Laboratory experiments and modelling studies of leaching of intermittently drained columns

Abstract

This paper examines the intermittent leaching of columns of porous ceramic spheres, as analogues of soil aggregates, in 3 ways: theoretically, by laboratory experiments, and by simulation. The work extends earlier investigations of continuously saturated columns to the case where columns drain during breaks in the leaching process.Solute movement during displacement is described by the mobile–immobile convection–dispersion equation but, because air replaces solution as the soil drains, it is considered to be immiscible rather than miscible displacement. During the rest period solute redistributes within the aggregates with the concentration at the surface of aggregates increasing. During the next leaching period transfer of solute into macropores is consequently faster. This results in a greater leaching efficiency.

The intermittent drained leaching is less efficient than the continuously saturated intermittent leaching (by 16% under the conditions of our experiment) as, in the latter system, solute can diffuse from within aggregates into macropores during rest periods. Nevertheless there is a benefit compared with continuous leaching.

Modelling reveals (i) that the first-order mass-transfer rate coefficient increases as the number of leaching cycles increases, (ii) that the concentration profiles of solute down the column of aggregates differ substantially between the saturated and drained situations, and (iii) that the concentrations in the mobile and immobile solutions at the base of the column converge towards the end of each leaching cycle.