Green and Continuous Synthesis of Nanoscale Zero-Valent Iron on Activated Carbon toward Ultrafast Heavy Metal Removal

Nanoscale zerovalent iron (nZVI) is a well-recognized candidate for heavy metal remediation, yet its large-scale application is hampered by intrinsic aggregation, reliance on toxic reductants, and lack of continuous production in conventional synthesis. Herein, we report a high-gravity-assisted green and continuous strategy for fabricating activated carbon (AC)-supported well-dispersed nZVI (nZVI/AC), with orange peel (OP) extract as a sustainable reductant and modifier. The as-prepared nZVI/AC achieves >99% removal of hexavalent chromium (Cr(VI)) at a high concentration of 120 mg L^-1 in merely 2 min, 4.05- and 31.97-fold higher than pristine nZVI and AC, respectively, ranking it among the top-performing materials reported. It retains nearly 90% efficiency after five cycles and exhibits robust versatility in removing As(V), Pb(II), Cu(II), and Ag(I) (>90% efficiency). The superior performance stems from synergistic merits: AC inhibits nZVI aggregation and prevents active site overloading, while OP polyphenols reduce iron ions and functionalize nZVI surfaces. Mechanistically, Cr(VI) removal follows electrostatic adsorption, chemical reduction, and coprecipitation: Fe⁰/Fe(II) and OP polyphenols synergistically reduce Cr(VI) to Cr(III), immobilized as stable FeCr₂O₄ and surface complexes to avoid secondary pollution. This work highlights high-gravity technology's pivotal role in solving green nZVI's continuous production dilemma, providing a scalable, eco-friendly strategy for high-performance heavy metal remediation.