Surfactant-Enhanced Crystallinity in Lanthanum-Doped Magnetite for High-Capacity, Regenerable Phosphate Adsorption from Wastewater

Adsorbent regenerability is critical to ensuring the economic viability of phosphate removal from wastewater. This study reports the development of surfactant-modified lanthanum-based magnetite (La-Mag) adsorbents using a hydrothermal synthesis method, achieving excellent regenerability and an outstanding maximum phosphate adsorption capacity of 121.6 mg P·g–1, which is nearly twice that of unmodified La-Mag. The addition of surfactants─cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), or polyethylene glycol (PEG)─significantly improved the adsorbents’ crystallinity. The optimal material, La-Mag-CTAB (containing a low 3.2 wt % La), exhibited a uniform octahedral morphology, minimal particle agglomeration, and a high saturation magnetization of 61.3 emu·g–1. La-Mag-CTAB demonstrated superior regenerability, maintaining a desorption efficiency of approximately 90% over five consecutive cycles. Spectroscopic analyses revealed that inner-sphere complexation is the dominant removal pathway, a mechanism that directly contributes to the material’s facilitated regenerability. The combination of high capacity, robust regenerability, efficient magnetic separation, and low La content highlights surfactant-mediated crystallinity control as an essential strategy for designing cost-effective and sustainable adsorbents for wastewater phosphate remediation.