Effect of pH on the reductive dissolution rates of iron(III) hydroxide by ascorbate

The kinetics of reductive dissolution of iron(III) hydroxide by ascorbate have been investigated as a function of pH and temperature. The reductive dissolution rate is relatively constant in the pH range of 4.3-6.0 and decreases markedly from pH 6.0 to 7.6. This pH effect is examined (1) by evaluating the dependence of the reductive dissolution rate of iron(III) hydroxide on the extent of ascorbate adsorption and on temperature and (2) by investigating the adsorption of iron(II) onto the surface of iron(III) hydroxide as a function of pH in the absence and in the presence of ascorbate. The experimental results demonstrate that the rate of reductive dissolution of iron(III) hydroxide is proportional to the extent of ascorbate adsorption over the pH range of 4.3-7.6. The dissolution rate shows the same pH-dependence trend as the surface coverage of ascorbate. The adsorption of iron(II) onto the surface of iron(III) hydroxide occurs above pH 7.0 in the absence of ascorbate but is pH independent in the presence of ascorbate. Thus, the pH effect on the reductive dissolution rate of iron(III) hydroxide can be interpreted by the pH-dependence of ascorbate adsorption. Using the data of dissolution rate and temperature, the apparent activation energies (E(a)) of the overall dissolution reaction are estimated as 27 kJ/mol at pH 6.0 and 47 kJ/mol at pH 7.5. The high values of the activation energy suggest that the overall dissolution process is controlled by the surface reaction. A higher activation energy barrier must be overcome when the dissolution reaction shifts from pH 6.0 to 7.5.