Dissolution of natural and commercial melanterite generates a significant decrease in pH that is not thermodynamically explained by Fe2+ hydrolysis. It has been recently proposed that the production of acidity is actually caused by hydrolysis of Fe3+ occurring in trace amounts in melanterite. Following this finding, the experiments of melanterite dissolution previously conducted by the author have been reviewed and modeled with PHREEQC. Without invoking oxidation of Fe2+ to Fe3+, modeling results indicate that the amount of Fe3+ needed to significantly decrease solution pH is low (0.16–0.20 wt%) and already contained in melanterite in the form of contaminant.

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