Enhanced adsorption capacity of tetracycline on tea waste biochar with KHCO3 activation from aqueous solution

Activation is an important pathway that can enhance the adsorption capacity of biochar. In this study, a modified tea waste biochar (MTWBC) was prepared via a two-step pyrolysis approach with KHCO₃ activation. Pristine tea waste biochar (TWBC) was also produced as control via one-step pyrolysis without activation. Various characterizations were undertaken to investigate the influence of modification on the morphology, composition, carbon structure, surface area, and functional group of biochar, including scanning electron microscope (SEM), surface area and pore analyzer, element analysis, point of zero charge (pHPZC), X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). After KHCO₃ activation treatment, the surface area, total pore volume, and micropore volume of MTWBC reached 1981 m²·g⁻¹, 0.8547 cm³·g⁻¹, and 0.6439 cm³·g⁻¹ which were 7.34-fold, 7.27-fold, and 7.30-fold increases, respectively, compared with TWBC. The aromaticity, hydrophilicity, and polarity of the MTWBC increased after modification. More graphitization with less defective structures occurred in MTWBC after modification. The C-, O-, and N-containing groups in MTWBC also changed after the reaction of KHCO₃. The pseudo-second-order and Freundlich models best described the adsorption process on biochar. The maximum adsorption capacity of tetracycline (TC) on MTWBC reached 293.46 mg·g⁻¹, which was 15-fold more than that of TWBC (19.68 mg·g⁻¹). An alkaline environment decreased the TC adsorption on biochars. The presence of Na⁺, K⁺, Ca²⁺, and Mg²⁺ inhibited TC adsorption onto biochars. The influence of Cu²⁺ on TC adsorption by biochars depends on its initial concentration. The enhanced adsorption capacity of TC on MTWBC was mainly attributable to the large surface area, the improved pore volume, and more aromatic structure. The adsorption mechanism was based on pore filling and π-π EDA interaction. Therefore, KHCO₃ activated biochar has the potential to remove TC from aquatic environments.