Retention of phosphorus on calcite and dolomite: speciation and modeling

The intensive application of phosphate fertilizers in agriculture has created an important source of diffuse phosphorus pollution. The interaction of phosphorus with carbonate minerals plays a role in the fate and transport of phosphorus in soil. The object of the present study was to investigate the speciation of phosphorus on two common carbonate minerals, calcite and dolomite, using a combination of batch experiments, ATR-FTIR spectroscopy, XANES analysis, and diffuse layer modeling. Within the pH range 6.0–7.0, the retention of phosphorus by calcite and dolomite is mainly attributed to the formation of amorphous calcium phosphate (Ca3(PO4)2, ACP), dibasic calcium phosphate (CaHPO4·2H2O, DCP), and hydroxyapatite (Ca5(PO4)3OH, HAP). At pH ≥ 8.0 the immobilized phosphorus takes the form of complexes =CaPO4Ca0/=sCaPO4Ca0 on the surface of calcite, followed by the formation of Ca–P phases, including ACP, DCP, and HAP, with increasing phosphorus levels (>2 mg L−1). However, the dolomite surface is initially dominated by the adsorption complex =MgHPO4Ca+ at =Mg sites, and at higher phosphorus levels it then grows due to Ca–P phases and the formation of newberyite (MgHPO4). It is interesting to note that the Mg content in dolomite favors the rapid growth of DCP at phosphorus levels >200 mg L−1. As a result, at pH ≥ 8.0, dolomite shows a stronger capacity for immobilizing phosphorus than does calcite. Dolomite therefore serves as a better phosphorus sink than calcite in calcareous soil environments.