Abstract:
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.
