KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER

Abstract

Summary: The rate of ion exchange of humic substances (humic acid and peat in the hydrogen form) with lead ions was investigated. As in processes occurring in nature, the lead solution was added in all our experiments slowly and continuously at a constant feed rate to the well‐stirred water‐suspended humic substances. Control experiments were also performed with a well‐defined synthetic ion‐ exchange resin (carboxylic acid type). The uptake of divalent lead ions by the humic substance and by the ion‐exchange resin, which is coupled with the release of an equivalent amount of hydrogen ions, proceeds as a function of time according to a characteristic sigmoidal curve. During the initial phase, the rate of ion exchange increases continuously with time, even though the humic substances become more saturated with lead ions.Theoretical calculations, based on a film diffusion process as the rate‐determining step, show that this behaviour should indeed be observed if any solution, containing exchangeable counter ions, is added continuously to an ion exchanger suspended in a solvent.The theory predicts further, in agreement with the experimental results, that in the initial phase of the ion‐exchange reaction, the uptake of the counter ions is proportional to the square of the time. If experiments are performed with different rates of addition of the counter ions, the time required to exchange a certain amount of counter ions should be proportional to α^‐1/2, where α is the rate at which the ions are added. This prediction is also in accordance with the experimental results.