Synergistic adsorption and reduction of Cr(VI) by ferrocene-functionalized polyethyleneimine-crosslinked lignin composite.

To overcome the limitations of low specific surface area and functional group reactivity of lignin, a novel ferrocene-loaded, polyethyleneimine (PEI)-crosslinked lignin composite (FLP) was developed via glutaraldehyde-mediated grafting of PEI and strategic immobilization of ferrocene (Fc), which effectively improved the pore structure, surface accessibility, and redox-assisted reduction capability of the material. FLP achieved a Cr(VI) removal efficiency of 96.53%, and the maximum adsorption capacity was 536 mg g-1. The adsorption behavior followed the pseudo-second-order kinetic model, while the Freundlich and Temkin isotherm models jointly suggested that the Cr(VI) removal process involved heterogeneous surface adsorption and adsorbate-adsorbate interactions. Thermodynamic analysis demonstrated that the adsorption process was spontaneous and endothermic. Furthermore, FLP exhibited exceptional stability and reusability. The removal of Cr(VI) by FLP was the synergistic effect of "capture-reduction-fixation": Cr(VI) was initially captured through electrostatic attraction by the protonated amino groups of PEI, after which the incorporation of the redox-active Fc component facilitated interfacial electron transfer, reduced the charge-transfer resistance, and promoted the in-situ reduction of Cr(VI) to Cr(III), and finally Cr(III) was immobilized through multi-site chelation with amine groups from PEI and oxygen-containing functional groups on the lignin skeleton. This research provides a novel strategy for the high-value application of lignin-based materials in Cr(VI) wastewater remediation.