Green Synthesis of Fe2O3/PEI/MIL-53(Al) from Upcycled Linkers for Organophosphate Antiscalant Cleanup

An environmentally friendly magnetic metal–organic framework (MMOF) based on MIL-53(Al) was synthesized using terephthalic acid (TPA) recovered from polyethylene terephthalate (PET) waste as the organic linker. The Fe2O3@MIL-53(Al) adsorbent was fabricated through a layer-by-layer assembly, with polyethylenimine (PEI) bridging the Fe2O3 core (∼200 nm) and MIL-53(Al) shell (∼25 nm). A uniform MOF layer was achieved in a single cycle under green synthesis conditions with PEI concentrations ≥ 10 g/L. The MMOF efficiently removed organophosphate pollutants, particularly 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), a common scale and corrosion inhibitor. Adsorption kinetics across a pH range of 3–10.5 closely match the pseudo-second-order kinetic model, indicating chemisorption. The adsorption capacity ranged from 250 to 460 mg/g, with a maximum surface-normalized capacity of 0.44 mg/m2 at pH 5.5, attributed to favorable electrostatic interactions. Adsorption performance was strongly influenced by both the MMOF zeta potential and the ionization state of PBTC. The well-fitted adsorption isotherms (R2 > 0.9) to the Freundlich, Temkin, and Henry models indicate heterogeneous surface adsorption mechanisms. Electrostatic attraction, hydrogen bonding, and surface complexation are proposed as the dominant mechanisms. Overall, the Fe2O3/PEI/MIL-53(Al) composite exhibits high adsorption capacity, broad pH stability, and low toxicity, highlighting its promise for efficient PBTC antiscalant removal applications.