Enhanced reduction and in-situ stabilization of Cr(VI) by Fe3O4@polydopamine magnetic microspheres embedded in sludge-based carbonaceous matrix

Highlights: • Excess sludge was recycled to act as a porous carbonaceous matrix for anchoring magnetic microspheres. • Polydopamine was evenly coated onto magnetic microsphere and formed a core–shell structure. • The electron donating capacity of surface nanolayered coating was quantitatively characterized. • Cr(VI) reduction–stabilization mechanism based on the molecular level was proposed in detail. In this study, well-defined sludge-based magnetic polydopamine (SMP) was synthesized by a facile and green approach. Pristine activated sludge (PAS) was recycled to be a carbonaceous matrix, represented as sludge-based support (SBS) and used to support bare magnetic Fe₃O₄ microspheres (BM), which finally gave sludge-based magnetic Fe₃O₄ microspheres (SM). A lot of BM were further transformed to core–shell structured magnetic Fe₃O₄@polydopamine microspheres (MP) by polymerization of dopamine. The specific surface areas of SMP were 30.9 m² g⁻¹, far larger than those of the BM (5.5 m² g⁻¹) and SBS (2.1 m² g⁻¹). The maximum adsorption capacity was calculated to be 118 mg g⁻¹ at 323 K. More significantly, reduction-stabilization process of Cr(VI) was found to occur on the surface nanolayered coating. Specifically, after being protonated, the nitrogen atoms contained in SMP were confirmed to attract negatively charged Cr(VI) by electrostatic attraction. The redox system promoted electron transfer from polydopamine (PDA) to Cr(VI), and then the non-protonated nitrogen atoms in-situ chelated the reduction product Cr(III) as coordination atoms. In short, BM supported by the SBS and coated by PDA possessed excellent capability for magnetic separation and reduction–stabilization of Cr(VI), and proved to be a stable and green adsorbent.