Role of sulfonation in lignin-based material for adsorption removal of cationic dyes.

Sulfonate lignin-based hydrogels (SLG) were prepared through simple and effective method and successfully applied to the adsorption of cation dye methylene blue (MB). Samples were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Sulfonation degree and zeta potential of the samples were also measured by automatic potentiometric titrator and zeta potential analyzer, respectively. Adsorption results show the adsorption capacity of SLG for MB increased fivefold from raw lignin to 495 mg g-1 at 30 °C. Various factors affecting the adsorption were investigated such as temperature, pH, contact time, ionic strength and initial MB concentration. Results of kinetics, isotherm and thermodynamics reveal the adsorption process is consistent with Langmuir model and conforms to pseudo-second-order kinetics model. Adsorption process is mainly a spontaneous physisorption of monolayer. Further, results suggest sulfonation improved electrostatic interactions as well as π-π stacking in the adsorption process, and the revelation of the role of sulfonation is expected to consummate the selection and design of the lignin-based adsorbent in the specific adsorption process of cationic dyes with aromatic nucleus.