Hierarchically porous bacterial cellulose-derived biochar/PAA hydrogel beads for cationic dye adsorption:...

The emergence of dye pollutants into aquatic environments necessitates the development of efficient, stable, and reusable adsorbents for wastewater treatment. In this study, hierarchically porous bacterial cellulose-derived biochar/poly(acrylic acid) (BCC-PAA) hydrogel beads were synthesized via gradual pH adjustment, constructing a three-dimensional network containing PAA-derived -COOH groups, oxygen-containing functionalities associated with the BCC-derived carbonaceous component and hydrogel network, and distinct BCC-derived carbonaceous domains. Physicochemical analyses revealed a highly porous texture, abundant oxygen-containing groups, and a pH-responsive surface with a point of zero charge of ∼5.5. The BCC-PAA beads achieved excellent adsorption capacities of 676.58 mg g^-1 and 725.74 mg g^-1 toward methylene blue (MB) and rhodamine B (RB), respectively. Adsorption kinetics were best described by the pseudo-first-order and Bangham models, indicating rapid surface adsorption followed by pore diffusion. Equilibrium data were well fitted by the Langmuir and Koble-Corrigan isotherms, suggesting predominantly monolayer adsorption with minor surface heterogeneity. Thermodynamic analysis revealed spontaneous and exothermic adsorption, with negative ΔG° values ranging from -26.77 to -26.88 kJ mol^-1 for MB and from -27.72 to -27.53 kJ mol^-1 for RB, and negative ΔH° values of -24.34 and -31.35 kJ mol^-1, respectively. Mechanistic evaluation suggested that adsorption may be governed by the synergistic effects of electrostatic attraction (E-attraction), π-π interactions, hydrogen-bonding (H-bonding), and pore filling phenomena. The beads retained 76.42% (MB) and 77.34% (RB) of their initial removal efficiencies after nine adsorption-desorption cycles. These findings demonstrate the potential of BCC-PAA hydrogel beads as a promising platform for further research on the efficient adsorption of cationic dyes from aqueous systems.