Enhanced Cr(VI) removal induced by electron donor in magnetic iron-nickel sulfides biochar composites

In this study, magnetic iron-nickel sulfides biochar composites (MINBs) were successfully prepared via one-step solvothermal method and applied to Cr(VI)-containing wastewater treatment. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that synthesized iron-nickel sulfides anchored and dispersed on biochar surface. Cr(VI) removal efficiency and capacity by MINB-5 (molar ratio of Ni to Fe was 5%) were more than 97% and 24.8 mg g -1 within 20 min respectively, when the initial concentration of Cr(VI) was 20 mg L -1 . Effects of different operational parameters on Cr(VI) removal efficiency were investigated, including molar ratio of Ni to Fe, dosage of catalyst, initial concentration of Cr(VI), pH value of solution, coexisting ions, natural organic matters (NOMs) and temperature. X-ray photoelectron spectroscopy (XPS) and flame atomic absorption spectrometric (FAAS) analysis demonstrated that Cr(VI) was removed through reduction process by Fe(Ⅱ), which was released from MINBs. Persistent free radicals (PFRs) of biochar, Ni(Ⅱ) and S(-Ⅱ) in MINBs jointly accelerated Fe(Ⅱ)/Fe(III) circulation, instead of direct reduction of Cr(VI) directly. These novel findings provide a new prospect of application of magnetic iron-nickel sulfides biochar composites for Cr(VI)-polluted wastewater remediation. • MINBs were synthesized by one-step solvothermal method. • Cr(VI) was immobilized on MINBs via absorption, reduction and precipitation. • Fe(Ⅱ) dissolved from MINBs was the critical role in Cr(VI) removal process. • PFRs, Ni(Ⅱ) and S(-Ⅱ) in MINBs accelerated the surface Fe(Ⅲ)/Fe(Ⅱ) circulation.