Abstract
Adsorbents that capture both cations and anions were prepared by addition of zeolite powders to aqueous dispersions of colloidal layered double hydroxide (LDH). Hybrid adsorbents were obtained from the mixtures by removal of water via mild heating. Scanning electron microscope observations revealed that zeolite particles were uniformly coated with LDHs. Targeted ions (IO_3^-, I^-, and Cs^+) were selected from among the probable radioactive substances released in nuclear power plant accidents; non-radioactive counterparts were used in the experiments. The zeolites used in the hybrid adsorbents were Na-type zeolites that produced a high pH environment in aqueous solutions. The hybrid adsorbents were conditioned before adsorption experiments by anion exchange for Cl^- with a NaCl solution. Arising of OH^- anions from zeolites, onto which LDHs closely attached, affected both conditioning and subsequent tests and resulted in poor uptake of IO_3^- and I^-. However, after treatment with a MgCl_2 solution, Na^+ ions in the zeolite moieties in the hybrid adsorbents were partially exchanged for Mg^<2+>, and generation of OH^- anions in aqueous solutions was suppressed. Additionally, Cl contents in the hybrid adsorbents increased probably because of suppression of OH^- anion generation during the treatment. As a result, uptake of IO_3^- and I^- by the hybrid adsorbents was improved. Sufficient adsorptive removal of Cs^+ was also attained, despite the fact that the surfaces of the hybrid adsorbents were covered with a crust of LDH.
