Dependence of the mechanism of phase transformation of Fe(III) hydroxide on pH

The phase transformation from Fe(OH)3 gel to α-Fe2O3 particles at different initial pHs at about 100 °C was studied. The time necessary for completing the above process was determined. The results showed that the time of phase transformation from Fe(OH)3 gel to α-Fe2O3 particles shortened with the increase of initial pH at pH < 4.5. In this pH range, β-FeOOH, as an intermediate product, was obtained and hematite was formed by dissolution/reprecipitation mechanism. However, in the pH range from 4.5 to 9.0, the transformation time prolonged with increasing pH. In this pH range, no intermediate product was found. From Fe(OH)3 gel to hematite, there are two transformation pathways. One is the dissolution/reprecipitation mechanism and the other is the solid state transformation mechanism. With the pH close to the point of zero charge (pzc) of Fe(OH)3 gel, the later mechanism gradually predominated.