MECHANISM OF ADSORPTION OF Mn2+ ONTO A SYNTHETIC BIRNESSITE

Adsorption of Mn2+ onto birnessite is very rapid and amounts adsorbed are several times higher than its cation exchange capacity. The nature and fate of the adsorption, however, were not well understood. Thus experiments were conducted to investigate the mechanism of adsorption of Mn2+ onto a synthetic birnessite and effects of three Mn-salts (MnSO4, MnCl2 and Mn(NO3)2) on mineralogical alteration of birnessite.The results indicated that adsorption of Mn2+ onto birnessite included both specific and non specific adsorption. The adsorbed Mn2+ was incorporated into the structure of birnessite and its extrability by salts decreased with time of aging. After reaction of birnessite with MnCl2 or Mn (NO3)2 solution, incorporation of Mn2+ into he structure caused transformation of birnessite into nsutite (Mn oxide hydroxide, -Mn (O, OH)2). In the solution of MnSO4, birnessite was convened into nsutite and a larger quantity of ramsdellite (Mn oxide, MnO2) . Both newly formed minerals are more stable and better crystallized than birnessite. Thus the Mn2+ initially adsorbed onto birnessite became the component of the newly formed minerals and was fixed in the structure after structural conversion of birnessite and finally lost its extractability and availability.