Suppression of the conversion process of Fe(OH)₃ to β-FeOOH and α-Fe₂O₃ by silicate ions

The influence of the addition of silicate ions on the conversion processes of condensed ferric hydroxide Fe(OH)₃ gel to ferric oxyhydroxides and iron oxides was investigated under constant initial [Fe₃₊]/[OH ^-] ratio or constant initial pH. Fe(OH)₃ gel, prepared from FeCl₃ and NaOH solution, was aged at 333 K or 373 K in the presence and absence of silicate ions. The aged suspensions of each stage with colloidal state were separated into solid particles and supernatant solutions by centrifugation. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and infrared spectroscopy (IR) were used to characterize the resultant solid particles. pH measurement and inductively coupled plasma (ICP) analyses were also carried out to obtain pH value and concentration of ferric and silicate ions in the supernatant solutions. XRD patterns and TEM micrographs revealed that silicate ions suppressed the conversion of the Fe(OH)₃ gel to β-FeOOH and α-Fe₂O₃ particles in both cases. Also, a decrease in nucleation number of α-Fe₂O ₃ particles resulted in larger particle size of final stage and the morphology became ellipsoidal with increase of aspect ratio by the specific adsorption of silicate ions. Analysis of pH curve indicated that silicate ions inhibited both dissolution from Fe(OH)₃ gel to β-FeOOH and crystallization of β-FeOOH particles. ICP and IR analyses showed that silicate ions had remarkably strong affinity with ferric ion by forming adsorbed silicon species on solid particles and this was a decisive factor for the suppression of the conversion process.