Manganese oxides are important scavengers for heavy metals in natural environments. Electron transfer and atomic exchange between aqueous Mn(II) and Mn(III/VI) oxides induce dissolution and transformation that affects the fate and transport of heavy metals. In the present work, three hexagonal birnessites previously reacted with Cd(II) (Cd240-Bir, Cd120-Bir, and Cd60-Bir) were reacted with Mn(II) at pH 7 under anoxic conditions. The results indicated that the reaction with Mn(II) reached a steady state within 30 min. The adsorbed Cd(II) had a slight effect on the removal of Mn(II). The transformation of birnessites and the release of adsorbed Cd(II) increased with longer reaction time up to 6 days. The concentration of the released Cd(II) in the solution increased with an increase in Cd(II) in the birnessites and Mn(II) concentration. For the Cd240-Bir samples, no other manganese oxide formed after reacting with 0.2-0.8 mmol/L of Mn(II) for 6 days, and only a small quantity of hausmannite (Mn3O4) as a product resulted after reacting with 2 mmol/L of Mn(II). Increasing the concentration of Mn(II) to 4 mmol/L resulted in the birnessites transforming to feitknechtite (β-MnOOH). However, only Mn3O4 formed with the birnessites reacted with lower Cd(II) contents (Cd120-Bir and Cd60-Bir) after reaction with 0.8-4 mmol/L of Mn(II) for 6 days. These results indicated that Mn(II) could catalyze the transformation of birnessite and the adsorption of heavy metals would influence the transformation in natural environments.

Effects of adsorbed Cd(II) on the Mn(II)-catalyzed transformation of hexagonal birnessite

251st National Meeting (2016)

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