Determination of the adsorption process of tributyltin (TBT) and monobutyltin (MBT) onto kaolinite surface using Fourier transform infrared (FTIR) spectroscopy

In the present study, the interactions between kaolinite surfaces and adsorbed organotin molecules were investigated. The bonding mechanism and the effects to the crystal lattice were determined by means of Fourier transform infrared (FTIR) spectroscopy. Adsorption experiments were conducted at pH 4, 6 and 8. In order to examine the influence of the molecular structure on the adsorption process, two organotin compounds were used, namely tributyltin (TBT) and monobutyltin (MBT). Attraction of TBT to the kaolinite surface caused a strong decrease of OH bending modes as well as a decrease of the stretching vibrations of Al(Si)O lattice groups at wavenumbers between 1400 and 800 cm−1, while the OH stretching vibration modes at wavenumbers 4000 to 3000 cm−1 were not affected significantly. In contrast to this, the attraction of MBT molecules influenced the stretching modes at 3667 and 3651 cm−1. These bonds are related to so-called “outer hydrogen” OH groups located at the surface and along broken edges of monocrystals. In addition, a pH-dependent increase of HOH vibrations at 1637 and 3470 cm−1 could be observed. ing on the molecular structure of the butyltin compounds, different surface complexes were found. According to our results, TBT occurring with trigonal–bipyramidal molecule structure are bonded by the formation of a mononuclear complex having monodentate configuration. In this case, one oxygen atom of a Al(Si)O lattice group is directly bonded to the Sn atom of a TBT molecule. The adsorption of TBT molecules showing a tetrahedral configuration results from hydrogen bonding. MBT forms octahedral units in aqueous solution and the molecule is bonded to the kaolinite surface by reaction with the “outer hydrogen” groups of the clay minerals.