Fe²⁺ adsorption on iron oxide: the importance of the redox potential of the adsorption system

We have demonstrated that the redox potential of the solution containing an inert electrolyte, ferrous ions, and metal oxide allows a much better understanding of the process of Fe(II) adsorption on oxide with the increase of pH. We tested two oxides: γ-Fe₂O₃ (maghemite) and TiO₂ (the mixture of anatase and rutile). For both of them, we determined proton surface charge, Fe(II) uptake curves, electrokinetic potential, and redox potential in solution. The all measured quantities except the last, behaved in an almost identical manner for ferric oxide and titanium dioxide. The redox potential strongly depends on the pH of the solution and stabilizes when the adsorption process is finished (pH above 6). We observed that for the solution consisting only of 0.2 mM Fe(II) and 0.1 KCl the redox potential in the pH range 3–6 can be expressed by the formula: $Eh\left[mV\right]=1120-3\times 59\times \text{pH}-59\times \log \left(a\_\text{Fe}\right(\text{II}\left)\right)$.