Experimental investigation of transport of strongly retained species by soil columns

Column experiments have been extensively used in transport studies of major cations but few investigations are available on migration through soils of strongly retained species that are environmentally relevant (like heavy metals). By presenting some selected experiments (lead and proton step-break-through tests in different conditions), this study shows that the soil-column technique is also applicable in the case of species which exhibit very large retention factors. The use of very small soil columns (about 0.4 mt of pore volumes) combined with relatively high flow rates (0.1-0.3 mL min(-1)) allows to observe the entire breakthrough curve (adsorption and desorption front up to 5000 pore volumes) in reasonable experimental time, in reproducible conditions and without experimental drawbacks. Zn the adopted experimental conditions no kinetic effects, related to diffuse transport and sorption reaction were recognized; moreover, Peclet number was higher than 60. Consequently, it was possible to calculate the equilibrium isotherms from the diffuse fronts of the breakthrough. Knowledge that can be derived, concerning the reversibility of the adsorption process, the influence of complexation on the adsorption, the kinetics of complex formation, and the effect of dissolution on proton transport, is also discussed.