FeII oxidation by molecular O2 during HCl extraction

Environmental contextIn the environment, iron exists mainly as FeII and FeIII and plays an important role in biogeochemical processes. The FeII and FeIII content is often quantified by hydrochloric acid extraction and the acid is thought to prevent FeII oxidation by oxygen. However, we found that with increasing HCl concentration and temperature, oxidation of FeII by oxygen is accelerated. Therefore, in order to obtain reliable results extractions should be performed with dilute HCl or in the absence of oxygen. AbstractHCl is commonly used to stabilise FeII under oxic conditions and is often included in Fe extractions. Although FeII oxidation by molecular O2 in HCl is described in the field of hydrometallurgy, this phenomenon has not been systematically studied in environmentally relevant systems. The extent of FeII oxidation by O2 during extraction of soils and magnetite by HCl and in HCl/FeCl2 solutions was therefore quantified. FeII was stable in 1 M HCl at room temperature for several days, whereas in 6 M HCl at 70°C, 90% of the FeII was oxidised within 24 h. In the absence of O2, no FeII oxidation occurred. Experiments at low pH with increasing H+ or Cl– concentration alone and geochemical modelling suggested that the formation of complexes of FeII and HCl may be responsible for the observed FeII oxidation. The use of strictly anoxic conditions for Fe extraction by HCl to obtain reliable Fe redox speciation data is therefore recommended.