Testing a biexponential kinetic model for the heterogeneous isotopic exchange of phosphate ions on several types of soil

In this study, a new kinetic equation was applied for the heterogeneous isotopic exchange of phosphate ions on soil. Phosphate sorption on chernozem, marshy meadow and meadow soil was studied by P-32 heterogeneous isotopic exchange in steady-state. The soil samples were incubated with 5 different amounts of KH₂PO₄ for 1, 3, 13, 10, 12 and 22 weeks. Then they were equilibrated with destilled water, and after the equilibrium was reached, H₃³²PO₄ radiotracer was added to the soil solution. At different times samples were taken from the soil solution, and radioactivities were determined by liquid scintillation (LSC) technique. The concentration of phosphate ions was determined by photometry. The relative radioactivity of soil vs time could be fitted well by the biexponential kinetic equation, which assumes two types of weakly bonded phosphate. From the fitting parameters of the biexponential equation and the phosphate concentration determined by photometry the amounts of the two types of weakly bonded phosphate and their steady-state rates were determined. The amounts of weakly bonded phosphate were plotted vs phosphate concentration - data points could be fitted by a Langmuir-like representation or the S-shape sorption isotherms. The rate orders of the desorption proved to be 1 or 2; rate constants were determined. It can be concluded that in some cases cooperative sorption can occure, which can be due to hydrogen bonding between the hydrogenphosphate ions on the surface.