Facile synthesis of alginate hydrogel beads activated by La3+/ graphene oxide for enhanced phosphate removal from aqueous environment

Abstract

La-based nanoparticles encapsulated within a host matrix exhibit enhanced phosphate removal efficiency and improved stability compared to their bulk counterparts. The optimization of La-based adsorbents, balancing adsorption capacity and separation efficiency, is of great significance. In this study, we developed a three-dimensional layered skeleton network of La-modified graphene oxide/sodium alginate beads (La-GO/SA) by uniformly embedding La(OH)₃. Original GO maintained a high specific surface area (2630 m²/g), which boosted the surface area, electrical crosslinking, and affinity towards oxygen–donor compounds of La-GO/SA. The crosslinked hydrogel exhibited enhanced mesoporous and microporous structures, as confirmed by scanning electron microscopy (SEM) and surface structural analyses. Batch experiments demonstrated that La-GO/SA achieved stable phosphate removal (>80 %) across a broad pH range of 3.0–10.0, with a maximum phosphate uptake of 34.8 mg/g at pH 4.0. Notably, La-GO/SA maintained high selectivity for phosphate even in the presence of competing anions such as Cl⁻, HCO₃⁻, SO₄²⁻, and NO₃⁻. The experimental data were well-fitted to Freundlich and pseudo-second-order models, indicating a multilayer chemisorption mechanism. Additionally, multi-instrument characterization analysis elucidated the phosphate removal mechanisms, including electrostatic interactions, surface precipitation, ligand exchange, and Lewis acid-base interactions. The La-GO/SA hydrogel provided attachment sites for LaPO₄ precipitates, which contributed to a decrease in pore volume after adsorption. Our research on the synthesis, properties, and adsorption mechanisms of La-GO/SA hydrogel laid a scientific foundation for practical phosphate immobilization and recycling applications.