PHOSPHATE SORPTION BY NATURAL HEMATITES

Iron (hydr)oxides are active phosphate sorbents in soils and sediments , but no information exists on phosphate (P) sorption by natural hemat ites. In this study, we characterized the chemical, mineralogical and P-sorption properties of 14 hematite-rich natural materials of differe nt origins. Phosphate sorption was described by a modified Freundlich equation including a time term. Phosphate sorbed at 1 d at an equilibr ium concentration of 1 mg P dm-3 ranged widely (0.2-1.7 mumol m-2). Af ter 1 d, hematites showed a marked slow sorption. At 75 d, and an equi librium concentration of 6 mg P dm-3, the total amount of P sorbed ran ged between 0.8 and 4.1 mumol m-2. Phosphate-sorption capacity was inf luenced by the morphology of the coherently scattering domains: the sh orter the domains in the crystallographic c relative to the a directio n, the lower the P-sorption capacity. This has been observed by other authors in synthetic hematites and agrees with the idea that the prism atic faces, which have singly co-ordinated Fe-OH groups, are more acti ve in P-sorption than other faces. The average P sorption capacity of hematites was similar to that of natural goethites (2.6 mumol P m-2) b ut, in contrast to these, variability among samples was high. In compa rison with goethites, hematites show slower sorption and lower affinit y for phosphate.