Hierarchical polydopamine coated cellulose nanocrystal microstructures as efficient nanoadsorbents for removal of Cr(VI) ions

Cellulose nanocrystals (CNCs) have been extensively explored as adsorbents for removal of heavy metals, dyes and other pollutants, but the pristine CNCs only show very limited adsorption capacity for Cr(VI) and usually demand complicated surface modification strategies to achieve acceptable Cr(VI) adsorption performance. In this work, we report a facile and efficient strategy to develop CNC-based nanoadsorbents with excellent Cr(VI) adsorption performance through polydopamine (PDA) surface modification. By controlling dopamine polymerization on the surfaces of CNCs and in the bulk solution, we achieved a hierarchical PDA coated cellulose nanocrystal (CNC@PDA) microstructure wherein PDA nanoparticles of less than 30 nm are uniformly attached on PDA coated CNCs. This special hierarchical microstructure endows the developed CNC@PDA adsorbents with a high specific surface area 101.88 m 2 /g thereby affording high Cr(VI) adsorption capacity 205 mg/g, which outperformed most adsorbents based on cellulose materials. Moreover, the Cr(VI) adsorbed CNC@PDA nanoadsorbents precipitate out from the solution and thus can be easily removed to reclaim the purified water. This mussel-inspired strategy to fabricate hybrid nanomaterials with hierarchical microstructures might open up new opportunity to modify cellulose-based agriculture by-products for their application in sewage treatment. Graphic abstract