Enhancing biochar production: A technical analysis of the combined influence of chemical activation (KOH and NaOH) and pyrolysis atmospheres (N₂/CO₂) on yields and properties of rice husk-derived biochar

The production of biochar from biomass has received considerable interest due to its potential in environmental applications; however, optimizing biochar properties remains a major challenge. The objective of the present study was to investigate the synergistic effects of pyrolysis atmospheres (N₂ and CO₂) and chemical activation (pre- and post-pyrolysis) with NaOH and KOH on the properties of biochar useful for its environmental applications. In this study rice husk and biochar were impregnated with KOH and NaOH before and after pyrolysis, which was carried out at 600 °C under N₂ and CO₂ atmosphere. The pyrolytic yields (biochar, liquid and gas) and detailed characterization of biochar were performed. The results showed that pre-activation with both alkalis under a CO₂ atmosphere slightly decreased the biochar yield and carbon contents while increasing oxygen in biochars compared to N₂ atmosphere. Alkali pre-activation in the CO₂ atmosphere considerably increased the specific surface area and pore volume of biochars compared to the N₂ atmosphere, with KOH being more effective than NaOH. The maximum specific surface area (SSA) and pore volume (PV) of biochar obtained were 178.4 m²/g and 0.60 cm³/g for KOH activated biochar under CO₂, which were 3.2 times and 30 times higher than the untreated biochar. The post-activation of biochars with both alkalis resulted in moderate improvements in textural properties. Overall, chemical activation under CO₂ pyrolysis facilitated a higher level of chemical activation reactions leading to increased formation of oxygen functional groups and contributed to enhanced SSA and PV of the biochar useful for adsorption.