Reactions of ferrous iron with hematite

The adsorption of Fe(II) onto hematite was measured as a function of pH, su rface area, and time. The effects of anions (chloride, sulfate, or nitrate) and of Zn(II) were also determined. All experiments were conducted under s trict anoxic, conditions with 5 or 30 days for equilibration. Results showe d that immobilization of Fe(II) on hematite consists of a fast sorption pro cess and one or more slow processes, which probably include both sorption a nd formation of new phases. Sorption occurred at pH values as low as 4, whi ch has not been reported in existing literature. Some Fe(II) could not be e xtracted after 20 h with 0.5 N HCL In the presence of 0.01 M NaCl, all of t he 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 re sults are consistent with auto-catalytic formation of magnetite, which was stable relative to hematite for pH above 5.9. However, when sulfate was gre ater than 1 mM, unextracted Fe(II) was observed at pH above 5 where only ap proximately 15% of added Fe(II) was recovered by a 0.5 N HCl extraction; th ese results could not be explained by precipitation of magnetite nor of kno wn sulfate phases. Based on these results, existing models for adsorption o f Fe(II) onto ferric oxides (based on experiments of several hours to a day ) are not accurate for prediction of environmentally significant Fe(II) rea ctions with ferric oxides, when much longer times are available for reactio n. There was no competition between Zn(II) and Fe(II) for 0.25 mM or less a nd 90 m(2) 1(-1) hematite. Zn(II) was completely recovered using 0.5 N HCl for every condition that was tested. (C) 2001 Elsevier Science B.V. All rig hts reserved.