ATR–FTIR Spectroscopic Investigation on Phosphate Adsorption Mechanisms at the Ferrihydrite–Water Interface

We investigated the phosphate(P) adsorption mechanisms at the ferrihydrite–water interface as a function of pH, ionic strength (I), and loading level, using a combination of adsorption envelopes, electrophoretic mobility (EM) measurements, and attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy. The P adsorption envelopes show that: (1) adsorption decreases with increasing pH 3.5–9.5 and (2) adsorption is insensitive to changes in I at pH 4–7.5, but it slightly increases with increasing I from 0.01 to 0.8 at pH>7.5. The EM in 0.1 M NaCl decreases with increasing P concentration from 0 to 50 μmol L−1 at pH 3–9. The results of these macroscopic studies suggest the formation of inner-sphere complexes. The ATR–FTIR investigation shows that inner-sphere surface complexes are nonprotonated, bidentate binuclear species (≡Fe2PO4) at pH≥7.5 and could be associated with Na+ ions at P loading levels of 0.38 μmol m−2. At pH 4–6, protonated inner- sphere complexes are proposed at the loading levels between 0.38 and 2.69 μmol m−2.