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Summary Band 33

Spang, Günter: Measurement and modelling of sorption and diffusion processes in a loess soil – Batch- and column-experiments with ions of different matrix affinity. Bonner Bodenkundl. Abh. 33 (2000), 272 p.


Laboratory experiments were used to measure the processes of sorption and diffusion for se-lected ions in a loess soil. The work was conducted to characterize the influence of these pro-cesses on the transport of the ions in soil columns. The data obtained in the experiments were used for calibration and validation of new simulation models ESORP and ESTIM.
An extended version of the Freundlich sorption isotherm (s = c1/n – Q) gave the best description of the data from the ad-/desorption batch experiments (24 h agitation). The ad-sorption of the different ions in the Ah soil horizon increased in the following manner: SO42- < Na+ < Mg2+ < K+ < phosphate-P < MoO42- < Zn2+ < Cd2+. In the subsoil, stronger adsorption was observed for all ions - with exception of MoO42- - compared to Ah horizon. All ions in-vestigated - with exception of SO42- and Na+ - revealed a sorption hysteresis.
The batch experiments showed bimodal sorption kinetics, which demonstrated the temporal dependence of  phosphate and zinc sorption. This sorption behavior could be de-scribed by a sorption/diffusion model that was developed, i.e. ESORP, which assumed spon-taneous adsorption on outer surfaces and, subsequently, a long term solid state diffusion to the inner surfaces of soil particles, especially iron oxides. The sorption hysteresis observed during desorption of phosphate  was also accurately described by the applied diffusion equations.
In column percolation experiments breakthrough curves (BTC’s) were derived. The BTC’s for the conservative tracer Br, obtained in water saturated flow experiments, were simulated using the transport model ESTIM. The model calculations were based on the con-vective-dispersive equation and proved to be successful. In the column experiments that were conducted with higher flow velocity, a physical nonequilibrium was observed and could be described with ESTIM, considering the concept of mobile/immobile pore regions in soil. The simple linear adsorption of sulfate in the soil columns was very accurately simulated with ESTIM using a Freundlich type sorption isotherm. The sorption nonequilibrium observed during phosphate transport could be adequately described in most of cases by the model,
ESTIM, using coupled sorption/diffusion equations. The BTC’s for Zn, Ca and Mg showed a strong impact of cation exchange processes. The Zn-BTC’s revealed snowplow effects as a reflection of the fast displacement of the adsorbed ions from outer surfaces.
 The sulfate sorption parameters determined in batch- and column-experiments differed only by a small degree. However, Freundlich sorption parameters derived from phosphate BTC’s showed the strong influence of transport conditions and revealed a wider range than the Freundlich coefficients determined in batch studies.