Acid Extraction Overestimates the Total Fe(II) in the Presence of Iron (Hydr)oxide and Sulfide Minerals

Acid extraction followed by a colorimetric assay is a widely used approach for the determination of the total Fe(II) in samples from suboxic and anoxic environments. Under sulfidogenic conditions, these samples can contain both metal sulfides and iron (hydr)oxides. Our comparisons between 0.5 N HCl extraction and X-ray absorption fine structure (XAFS) analysis of systems containing 40 mM Fe(III) as specific (hydr)oxides and 10 mM FeS showed that the presence of sulfide can result in significant overestimation of the total Fe(II) by acid extraction. The total Fe(II) determined by XAFS was consistent with the added FeS. However, the total Fe(II) determined by extraction was significantly higher and dependent on the nature of the Fe(III) (hydr)oxides; the rates and extent of Fe(II) production were higher with ferrihydrite and lepidocrocite than with goethite. Total Fe(II) concentrations determined following acid extraction might be overestimated when iron (hydr)oxides and sulfide minerals are present in samples.