Preparation of thermosensitive, calix[4]arene incorporated P(NIPAM‐co‐HBCalix) hydrogel as a reusable adsorbent of nickel(II) ions

A calix‐conjugated thermo‐responsive hydrogel containing 15% tetra(5‐hexenyloxy)‐p‐tert‐butylcalix[4]arene (HBCalix), P(NIPAM‐ co ‐HBCalix), was used to remove nickel(II) ions from water. Both thermo‐sensitive properties and the Ni 2+ ‐adsorption capabilities of the prepared P(NIPAM‐ co ‐HBCalix) hydrogels are investigated. Introduction of the monomer HBCalix considerably enhanced the adsorption of Ni 2+ onto the hydrogel by chelation between hexenyloxy groups and metal ion. When HBCalix units capture Ni 2+ and forms HBCalix/Ni 2+ host–guest complexes, the lower critical solution temperature of the hydrogel shifts to a higher temperature due to both the repulsion between charged HBCali/Ni 2+ groups and the osmotic pressure within the hydrogel. Adsorption studies were carried out by varying contact time, counter ion and initial concentration of Ni 2+ . The evaluation of adsorption properties showed that the hydrogel exhibited better correlation with Langmuir isotherm model. P(NIPAM‐ co ‐HBCalix) could be used repeatedly with little loss in adsorption capacity by simply changing the environmental temperature. This kind of ion‐recognition hydrogel is promising as a novel adsorption material for adsorption and separation of Ni 2+ ions.