Objectives The aim was to overcome the problems such as the reduction of functional groups and the destruction of pore structure caused by high-temperature pyrolysis of biochar. This study adopted physical, chemical and composite modification methods to improve the properties of biochar and evaluate the quality.

Methods A total of 32 modified biochars were synthesized using 8 distinct methods (HCl, HNO₃, NaOH, KOH, ultrasound, microwave, HCl + ultrasound, NaOH + microwave), utilizing crop residues (corn stover and peanut hulls) and livestock manure (swine and cow manure) as raw materials. The specific surface area, average pore diameter, total pore volume, functional group content, ash content, cation exchange capacity, and primary nutrient content were quantified. A comprehensive evaluation of biochar quality was conducted through factor analysis.

Results Acid modification significantly increased the specific surface area and pore volume, of which HCl was more effective, with a maximum increase of 200.30%. Alkali modification significantly increased the cation exchange capacity, and the effect of KOH was most prominent, with an increase of 136.09%. The effect of chemical modification in reducing the ash was better than that of physical modification, with a maximum decrease of 29.69%. Acid modification significantly enhanced the adsorption peak of functional groups, and physical modification had little effect. All modifications enhanced the carbon content, with KOH modification being the most significant, increasing 24.67%. Acid modification increased the nitrogen content, with HNO₃ having a better effect, but significantly reduced the phosphorus and potassium content. Alkali modification, on the other hand, enhanced phosphorus (maximum + 21.97%) and potassium content (maximum + 466.15%). The results of comprehensive factor analysis showed that alkali modification > acid modification > compound modification > physical modification. The quality of KOH-modified corn stover biochar was better, with a composite score of 0.88; the quality of microwave modification of pig manure biochar was worse in the effect of physical modification, with a composite score of 0.12.The quality of biochar from crop waste was better than those biochars from livestock and animal manure, in which the biochar from corn stover performed the better.ConclusionsThe KOH-modified corn stover biochar had the highest composite score (0.88) and was the best quality modified biochar in this study.