Polyethyleneimine-modified chitosan stabilized nano zero-valent iron for enhanced removal of Cr(VI) from acidic aqueous solution: Performance and mechanism.

In this study, a novel composite material (S-nZVI@CP) with ultra-high adsorption and reduction capability was prepared by modifying chitosan (CS) with polyethyleneimine (PEI) and incorporating sulfurized nanoscale zero-valent iron (S-nZVI). The -NH stretching vibration peak at 3535-3433 cm-1 and the FeO bending vibration peak at 613 cm-1 in the FTIR spectrum, along with the characteristic nZVI peak in the XRD pattern, confirmed the successful synthesis of the composite. The morphological results revealed that S-nZVI@CP particles had a core-shell structure, which addressed the inherent agglomeration issue of nZVI. Electrochemical characterization demonstrated that S-nZVI@CP exhibited enhanced electron transport efficiency. When the PEI content, Fe:C mass ratio and S:Fe molar ratio were 4 wt%, 3:1 and 0.09, respectively, S-nZVI@CP achieved 100 % Cr(VI) removal rate. The analysis results demonstrated that PEI modification enhanced the adsorption effectiveness, reduction effect and chemical stability of CS, while sulfurized nZVI addressed its susceptibility to oxidation and agglomeration, and significantly improved its reactivity. The optimal experimental conditions for removing Cr(VI) by S-nZVI@CP are: pH = 2.0, adsorbent dosage = 0.6 g L-1, C0(Cr(VI)) = 100 mg L-1. The adsorption process was classified as a spontaneous endothermic reaction, consistent with pseudo-second-order kinetic model and Langmuir adsorption isotherm. The maximum adsorption capacity of S-nZVI@CP for Cr(VI) was 574.69 mg g-1 at 318 K. S-nZVI@CP exhibited strong tolerance to coexisting anions, maintaining removal rates of Cr(VI) over 80 % and 90 % after 6 cycles and 30 d of aging, respectively. The main pathways for Cr(VI) elimination via S-nZVI@CP involved electrostatic attraction, surface complexation, reduction and coprecipitation, among which the contribution rate of reduction was 94.7 %. Fe(II) played a crucial role in the reduction of Cr(VI), and FeSx layer facilitated the conversion of Fe(III) to Fe(II). This research provides a new approach for the efficient treatment of Cr(VI) wastewater.