Stability Mechanism of Hexavalent Chromium Reduction by Nano-zerovalent Iron Under Different Environments

Nanoscale zero-valent iron (NZVI) has a promising application in the remediation of hexavalent chromium in aqueous environments, but its stability in the remediation environment has rarely been investigated. In this study, firstly, natural NZVI(N-NZVI) and sodium alginate–modified NZVI (S-NZVI) were used to reduce hexavalent chromium in water. The results revealed that S-NZVI showed the highest removal rate of Cr(VI) under the same conditions. Then, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to characterize the reduced solid product (Cr-NZVI), which proved to be composed of chromite, chromferide, maghemite, magnetite, hematite, and lepidocrocite. To evaluate the stability and associated risks of Cr-NZVI, this study also simulated the corrosive effects of Cr-NZVI under different environments. The results found that natural ageing (0–60 days) had only a minor effect on the stability of the reduced product. However, the structure of the reduction products may be damaged under strong acidic conditions and in the presence of large amounts of SiO₃²⁻, HPO₄²⁻, Al³⁺, Co²⁺, organic acids (humic acid (HA), citric acid (CA), diethylenetriaminepentaacetic acid (DTPA), and oxalic acid (OA)) and H₂O₂. Organic acids showed significant effects (p < 0.001), especially 0.5 M CA dissolved 90% of the iron and 50% of the chromium in the reduction products. Thus, our results suggest that hexavalent chromium reduced with NZVI cannot be left without attention and that its stability in the environment and possible safety issues are of concern.