HRTEM study of freeze-dried and untreated synthetic ferrihydrites: consequences of sample processing

Two-line ferrihydrite samples were synthesized following conventional procedure. Detailed characterizations of freeze-dried and untreated samples – prepared from direct inclusion of the fresh precipitates in hydrophilic resin – were made by high resolution transmission electron microscopy to investigate in detail their structural organization and to compare the two types of preparations. Only highly disordered chain-like aggregates of 2–5 nm diameter size nano-particles were revealed within the untreated samples. Conversely, in freeze-dried samples, domains with different degrees of order were recognized, from poorly ordered 2-line ferrihydrite to 3-line ferrihydrite particle aggregates that revealed lattice fringes. Associated with these aggregates, a goethite phase with a modified crystal morphology was also observed. These HRTEM observations revealed that freeze-drying techniques induce artifacts by disturbing ferrihydrite aggregates while direct inclusion in hydrophilic resin preserves the primary organization of such poorly ordered phases.