Acid production by FeSO₄·nH₂O dissolution and implications for terrestrial and martian aquatic systems

Combined experimental, modeling, and analytical results indicate that the rapid acidification of dilute waters in contact with nominally Fe ²⁺-sulfate minerals (FeSO₄·nH₂O) is caused by Fe³⁺ hydrolysis, which occurs when low levels (<1 mol%) of a contaminant Fe³⁺-sulfate phase are dissolved along with the FeSO₄·nH₂O. This rapid acidification has previously been attributed to hydrolysis by Fe²⁺. However, dissolution experiments performed using ZnSO₄·nH₂O, in which the Zn²⁺ cation has a higher hydrolysis constant (log K = -8.96) than Fe²⁺ (log K = -9.5), failed to produce significant changes in solution pH. We present the results of geochemical modeling simulations confirming that FeSO₄·nH₂O dissolution alone cannot explain the experimentally observed change in pH from 5.65 to 3.50. Nor can the experimental observations be explained by oxidation of Fe²⁺ to Fe³⁺ in solution. Instead, our experimental results can be best explained by modeling the incorporation of <1 mol% Fe³⁺ contamination from any number of Fe³⁺ or mixed valence Fe-sulfate phases, including anhydrous Fe³⁺₂(SO₄) ₃, coquimbite, kornelite, römerite, bilinite, copiapite, or ferricopiapite, all of which are reasonable candidate phases for oxidative breakdown products of FeSO₄·nH₂O. Laboratory Mössbauer spectra are consistent with up to 0.6 mol% of the total Fe in the sample to be present as Fe³⁺. Although the doublet has parameters that are not diagnostic of any specific Fe³⁺-sulfate, they do help constrain its identification. These results demonstrate that minor contamination of labile Fe²⁺ sulfates by Fe³⁺ can have dramatic effects on solution chemistry that should be considered when studying reactions relevant to acid mine drainage waste sites and other localities where Fe-sulfate minerals occur, such as the surface of Mars.