Molybdate adsorption by birnessite

Molybdate (MoO42-) adsorption by manganese (Mn) oxide was investigated using a synthetic birnessite. Experiments were carried out in a batch experiment as a function of time (1 min to 28 d), pH (2-10) and the competitive anions sulphate (SO42-), phosphate (PO43-), selenate (SeO42-) and selenite (SeO32-). Furthermore, MoO42- adsorption was described as a function of equilibrium concentration at pH 4-7 and the data were evaluated with the Freundlich and Langmuir equations. The amount of adsorbed MoO42- was strongly dependent on time and reached roughly an equilibrium after three days. An increase in the pH by 1.15 units within 28 days indicates ligand exchange of surface hydroxyls with MoO42-. Molybdate adsorption showed high sensitivity to pH and reached a maximum at pH 3, near the pK(a1) and pK(a2) for molybdic acid. The Freundlich equation adequately reconstructed the adsorption data. Molybdate adsorption also conformed to the Langmuir equation for the investigated pH values. The competition sequence of anions for MoO42- adsorption by Mn oxide was SeO32- > SeO42- > PO43- > SO42-, assuming a strong adsorption mechanism of MoO42- onto the surface of birnessite. These results indicate that Mn oxides have a strong effect on the adsorption of MoO42- especially due to their higher specific surface area compared to aluminium or iron oxides. This is important regarding the availability of MoO42- in soils under agricultural conditions (pH 4-7) as well as for the treatment of soils and groundwater affected by elevated intake from industries. (C) 2015 Elsevier B.V. All rights reserved.