Microwave-hydrothermal synthesis was employed to produce Na-birnessites. Crystalline, single-phase materials were obtained at temperatures as low as 120 degrees C and times as short as 1 min. X-ray diffraction and Raman spectroscopy were used to characterize the structural features of the nanostructured powders. Bimessites possessed a monoclinic structure in space group C2/m with nine Raman-active bands, all of which were observed for the first time due to optimized acquisition of the spectroscopic data. The highly reactive materials produced were submitted to sorption experiments with As(III). An oxidative precipitation occurred with the production of Mn(II) arsenate at higher arsenic concentrations. In addition, the formation of hausmannite (Mn3O4) was confirmed by X-ray diffraction and Raman analyses of the reacted solid phase. The observed 14 Raman-active modes were adjusted according to the tetragonal I4(1)/amd space group for hausmannite. An additional band related to the breathing mode of the arsenate was observed, leading to the conclusion that adsorption onto hausmannite takes place in addition to the oxidative precipitation of manganese arsenate. (C) 2007 Elsevier Ltd. All fights reserved.