<p>Organic and inorganic phosphonates often co-exist in municipal sewage, and it is a challenge to remove them simultaneously. Nanoparticle Fe₃O₄ (Fe₃O₄ NPs) has attracted significant attention due to its high adsorption activity, low cost, environmental friendliness, and magnetic separation. Herein, the adsorption performance and mechanism of hydroxyethylidene diphosphonic acid (HEDP) and orthophosphate (PO₄³⁻) onto Fe₃O₄ NPs were systematically investigated. When the dosages were 0.4 g/L, the removal efficiencies of HEDP and PO₄³⁻ reached 96.3% and 95.1%, respectively. pH had no significant impact on the adsorption, whereas the presence of HCO₃⁻/CO₃²⁻ markedly suppressed the removal of HEDP and PO₄³⁻. The adsorption of HEDP and PO₄³⁻ onto Fe₃O₄ NPs conformed to the pseudo-second-order kinetics and Langmuir isotherm models in single and binary P systems. HEDP consistently inhibited the removal of PO₄³⁻ in the binary P system. The adsorption mechanisms were primarily driven by the combined effect of electrostatic attraction, hydrogen bonding, and coordination complexation. DFT molecular simulation showed higher adsorption energy between HEDP and Fe₃O₄ NPs, and the simulation outcomes were in excellent agreement with the experimental data. Although the adsorption of HEDP and PO₄³⁻ was competitive, total phosphorus in the effluent of municipal sewage could still meet the discharge standard.</p>