From marine biomass to sustainable adsorbents: comparative evaluation of polyaniline, Posidonia oceanica-derived activated carbon, and their composite for Murexide removal.

The development of sustainable and high-performance adsorbents remains a critical challenge in wastewater treatment. In this work, polyaniline (PANI), activated carbon derived from Posidonia oceanica (ACPO), and a PANI/ACPO hybrid composite were synthesized and evaluated for Murexide adsorption from aqueous solutions. The approach combines abundant marine biomass valorization with a nitrogen-rich conducting polymer to develop efficient adsorbents for water treatment. Physicochemical characterization confirmed the successful conversion of Posidonia oceanica biomass into hierarchically porous activated carbon (ACPO) and the homogeneous incorporation of PANI in its doped emeraldine state within the hybrid composite framework. Strong interfacial interactions, mainly π-π stacking and hydrogen bonding, ensured good dispersion of the polymer phase and preservation of accessible adsorption sites. PANI and PANI/ACPO exhibited comparable adsorption capacities (333.33 and 322.58 mg·g⁻1, respectively), both significantly higher than ACPO (149.25 mg·g⁻1), highlighting the dominant contribution of the polymer phase within the composite system. Equilibrium data fitted the Langmuir model for PANI-based materials, while ACPO followed the Freundlich isotherm (R2 ≈ 0.966), indicating surface heterogeneity and multilayer adsorption. Experimental kinetic data were consistent with the pseudo-second-order model. Evaluation of the thermodynamic parameters (ΔG° < 0, ΔH° > 0, and ΔS° > 0) indicated favorable adsorption behavior promoted at elevated temperatures and dominated by physical interactions. Intraparticle diffusion and boundary layer effects contributed to the overall adsorption rate, particularly for ACPO, highlighting the role of pore diffusion in mass transfer. Mechanistically, adsorption is governed by electrostatic forces, π-π donor-acceptor associations between aromatic moieties, hydrogen bonding, and diffusion within the porous network, indicating the coexistence of multiple uptake pathways. Overall, the PANI/ACPO composite combines structural porosity and surface functionality, making it a sustainable adsorbent for efficient Murexide removal from aqueous systems.