Nitrogen and Phosphorus Leaching Characteristics and Adsorption Properties of Hardwood Biochar in Stormwater Runoff

Abstract

At present, the fillers used in bioremanent systems are limited by problems such as unstable leaching and adsorption of nitrogen and phosphorus. To assess the feasibility of using hardwood biochar as a filter filler material in bioretention systems, it was compared to traditional filter filler materials, namely coconut chaff, compost, ceramsite, and volcanic stone. Experiments and tests were conducted to study the physico-chemical properties, leaching, and adsorption characteristics of hardwood biochar, and to explore the optimizing effect and mechanism of hardwood biochar in bioretention systems. Hardwood biochar created through high temperature pyrolysis was found to be porous and loose, with a saturated moisture content of 195.65% and good water retention capacity. After pyrolysis, the nitrogen and phosphorus elements on the hardwood biochar surface were converted into stable compounds. In a batch leaching experiment, the quantity of leached nitrogen was low, and the leaching speed was fast; phosphorus leaching was slow, although the linear negative value increased during artificial rainwater runoff leaching, and the adsorption effect was stable. When exposed to typical nitrogen and phosphorous concentrations in rainwater runoff (2 mg·L-1 of NH4+ and 2 mg·L-1 of PO43-), hardwood biochar adsorbed 34.6 mg·kg-1 NH4+ and 59.5 mg·kg-1 PO43-, showing outstanding comprehensive adsorption capacity. After reaching adsorption equilibrium, the average desorption rates of the hardwood biochar in deionized water were 21.23% and 17.43%, proving that the adsorption effect was stable. In conclusion, the application of hardwood biochar can mitigate excessive leaching of nutrients from fillers, as it has a better adsorption capacity for nitrogen and phosphorus. Therefore, hardwood biochar can be used as the filler material in bioretention systems, to reduce pollution associated with rainwater runoff.