Redox-Active PEI/Polydopamine-Functionalized Wood for Coupled Hexavalent Chromium Adsorption–Reduction and Multicontaminant Water Purification

Abstract

Hierarchically structured bioderived materials with integrated interfacial functionalities offer new opportunities for advanced water purification, yet coupling redox activity with selective adsorption in macroscopic scaffolds remains challenging. Herein, we engineer a multifunctional wood-based adsorbent by integrating polydopamine (PDA) and polyethylenimine (PEI) within a delignified wood (DW) framework, enabling synergistic control over surface chemistry and transport pathways. Delignification generates an interconnected porous architecture that facilitates mass transfer, while conformal PDA deposition and PEI grafting introduce redox-active catechol groups and high-density amine functionalities, yielding a pH-responsive, positively charged interface (ζ of +28 mV). Detailed mechanistic analysis was conducted only for hexavalent chromium (CrVI), whereas AsV, NiII, and dyes were examined as proof-of-concept contaminants. Their interactions with PEI/PDA-W were interpreted based on the observed adsorption trends. The resulting material exhibits coupled adsorption–reduction behavior toward CrVI, achieving ∼95% removal under acidic conditions, with spectroscopic evidence confirming partial in situ reduction to CrIII mediated by PDA redox chemistry. Adsorption follows Langmuir-type behavior (qm = 23.3 mg/g) and pseudo-second-order kinetics, indicating surface-controlled interactions on energetically uniform sites. Beyond chromium, the system demonstrates mechanism-dependent selectivity toward coexisting contaminants: arsenate removal is governed by electrostatic interactions and pH-dependent speciation, whereas nickel uptake is dominated by coordination with amine/catechol ligands. Dye adsorption further reveals structure-sensitive behavior, where π–π and hydrophobic interactions enable near-complete removal of aromatic cationic dyes, while weakly interacting species exhibit limited uptake. This work establishes a design strategy for integrating redox functionality and multivalent interfacial interactions within a hierarchical wood scaffold, providing a versatile platform for selective and multimodal water purification.