Enhanced adsorption of bio-oil on activated biochar in slurry fuels and the adsorption selectivity

In this study, activated biochars were prepared by CO2 and KOH activations for enhanced bio-oil adsorption in the slurry system. The results showed that activated biochars show better adsorption capacities on bio-oil than unactivated biochars due to the improved pore structure, and the highest adsorption rate reached 5.49 g bio-oil/g biochar by CO2 activated biochar prepared at 900 °C. The biochars especially activated biochars showed the best selectivity over aromatic compounds in bio-oil, followed by the selectivity over acids and ketones, and relatively weak selectivity for alcohols, aldehydes and monosaccharides. The average molecular weights of adsorbed bio-oil samples were 476–578 g/mol, higher than the 436 g/mol of bio-oil, implying the selective adsorption over heavy components. Biochar with large specific surface area, high O content or highly aromatized structure can facilitate the adsorption of monophenols and fused rings through pore filling, hydrogen bonding, π-π interactions and hydrophobic effect. Biochars with high O content can also have better adsorption selectivity on the acids and ketones in bio-oil via hydrogen bonding and electrostatic force. In summary, the developed pore structure, abundant O-containing functional groups, and high graphitization degree of biochar are beneficial for bio-oil adsorption.