From waste to resource: circular valorization of spent mushroom substrate into biochar and advanced materials

Spent mushroom substrate (SMS), also known as spent mushroom compost, is the material that is left over after a mushroom crop has been harvested. They typically contain materials like straw, sawdust, or manure, and are rich in mycelium and leftover nutrients. SMS represents a reliable as well as sustainable lignocellulosic feedstock to produce biofuels, hydrochars and value-added products. Evidently, as an abundant waste in the agricultural field, SMS contains high levels of organic matter, making it a promising candidate for conversion into biofuels such as bioethanol, biogas and biochar by different thermochemical and biochemical processes. Hydrothermal carbonization can convert SMS into hydrochars, which can be then used for various applications including but not limited to carbon-based materials and adsorbents. In biofuel production, SMS has shown that it is valuable in fermentation into bioethanol with enzyme action. Due to its lignocellulosic nature, SMS can be further processed into durable materials with exceptional adsorptive properties, can be used in water treatment, pollution control, and soil remediation. Furthermore, the conversion of SMS into biofuels and materials offers a well-round solution for waste management, reducing environmental pollution, promoting circular economies, and supporting the development of renewable energy systems. This review examines the diverse uses of SMS, focusing on its potential as a biochar, biofuel and material precursor, and highlights the technological challenges and opportunities for optimizing the conversion processes.