Synthesis of La-Fe3O4@polydopamine core–shell magnetic composites for enhanced phosphate removal from aqueous solutions

In this work, a lanthanum-doped Fe3O4@polydopamine (PDA) core–shell composite was prepared via a hydrothermal method. XPS depth profiling revealed that La is uniformly distributed throughout the Fe3O4 particles, with a constant La/Fe ratio from the surface to 60 nm depth. The material exhibits superparamagnetic properties, enabling rapid magnetic separation within 30 s. Phosphate adsorption reaches equilibrium in 180 min, following pseudo-second-order kinetics (R2 > 0.999) and the Sips isotherm, with a maximum capacity of 123.44 mg P per g. XPS analysis revealed shifts in the La 3d spectra consistent with inner-sphere La–P complexation, providing supportive evidence for chemisorption. The adsorbent retained >85% of its capacity after five cycles using 1 M NaCl as the eluent. Negligible La leaching (<0.01 µg L−1) and low aquatic toxicity to Chlorella sp. further support its environmental safety. These results demonstrate that 2.5%La-Fe3O4@PDA provides a robust, magnetically separable, and reusable platform for phosphate removal, offering deeper mechanistic insights into La-doping and PDA synergy in established magnetic core–shell systems.