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 mu mol 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 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 loading levels between 0.38 and 2.69 mu mol m(-2).