Effect of iron oxide on phosphate sorption by calcite and calcareous soils

Abstract

SUMMARY Pure calcite (AR grade CaCO 3 ) was treated with ferrous perchlorate solution to give a surface coating of iron (Fe) oxide. Maximum sorption ( x m ) of phosphate (P) by the calcite increased from 18.2 to 160 mg P kg −1 as the amount of coating increased from 0.00 to 16.0 g Fe 2 O 3 , kg −1 CaCO 3 . Evidence for Fe oxide coatings on carbonate minerals in two Sudanese soils was obtained by optical microscopy and electron‐probe microanalysis. The relative contributions of carbonate and Fe oxide minerals, and Fe oxide coatings to P sorption in these soils were calculated, based on an assumed model of oxide distribution. Separate‐phase Fe oxide was the major contributor (30–40%) to P sorption in the soils; the Fe oxide coatings on carbonate minerals were only minor contributors (< 6%), and the contribution of uncoated carbonate minerals was found to be negligible (<1 %). These results suggest a very minor role for carbonate minerals, even when coated with Fe oxide, in the sorption of P by these calcareous, Sudanese soils.