Research on the Adsorption Mechanism and Performance of Cotton Stalk-Based Biochar

In this research, we produced two types of biochar (BC) using cotton stalks as raw material and KOH as an activator, and compared their performance and adsorption mechanisms in the removal of tetracycline (TC) and methylene blue (MB) from wastewater. The results showed that the biochar generated using both procedures formed pores that connected to the interior of the biochar and had extensive microporous and mesoporous structures. The molten salt approach produces biochar with a higher specific surface area, larger pore size, and higher pore volume than the impregnation method, with a maximum specific surface area of 3095 m²/g. KBCM-900 (the BC produced using the molten salt method at 900 °C) had a better adsorption effect on TC, with a clearance rate of more than 95% in 180 min and a maximum adsorption amount of 912.212 mg/g. The adsorption rates of the two BCs for MB did not differ significantly at low concentrations, but as the concentration increased, KBCI-900 (the BC generated by the impregnation method at 900 °C) exhibited better adsorption, with a maximum adsorption of 723.726 mg/g. The pseudo-second-order kinetic model and the Langmuir isotherm model may accurately describe the TC and MB adsorption processes of KBCI-900 and KBCM-900. The KBCI/KBCM-900 adsorption process combines physical and chemical adsorption, with the primary mechanisms being pore filling, π-π interactions, hydrogen bonding, and electrostatic interactions. As a result, biochar generated using the molten salt method is suitable for the removal of large-molecule pollutants such as TC, whereas biochar prepared using the impregnation method is suitable for the removal of small-molecule dyes such as MB.