Fourier transform infrared spectroscopy study of acid birnessites before and after Pb2+ adsorption

To provide fundamental knowledge for studying the relative content of vacant sites and exploring the mechanism of interaction between Pb2+ and birnessite, Fourier transform infrared spectroscopy (FTIR) of birnessites with different Mn average oxidation states (AOS) before and after Pb2+ adsorption were investigated. The number of absorption bands of FTIR spectra was determined by using the second derivatives of the original spectra. The band at 899-920 cm(-1) was assigned to the bending vibration of -OH located at vacancies. The bands at 1059-1070, 1115-1124 and 1165-1171 cm(-1) could be attributed to the vibrations of Mn(III)-OH in MnO6 layers, and the intensities of these bands increased with decreasing Mn AOS. The bands at 990 and 1023-1027 cm(-1) were ascribed to the vibrations of Mn(III)-OH in the interlayers. Mn(III) in MnO6 layers partially migrated to interlayers during Pb2+ adsorption, which led to an increased intensity of the band at 990 cm(-1). The band at 564-567cm(-1) was assigned to the vibration of Mn-O located at vacancies. This band could split by coupling of vibrations due to Pb2+ and/or Mn2+ adsorbed at vacant sites. The large distance between the band at 610-626 cm(-1) and that at 638-659 cm(-1) might reflect small Mn(III) ions located in Mn(III)-rich rows.