Bio-based iron oxide nanoparticles forming bi-functional chitosan composite adsorbent for Cr(VI) decontamination

Abstract

Iron oxide nanoparticles (IONPs) synthesized in a bio-based route are viable contenders for the removal of Cr(VI) from contaminated water. However, recovery and reuse of these IONPs are challenging and could limit their practical applicability. Herein, reusable and bi-functional composite chitosan (CS) beads (CS/IONPs) are developed by infusing IONPs in the CS matrix and applied for Cr(VI) adsorption and detoxification from synthetic wastewater. CS/IONPs beads were characterized through various analytical techniques: X-ray diffraction, X-ray photoelectron spectroscopy and Scanning electron microscopy. The loading of IONPs varied from 0.1 to 0.25 (% w/v) with Fe(III)/Fe(II) in a molar ratio of 2. Mesoporous (11.65 nm) CS/IONPs beads with a size from 6.6 − 4.31 mm were formed with a specific surface area of 63.67 m2/g compared to 25.42 m2/g for CS beads. Cr(VI) removal of 98.71 % was achieved by CS/IONPs adsorbent within 480 min at the optimal conditions of initial pH 3, CS/IONPs dosage 2.5 g/L, temperature 25 °C, and initial Cr(VI) concentration 150 mg/L. The co-existing cations didn’t affect Cr(VI) adsorption capacity of CS/IONPs beads, though PO43− and SO42− competed slightly with Cr(VI) adsorption. Cr(VI) removal process was spontaneous and exothermic and followed the pseudo-first-order kinetic. CS/IONPs composite beads could adsorb a maximum of 345.53 mg Cr(VI) per g of IONPs as determined by Langmuir isotherm. The used CS/IONPs beads could be regenerated using NaOH for the initial 4 cycles, but about 15.7 % of active sites were reduced afterwards. Cr(VI) removal involves electrostatic interaction, Cr(VI) reduction to Cr(III), Cr(III) adsorption, Cr(III) surface complexation through abundant –NH2 group, and Cr(III) co-precipitation with Fe(III).