ATR-FTIR spectroscopic investigation on phosphate adsorption mechanisms atthe 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) adsorpti on decreases with increasing pH 3.5-9.5 and (2) adsorption is insensitive t o changes in I at pH 4-7.5, but it slightly increases with increasing I fro m 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 mu mol L-1 at pH 3-9. The results of these macr oscopic studies suggest the formation of inner-sphere complexes. The ATR-FT IR investigation shows that inner-sphere surface complexes are nonprotonate d, bidentate binuclear species (=Fe2PO4) at pH greater than or equal to 7.5 and could be associated with Na+ ions at P loading levels of 0.38 mu mol m (-2). At pH 4-6, protonated innersphere complexes are proposed at the loadi ng levels between 0.38 and 2.69 mu mol m(-2). (C) 2001 Academic Press.