An ATR-FTIR study of different phosphonic acids adsorbed onto boehmite

An ATR-FTIR study of the vibrational spectra of N,N-bis(2-hydroxyethyl) aminomethylphosphonic acid (BHAMP), 1-hydroxyethane-1,1′-diphosphonic acid (HEDP) and nitrilotris(methylenephosphonic acid) (NTMP) adsorbed onto boehmite is presented. The study was performed in the pH range from 5 to 9, and bands assignments are given in the 1200–900 cm−1 wavenumber range, where the bands associated with various P–O(H) vibrations can be found. The three phosphonic acids adsorb onto boehmite by forming inner-sphere surface complexes. ATR-FTIR data indicates the presence of both protonated and deprotonated mononuclear surface species. In all cases, the surface-bound ions undergo protonation reactions as pH is decreased. The results are in good agreement with previously proposed surface complexation models.