Synthesis of Acidic Phosphonic Chitosan and the Complexation of La(III) in Acidic Aqueous Solution

Due to the similar physicochemical properties of rare earth ions, their separation presents significant challenges. In this study, acidic phosphonic chitosan (aPCS) was prepared by modifying chitosan with phosphite and formaldehyde for improving the water solubility and complexing ability of rare earth ions in acidic aqueous solutions. DFT calculations revealed that its phosphonic groups serve as preferred reaction sites, forming stable bidentate complexes with rare earth cations. The complexation abilities of aPCS and phosphorylated chitosan (PCS) for La(III) were compared at various pHs by complexation–ultrafiltration. The results showed that aPCS achieved a 97% rejection for La(III), while 70% for PCS at pH 5 and P/RE 10. Furthermore, aPCS maintained higher rejection than PCS at pH of 3 to 7. In conclusion, aPCS demonstrates excellent potential for the selective extraction and purification of rare earth ions.