Nitrogen-doped chitosan-Fe(III) composite as a dual-functional material for synergistically enhanced co-removal of Cu(II) and Cr(VI) based on adsorption and redox

Efficient synergistic co-removal of Cu(II) and Cr(VI) was accomplished with a novel nitrogen-doped chitosan-Fe(III) composite (NFEC). Coexistent Cu(II) and Cr(VI) elevated mutual adsorptive capacities by 83.05% and 100.27%, and accelerated initial uptake rates by 23.58% and 123.10%, respectively. Moreover, common anions including NO3−, Cl−, SO42−, CO32− and PO43− all promoted Cu(II) adsorption, and toxic heavy-metal cations such as Ni(II) and Cr(III) also enhanced Cr(VI) removal. Synergistic adsorption and redox interactions occurred during the co-removal process based on Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The large enhancement of Cu(II) adsorption onto NFEC in the presence of Cr(VI) was driven by the electrostatic shielding of Cr(VI) against the repulsion force between Cu(II) cations. Meanwhile, the redox between Cr(VI) and chitosan matrix could significantly consume protons and thus weaken the site competition from protons with Cu(II). The preloaded Cu(II) conversely served as cation bridge for Cr(VI) enrichment via electrostatic attraction. Cu(II) also replaced protons of hydroxyl and carboxyl groups in NFEC via ion exchange, facilitating the redox of Cr(VI) and avoiding the unfavorable species conversion of HCrO4− to CrO42−. In addition, NFEC could be well regenerated for repeated use without significant capacity loss in continuous 5-cycle adsorption-regeneration runs. Overall, the facile fabrication, synergistic performance and excellent reusability suggest NFEC is qualified for practical application in integrated treatment of combined inorganic cation-anion pollutants.