Coordination of Adsorbed Boron: A FTIR Spectroscopic Study

We studied B adsorption on amorphous aluminum and iron hydroxides, allophane, and kaolinite as a function of pH and initial B concentration. Boron adsorption lowered the point of zero charge of all four adsorbents, implying specific adsorption (inner-sphere complexation) of B. We provided novel information on the coordination of B adsorbed at mineral-water interfaces by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The ATR-FTIR spectra of interfacial B species were influenced by pH and mineral type. Strong trigonal B and weak tetrahedral B bands of the asymmetric stretching mode were observed on the difference spectra at pH ≈7 for amorphous iron hydroxide, whereas both strong trigonal and tetrahedral B bands were found at pH ≈10 . A strong IR band of asymmetric stretching of tetrahedral B shifted to higher frequencies in am-Fe(OH)3 paste at both pH’s relative to that of boric acid solution at pH 11. Trigonal B asymmetric stretching bands shifted to higher frequencies on the difference spectra for am-AI(OH)3 and allophane at both pH’s compared to that of boric acid solution at pH 7. Polymerization of B on mineral surfaces is shown to be possible. The results provide spectroscopic evidence that both B(OH)3 and B(OH)4- are adsorbed via a ligand exchange mechanism.