Counterion-engineering viologen-based mesoporous microspheres for ultra-high Cr(VI) removal via a synergistic triple-action mechanism

The effective removal of highly toxic Cr(VI) from industrial wastewater necessitates the development of an exceptionally efficient and stable adsorbent. Herein, we designed an innovative strategy for constructing a multifunctional polymer adsorbent, wherein cationic viologen served as both the cross-linker and the immobilization carrier for polyethyleneimine (PEI) through the Zincke reaction. Moreover, the incorporation of Pluronic F127 as a soft template induced the formation of well-defined mesoporous microspheres, which significantly improved the accessibility of active sites and the surface hydrophilicity of the adsorbent. Consequently, the synthesized adsorbent, VLF-PEI 10,000 -Cl, exhibited an ultra-high adsorption capacity of 1304 mg/g for Cr(VI) under optimal conditions (pH 2, 318 K), surpassing that of most reported materials. The adsorption process followed the pseudo-second-order kinetic model and the Langmuir isotherm, indicating a monolayer chemical adsorption process. Meanwhile, a synergistic triple-action mechanism, encompassing "capture-detoxification-complexation" processes, was identified as contributing to the superior adsorption capacity. Notably, substituting the counterions with nitrate ions (NO 3 − ) resulted in a 5% enhancement in removal efficiency within nitrate-rich water. This result demonstrates the great potential of tailoring the adsorbent's counterion composition to enhance its adsorption performance in various aquatic environments.