Selective adsorption of uranyl and potentially toxic metal ions at the core-shell MFe2O4-TiO2 (M=Mn, Fe, Zn, Co, or Ni) nanoparticles

Abstract Potentially toxic metal ions (X n+ : Rb + , Sr 2+ , Cr 3+ , Mn 2+ , Ni 2+ , Zn 2+ , Cd 2+ ) usually coexist with uranyl (UO 2 + ), which will have a great influence on the selective adsorption process. Here, the core-shell MFe 2 O 4 -TiO 2 (M = Mn, Fe, Zn, Co, or Ni) nanoparticles were synthesized and assessed as new selective adsorbents. The results reveal that TiO 2 (101) preferentially grows along the MFe 2 O 4 (311)/(111) orientation. The M 2+ ions as the mediators transfer the holes from MFe 2 O 4 to TiO 2 , at the conduction bands. On the TiO 2 (101) surfaces and TiO 2 (101)-TiO 2 (101) gaps, the paired active electrons mainly complex with water molecules as hydroxyl radicals to capture X n+ ions, forming an ion layer to block UO 2 2+ from being adsorbed. Simultaneously, it should be noted that an interesting adsorption pathway was UO 2 2+ being horizontally and irreversibly adsorbed in the MFe 2 O 4 (311)/(111)-TiO 2 (101) interface, and therein, the stable adsorption capacity was found to be 66.78 mg g −1 in the MnFe 2 O 4 (311)/(111)-TiO 2 (101) interface. Finally, a mechanism of hybrid orbitals between MnFe 2 O 4 -TiO 2 and UO 2 + -X n+ was proposed.