Phosphorus sorption-desorption properties of Luvisols and Vertisols, sorption kinetics and models comparison

Phosphorus sorption-desorption properties of acid soils collected from the highlands of Arsi zone, Ethiopia, were assessed using batch experiment methods and five sorption isotherm models in 2018. Fourteen out of 39 composite samples from Vertisols and 31 out of 126 from Luvisols were categorized into six groups based on selected physicochemical properties. The Langmuir, Freundlich, and Freundlich-Langmuir equations were used to evaluate the equilibrium-based sorption properties; and, the Elovich and modified Freundlich for time-dependent sorption properties. The first three models were analyzed using MS Excel sorption_spreadsheet; and, the latter two using MS Excel and the solver in it. The Freundlich-Langmuir equation provided better fits at all concentration ranges than the other two; and, the modified Freundlich provided better fits and description for time-dependent sorption data. The soil with the highest phosphorus binding energy and partitioning coefficient was G4 (Luvisols); and, the order decreased constantly to G1. Sorption maxima decreased from group1 to group6. A strong positive hysteresis phenomenon was observed in G4 and a negative hysteresis in the soil with high-initial P content. Generally, Luvisols displayed higher sorption maxima, stronger affinity, and very low desorption rate than Vertisols.