Biochar and biochar-bentonite composite from Carnauba (Copernicia prunifera) straw modified by high-energy ball milling: preparation, characterization, and methylene blue adsorption

Abstract

The conversion of agrowaste into engineered adsorbent materials represents a sustainable strategy for wastewater treatment. Biochar and biochar–clay composites were prepared from raw and dewaxed carnauba (Copernicia Prunifera) straw through high-energy ball milling (HEBM) and evaluated for methylene blue (MB) adsorption. Carbonization at 550 °C enriched fixed carbon and ash while reducing volatile matter. Dewaxed biochar (DCSB) exhibited higher carbon content (45.63 wt%) and a higher degree of carbonization compared to non-dewaxed biochar (CSB), leading to superior adsorption performance (Langmuir qm = 328.13 mg g-1 for

DCSB vs. 255.70 mg g-1

for CSB). SEM analysis revealed that HEBM produced highly fragmented materials with a fine powder-like morphology, while FTIR and XRD analyses confirmed the preservation of the fundamental chemical structure and the predominantly amorphous character of the biochars. A material-dependent response to HEBM was observed: CSB required higher specific energy (26.46 kJ g-1

) to reach its maximum adsorption capacity,

whereas DCSB achieved its optimum at a much lower energy (7.86 kJ g-1

), indicating that

dewaxing alters the biochar's susceptibility to mechanochemical activation. Notably, bentonite- containing composites exhibited broader energy tolerance, maintaining high adsorption

performance across a wide range of milling conditions, indicating greater robustness of the composites to variations in HEBM conditions. FTIR, EDS, and XRD analyses confirmed the successful incorporation of bentonite into the composites. HEBM enhanced adsorption kinetics, while bentonite improved removal efficiency (up to 97.75%) and surface heterogeneity. Among modified samples, adsorption followed the Elovich model (R2 up to 0.99), suggesting the presence of heterogeneous adsorption sites, while equilibrium data were best described by the Freundlich isotherm (R2 up to 0.97). Maximum adsorption capacities calculated from the Langmuir model reached 307.86 mg g-1

(BDCSB4002) and 274.61 mg g-1

(DCSB2006), although the Langmuir model did not provide the best fit for the modified materials reflecting the heterogeneous nature of the adsorption process. The adsorption mechanism involves π–π interactions, electrostatic attraction, hydrogen bonding, and intraparticle diffusion. These findings demonstrate that DCSB exhibits the highest Langmuir maximum adsorption capacity, HEBM and bentonite modification offer improvements in adsorption kinetics and removal efficiency, supporting the potential of engineered carnauba biochar and biochar–clay composites as sustainable adsorbents dye removal in wastewater treatment.