Phosphate sorption by natural hematites

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Abstract
Summary: Iron (hydr)oxides are active phosphate sorbents in soils and sediments, but no information exists on phosphate (P) sorption by natural hematites. In this study, we characterized the chemical, mineralogical and P‐sorption properties of 14 hematite‐rich natural materials of different origins. Phosphate sorption was described by a modified Freundlich equation including a time term. Phosphate sorbed at 1d at an equilibrium concentration of 1 mg P dm−3 ranged widely (0.2–1.7 μmol m−2). After 1 d, hematites showed a marked slow sorption. At 75 d, and an equilibrium concentration of 6mg P dm−3, the total amount of P sorbed ranged between 0.8 and 4.1 μmol mp−2. Phosphate‐sorption capacity was influenced by the morphology of the coherently scattering domains: the shorter the domains in the crystallographic c relative to the a direction, the lower the P‐sorption capacity. This has been observed by other authors in synthetic hematites and agrees with the idea that the prismatic faces, which have singly co‐ordinated Fe‐OH groups, are more active in P‐sorption than other faces. The average P sorption capacity of hematites was similar to that of natural goethites (2.6 μmol P m−2) but, in contrast to these, variability among samples was high. In comparison with goethites, hematites show slower sorption and lower affinity for phosphate.