Tunable synthesis of polyethylene polyamine modified lignin and application for efficient adsorption of Fe²⁺ in super acid system

In this study, a lignin-based adsorbent modified with epichlorohydrin and pentaethylenehexamine (LO-PEHA) was controllably synthesized via epoxidation and Mannich reaction to scavenge ferrous (Fe²⁺) ions in the super acidic solution. Various kinds of characterization were executed to investigate the structural morphological characteristics of the resulting adsorbent. The well-designed adsorbent showed a rich mesoporous structure with intricate channels. The ferrous ions entrapment by the adsorbent followed the Langmuir model with the maximum adsorption capacity of 862.9 mg/g at dosage of 0.05 g/20 mL under 298 K. And the adsorption kinetics complied with the pseudo-second-order model. The adsorbent possessed substantial adsorption efficiency (q_e: 188.9–198.3 mg/g) for ferrous ions with sulfuric acid concentration of 5–250 g/L. It also demonstrated reliable performance with ferrous ions removal of 92.23% in the actual pickling waste liquor, slightly lower than that in the simulated system (99.24%). Furthermore, the mass transfer process of ferrous ions was predicted considerably well under the pore volume diffusion model (PVDM) with the effective pore volume diffusion coefficient of 4.5 × 10⁻¹⁰ m²/s, essentially commanded by the intraparticle resistance. Meaningfully, the tailor-made adsorbent is prospective as an efficient alternative for ferrous ions elimination, inspiring the future development of competent adsorbents for industrial effluent treatment.