Zeolite X derived from fly ash was modified with Cu(NO₃)₂∙3 H₂O solutions of various initial concentrations (0.01, 0.05, 0.10, 0.15, and 0.20 M). The obtained materials were investigated by means of XRD, XRF, N₂ adsorption-desorption, and pHpzc. The concentration of Cu(NO₃)₂∙3 H₂O significantly affected both the textural and chemical surface properties of the obtained zeolites. The materials were subjected to a series of adsorption experiments to remove phosphate ions from aqueous solutions. The highest adsorption capacity was obtained for zeolite X modified with 0.05 M Cu(NO₃)₂∙3 H₂O solution, and the obtained adsorption capacity was 87.7 mg (PO₄^3-) g⁻¹. The adsorption mechanism was investigated by means of XPS analysis and correlated with the assumptions of kinetic adsorption models. The interaction between phosphate ions and Cu species on the zeolite surface was found to be the guiding mechanism of adsorption. Additional mechanisms, including precipitation as calcium phosphate and electrostatic attraction, should also be taken into account. A single-point adsorption of phosphate ions was also studied to evaluate the effects of adsorbent dose (1, 2, and 3 g L⁻¹) and adsorption temperature (18, 25 and 40 °C). Increased adsorbent dose resulted in a significantly greater removal of phosphate ions for zeolite Cu0.05X (up to 65.4%). The temperature of 25 °C was found to be the most suitable for conducting adsorption of phosphate ions. Leaching test was performed in water to investigate the risk of applying the investigated zeolites in aqueous solutions as potential adsorbents.