Phosphonate antiscalants are commonly used in reverse osmosis (RO) membrane desalination to prevent fouling by mineral scale. However, due to its adverse effects on the ecosystem and the removal of hardness from RO concentrates, it is necessary to remove phosphonate antiscalants before RO concentrates disposal. In this work, using powder activated carbon (PAC) as carrier, La/FeOOH@PAC composite material was prepared via co-precipitation of lanthanum hydroxide and iron oxyhydroxide. The synergistic effect of La/Fe enhanced the adsorption capability of the composite material. Morphology and structure of the adsorbents were characterized by different methods. Adsorption behavior and properties of this composites for phosphonate antiscalant 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) in simulated reverse osmosis concentrates were also investigated. The effect of molar ratio of La to Fe, adsorption time, initial concentration of HEDP and temperature on the adsorption performance were investigated in details. The results showed that La/FeOOH@PAC composites was successfully synthesized by co-precipitation method. Under the adsorption conditions of 298 K, pH=8.0 and dosage of 0.4 g·L^-1, the adsorption isothermal was in accordance with the Langmuir model, and the theoretical maximum adsorption capacity was 65.359 mg·g^-1. Its adsorption kinetics could be fitted well with the pseudo-second-order kinetics equation, and adsorption behavior was an exothermic and spontaneous process. XPS analysis showed that the main adsorption mechanism of the adsorbent was the ligand exchange between hydroxyl groups linked with La/Fe and HEDP.