A Universal Dimensionless Phosphate Adsorption Isotherm for Soil

Abstract

Abstract A unique dimensionless phosphate adsorption isotherm, covering the solution concentration range 10 −6 to 10 −1 M P, which has been found to fit 17 materials including 15 soils, pure kaolinite and amorphous Al(OH) 3 , is derived. The soils shown to fit the isotherm include chromic luvisols, pellic vertisols, calcic luvisols, a tropeptic eutrorthox, a volcanic ash soil from New Zealand, and a number of English soils. The necessary parameters to derive, a complete adsorption isotherm for a particular soil, from the universal isotherm, requires three adsorption experiments in the high concentration range (10 −4 to 10 −1 M P) to define the linear portion of the isotherm and the critical concentration. Three other determinations in the range 10 −6 to 10 −4 M P are needed to obtain the Freundlich isotherm from the data after correction for the effect of the linear region of the isotherm at high concentration. The Freundlich constant a was shown to be a capacity factor, and the constant b to be related to the chemical potential of the phosphate in solution. For the soils studied the variation in the magnitude of b was found to be smaller (0.11 to 0.37) than in a (1.60 to 9.66). The advantage of this universal isotherm over conventional isotherms, is that it requires less experimental work to define and is applicable over a wide range of P concentrations such as might be found surrounding a fertilizer granule.