Synthesis of a chromium (III) hydroxide nanocomposite – KU-2-8 cation exchanger

Abstract

The study is devoted to the preparation of a chromium (III) hydroxide nanocomposite - cation ex-changer by the inclusion of Cr(OH)3 into the matrix of a strongly acidic cation exchanger. A hydroxide-containing hybrid cation exchanger was synthesized based on polymer carrier with KU-2-8 sulphonic acid ion-exchange groups, step by step transferring functional groups from sodium to Cr3+-form and precipitating chromium (III) hydroxide into the grain of the cation exchanger with an alkali NaOH solution. The chromi-um content in the nanocomposite was determined based on the concentration of counterions released into the solution by direct potentiometry. The surface morphology of hybrid chromium-polymer granules was inves-tigated by scanning electron microscopy. Elemental analysis of the composition of the cleavage of nanocom-posite was determined by the energy dispersive X-ray analysis. It was found that the surface of the hybrid granules was uniform. Sulphur and sodium were fairly evenly distributed throughout the entire volume of polymer granules, with the exception of the outer layer depleted in these elements. Chromium was found not only in the surface layer of the granules with the thickness of 10–15 μm, but also in the bulk of the grain, where its concentration was significantly lower. The distribution of chromium throughout the grain was une-ven. It was shown that the chromium concentration decreased exponentially from the grain periphery to the centre. Elemental mapping confirmed that in the cation exchanger chromium granules are in oxidized form. The mechanism for the formation of a chromium (III) hydroxide nanocomposite - KU-2-8 cation exchanger with sulphonic acid ion exchange groups was proposed. During the ion-exchange saturation reaction the tran-sition of the ion exchanger into the Cr3+-form takes place. A precipitate of chromium (III) hydroxide was localized near the surface of the cation exchanger during the alkalizing of the reaction medium. The for-mation of Cr(OH)3 precipitate in the nanosized pores of the cation exchanger prevents the diffusion transfer of Cr3+ ions towards its surface, while the Donnan exception does not allow hydroxyl ions in sufficient con-centration to penetrate deep enough into the volume of the ion exchanger granule. The possibility of obtain-ing nanoparticles of a modifier (chromium (III) hydroxide) with an uneven distribution in the volume of granules, combined into micrometer-sized agglomerates, has been shown.