Self-assembled magnetic bio-adsorbent from soluble starch, chitosan, and tannic acid for facilely recyclable and ultra-high capacity cationic dye removal from water.

Conventional biomass adsorbents often suffer from structural instability and difficult recovery. To address this, we report a novel strategy for fabricating a magnetic bio-adsorbent by harnessing the dual-role of tannic acid (TA) in a synergistic self-assembly process. The key novelty lies in using TA to first drive the formation of a 3D nanoporous network with chitosan (CTS) and soluble starch (SS) via non-covalent interactions, and second, to act as a molecular bridge that coordinates with and cross-links nano-Fe3O4 particles. This approach transforms the Fe3O4 from a simple magnetic additive into an inorganic structural hub, creating a stable composite (TA-CTS-SS-Fe3O4) with an enhanced specific surface area. This unique architecture, a direct result of the TA-mediated coordination, is responsible for the ultra-high theoretical maximum adsorption capacities for methylene blue (MB) and crystal violet (CV), reaching 904.42 and 1051.18 mg/g, respectively. The adsorption mechanism involves monolayer chemisorption driven by electrostatic interactions, hydrogen bonding, and π-π stacking. The integrated magnetism of this material enables facile magnetic separation and exhibits outstanding recyclability after five consecutive cycles. This work presents a new design principle for biomass adsorbents, where a strategic cross-linking mechanism simultaneously enhances structural stability, adsorption performance, and recoverability, offering a practical solution for industrial wastewater treatment.