EXPERIMENTAL INVESTIGATION OF TRANSPORT OF STRONGLY RETAINED SPECIES BY SOIL COLUMNS MARCO PETRANGELI PAPINI and MAURO MAJONE University of Rome “La Sapienza” – Department of Chemistry P. le Aldo Moro, 5–00185 Rome, Italy (Received 14 August, 1995; accepted 26 March, 1996) Abstract. 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-breakthrough 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 mL 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. In 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. Key words: transport, soil columns, breakthrough curves, lead, proton 1. Introduction The ability to predict the transport of hazardous chemicals in soils and subsoils is an important task either for the correct management of waste disposal and for the contamination assessment of polluted sites (e.g. uncontrolled dumps). The move- ment of a solute through soils is a complicated phenomenon which involves several different mechanisms. While the rate of percolation is influenced by hydrogeolog- ical parameters, physical and chemical characteristics of both soil and liquid phase are responsible for the partitioning of the solute between the two phases in contact and thus for its retardation during the transport. Generally, the evaluation of solute movement through soils should be performed by three strictly related steps, essentially comprising batch reactor experiments, laboratory column experiments and subsequent scaling up of the results to the natural soil of a given site if the preceding steps are satisfactory (Schweich and Sardin, 1981, Amoozegar-Fard et al., 1984). In this framework, batch experiments are used to obtain the equilibrium dis- tribution function of a solute between the solid and liquid phases while column experiments are performed to reproduce, as well as possible, the field conditions and to validate the transport modelling. Author for all correspondence. Water, Air, and Soil Pollution 95: 337–351, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.