Spectroscopic study on heavy metals stabilization by repetitive in situ iron oxide synthesis

In a spatially limited contaminated site, in situ synthesis of iron oxide would be an option for metal stabilization. We proposed the repetition of in situ iron oxide synthesis through additional simultaneous injection of iron oxide synthesizing solution, Fe(NO₃)_3, and NaOH to enhance the stability of heavy metals. In this study, iron oxide was repeatedly synthesized, and the behavior of immobilization of Cd, Zn, and As was investigated. Cd, Zn, and As were surface-adsorbed on the first synthesized iron oxide 69±0.97%, 38±1.6%, and 23±0.96%, respectively. The remaining metals were adsorbed into the ferrihydrite structure (i.e., incorporation). Repeating the synthesis resulted in reduced surface adsorption rates of Cd and Zn at 54±2.6% and 23±1.5%, respectively, while As adsorption remained constant at 23±0.54%. Meanwhile, the crystallinity of the second and third-synthesized iron oxides measured using X-ray diffraction (XRD) was similar to that of pure ferrihydrite. Encapsulation of the surface adsorbed metals occurred due to particle growth on pre-synthesized iron oxide by agglomeration on the surface. However, encapsulation of surface-adsorbed As was not observed, probably due to its inhibitory effect on ferrihydrite agglomeration. The incorporation and encapsulation of heavy metals were determined by scanning transmission electron microscopy (STEM). The zone axis [11¯0] fast Fourier transform (FFT) reveals that the interplaner lattice space (d-spacing) of the c-axis elongates by 0.001 – 0.012 nm compared to pure ferrihydrite. In addition, a common zone of axis of brighter spots was also found at multiple bulk sites of iron oxide of high-angle annular dark-field (HAADF) image, indicating metal incorporation within the iron oxide. The abundance of metals as an atomic ratio measured by TEM energy-dispersive X-ray spectroscopy (EDS) line-scanning from the edge to the bulk site of iron oxide showed encapsulation of metals by repetition. The metal abundance at the edge site decreases with additional synthesis due to an increase in the number of Fe in the surrounding area, while a constant abundance of stabilized metal from a single synthesis was consistently detected across the edge to the bulk site. However, As shows a constant abundance in both single and repetitive synthesis. The Fourier transform of Fe K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) was also able to determine the incorporation of heavy metals within the iron oxide by the single synthesis. As expected, the number of neighboring second and third shell-ions increased and those radial distances were shortened by 0.01 – 0.03 Å are distinctively observed for Zn and As. Electron probe micro-analyzer (EMPA) analysis results indicate a lower relative metal concentration within the third-synthesized iron oxide structure than the first synthesis. This result exhibits repetitive synthesis induces agglomeration and aggregation of previously synthesized iron oxide further encapsulates surface-adsorbed metals which explains the reduction of surface-bound metals extractability with an increasing number of the iron oxide synthesis. The findings suggest that repeated synthesis of iron oxide can enhance the stabilization of heavy metals by encapsulating Cd and Zn, which were previously adsorbed on the iron oxide surface synthesized by a single application.