Sequestration of molybdate during transformation of 2-line ferrihydrite under alkaline conditions

• Incorporation of Mo to hematite structure during transformation of ferrihydrite was investigated.
• EXAFS analyses showed that molybdate incorporated into hematite:
• Two Mo-O shells having a coordination number (CN) of 3 and average bond distances of 1.78 ± 0.01 Å and 2.08 ± 0.02 Å.
• Two Mo-Fe shells with a CN of 3 and average bond distances of 3.10 ± 0.02 Å and 3.44 ± 0.02 Å.

Hematite is a thermodynamically stable iron oxide under the aerobic conditions present in most natural surface soils and sediments. Most studies to date have focused on the capacity of hematite to adsorb trace metals and metalloids, but structural incorporation of trace metals within hematite is less recognized. This study assessed the incorporation of molybdenum within the structure of hematite during the phase transformation of 2-line ferrihydrite under alkaline conditions (pH ∼10). Extended X-ray absorption fine structure analyses show molybdenum incorporated into hematite, with two Mo-O shells having a coordination number (CN) of 3 and average bond distances of 1.78 ± 0.01 and 2.08 ± 0.02 Å, respectively, as well as two Mo-Fe shells with a CN of 3 and average bond distances of 3.10 ± 0.02 Å and 3.44 ± 0.02 Å, respectively. This observation suggests the tetrahedrally-coordinated Mo in the molybdate that adsorbs onto the 2-line ferrihydrite changes to an octahedrally-coordinated Mo within the hematite with Mo possibly substituting for Fe in the hematite structure. Our findings suggest that molybdenum partitioning (low concentrations) to iron oxides in the environment can occur due to structural incorporation as well as adsorption.