Valorization of sulfuric acid-extracted olive pomace lignin for phenol adsorption: adsorption behavior, modeling, and mechanism analysis

Olive pomace is a valuable by-product for creating materials for environmental applications. This study demonstrates the waste valorization of olive pomace through the production of lignin-based adsorbents for phenol adsorption from aqueous solutions. This study investigates lignin extracted from olive pomace through sulfuric acid hydrolysis as a sustainable adsorbent for phenol removal from water. It also evaluates CuO nanoparticles and a lignin–CuO composite to assess the effects of lignin modification on adsorption performance. The effects of contact time, pH, and initial phenol concentration on phenol removal were investigated using one-factor-at-a-time experiments. Phenol removal increased with longer contact times and higher initial concentrations, while alkaline conditions hindered performance due to pH effects. Under consistent conditions (120 min, pH 7, 50 ppm), CuO, lignin, and lignin–CuO achieved phenol removal efficiencies of about 50%, 35%, and 40%, respectively. Second-order empirical models were used to compare model-estimated and experimentally observed removal efficiencies under selected operating conditions, showing close agreement between the two. Adsorption kinetics were evaluated using nonlinear pseudo-first-order and pseudo-second-order models, together with the Weber–Morris intraparticle diffusion model. The pseudo-second-order model generally provided a better empirical description of the experimental uptake profiles, while the intraparticle diffusion results indicated that diffusion contributed to phenol uptake but was not the sole rate-controlling process. The adsorption capacities were 25.12 mg/g for CuO, 35.07 mg/g for lignin, and 42.04 mg/g for the lignin–CuO composite. The Freundlich model provided a better description of the equilibrium data, supporting the heterogeneous nature of the adsorption surfaces. FTIR revealed oxygen-containing functional groups and changes in the surface chemical environment following CuO incorporation. XRD and EDS further supported the presence of crystalline CuO in the composite. SEM revealed rough and heterogeneous surface morphologies for the investigated adsorbents. The combined characterization and adsorption results suggest that phenol uptake may involve contributions from surface interactions and diffusion within heterogeneous adsorption domains. These findings demonstrate the potential of sulfuric acid-extracted olive pomace lignin and its CuO-modified form as biomass-derived adsorbents for phenol removal from aqueous solutions.