Magnetic Fe3O4@polyaniline nanocomposites with a tunable core–shell structure for ultrafast microwave-energy-driven reduction of Cr(vi)

Abstract

Magnetic core–shell Fe₃O₄@polyanilines (Fe₃O₄@PANI) were controllably synthesized to harvest microwave (MW) energy for ultrafast Cr(VI) reduction. The nanocomposites are excellent MW acceptors due to the combination of the magnetic loss of Fe₃O₄ and dielectric loss of PANI. Under MW irradiation, abundant hot spots are generated on Fe₃O₄@PANI to accelerate the reduction of surface-bound Cr(VI). Moreover, Fe₃O₄@PANI is highly stable in an acidic solution and readily recoverable with an external magnet after use. The PANI shell possesses good enrichment ability for Cr(VI), creating a high Cr(VI) surface concentration which benefits the sequential reduction. Furthermore, an increase in the PANI coating amount from 24.78% to 55.15% increases the Cr(VI) adsorption capacity (q_e) from 0.63 mmol g⁻¹ to 1.68 mmol g⁻¹, which linearly enhances the removal rate constant (k) of Cr(VI) under an MW field as k = 0.30q_e − 0.03. The addition of oxalic acid as a sacrificial electron donor increases the Cr(VI) removal rate by 120.09% and protects the PANI shell from excessive oxidation by Cr(VI); this prolongs the lifetime of Fe₃O₄@PANI. In the presence of oxalic acid, the MW-driven Cr(VI) reduction on Fe₃O₄@PANI proceeds at a rate constant of 0.5773 min⁻¹, which is much higher than that of palladium-based chemocatalysis. Moreover, Fe₃O₄@PANI can be in situ regenerated by MW and directly reused in the next cycle, and no efficiency loss occurs in 5 continuous runs. The mechanism study based on Cr species tracking and solid analyses indicates that a synergistic adsorption and reduction cycle proceed consecutively, contributing to the rapid Cr(VI) removal. This study demonstrates that Fe₃O₄@PANI is a promising alternative to utilize MW for ultrafast Cr(VI) reduction and has the advantages of time-saving and energy efficiency.