Pleurotus pulmonarius Pretreated Lignocellulosic Feedstock as a Sustainable Precursor to Produce Biochar for Efficient Removal of NO₃–

Spent mushroom substrate (SMS) has traditionally been discarded as waste or combusted directly; this suggests a great potential for the integrated production of edible mushroom and biochar, provided that the SMS could be a promising carbon precursor to prepare high-quality porous biochar. Biological pretreatment using summer oyster mushroom (Pleurotus pulmonarius) was performed on birch and wheat straw; biochar was then prepared via pyrolysis at different temperatures (500 and 700 °C). Compared with birch, wheat straw-based substrate resulted in a comparable fruit body quality but slower fructification and a lower yield. Biological pretreatment formed pore structures and led to a 60% degradation of xylan, together with a good preservation of lignin and glucan in wheat straw-based SMS, facilitating surface area development during pyrolysis. Mushroom substrate preparation and cultivation resulted in a considerable accumulation of ash in SMS, which contributed to a negative correlation between the pyrolysis temperature and biochar microporosity. After modification with cetylpyridinium chloride, the biochar derived from wheat straw-based SMS with pyrolysis temperature of 500°C (C-WS-SMS-500) had a NO3– adsorption capacity of 0.37 mg/g biochar, which corresponds to a two-fold increase compared with that derived from raw wheat straw (C-WS-R-500).