Thermochemical Conversion of Food Waste into Biochar/Hydrochar for Soil Amendment: A Review

Current agriculture faces the challenge of producing sufficient food from diminishing land resources, due to deteriorating soil quality and accelerated population growth. Numerous studies have demonstrated that biochar/hydrochar can serve as efficient soil amendments by improving soil fertility and enhancing crop productivity. Various food wastes are promising raw materials for biochar/ hydrochar production due to their abundant organic matter. Recently, thermochemical techniques such as pyrolysis, hydrothermal carbonization (HTC), and microwave-assisted pyrolysis (MAP) have been widely proposed for converting food waste into biochar/hydrochar for soil amendment. However, the composition of food waste is complex and the parameters for its thermal treatment are highly variable, leading to uncertainties in the performance of the derived biochar/hydrochar for soil applications. This study aims to establish a structure–activity relationship linking food waste carbonization technology, the properties of the obtained biochar/hydrochar, and its functions as a soil amendment. Furthermore, the detailed mechanisms by which biochar improves plant growth or poses potential ecological risks to agricultural land are discussed. This review is intended to provide a guideline for the large-scale application of food waste-derived char for soil amendment.