Hierarchically porous bacterial cellulose-derived biochar/PAA hydrogel beads for cationic dye adsorption: performance, mechanistic insights, and reusability.

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.