Kinetics, equilibrium modeling, and thermodynamics on removal of Cr(VI) ions from aqueous solution using novel composites with strong base anion exchanger microspheres embedded into chitosan/poly(vinyl amine) cryogels

Abstract

Efficient removal of hexavalent chromium Cr(VI) is a stringent issue which requires efficient and, preferable, reusable sorbents. In this context, novel composite sorbents were designed in this work consisting of a combination between the chitosan (CS) and a synthetic polycation, poly(vinyl amine) (PVAm), in a ratio of 80:20, w/w, dual-cross-linked with glutaraldehyde (GA) and ethylene glycol diglycidyl ether (EGDGE), in which the microspheres of some porous strong base anion exchangers (IEx) having (vinylbenzyl diethyl 2-hydroxyethyl)ammonium chloride functional groups, and sizes in the range 90–200μm, were evenly dispersed. Macroporous CS/PVAm/IEx composites as cryobeads or monoliths were applied to the sorption of Cr(VI) from synthetic aqueous solutions, in batch mode. The results revealed that the sorption of Cr(VI) oxyanions was high in the range of pH 3–6, and abruptly decreased when pH was >6. The pseudo-first order kinetic equation well fitted the kinetics profiles obtained with all composites indicating that electrostatic interaction dominated the sorption process. Equilibrium sorption results of Cr(VI) onto composite cryobeads were the best described by the Sips, Langmuir, and Temkin isotherm models. The maximum sorption capacity evaluated by the Langmuir model at 25°C was in the range 200–320mg Cr(VI)/g adsorbent. The sorption process proved to be spontaneous and endothermic. The composite cryogels could be repeatedly used in the sorption of Cr(VI), a low decrease of the adsorption capacity being observed after five consecutive sorption/desorption cycles. The remarkable sorption capacities, and high reusability of the CS/PVAm/IEx composites suggest the promising potential of these novel sorbents in the removal of Cr(VI).