Synthesis of carbon-coated, porous and water-dispersive Fe3O4 nanocapsules and their excellent performance for heavy metal removal applications

Abstract

Porous Fe₃O₄@C nanocapsules with a diameter of about 120 nm (about 50 nm cavity) were synthesized by combining a sacrificial template method with solvothermal treatment. The N₂ adsorption–desorption isotherms reveals their mesoporous structure and large BET surface area (159.8 m² g⁻¹). The magnetic investigation indicates their superparamagnetic nature and high saturation magnetization (55.93 emu g⁻¹). The nanocapsules also exhibit negative zeta potential (−27.59 mV) and possess carboxyl groups on the outer carbon layer, which keeps them highly dispersive in aqueous solution and provides a chelating function for metal ions. The heavy metals removal test demonstrates the excellent affinity of nanocapsules, the high efficiency for different metals (>90%), 79 mg g⁻¹ adsorption capacity for Pb²⁺ and ultrafast removal process (Pb²⁺, 99.57% within 1 minute). Protected by a porous carbon layer, the nanocapsules display excellent acidic resistance and adsorption properties even in an acidic solution (pH = 3). Moreover, the metal ions can be easily adsorbed and desorbed through manipulating the pH value for adsorbent regeneration and heavy metal recycling.