Morphology and adsorption properties of chitosan sulfate salt microspheres prepared by a microwave-assisted method

Abstract

A simple and convenient microwave-assisted method was proposed for the synthesis of chitosan sulfate salt (CSS) microspheres with chitosan and sulfuric acid as reagents. The influence of different synthesis conditions such as the concentration of H₂SO₄ and the temperature and time of microwave-heating on the microsphere characteristics, was investigated using XRD, NMR, SEM and XPS. Novel multilayer CSS microspheres or discs with oblate and hollow shapes were observed. The morphology and size (diameter of 1.12–2.0 μm and thickness of 0.19–0.96 μm) of the CSS microspheres could be controlled by changing the temperature and time of microwave heating, and the H₂SO₄ concentration. Moreover, the morphology and size of the CSS microspheres were preserved even after the NaOH treatment. A complete ionic crosslinking of chitosan by sulfuric acid could be accomplished within 30 min with the help of microwave-heating. The CSS microspheres were found to be effective adsorbents for the removal of Cr(VI) from aqueous solutions. The adsorption capacity of CSS-30 for Cr(VI) was found to be 112 mg g⁻¹, and the equilibrium was attained within 9 min. The Cr(VI) adsorption process was described well by the Langmuir isotherm and the pseudo-second-order kinetics. The chromate Cr(VI) ion was partially reduced to Cr(III) during the adsorption by CSS microspheres. Cr-absorbed CSS-30 could be regenerated by H₂SO₄ aqueous solution. The reusability experiment showed that CSS-30 was a stable adsorbent for Cr(VI).