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

Birnessite (delta-MnO2) has swelling properties that resemble those of clay minerals. If birnessite is present in a sediment, its swelling nr shrinkin g may affect thr dispersivity of the sediment. Tn study the physical behavi or 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 fro m 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 diff erences in swelling behavior observed with X-ray diffraction at 0% relative humidity. After reduction of delta-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. (C) 1999 Elsevier Science B.V. All rights reserved.