Mechanisms of Phenol Adsorption on Banana Leaves and Coffee Husk Biochars

In this study, biochars were produced from banana leaves (BB) and coffee husk (BC) for phenol adsorption. The biochars were characterized using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, textural analysis, point of zero charge measurement, and determination of surface acidic and basic groups. For both biochars, a higher pyrolysis temperature led to losses of oxygenated groups as well as increases of graphitic structures and greater basic character. For biochars produced at 400 °C, phenol adsorption kinetics was best described by the pseudo-second-order model. Chemisorption involving π–π interactions was identified as the main adsorption mechanism. For biochars produced at 500 °C, a smaller pore size resulted in limited adsorption by intraparticle diffusion. The Freundlich model provided the best fit to the isotherm data due to the high surface heterogeneity. Moreover, the results also suggested the formation of multilayers or pore filling as adsorption mechanisms for the obtained biochars. The maximum adsorption capacity values (q_e) were 13.8 and 21.2 mg g^–1 for phenol adsorption on BB400 and BB500, and 17.3 and 19.1 mg g^–1 for BC400 and BC500, respectively. The results showed that the agroindustrial residues are suitable for phenol adsorption in aqueous solutions.