Enhanced hydrologic performance and nutrient removal of bioretention systems modified with polyurethane-biochar...

Bioretention systems are widely used for stormwater management due to their ability to retain and purify runoff; however, their nutrient removal performance, particularly for nitrate (NO₃^--N), remains inconsistent. To address this limitation, this study proposed an integrated approach that incorporated a novel polyurethane-biochar crosslinked material (PCB) into the filler soil, along with a zero-valent iron (ZVI) amended internal storage layer. PCB is a porous polymer sponge that has been demonstrated in previous studies to possess a high water holding capacity and strong ion exchange ability. Three rounds of 12-h infiltration experiments, involving injections of synthetic runoff with high and typical nutrient concentrations, as well as a bromide tracer, were conducted on two parallel pilot-scale bioretention cells (Control Cell and PCB Cell, 30 × 30 × 70 cm³, both without vegetation) to evaluate their hydrologic performance and nutrient removal. The filler soil in both cells consisted of a mixture of river sand, clay, and wood chips (88:6:2), with the PCB Cell amended with 4 % PCB. Compared to the Control Cell, PCB Cell increased the permeability coefficient by 1.46 times (from 4.52-3.59 × 10^-3 cm/s to 5.53-6.61 × 10^-3 cm/s), water holding capacity by 1.7 times (from 6.56 ± 0.10 L to 11.21 ± 0.30 L), and residence time based on bromide tracer concentration changes (from 6.56 h to 7.00 h). The PCB Cell consistently demonstrated higher nutrient removals: 91.18 ± 1.52 % for NO₃^--N, 64.83 ± 7.46 % for ammonium, 53.47 ± 12.06 % for total nitrogen, and 98.07 ± 0.37 % for phosphates. The corresponding removals in the Control Cell were 69.59 ± 3.48 %, 48.83 ± 8.37 %, 20.12 ± 19.07 %, and 86.52 ± 8.76 %, respectively. PCB-enhanced denitrification was achieved by creating microscale anoxic zones and increasing dissolved organic carbon. ZVI further promoted autotrophic denitrification and phosphate adsorption. The redox potential and dissolved oxygen content also indicated more favorable denitrification conditions with the addition of PCB and ZVI. These results demonstrate that the integrated approach, combining PCB and ZVI, can enhance the hydrologic performance and nutrient removal of bioretention systems. However, potential iron leaching could pose secondary environmental risks. Additionally, long-term performance under varying climatic conditions, material durability, and potential clogging risks remain to be evaluated for broader implementation.