Rapid adsorption of dyes from aqueous solutions by modified lignin derived superparamagnetic composites

• Self-assembly of lignin with γ-Fe2O3 was achieved under simple ultrasonication • Modification of lignin by protic ionic liquid increases its adsorption capacity • The prepared adsorbents have good adsorption effect on both anionic and cationic dyes • Lignin-based composites enable rapid separation from solution and stable recovery In recent years, lignin has gotten a lot of press as an adsorbent for treating organic dyes. However, lignin adsorption has problems such as low adsorption capacity, slow adsorption and difficult recovery. In this study, a simple method was used to prepare superparamagnetic composites by self-assembly of enzymatic lignin (EL) or its modification products with γ-Fe 2 O 3 nanoparticles under electrostatic interactions. Then the effect of the adsorbents in removing the methylene blue (MB, as a model cationic dye) and Congo red (CR, as a model anionic dye) from aqueous solutions was investigated by detailed characterization and experiments. The samples were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibration sample magnetometer (VSM). In the process of adsorption, the effects of the initial pH and inorganic ions concentration of the dyes were discussed. And the adsorption mechanisms such as kinetics, isotherm were explored. The results showed that the synthesized magnetic acetylated modified lignin (MADL) has a well-defined spherical structure. In the adsorption kinetics study, MADL demonstrated a fast adsorption capacity. it took less than 50 min to reach the equilibrium. MADL maintained a stable adsorption capacity at a wide pH range and showed >90% removal efficiency in higher concentration (0.5 mol/L) of inorganic ions solutions. Moreover, MADL has good recyclability, can be separated from water quickly by the action of the applied magnetic field and still has >94% regeneration efficiency after three times of desorption and regeneration. In summary, the novel magnetic lignin-based adsorbent material prepared in this study has the advantages of simple preparation process, rapid adsorption, stability and good recyclability, which provides a new way for the removal of dyes in practical applications.