Betaine-modified La-doped ferrihydrite for efficient phosphate removal to ultralow levels

Eutrophication control requires efficient removal of phosphate in wastewater treatment plants to very low levels (<0.1 mg P/L). This cannot be met by common low-cost adsorbents like hydrated ferric oxide (HFO) due to inefficient mass transfer at low P concentrations and steric hindrance posed by hydration shell of phosphate impeding inner-sphere complexation. We developed a betaine-modified lanthanum-doped hydrated ferric oxide (B_me-LaHFO) adsorbent by regulating the interfacial hydrogen bonding between surface functional groups and water molecules. This approach achieved interfacial phosphate dehydration, leading to unprecedentedly high performance for the removal of low P concentrations. La doping facilitated the modification of betaine on the adsorbent. The quaternary ammonium group (-(CH₃)₃N⁺) of betaine enriches phosphate via electrostatic adsorption and facilitates phosphate dehydration by forming hydrogen bonds (-(CH₃)₃N⁺···H-O) with water molecules. Concurrently, the electron-donating effect of the -COO⁻ synergizes with La doping to optimize the electronic structures of the Fe and La sites, lowering the -OH desorption energy barrier and promoting formation of stable La/Fe-O-P inner-sphere complexes. This endows B_me-LaHFO with superior adsorption capacity (98.6 mg P/g, 4.6-fold higher than HFO), broad pH adaptability (pH 3-11), high selectivity, and high regeneration performance (>90% efficiency after 5 cycles). Flow-through columns packed with immobilized B_me-LaHFO effectively treated 6300 bed volumes of secondary effluent from a wastewater treatment plant (from 0.6 mg P/L in influent to <0.1 mg P/L in effluent), with a treatment capacity 8-fold higher than that of immobilized HFO. Techno-economic analysis demonstrates the feasibility of this new material. Overall, this study informs the design of high-efficiency phosphate removal adsorbents to meet stringent discharge requirements, and of high-performance adsorbents for removal of other oxyanions.