Novel nitrogen-doped KFeS2/C composites for the efficient removal of Cr(vi)

Adsorbents with excellent redox or adsorption properties are beneficial for the efficient removal of heavy metals. In this work, a novel nitrogen-doped KFeS2/C composite (NSC-1.3) was first prepared by a simple one-step pyrolysis method and used to effectively remove Cr(VI) from aqueous solutions. The kinetics and adsorption isotherms of Cr(VI) were in accordance with pseudo-second-order kinetic and Langmuir models, respectively. The maximum adsorption capacity of NSC-1.3 was 575.68 mg g−1 at 318 K and 326.8 mg g−1 at 298 K. The removal mechanism of Cr(VI) by NSC-1.3 was first discussed by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) simulation. The XPS analysis showed that nitrogen or carbon functional groups provide abundant active sites for Cr(VI) adsorption, and Sn2− or S8 in NSC-1.3 reduces highly toxic Cr(VI) to less toxic Cr(III). The XPS analysis showed that the removal mechanism of Cr(VI) was through adsorption and redox reaction, but the dominant one was the redox reaction. DFT simulations showed that N atoms doping was feasible and thermodynamically stable. In addition, H2CrO4 makes stronger intermolecular hydrogen bonds and larger adsorption energies with the N-doped KFeS2 layer during the adsorption process, indicating that the N-doped system has a higher adsorption capacity for Cr(VI). This study confirms the importance of the adsorption and redox reaction mechanism of this new composite material, and has promising potential for the removal of highly toxic Cr(VI) from aqueous solutions.