CATION-TRANSPORT IN NATURAL POROUS-MEDIA ON LABORATORY-SCALE - MULTICOMPONENT EFFECTS

Multicomponent transport experiments were performed with four major ca tions, Na+, K+, Ca2+ and Mg2+, in laboratory columns packed with a non -calcareous soil. The breakthrough curves are explained quantitatively with a box model including cation exchange. We use a single set of se lectivity coefficients, an independently verified value of the cation- exchange capacity (CEC), and an adjusted value of the Peclet number. T his Peclet number is smaller than the value determined from independen t tracer experiments. The model is able to predict all experimentally observed breakthrough curves quite well. The selectivity coefficients determined from binary exchange experiments prove unreliable for the p rediction of multicomponent experiments. We propose to estimate the se lectivity coefficients by directly fitting the multicomponent breakthr ough curves. Their shape is a very sensitive function of the values of these coefficients. Concepts from non-linear chromatography can be us ed in order to interpret several qualitative features of the breakthro ugh curves.