Quantitative Analysis of Iron Oxides and Investigation of Ferrihydrite Phase Transformation Using Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy

Ferrihydrite, as a metastable poorly crystalline iron oxide, spontaneously transforms into more stable iron oxides. Identifying intermediate and final products during its transformation is significant for understanding environmental geochemical processes such as contaminant fate and nutrient cycling. In recent years, quantitative analysis methods based on Fourier transform infrared (FTIR) spectroscopy have been applied in iron oxide research. During ferrihydrite transformation, coexistence of multiple minerals may cause mutual interference in infrared spectral characteristics; the type and proportion of coexisting minerals significantly affect characteristic peak intensities, leading to reduced accuracy of quantification models established solely on binary systems in multi-mineral systems. Therefore, this study employed total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to observe infrared absorption characteristics by preparing binary and ternary standard mineral mixtures with varying proportions. Results show that characteristic absorption peaks of lepidocrocite are located at 748cm⁻¹ and 1020cm⁻¹, while goethite peaks occur at 792cm⁻¹ and 897cm⁻¹. In different systems, relative content of goethite exhibits strong linear relationships with characteristic peak intensity (R²>0.987), but its linear regression coefficients are affected by coexisting minerals with differences reaching 14.4%. Relative content of lepidocrocite shows linearity with characteristic peak intensity in binary systems, but in ternary systems, a second-order polynomial provides better fitting performance, demonstrating that coexisting mineral types directly affect quantitative model establishment. Further detection of relative contents of secondary minerals (including lepidocrocite and goethite) during anaerobic 1mmol/L Fe(Ⅱ)-catalyzed ferrihydrite transformation reveals that in the initial reaction stage, ferrihydrite simultaneously transforms into lepidocrocite and goethite. Subsequently, ferrihydrite is completely consumed, transforming the system into a binary mixture of lepidocrocite and goethite. Ultimately, lepidocrocite is completely converted to goethite. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202504280109.