Composite cryo-beads of chitosan reinforced with natural zeolites with remarkable elasticity and switching on/off selectivity for heavy metal ions.

Combining ion-imprinting technology with pH-dependent adsorptive features of acid- or salt-activated zeolites brings up the opportunity to develop composite polymer materials with 'desired' sorption properties and performances. In this respect, we present here Co2+-imprinted composite cryo-beads with switching on/off selectivity towards the template ions, engineered by selecting the appropriate zeolite-treatment conditions and/or controlling the initial sorption pH values. Co2+ chelating efficiency of all cryo-beads was investigated either at pH 4 or 6 depending on zeolite conditioning strategy. The maximum sorption capacity values of ion-imprinted cryo-beads were from about 5 up to 7 times higher compared with those of non-imprinted ones. Under competitive conditions (Cu2+, Ni2+, Fe2+ and Cd2+ ions), the change of pH value from 4 to 6 resulted in a remarkable quenching of Co2+ selectivity generated by the zeolite shift from the H+-form to the Na+-form. The presence of zeolites within cryogel matrix generated composites with outstanding elasticity that allows the instant recovery of gels after full compression. These results indicate that the cryogel-type composites can be successfully re-used in separation processes for several times without losing their features.