Kinetics and mechanisms of monoammonium phosphate-induced potassium release from selected potassium-bearing minerals

Abstract

A high concentration of phosphate in the immediate vicinity of the phosphate fertilizer zone of soil could induce K release from K-bearing minerals to soil solution. In this study, the kinetics of K release by monoammonium phosphate (NH4H2PO4) from biotite, muscovite, and microcline were investigated. The results indicate that K release from all of these minerals was promoted by 1 M NH4H2PO4 (pH 4.0). The release of K from these minerals can be described by the zero-order rate equation. The rate coefficients of K released from the minerals (2–5 µm) in 1 M NH4H2PO4 solution ranged from 0.20 to 2.09 mg kg-1 h-1 at 25°C and from 1.22 to 9.00 mg kg-1 h-1 at 45°C, respectively. The amount of K release from the same mineral samples in 1 M NH4Cl solution (pH 4.0) at 25°C was not detectable and was too low to be determined accurately at 45°C. The dfferences in the rate of NH4H2PO4-induced K release from the different minerals were explained by the chemical composition, crystal structure, and the crystallization sequence of the minerals. The activation energies of the phosphate-induced K release from these minerals ranged from 37 to 72 kJ mol-1 and the pre-exponential factor values (Arrhenius equation) ranged from 1.2 × 106 to 7.2 × 1011 mg kg-1 h-1. The examination of the phosphate-reacted minerals shows the formation of crystalline NH4-taranakite in the muscovite and microcline systems and noncrystalline products in the biotite system. The absence of the formation of crystalline NH4-taranakite in the biotite system is attributed to the Fe perturbation. These data indicate that the major mechanism of NH4H2PO4-induced K release from the K-bearing minerals was the combined effect of phosphate and protons on the alteration of minerals. Therefore, the K-supplying rate of soils in the immediate vicinity of fertilizer zones may be enhanced through phosphate fertilization. Key words: Potassium release, kinetics and mechanisms, phosphates, potassium-bearing primary minerals