Fe(II)-induced phase transformation of La-substituted ferrihydrite significantly influences the reduction and immobilization of Cr(VI) in soils

Ferrihydrite (Fh) can usually transform into phases with higher crystallinities. This may greatly alter Fh adsorption capacity for chromium ions, thereby affecting their immobilizations and fates. Foreign elements [e.g., lanthanum (La)] as impurities can become incorporated into Fh. Herein, the effect of La-substitution into Fh and the influence of the presence of Cr(VI) on Fh phase transformation and fate of Cr(VI) were investigated. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Electro-chemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS) and other test methods were employed to probe phase changes after the Fe(II)-induced La-substituted Fh (La-Fh) transformations. At pH 5.5, no neoformed phase was observed until 7 d for La-Fh or pure Fh at 1 mM Fe(II) aq, but all La-Fh systems underwent transformations at 5 mM Fe(II)aq. A low pH and Fe(II)aq level favored lepidocrocite (Lp) to form, whereas goethite (Gt) was the predominant phase rather than Lp as pH increased to 7.2 with higher level Fe(II)aq. However, the La-substitution inhibits the La-Fh transformation rate and path. This made 5 % La-Fh transformed to Lp after 30 d at pH 7.2 and 1 mM Fe(II)aq. The La-release was present to some extent during the transformation process. Either the inhibition to La-Fh transformation or the La-slow release favors the retention of early-adsorbed Cr(VI) on iron-mineral surface. The added Cr(VI) might promote the La-Fh transformation even under a lower-level Fe(II), primarily attributed to the rapid reduction of Cr(VI) to Cr(III) by Fe(II), which produced more liable Fe(III) in addition to the Cr(III) that can improve the mineral conductivity, thereby enhancing the La-Fh transformation. Mechanisms responsible for the Fe(II)-induced La-Fh transformation companying the La slow-release are proposed to provide insights into the reduction and immobilization of Cr(VI) under anaerobic conditions, thus demonstrating that La-Fh is a promising material for application in Cr-contaminated site remediation.