Engineered tea-waste biochar for the removal of caffeine, a model compound in pharmaceuticals and personal care products (PPCPs), from aqueous media

This study aimed to synthesize engineered tea-waste biochar, pyrolyzed at 700 °C using steam activation (TWBC-SA) for caffeine (CFN) removal from aqueous media. The morphological features and available functional groups on the surface of biochar were characterized using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Adsorption batch experiments were carried out at various pH values (3–10), contact time (up to 24 h), and initial concentration of CFN (10–300 mg L⁻¹) using 1 g L⁻¹ of TWBC-SA at 25 °C. SEM images showed the distribution of well-developed pores on the surface of biochar. FTIR spectra revealed that the surface of TWBC-SA provided extra aromatic character, which was further confirmed by XPS analysis. pH-adsorption edge data showed a maximum adsorption capacity of 15.4 mg g⁻¹at pH 3.5. The experimental data were best-fitted to the non-linear Elovich kinetic model, demonstrating the contribution of chemical forces for adsorption of CFN onto the heterogeneous surface of TWBC-SA (initial rate of adsorption = 55.6 mg g⁻¹min⁻¹). Non-linear forms of Freundlich and Temkin isotherm models were fitted with the experimental data, describing favorability of chemical interactions between CFN and TWBC-SA. Finally, it is demonstrated that the adsorption of CFN by TWBC-SA is mainly governed by the chemisorption mechanism via electrostatic interactions and nucleophilic attraction. Thus, the engineered steam-activated tea-waste biochar has a high potential for adsorbing CFN from water.