first simulated rainfall event.

The estimated and detected values of the PO4-P concentration and total release quantities from PCB-Column and HB-Column are listed in Table 6. The estimate was based on the assumption that the PCB (626.73 g) and HB (834.94 g) in the soil columns would release the same quantities of phosphorus as in the leaching tests. Comparisons in Table 6 illustrated the wide gaps between the estimated and the detected values. These gaps were caused by the differences in the contact ways and the hydrophilicity of materials, which could also be predictable. In column infiltration experiments, the DW inflow had a shorter contact time and a smaller contact surface with the additive PCB and HB and reduced the leaching quantities. Gupta drew a similar conclusion through the observation of heavy metal batch leaching experiments and column experiments [49]. HB was pyrolyzed at 600 °C and oxygenated functional groups on HB's surface made it possess a lower hydrophilicity [50]. The percentage of polyol used as a raw material in polyurethanes is positively associated with hydrophilicity [51] and the molar ratio of urethane linkage:polyol in PCB was 1:1. Combined with the water retention capacity of PCB, we could infer that PCB had a higher hydrophilicity. Additionally, the volume of PCB was double that of HB under same ratio in column experiments and PCB had more contact time and space in the stormwater. Hence, it made sense that there were more PO4-P leaching quantities in the PCB-Column than in the HB-Column.

**Table 6.** Comparison of predicted and detected concentrations and total release of PO4 3- in column experiments.


DW was replaced by AS in the second simulated rainfall event, while the other conditions remained. Considering the influence of the first rainfall event, the PO4-P

concentration of the outflow was detected after the infiltration for 1 h and the results are shown in Figure 8. The PO4-P concentration of the outflow from the PCB-Column and HB-Column remained stable at lower levels, while it increased sharply from the Sand-Column during the AS infiltration process. MeanPO4-P removal rates for the three columns during the AS flushing are also shown in Figure 8. Compared with the control group (Sand-Column), the experimental groups (PCB-Column and HB-Column) had a significantly higher PO4-P filtration capacity, with removal rates of 93.84% and 90.00%, respectively. This confirmed the feasibility and superiority of PCB as a filter additive in bioretention systems for removing PO4-P in stormwater treatments.

**Figure 8.** Concentration and mean removal rates of PO4 <sup>3</sup><sup>−</sup> in outflows from the three columns in the second simulated rainfall event. (**a**) The outflow concentrations of PO4 <sup>3</sup><sup>−</sup> from the three columns; (**b**) the mean removal rates of PO4-P from the three columns.

In addition, it should be noted that, in the effluent concentration detection, the concentration of PO4-P was kept at a low and stable state with a downward trend, which confirmed the influence of hydrophilicity on the adsorption effect of filter additives. With the infiltration and scouring of water inflow, oxygenated functional groups carried on the surface of HB were gradually washed away, so its hydrophilicity was improved to some extent. For polyurethane, its surface roughness changed during the infiltration process, which affected its adsorption capacity [52]. After the increase in hydrophilicity and surface roughness, the contact paths and time between stormwater inflow and filter materials increased, leading to the enhancement of adsorption. Although the improvement of hydrophilicity and surface roughness led to the enhancement of the adsorption capacity, the adsorption capacity tended to be saturated as the adsorption process continued. Therefore, the concentration of the outflow did not decrease significantly and it maintained a relatively stable trend in later stages.
