Comparison of methods for measuring metal desorption from soils for parameterizing rhizosphere models

Sorption curves are commonly used to predict the partitioning of ions between the soil solid and solution in ion transport and rhizosphere models. We review methods for measuring metal desorption for such purposes, and we illustrate the problems involved by means of measurements of Ni desorption from a contaminated soil as an example. We find that measurements of metal concentration in shaken soil suspensions at a range of solution:soil ratios give similar desorption curves to the more tedious method of repeated extraction at a constant, smaller and therefore more realistic solution:soil ratio. We also find that desorption is sensitive to the nature and concentration of the background electrolyte used, and to interactions between these and additions of organic chelating agents of the sort excreted by roots and rhizosphere microbes.