A meta‐analysis on the impacts of different oxidation methods on the surface area properties of biochar

Abstract Biochar has a beneficial impact on agricultural prosperity, according to numerous studies, and the key dispute presently is how to maximize that impact. Biochar oxidation is well‐known as one of the accepted methods for increasing biochar efficiency. However, due to contradictory data presented in studies and because biochar is derived from diverse oxidation procedures, as well as, the lack of an analytical comparison between different methods, oxidation data sets require a thorough assessment that can be covered by meta‐analysis. In this study, the effects of the five most common oxidation methods in the literature, namely oxidation by acids, alkaline, metal oxides, physical, and natural oxidation, on the cation exchange capacity (CEC), micro‐pores (MP), specific surface area (SSA), and oxygen‐content functional groups (OFGs) of biochar, were meta‐analyzed. When pyrolysis conditions varied, the efficiency of various operations was also addressed. The most efficient method was acidic oxidation, which increased CEC, SSA, MP, and OFGs by 46%, 43%, 55%, and 72%, respectively. Additionally, increasing the pyrolysis temperature above 550°C is critical for lowering OFGs, which has an impact on biochar sorption characteristics. According to the findings, biochar oxidation (post‐pyrolysis) is important since it creates more oxide ions on the surface area than feedstock oxidation (pre‐pyrolysis). Due to their high inorganic nutrient content, crop residues such as rice husk, maize stalk, and rapeseed stem, which were classed in the intermediate group in this study, are promising feedstocks for synthesizing biochars with high SSA, MP, and CEC. Overall, when compared to other approaches, the acidic process significantly improves the surface properties of biochar.