Adsorption of Co(II) ions using Zr-Ca-Mg and Ti-Ca-Mg phosphates: adsorption modeling and mechanistic aspects

The environmental pollution by toxic Co(II) ions had a negative impact on living organisms and water resources. The amorphous Zr-Ca-Mg and Ti-Ca-Mg phosphates with varied Zr and Ti content with the mesoporous structure (A_BET = 19-232 m²/g, V_des. = 0.075-0.370 cm³/g, D_des. = 6.2-10.9 nm) were synthesized. The effect of adsorbent chemical composition, the presence of competing ions (0.1-1.0 M NaCl and 0.01-0.1 M CaCl₂ backgrounds), and pH (3.0-7.0) of aqueous solution on adsorption removal of Co(II) ions by Zr-Ca-Mg and Ti-Ca-Mg phosphates was studied. The highest adsorption capacity of Zr-Ca-Mg-1 and Ti-Ca-Mg-1 samples reached 253.3 and 212.8 mg/g. The prepared adsorbents demonstrated high efficiency at pH in the range of 3.0-7.0 and the presence of 0.1-1.0 M NaCl and seawater with a salinity of 35.0 g/L backgrounds. The chemisorption and ion-exchange mechanisms of Co(II) ions removal for Zr-Ca-Mg and Ti-Ca-Mg phosphates were proposed. The adsorption isotherms were well fitted with Sips and Langmuir models that proved the heterogeneous nature of adsorption sites as well as assumed the monolayer adsorption that occurs at specific homogeneous sites within the adsorbent without any interaction between the adsorbed substances. The kinetic data was well described by the pseudo-second-order model that is suitable for chemisorption processes as liming adsorption stage. The presented results shown the prospects of developed adsorbents for the investigation of real wastewater treatment from heavy metal ions and liquid radioactive waste purification.