**1. Introduction**

Phosphorus is the main factor of eutrophication in urban rivers [1] and comes from industry, agriculture and transportation activities [2], being mainly spread by urban stormwater runoff, a kind of non-point pollution of surface water. To manage stormwater runoff, developers typically use bioretention facilities, whose primary goal is to reduce floods by reducing the volume of overland flow during a storm event and reinstating natural stormwater infiltration in the developed area to its pre-developmental capacity [3]. The filtration layer in bioretention facilities takes on the role of purification, which has been proven to be efficient in removing oil [4], heavy metal [5] and pathogenic bacteria indicator species [6] from stormwater runoff. However, the performance of traditional filtration layer is not effective at removing phosphorus [7], mainly due to the leaching of phosphorus from compost (a typical filter additive in the filtration layer) [8].

Attempts have been made to improve phosphorus removal. In recent studies, many natural and artificial materials have been investigated to determine their feasibility as filter additives in the filtration layer of bioretention facilities and they can be generally divided into three types: biological waste materials, mineral materials and biochar. Biological waste materials (e.g., coconut [9], peat [10] and livestock manure [11], etc.) still have high leaching quantities of phosphorus, due to the accumulation of a large number of nitrogen

**Citation:** Meng, Y.; Wang, Y.; Wang, C. Phosphorus Release and Adsorption Properties of Polyurethane–Biochar Crosslinked Material as a Filter Additive in Bioretention Systems. *Polymers* **2021**, *13*, 283. https://doi.org/10.3390/ polym13020283

Received: 11 December 2020 Accepted: 13 January 2021 Published: 17 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and phosphorus nutrients in the growth process, leading to the limitations of phosphorus removal. Mineral filter additives (e.g., volcanic stone [12], montmorillonite [13] and zeolite [14], etc.) have relatively low removal rates and water retention capacities compared to biological waste materials, in spite of their reduced nutrient-leaching quantities. Biochar, a thermal decomposition product of biomass, is suitable as a filter additive in bioretention facilities due to its cleanness [15] and it can reduce the concentration of both nitrogen and phosphorus in runoff [16]. As a popular soil amendment, biochar can also sequester carbon and retain nutrients [17] and this is significant for additive materials to support the growth of plants in the vegetation layer of bioretention facilities. However, pyrolysis brings brittleness to the pore structure of the biochar, which is destroyed by the hydropower of stormwater during long term operation [18], while the saturated hydraulic conductivity of bioretention facilities is significantly reduced [19]. This does not meet the primary goal of bioretention facilities. If this shortcoming of biochar can be improved, it would be a big step forward for bioretention systems.

Polyurethane materials provide a potentially feasible solution to this problem. On the one hand, the water retention capacity of polyurethane improved under multi-field coupling, due to the broken molecular chain and higher connectivity of the pore structure [20]. When subjected to soil, water and air, the structure of polyurethane showed no significant changes, indicating the good durability of its mechanical properties in the long term [21]. On the other hand, it has been widely recognized that polyurethane foams can be employed as highly efficient adsorbents in removing heavy metals [22], ammonium [23], nitrate [24] and some organic pollutants (e.g., dialkyl phthalates [25], oils and trichloromethane [26], etc.). All these properties meet the requirements of filter additives in bioretention systems: a high hydraulic conductivity to reduce overland stormwater, a high retention volume to minimize peak flow, a good endurance to multi-field coupling effects and a high removal capacity of many contaminants from stormwater. However, due to the limitation of raw material composition, the nutrients needed for vegetation growth cannot be provided by polyurethane alone. Considering the characteristics of biochar, it seems that a combination of polyurethane and biochar may achieve acceptable results.

Additionally, polyurethane, as a coating material, will prolong the nutrient release period of inner fertilizers in agriculture and reduce their leaching quantities [27]. Combined with fertilizers, polyurethane composite material has a high potential to preserve moisture and fertility for the amelioration of desertification [28]. Moreover, this advantage could be applied to biochar in the form of a polyurethane–biochar composite material, helping to release phosphorus more slowly.

Some work has been done regarding polyurethane composites in order to relieve the eutrophication crisis in urban rivers caused by phosphorus—Sasidharan developed silver/silver oxide nanoparticles impregnating polyurethane foam with a 61.24% phosphate and this system was still effective in removing 20.58% of phosphate after 7 cycles of reuse [29]. Nie also conducted a column experiment to purify the septic tank effluent, which was mixed with soil and polyurethane and found that the column had a phosphorus removal rate of 96% [30]. While demonstrating above the positive results in the phosphorus removal of polyurethane composites, these studies are limited and do not estimate the phosphorus leaching quantities of polyurethane composites, nor do they try to apply them to bioretention facilities.

Based on the current research, we assume that, if polyurethane and biochar could be combined together as a composite material with both of their advantages, this composite may have an outstanding hydraulic and environmental performance as a filter additive in bioretention systems. Hence, the present study tried to explore the feasibility of a novel composite material, polyurethane-biochar crosslinked material (PCB), as a filter additive in bioretention systems. We estimated the water retention capacity, phosphorus leaching quantities and adsorption capacity of PCB and aimed to improving the performance of bioretention facilities and avoiding eutrophication. This composite material is a sponge structure in which polyurethane interpenetrates and crosslinks the biochar. Hardwood

biochar (HB) was selected as a raw material for the production of PCB because of its low nutrient concentrations [31] and high specific surface area [32] and it was also compared with PCB in this study.
