Enhanced abundance of oxygen-containing intermediates from pre-oxidation of poplar sawdust facilitates generation of porous structures in activation with phosphoric acid

Pretreatment of biomass in oxidative atmosphere could induce partial oxidation of organic components on surface of biochar, which might affect evolution of pore structures in subsequent activation of biochar. This was investigated herein by pretreatment of sawdust at 150 to 300 °C in an air or nitrogen flow and the followed activation of the resulting biochar with phosphoric acid. The results indicated that the oxidation in the pretreatment in air became substantial at 250 °C, reducing the yield of biochar, crystallinity of cellulose while introducing abundant oxygen-containing functionalities in form of OH, COC, CO, etc. Some of these oxygen-containing species even can be reserved in the activated carbon (AC) formed, making the oxygen-rich AC with high hydrophilicity. More importantly, these abundant oxygen-containing functionalities were beneficial for the subsequent activation with phosphoric acid for creating developed pore structures (1314.3 m2g−1 of AC-250-air versus 1056.5 m2g−1 of AC-250-N2). This resulted in superior adsorption capacity towards tetracycline over AC-250-air. The in-situ IR characterization suggested that abundant oxygen-containing species bearing ester/carboxyl functionalities were generated during activation of the sawdust pretreated in air, which played essential roles in generation of especially micropores. The deficiency of these oxygen-containing species in activation of the sawdust pretreated in nitrogen negatively affected development of pores.