La-Al modified Fe3O4 adsorbent for adsorption performance and mechanism of low P concentration in water

Adsorbents of LaAl hydroxide (oxygen) species have been widely used for removal of phosphate (P) from water, but fewer studies were conducted on the removal of low P concentration. This study firstly fabricated a new composite material of LaAl modified Fe₃O₄ by co-precipitation method applied for removal of low P concentration in water. Results showed that the optimal adsorbent performance was at a La/Al molar ratio of 2:1 and an optimum dosage of 0.1 g/L. P removal was >99 % with a low initial P concentration (2 mg/L), and the residual P was only 0.02 mg/L. La₂-Al@Fe₃O₄ adsorbent was an irregular magnetic particle with a large specific surface area of 101.35 m²/g. Adsorption process primarily involved chemisorption and formation of multiple adsorption layers. The maximum adsorption capacity was 71.75 mg/g when the initial P concentration was 50 mg/L. Meanwhile, it showed good adsorption performance at pH 4.0–9.0, exhibited notable selectivity for P despite competing ions such as Cl⁻, HCO₃⁻, SO₄²⁻, and NO₃⁻, and was influenced by coexisting citric acid and humic acid. Also, La₂-Al@Fe₃O₄ showed good reproducibility and maintained 85.6 % of the original adsorption capacity after five adsorption and desorption cycles. Additionally, P concentration could be reduced to below 0.02 mg/L in actual water, and La₂-Al@Fe₃O₄ was friendly to the water environment based on the toxicity analysis of Chlorella vulgaris. The adsorption mechanism of P by La₂-Al@Fe₃O₄ mainly involved electrostatic adsorption and ligand exchange. This novel adsorbent shows great potential for application in low P concentration water treatment.