Intersection of isotherms for phosphate adsorption on hematite.

Adsorption isotherms for phosphate on hematite were prepared at pH 3.39, 4.16, 5.10, 5.63, and 6.71 in this study. It was found that the adsorption isotherms at pH 5.63 and 6.71 intersected those at pH 4.16 and 5.10. Using surface complexation theory, this study demonstrates that the intersection of adsorption isotherms results from (1) phosphate being adsorbed mainly as protonated complexes at pH 4.16 and 5.10 but as nonprotonated complexes at pH 5.63 and 6.71; (2) the electric potential (psi) at the surface of hematite changing with pH at a rate less than 29.5 mV per pH unit (-d psi /dpH approximately equal 8.9 mV/pH). Fundamentally, however, it seems that the dominance of an imperfect (001) crystal face in the hematite sample is responsible for a low value of -d psi/dpH and the intersection of adsorption isotherms. The adsorption behavior may be regarded as characteristic behavior of protonation of adsorbed phosphate on an oxide with a small value of -d psi / dpH.