Reactions of ferrous iron with hematite

The adsorption of Fe(II) onto hematite was measured as a function of pH, surface area, and time. The effects of anions (chloride, sulfate, or nitrate) and of Zn(II) were also determined. All experiments were conducted under strict anoxic conditions with 5 or 30 days for equilibration. Results showed that immobilization of Fe(II) on hematite consists of a fast sorption process and one or more slow processes, which probably include both sorption and formation of new phases. Sorption occurred at pH values as low as 4, which has not been reported in existing literature. Some Fe(II) could not be extracted after 20 h with 0.5 N HCl. In the presence of 0.01 M NaCl, all of the added Fe(II) was recovered when pH was below 6, but either 100% or less than 25% of added Fe(II) was recovered when pH was greater than 6. These results are consistent with auto-catalytic formation of magnetite, which was stable relative to hematite for pH above 5.9. However, when sulfate was greater than 1 mM, unextracted Fe(II) was observed at pH above 5 where only approximately 15% of added Fe(II) was recovered by a 0.5 N HCl extraction; these results could not be explained by precipitation of magnetite nor of known sulfate phases. Based on these results, existing models for adsorption of Fe(II) onto ferric oxides (based on experiments of several hours to a day) are not accurate for prediction of environmentally significant Fe(II) reactions with ferric oxides, when much longer times are available for reaction. There was no competition between Zn(II) and Fe(II) for 0.25 mM or less and 90 m² l^-1 hematite. Zn(II) was completely recovered using 0.5 N HCl for every condition that was tested.