Heating rate effect on the sorptive properties of biochars from NaOH-modified sugarcane bagasse residues

The modification of the operational parameters of pyrolysis, such as the heating rate (HR), to produce adsorbent biochar (BC) has been little explored, especially regarding materials obtained from premodified biomass. Consequently, some fundamental aspects in this field remain not understood, which can limit more targeted syntheses of these materials in environmental application. This study investigated the effect of HR on the pyrolysis of sugarcane bagasse premodified with NaOH. BC was produced in a muffle furnace with HR of 5, 10, 15, or 20 °C min⁻¹, at final temperature of 400 °C, and characterized by FTIR, N₂ adsorption, SEM, EDS, conductometric titration, and determination of the point of zero charge. The methylene blue (MB) adsorption kinetics and equilibria on the produced materials were studied to elucidate the influence of HR on the adsorption. The results showed that the surface chemistry and physical properties of the materials were not influenced by different HR. However, equilibrium times for MB adsorption of 10 and 5 h were observed for HR of 5 and 10 °C min⁻¹, respectively, while an equilibrium time greater than 48 h was observed for the highest HR; MB intraparticle diffusion played an important role in these different behaviors. The adsorption isotherms were better fit by the Konda model. The fitted parameters suggested that the distribution of sites within pores of different sizes was also determined by the HR. The results showed that the use of MB can help identify structural changes induced by pyrolysis parameters that have a minor effect on the properties of adsorbent BC, such as HR.