XRD, FT-IR, and thermal analysis of bauxite ore-processing waste (red mud) exchanged with heavy metals

In the present work we show the results of XRD, FT–IR and thermal analysis of non-treated (RMnt) and acid-treated red mud (RMa), a bauxite ore processing waste, exchanged with Pb2+, Cd2+ and Zn2+ cations. These studies were performed to investigate the changes in the sorbent structure caused by the exchange with metals of different ionic radii. The XRD pattern of RMnt, analysed according to the Rietveld method, showed a mixture of eight different phases. However, 78 wt % of the RMnt was made by just 3 phases: cancrinite (33 wt %), hematite (29 wt %), and sodalite (16 wt %). XRD patterns of RMnt exchanged with Pb2+ and Cd2+ cations revealed two additional phases, namely hydrocerussite [Pb3(CO3)2(OH)2 (10% wt)] and octavite [CdCO3 (8% wt)], respectively. These two phases have been likely originated by the carbonate precipitation processes which were due to the decarbonation of cancrinite. Hydrocerussite and octavite were not found in the case of acid–treated red mud samples. In the FT–IR spectra the introduction of cations caused a distinct shift to higher wavenumbers in the peak at ~ 1100 cm-1, which is attributed to the asymmetric stretch of the Si–O–Al. This effect may be associated to the Pb2+, Cd2+ and Zn2+ adsorbed by the red muds which caused a deformation of the initial structure. Thermal analysis data of the red mud samples were obtained by TG/DTG, and they were employed to evaluate the desorption behavior of water, and to clarify the thermal stability of the chemical phases of the different red mud samples. The loss of metal-bound water in the red mud samples was found to depend on the size of non-framework cations and follow always the order: Zn2+>Cd2+>Pb2+.