The maximum of phosphate adsorption at pH 4.0: why it appears on aluminum oxides but not on iron oxides.

We conducted a series of phosphate adsorption experiments with alumina and hematite at a range of initial P concentrations and pH values. We found that at low initial P concentration of <323 micromol/L for alumina and <194 micromol/L for hematite, phosphate adsorption showed a plateau from pH 2.5 to pH 6.5, followed by an abrupt decrease at pH 6.5-7.0. However, as initial P concentration increased, adsorption on alumina displayed a sharp maximum at pH 4.0, while adsorption on hematite exhibited a steep increase with decreased pH but did not seem to reach a maximum even at pH<3.0. Using surface complexation theory, this study proves that the adsorption maximum at pH 4.0 for alumina results from (1) phosphate being adsorbed preferentially as protonated complexes at low pH and high surface P coverage and (2) surface electric potential approaching a plateau with decreased pH due to the absence of proton-reactive but phosphate-nonreactive triply coordinated hydroxyls at transformed surfaces of Al oxides. The absence of triply coordinated surface hydroxyls at the hydrated oxide surface seems to be the reason why the adsorption maximum appears on Al oxides but not on Fe oxides.