Comprehensive understanding of tetracycline hydrochloride adsorption mechanism onto biochar-based gel pellets based on the combination of characterization-based and approximate site energy distribution methods

The application of sludge-based biochar and its derivatives to remove antibiotics from aquatic media has long been recognized as an environmentally-friendly approach since it delivers the scenario of waste treatment by wastes. Despite the existence of various proposed adsorption mechanisms, a comprehensive analysis that combines macroscopic and microscopic perspectives remains insufficient. Herein, surface modification (Fe3O4) and spherization (chitosan) were employed to improve the adsorption capacity of sludge-based biochar toward tetracycline hydrochloride (TC). The as-prepared gel pellets (FeB-11) exhibited high adsorption capacity toward TC in aqueous with maximum uptake of 206.98 mg⋅g−1 under 298K. The combination of characterization-based and approximate site energy distribution (ASED) methods concluded that surface complexation played the primary role in enhancing the adsorption capacity of FeB-11 for TC, while space filling, hydrogen bonding, π-π interaction, electrostatic interaction and ion exchange also contributed to the adsorption process. The introduction of chitosan and Fe3O4 weakened physisorption but strengthened chemisorption, and Fe3O4 was proposed as the primary promoter for TC adsorption due to the better affinity between TC molecules and FeB-11 which Fe3O4 brought. Concentrated and high distribution frequency of adsorption sites with high adsorptive energy were the primary surface property responsible for the remarkable TC adsorption capacity exhibited by FeB-11 in the macroscopic perspective. These findings underscored the importance of combining macroscopic and microscopic perspectives to understand the prioritization among various adsorption mechanisms, other than solely proposing various mechanisms, which was useful to help design efficient adsorbents.