A magnetically recyclable lignin-based bio-adsorbent for efficient removal of Congo red from aqueous solution

It has been always attractive to design a sustainable bio-derived adsorbent based on industrial waste lignin for removing organic dyes from water. However, existing adsorbent strategies often lead to the difficulties in adsorbent separation and recycling. Herein, we report a novel magnetically recyclable bio-adsorbent of Mg(OH)2/Fe3O4/PEI functionalized enzymatic lignin (EL) composite (EL-PEI@Fe3O4-Mg) for removing Congo red (CR) by Mannish reaction and hydrolysis-precipitation. The Mg(OH)2 and PEI functionalized EL on the surface act as active sites for the removal of CR, while the Fe3O4 allows for the easy separation under the help of a magnet. As-obtained EL-PEI@Fe3O4-Mg forms flower-like spheres and has a relatively lager surface area of 24.8 m2 g-1 which is 6 times that of EL. The EL-PEI@Fe3O4-Mg exhibits a relatively high CR adsorption capacity of 74.7 mg g-1 which is 15 times that of EL when initial concentration is around 100 mg L-1. And it can be easily separated from water by applying an external magnetic field. Moreover, EL-PEI@Fe3O4-Mg shows an excellent anti-interference capability according to the results of pH values and salt ions influences. Importantly, EL-PEI@Fe3O4-Mg possesses a good reusability and a removal efficiency of 92 % for CR remains after five consecutive cycles. It is illustrated that electrostatic attraction, π-π interaction and hydrogen binding are primary mechanisms for the removal of CR onto EL-PEI@Fe3O4-Mg. This work provides a novel sustainable strategy for the development of highly efficient, easy separable, recyclability bio-derived adsorbents for removing organic dyes, boosting the efficient utilization of industrial waste lignin.