Effect of Mg²⁺-, Sr²⁺-, and Fe³⁺-substitution on ⁸⁵Sr and ⁶⁰Co adsorption on amorphous calcium phosphates: Adsorption performance, selectivity, and mechanism

Hydroxyapatite Ca₁₀(OH)₂(PO₄)₆ is a well-known efficient adsorbent of dyes, heavy metal ions, and radionuclides. Its adsorption efficacy strongly depends on crystalline/amorphous structure, defectiveness, texture characteristics, and morphology. Herein, we synthesized Mg²⁺-, Sr²⁺-, and Fe³⁺-substituted (5 mol%) amorphous calcium phosphates as an effective ⁸⁵Sr and ⁶⁰Co radionuclides adsorbents. The introduction of Mg²⁺-, Sr²⁺-, and Fe³⁺ ions led to the formation of amorphous calcium phosphates with particles size in the nanoscale range of approximately 10–50 nm. The features of the adsorption behavior of amorphous calcium phosphates were determined depending on the variation of the pH of aqueous, NaCl and CaCl₂ solutions. Fe³⁺-substituted samples demonstrated the superior adsorption efficiency to ⁸⁵Sr (K_d 7.77 ×10³ cm³/g) and ⁶⁰Co (K_d 6.84 ×10⁴ cm³/g) radionuclides at pH of 10.0 and 4.0, respectively. The adsorption performance of obtained adsorbents slowly decreased at 0.1 M NaCl backgrounds and K_d⁸⁵Sr and ⁶⁰Co reached 1.67 × 10³ and 2.78 × 10⁴ cm³/g for non-substituted calcium phosphate. The dramatic decrease of calcium phosphates adsorbents efficiency to ⁸⁵Sr (K_d <150 cm³/g) and ⁶⁰Co (K_d <1.34 ×10³ cm³/g) for 0.05 M CaCl₂ model solution was established. The adsorption mechanism of dissolution-precipitation (for ⁸⁵Sr) and chemisorption (for ⁶⁰Co) was proposed. Fe³⁺-substituted calcium phosphate showed the competitive affinity and selectivity to ⁸⁵Sr and ⁶⁰Co comparing with described inorganic adsorbents and to be considered as prospective adsorbent for wastewater treatment in nuclear industry.