Removal of iodine (I− and IO3−) from aqueous solutions using CoAl and NiAl layered double hydroxides

The treatment of radioactive iodine released from nuclear power plants and radiological waste disposal sites is of great concern due to its high mobility and toxicity. In particular, iodide (I−) and iodate (IO3−) are the major iodine species of concern under various pHs and groundwater conditions. Herein, CoAl and NiAl layered double hydroxides (LDHs) were synthesized and investigated to identify the iodine removal mechanism and efficiency. Both CoAl and NiAl LDHs exhibited rapid iodine removal processes within 20 min, following the pseudo-second-order model via ion-exchange with parent NO3− anion in the LDHs. The CoAl LDH's maximum sorption capacities for I− and IO3− were about 1.67 and 2.16 mmol g−1, respectively, whereas for the NiAl LDH, these were about 2.10 and 2.26 mmol g−1, and they followed the Langmuir isotherm model. Interestingly, both the CoAl and NiAl LDHs showed a preferential ion-exchange affinity for IO3− over I−, which was attributed to the structural similarity of the IO3− and NO3− as well as new formation of secondary Co(IO3)2·2H2O or Ni(IO3)2·2H2O phases. In addition, a desorption study indicated that the selectivity order was SO42− ≥ IO3− ≥ OH− > HCO3− > Cl− > NO3− ≥ I− and demonstrated the higher retention of the IO3− than I− anion. This study provides insights into promising iodine sorbents and the different removal mechanisms of I− and IO3− using CoAl and NiAl LDHs.