Reject desalination brine is a rich source of salts and valuable materials. But, its disposal into the external environment generates a major source of pollution. This paper investigated the production of reactive CaO from reverse osmosis (RO) reject brine and its use as adsorbent for phosphate removal from contaminated seawater and RO industrial waste. This study was realized via two steps. Firstly, the recovery of Ca-phase from reject brine via oxalic acid at optimized conditions resulting clcium oxalate monohydrate, and its calcination at 900°C for 2 h to produce the reactive CaO. Secondly, the produced reactive CaO was doped by Fe3O4 via co-precipitation method to produce reactive CaO@Fe3O4 composites. The reactive CaO@Fe3O4 was explored as a potential adsorbent with enhanced capacity for phosphate ions (PO43−) removal. The estimated maximum reactive CaO@Fe3O4 uptake capacity for PO43− (106.3 mg/g) is comparatively higher than the identified values for the reactive CaO (72.8 mg/g) and Fe3O4 (41.6 mg/g). The kinetics of the PO43− uptake reaction via reactive CaO@Fe3O4 obey the Pseudo-Second order model (R2 > 0.98) and the equilibrium time was determined after 450 min. The equilibrium study of the demonstrated reaction exhibited excellent agreement with the isotherm assumption of the Freundlich model implying multilayer and heterogeneous adsorption processes. The thermodynamic aspect of PO43− adsorption reaction is favorable and exothermic. The adsorbent selectivity, reusability experiments and realistic study were also discussed . Furthermore, the computational study using DMs was applied to better understand the interaction for {PO43−/CaO(111)&CaO@Fe3O4(111)} systems. The simulation results demonstrate favorable, more stable, spontaneous adsorption and exothermic for PO43−/CaO@Fe3O4(111) than PO43−/CaO(111)complex. Overall, CaO@Fe3O4(111) could serve as a effective and reusable adsorbent for phosphate ions recovery from aqueous solutions.