The Transformation of Two-Line Ferrihydrite into Crystalline Products: Effect of pH and Media (Sulfate versus Nitrate)

Two-line ferrihydrite (Fh), ubiquitous in soils, groundwater, and aquatic sediments, may serve as an important sink for sequestering trace metals, metalloids, and organic matter via adsorption/coprecipitation due to its high surface area and reactivity. Although considerable attention has been paid to the transformation process of this thermodynamically metastable solid, little is known about the transformation products, the crystallization rates, or the transformation routes of two-line Fh in sulfate- and calcium-rich environments. This work systematically investigates the transformation of 2-line ferrihydrite produced by using different neutralization reagents (CaO vs NaOH) at different pHs (4 and 8), temperatures (25 degrees C, 40 degrees C, and 80 degrees C), and media (sulfate vs nitrate). X-ray diffraction, Raman, Fourier transform infrared spectroscopy, and chemical extraction were employed to characterize the transformed solids as well as the crystallization rate of 2-line Fh. The results show that the crystallization products in nitrate media include hematite (major) and goethite (minor) under acidic conditions (pH 4) and only hematite under alkaline conditions (pH 8) and are independent of the neutralization reagent used. By contrast, in sulfate media, goethite is the dominant product under slightly acidic conditions (pH 4) with 6-line ferrihydrite as the intermediate product, whereas under slightly alkaline conditions the formation of hematite is favored. Chemical extraction results indicate that the transformation process of those CaO-neutralized solids precipitated at pH 8 is intensely retarded by calcium ion. The results suggest that neutralization reagents (calcium ion), as well as the reaction media (sulfate ion), play an important role in the 2-line ferrihydrite crystallization process.