The impact of feedstock type and pyrolysis parameters on the physical and chemical properties of biochars for sorption, agricultural and carbon sequestration applications: A meta-analysis

Understanding the physiochemical properties of biochar is fundamental for matching biochar to its respective environmental use. We present updated values for key biochar physiochemical properties and relationships with pyrolysis parameters based on 832 articles, leading to a dataset that encompasses 5004 entries related to 29 unique biochar properties (53,700 experimental data points). Since feedstock composition causes the greatest variation in biochar properties, we classified into ten distinct feedstock types. This allows us to develop a unique understanding of how different feedstock types impact biochar properties and subsequent functions for different applications. Biochars from soft wood, hard wood, and nut shell feedstocks exhibited greater mean surface area, total carbon content, and fixed carbon content and lower mean pH, cation exchange capacity, ash content, nitrogen content, biochar yield, H/C ratio, and O/C ratio compared to other biochars. These are best suited for sorption applications or carbon sequestration given their high carbon stability and associated conversion of feedstock into stable carbon. In contrast, high-ash feedstock, primarily manures and sludges, produce more stable carbon per biomass (ash-free feedstock) input due to minerals within that catalyze and retain carbon during pyrolysis and hence increase conversion efficiency of biomass into stable carbon. This challenges the prevailing assumption that woody biomass is the most suitable feedstock for biochar-based carbon sequestration. Crop waste-derived biochar demonstrated the highest mean average pore diameter that was associated with the highest water holding capacity emphasizing suitability for agricultural soil applications to increase water retention. The cation exchange capacity was highest in biochar derived from aquatic biomass, which, however, could lead to salinity issues in soil due to its high electrical conductivity. This study emphasizes the highly variable properties of biochar and the need to match the biochar type to its intended application based on feedstock and pyrolysis conditions.