Room-temperature Mössbauer spectra of precipitation-obtained β-FeOOH synthesized at 0.10 M, 0.37 M, and 0.55 M concentrations of FeCl₃·6H₂O in the solution

The samples of of β-FeOOH were synthesized using a procedure based on the hydrolysis of trivalent iron (with FeCl₃·6H₂O as the iron precursor) followed by precipitation, with the main objective being to evaluate how the molar concentration of the precursor affects the structural and morphological characteristics of the resulting material. In a typical synthesis, a 1 M aqueous NaOH solution was added dropwise to an aqueous FeCl₃·6H₂O solution under continuous stirring, while maintaining the reaction temperature in the range of 55–65 °C. The resulting colloidal sols were then kept at 65 °C to initiate gelation and to allow gel aging for 24 h. After aging, the precipitate was separated by centrifugation and washed several times with distilled water until the pH of the supernatant reached near-neutral values (pH 5.5–6.0). Finally, the products were dried at 100 °C overnight. Three systems (F1, F2, and F3) were prepared using different initial molar concentrations of FeCl₃·6H₂O in the precursor solution, namely 0.10 M, 0.37 M, and 0.55 M, respectively.