Variable dispersivity in a column experiment containing MnO2 and FeOOH-coated sand

Birnessite (δ-MnO2) has swelling properties that resemble those of clay minerals. If birnessite is present in a sediment, its swelling or shrinking may affect the dispersivity of the sediment. To study the physical behavior of birnessite, a column packed with birnessite-coated sand was injected with a sequence of CaCl2, MgCl2, FeCl2 and KBr solutions.

The dispersivity of the sediment as a function of the solution composition was evaluated from breakthrough curves. The birnessite-coated sediment showed in the Mg-form a 4–8 times higher dispersion than in the Ca-form. This is related to differences in swelling behavior observed with X-ray diffraction at 0% relative humidity.

After reduction of δ-MnO2 with FeCl2 and precipitation of Fe(OH)3, the dispersion is 10–15 times higher than in the original sediment in the Ca-form.