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Two sodalite phases (potassium sodalite (K.SD) and sodium sodalite (Na.SD)) were prepared using alkali fusion of kaolinite followed by a hydrothermal treatment step for 4 h at 90 °C. The synthetic phases were characterized as potential adsorbents for PO₄³⁻ from the aqueous solutions and real water from the Rákos stream (0.52 mg/L) taking into consideration the impact of the structural alkali ions (K⁺ and Na⁺). The synthetic Na.SD phase exhibited enhanced surface area (232.4 m²/g) and ion-exchange capacity (126.4 meq/100 g) as compared to the K.SD phase. Moreover, the Na.SD phase exhibited higher PO₄³⁻ sequestration capacity (Q_max = 261.6 mg g⁻¹ and Q_sat = 175.3 mg g⁻¹) than K.SD phase (Q_max = 201.9 mg g⁻¹ and Q_sat = 127.4 mg g⁻¹). The PO₄³⁻ sequestration processes of both Na.SD and K.SD are spontaneous, homogenous, and exothermic reactions that follow the Langmuir isotherm and pseudo-first-order kinetics. Estimation of the occupied active site density validates the enrichment of the Na.SD phase with high quantities of active sites (Nm = 86.1 mg g⁻¹) as compared to K.SD particles (Nm = 44.4 mg g⁻¹). Moreover, the sequestration and Gaussian energies validate the cooperation of physisorption and weak chemisorption processes including zeolitic ion exchange reactions. Both Na.SD and K.SD exhibit significant selectivity for PO₄³⁻ in the coexisting of other common anions (Cl⁻, SO₄²⁻, HCO₃⁻, and NO₃⁻) and strong stability properties. Their realistic application results in the complete adsorption of PO₄^3- from Rákos stream water after 20 min (Na. SD) and 60 min (K.SD).