High-energy ball milling-driven La/Zr doping of magnetic Na-A zeolite derived from coal gangue for efficient phosphate removal

Abstract

To overcome the cage-energy barriers of zeolites that limit the incorporation of large functional cations, coal gangue (CG) was calcined and alkali-activated to synthesize Na-A zeolite. High-energy ball milling (HEBM) of Fe₃O₄ with Na-A produced a magnetic zeolite (MZ), and a mechanochemically driven in situ ion-exchange process enabled rapid La³⁺ incorporation to form lanthanum magnetic zeolite (LMZ) for phosphate removal. Fe₃O₄ and Na-A were tightly interlocked at the nanoscale, forming 150–800 nm aggregates composed of 10–20 nm nanocrystallites. The disappearance of NaCl diffraction peaks after washing confirmed in situ Na⁺/La³⁺ exchange during milling. LMZ showed a specific surface area of 91.32 m² g⁻¹, over four times that of pristine Na-A zeolite, and a saturation magnetization of 25.6 emu g⁻¹. At pH 3.0, the maximum phosphate uptake reached 44.47 mg P g⁻¹ and remained stable over five adsorption–regeneration cycles. Under identical conditions, Zr⁴⁺-doped zirconium magnetic zeolite (ZMZ) displayed a similarly favorable composite structure, highlighting the generality of this strategy. This work offers a scalable mechanochemical route for introducing large functional cations into solid-waste-derived zeolites and developing recyclable phosphate sorbents.