Influence of pH on Formation of Taranakites
摘要
Monoammonium phosphate (1 mol L-1), monopotassium phosphate (1 mol L-1) and aluminum ohloride (5×10-3 mol L-1) were used to investigate the influence of pH on the formation of NH4-taranakite and K-taranakite in systems with a high NH4H2PO4 or KH2PO4 concentration. The experimental data indicated that the reaction products of aluminum with NH4H2PO4 or KH2PO4 changed with the pH in the systems. In a pH range of 2.5 to 10.0, as the pH increased, the products in either ammonium or potassium system followed the sequence: taranakite, amorphous (NH4, K)2AlH(PO4)2 .4H2O, and then crystalline (NH4, K)AlPO4OH.2H2O. However, the pH ranges of the formation of these products in ammonium system were different from potassium system. NH4-taranakite formed from pH 2.75 to PH 5.75, whereas K-taranakite formed in the pH range of 3.00 to 5.00. From the theoretical calculation by a computer program (GEOCHEM version 2.0), NH4-taranakite and K-taranakite could form at the pH from 1.50 to 8.30 and from 1.25 to 8.45,respectively. These pH ranges were much wider than the experimental results. The difference between the experimental data and theoretical data was attributed to the lack of kinetic data and/or the incompleteness and inaccuracy of the thermodynamic data in the data base of the program. The PH ranges of the formation of the taranakites indicated that the taranakites could exist in the immediate vicinity of phosphate fertilizer zone as reaction products of phosphate fertilizers with soils, especially acidic soils, resulting in the fixation of not only phosphate but also nitrogen and/or potassium in soils.