Phosphate desorption from flooded and reoxidized soils as compared with adsorption characteristics

Abstract

The purpose of this study was to show how phosphorus (P) desorption can bring further information to what is inferred from adsorption experiments. A simple calculation procedure is proposed to predict the P amount that would be desorbed by washing with salt solution if adsorption was completely reversible. Consequently, the interpretation of P desorption can focus on the irreversible part of adsorption. This approach is applied to samples of acid sulfate soil from Vietnam that have been submerged for different periods of time, at 20°C and 30°C, and also to samples that were reoxidized after flooding. Phosphate desorption linearly increases with P concentration in solution and exponentially decreases with increasing adsorption capacity as expressed by the Freundlich coefficient of adsorption isotherms. These two types of relationships are correctly predicted by our calculation procedure. As far as reversibility is concerned, we find that with respect to calculated desorption, the proportion of irreversible adsorption greatly differ according to treatments. In relative terms, adsorption reversibility is lowest in the reoxidized soils and highest in the wet soils incubated at 30°C. This is related to the type and crystallinity of Fe‐oxihydroxides and consequent differences in P‐bonding energies.