Uptake and release of cerium during Fe-oxide formation and transformation in Fe(II) solutions

Fe-oxides are ubiquitous in soils and sediments and form during Fe(0) corrosion. Depending on redox conditions and solution composition, Fe-oxides such as ferrihydrite, goethite, magnetite, and green rust (GR) may form. These phases typically have high surface area and large affinity for adsorption of trace components. Further, Fe(II)-Fe(III) (hydr)oxides are redox active. Cerium, a member of the lanthanide family, can be used as an analogue for the tri- and tetra-valent actinides found in radioactive waste, expected to be stored in subsurface repositories. In experiments with ferrihydrite, Ce(III) was effectively scavenged from Fe(II)-bearing solutions within 5 min at pH 7. During transformation of ferrihydrite to green rust, however, all Ce(III) was released to solution. By varying initial solution Fe(II):Fe(III) ratio, magnetite and goethite formed together with GR_Na,SO₄, resulting in decreased Ce(III) release. X-ray photoelectron spectroscopy revealed Ce(III) adsorbed on magnetite. When Fe-oxides were synthesized by air oxidation of Fe(II) solutions at pH 7, GR_Na,SO₄ played a catalytic role in the oxidation of Ce(III) to Ce(IV) by O₂, removing more than 90% of the dissolved Ce. Transmission electron microscopy revealed that it formed discrete nanocrystals of CeO_2(s). These results demonstrate that Fe-oxide interaction with radionuclides is likely to depend strongly on the local redox conditions. By analogy with Ce, the trivalent actinides are not expected to be sequestered by preformed GR in anoxic environments. Our results also suggest that trivalent actinides and lanthanides are released when dissimilatory iron reduction of Fe(III)-oxides leads to GR formation However, under oxidizing conditions, GR may influence radionuclide mobility by catalyzing their transformation to a higher oxidation state.