KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER

Abstract

SummaryThe rate of ion exchange reactions of humic substances (humic acid and peat) with lead ions was investigated at 25 °C. All experiments were carried out under finite solution‐volume conditions. To the well‐stirred aqueous suspensions of the above samples lead ions were added in such small amounts that, after attainment of the equilibrium, the initial equivalent fraction of lead in the humic substance had increased only by a small amount. The kinetics of such differentially small ion exchange processes were studied as a function of the initial lead content of the samples. The experiments showed that the absolute rates for the uptake of small amounts of lead ions decrease, if the initial lead content of the samples increase. The relative rates of ion exchange, i.e. the rates with respect to the extent of each reaction, showed a more complicated behaviour, as they can exhibit a minimum at a certain initial lead content of the sample. The rate‐determining step of the above ion exchange experiments was shown, by interruption tests, to be diffusion of the ions through the Nernst‐film surrounding the ion exchanger particles.Theoretical calculations of the rates of film‐diffusion controlled ion exchange processes were carried out and the results revealed that the rates of any differential ion exchange reaction should decrease with increasing initial content of the ion adsorbed, their exact magnitude depending, among other quantities, strongly on the separation factor. The relative rates for the attainment of the equilibrium, on the other hand, depend much less on the initial ionic composition of the ion exchanger. A comparison of the experimental and the calculated results yields satisfactory agreement if the experimentally determined dependence of the separation factor on the lead content of the samples is taken into account.