Sustainable biomass-derived activated biochar/polyaniline/alginate composite beads for enhanced methylene blue adsorption: experimental and theoretical investigations

This study investigates biomass-derived adsorbents in powder and bead forms for the efficient removal of methylene blue MB from wastewater. Activated biochar powders (ABC and ABCZ) were synthesized using H3PO4 and ZnCl2 as activators for the chemical treatment of Crataegus azarolus CAS seed waste, while polyaniline (PA) and sodium alginate (SG) were integrated to form two bead-shaped composites. Compared to ZnCl2, H3PO4 activation produced similar acidic site concentrations but resulted in improved mesoporosity and larger pore diameters. Untreated lignin-like ABC and PA-functionalized ABC were successfully encapsulated into uniform composite beads (ABC-SG and ABC-PA-SG) with negatively charged surfaces at neutral pH of solution as confirmed by FTIR, TGA, and pHpzc analyses. Adsorption efficiency depended on material composition, form, and texture. Encapsulation significantly enhanced MB adsorption capacity, with ABC-PA-SG beads achieving 821 mg/g, compared to 261 mg/g for ABC powder. While greater surface area improved MB adsorption in powders, PA incorporation in composite beads contributed to higher adsorption performance. Kinetic modeling showed that MB adsorption on ABC powder was governed by chemisorption and pores filling, while composite beads followed a physical interaction-driven process. Thermodynamic analysis confirmed that adsorption was spontaneous and endothermic. Statistical modeling and DFT calculations provided deeper insights into the adsorption mechanisms. π-π interactions dominated MB adsorption, with a horizontal molecular arrangement. ABC powders exhibited multilayer pore filling on a heterogeneous surface, whereas adsorption on ABC-PA-SG beads followed a double-energy double-layer mechanism. This study provides integrated insights into biomass-derived composite beads adsorbents, highlighting their potential for sustainable wastewater treatment applications.