Effect of bifunctional isocyanate linker on adsorption of chromium (VI) diphenylcarbazide complex onto β-cyclodextrin

Abstract

Two polymers has been synthesized using bifunctional reagents of isocyanate group as crosslinking agent, which are β-cyclodextrin polymer crosslinked hexamethylene diisocyanate (β-CDP-HMDI) and b-cyclodextrin polymer crosslinked toluene-2,4-diisocyanate (β-CDP-TDI). The adsorption of Cr(VI) ions from aqueous solution by β-cyclodextrin polymer (β-CDP) as an adsorbent was studied in batch adsorption experiments. The method was based on the complex of Cr(VI) with 1,5-diphenylcarbazide (DPC) in acidic solution loaded on β-CDP-HMDI and β-CDP-TDI adsorbents. The adsorbents were characterized using Fourier transform infrared spectroscopy. The adsorption isotherm and kinetics of Cr(VI)-DPC complex onto both of b-cyclodextrin polymer were studied. It showed that the Langmuir isotherm is better fit described the adsorption process for β-CDP-HMDI, while Freundlich isotherm is better fit for β-CDPTDI. The pseudo second order kinetic model represented the adsorption process for both of β-cyclodextrin polymer adsorbents. However, the rate constant of adsorption by β-CDP-TDI was lower than β-CDP-HMDI. The thermodynamic study showed that the enthalpy (DH°) value of β-CDP-TDI was higher than β-CDP-HMDI, indicated that β-CDP-TDI has strong chemical bond with Cr(VI)-DPC. Furthermore, the value of free energy (ΔG°) and and entropy (ΔS°) showed that β-CDP-TDI was more absorbable of Cr(IV)-DPC complexes than β-CDP-HMDI. From these results, it was revealed that cross-linking agent is the main factor that contribute in the adsorption process of Cr(VI)-DPC complex onto β-cyclodextrin polymer.