Synthesis of gallic acid functionalized magnetic hydrogel beads for enhanced synergistic reduction and adsorption of aqueous chromium

Hexavalent chromium (Cr(VI)) is one class of highly toxicant heavy metal ion that severely threatens the environmental quality and human health. Herein, a class of magnetic hydrogel beads (MHB) has been facilely developed with poly(ethylenimine)/chitosan (PEI/CS) hydrogel as the substrate material and Fe3O4 as the incorporated magnetic nanoparticles, followed by covalently chemical modification with gallic acid (GA). The well-designed MHB was then evaluated for their performance for synergistic reduction and adsorptive removal of Cr(VI) from aqueous environment. Results showed that electrostatic attraction played a crucial role in adsorption of Cr(VI) onto MHB; meanwhile, the GA grafting could not only significantly accelerate the Cr(VI) reduction, but also facilitate the chelating adsorption of resultant trivalent chromium (Cr(III)) onto PEI/CS substrate due to the proton consumption accompanied by Cr(VI) reduction, resulting in the synergistic enhancement of total chromium (Cr(T)) sorption and Cr(VI) detoxification. The batch adsorption experiments revealed that the Cr(T) adsorption capacity increased with temperature rising and the pseudo-second-order kinetic and Langmuir models could well describe the Cr(T) sorption behaviors, suggesting a single-layer, exothermal, and chemical sorption process. Maximal Cr(VI) removal capacity of GA-functionalized MHB was estimated to be 476.2 mg∙g−1, exceeding most of other related adsorbents. Moreover, the GA-functionalized MHB exhibited satisfactory reusability with no remarkable loss in Cr(VI) removal efficiency after five cycles. As compared with other reported adsorbents, the newly synthesized GA-functionalized MHB was promising for chromium wastewater treatment due to its synergistic reduction and adsorption process, ultra-high Cr(VI) removal capacity, facile magnetic separation and superior recycling performance.