Synchronous reduction and removal of hexavalent chromium from wastewater by modified magnetic chitosan beads

As one of the highly toxic heavy metal contaminants, hexavalent chromium (Cr(VI)) poses a serious hazard to the environment and public health due to its excellent solubility in aqueous environment. To synchronously reduce and remove Cr(VI) from wastewater efficiently, a class of functionalized magnetic chitosan (CS) beads (MCB-ECH-SH/SO3H) was synthesized with CS as the substrate material and in situ assembling Fe3O4 nanoparticles as the incorporated magnetic material, followed by epichlorohydrin (ECH) cross-linking and l-cysteine grafting. Results showed that Cr(VI) in aqueous solution could be rapidly adsorbed onto MCB-ECH-SH/SO3H via electrostatic attraction, meanwhile -SH groups introduced by l-cysteine grafting could not only significantly facilitate the reduction of Cr(VI) to less toxic trivalent chromium (Cr(III)), but also promote the resultant Cr(III) chelating onto adsorbent due to the proton consumption caused by Cr(VI) reduction, resulting in the synergistic enhancement of Cr(VI) detoxification and total chromium (Cr(Total)) removal. The batch adsorption experiments revealed that the well designed MCB-ECH-SH/SO3H could be applied over a wide pH range (initial pH of 1.0–7.0), and showed much higher remove efficiency (99.6 ± 0.2 %) of Cr(VI) than MCB and MCB-ECH at initial pH 5.0 (60 °C) due to the improvement of stability and reducing capacity. The Cr(Total) sorption behaviors were well according with the pseudo-second-order kinetic and Langmuir models, indicating a single-layer, exothermal, and chemical sorption process, while the maximum Cr(Total) removal capacity of MCB-ECH-SH/SO3H was estimated to be 293.46 mg∙g−1, exceeding most of previously reported related adsorbents. Moreover, MCB-ECH-SH/SO3H could still maintained strong adsorption performance after five cycles and exhibited excellent ability to remove 95 % Cr(VI) within 1 h for low concentration Cr(VI)-containing wastewater. The combination of synchronous reduction and adsorption property, high removal capacity, convenient magnetic separation and good recycling performance makes MCB-ECH-SH/SO3H an ideal candidate for chromium-containing wastewater treatment.