Tetracycline Removal from Water by Adsorption on Hydrochar and Hydrochar-Derived Activated Carbon: Performance, Mechanism, and Cost Calculation

The objective of this study was to investigate the adsorption performance and mechanisms of tetracycline (TC) on hydrochar and hydrochar-derived activated carbon. We also assessed the influence of the solution pH and ionic strength on the adsorption of these compounds and studied their removal by synthetic adsorbents. The maximum adsorption capacities of TC estimated by the Langmuir model in pH 5.5 solution at 25 °C were found to follow the order: ACZ1175 (257.28 mg/g) > MGH (207.11 mg/g) > WAC (197.52 mg/g) > MOPH (168.50 mg/g) > OPH (85.79 mg/g) > GH (75.47 mg/g). The pH value and ionic strength affected TC’s adsorption on the adsorbents. These results indicate that the electrostatic interaction plays a critical role in these adsorption processes. Moreover, adsorption kinetic curves and adsorption isotherms demonstrated that electrostatic interactions were not the only adsorption driving force. Except for electrostatic interactions, the main adsorption mechanisms involved hydrogen bonding and π-π interaction. In addition, the cost of oxidized hydrochar (USD 4.71/kg) is slightly higher than that of hydrochar-derived activated carbon (USD 3.47/kg). This production cost would be lower when it can be produced on a large scale. The outcomes of this study show that the modified-hydrochar and hydrochar-derived activated carbon had the potential for TC removal in wastewater.