Leaching can be defined as the process by which soluble constituents are dissolved and filtered through the soil by a percolating fluid, while percolation can be described as the movement of water downward and radially through subsurface soil layers, continuing downward to groundwater (US EPA, 1997 in ADEME, 1999). This led Tas & Van Leeuwen (1995) to define leachate as water or wastewater that has percolated through a column of soil or solid waste in the environment (in ADEME, 1999). The laboratory terminology describes the leaching test as a technique of leaching of solid products by an appropriate solvent in order to extract its soluble fraction (ADEME, 1999). Leaching tests are a kind of extraction technique.
Extraction may be achieved in a number of ways that may be usefully classified as follows:
• Static tests in which the solid specimen is placed in a container with a fixed volume of fluid (leachant) for a certain period of time during which a static equilibrium is reached between the solid and the solution. Such tests are carried out in few hours. Among them one can distinguish:
о Single batch tests in which the leaching solution is unique. Depending on the test method this may, or may not, involved agitation of some form to quickly reach steady-state conditions. Agitated tests focus on measuring the chemical properties of a material-leachant system rather than the physical, rate-limiting mechanisms. In non-agitated extraction tests the material and leachant are mingled but not agitated: these tests measure the physical, rate- limiting mechanisms.
о Serial batch tests in which a series of single leaching tests is carried out on the same solid specimen. Such an approach, by means of a succession of steady states, is intended to exhaust the total amount of removable pollutant or, at least, to monitor change in leaching with volume of water passing. Leaching tests are generally carried out in few hours. They are simpler to apply but are less realistic than the percolation tests described next.
• Dynamic tests in which, in a column, a continuous supply of fresh leachant is passed through the specimen and withdrawn after contact with the solid fraction. Contrary to static tests, they allow assessment of the release as a function of time. After a while a dynamic equilibrium can be reached generating a continuous release. Such tests can last up to several dozens of days. Among them one can distinguish:
о Up-flow percolation tests in which the column is fed from the bottom. This method implies saturated conditions and avoids preferential flows in the column. It may induce pressure migration. The flow through the column is easier to control than in the down-flow percolation test and this means that it is more often used despite being less representative of usual flow conditions in soils. о Down-flow percolation tests in which the leachant flows under gravity through a partially saturated column. These tests are especially useful in studying biochemical activity in the vadose zone (Fig. 7.10).
As many soils are rather impermeable, the test is often accelerated by applying large pressure differentials across the specimen of soil, but this reduces the contact time of the water with the solids, so careful interpretation of results is then necessary to ensure that the laboratory result can be applied to the in-situ conditions with meaning.
In each of these tests, the fluid can be water (often distilled and deionised) or it may seek to be representative of in-situ or “worst-case” groundwater (e. g. a weak acid). For static leaching tests, when the water content of the material is too high (sediments, sludge), interstitial water can be recovered by means of a centrifuge.
The centrifuged pellet is then used to carry out leaching. The centrifuge supernatant may also be subjected to testing.
Chemical mechanisms controlling the release of pollutants are dissolution, sorption and diffusion. Diffusion will be controlled by concentration differences and by the total available contaminant content (which can be far lower than the total content). The pH of the material and its environment (in the laboratory: the solvent) are most important as dissolution of most minerals and sorption processes are pH dependent. The oxidation/reduction state of the material and its environment influences the chemical form of the contaminant and its solubility. Complexed forms are generally more soluble than non-complexed ones. The presence of solid and dissolved organic matter or humic substances can enhance the leaching. High ion strength of the solution in the material or in its environment generally increases the leaching of contaminants. Temperature increase leads to higher solubility. Lastly, time of contact is an important factor for the release amount (van der Sloot & Dijkstra, 2004).
The form of the material (granular, monolithic or cemented) is an important physical factor influencing the transport of a contaminant from the material to the liquid phase. Indeed, the release behaviour of granular materials is percolation (advection) dominated, while for monolithic materials it is diffusion dominated. For granular materials, the particle size determines the distance between the centre of the particle and the surface area of exchange and also, for a given amount of material, the total exchange surface. The latter factor is also important for monolithic materials, considering the shape of the monolith. For granular (in column) and monolithic materials, the porosity and the permeability are important factors on release (van der Sloot & Dijkstra, 2004).
Several parameters can be controlled in leaching test protocols in order to highlight different leaching behaviours:
• the relative amount of solvent in contact with the material (expressed in litres/kilogram of dry material, or sometime in litres/sq. metre for monoliths) or the flow through columns;
• the nature of the solvent (generally de-ionised water);
• the time of contact;
• the pH of the solvent (natural or controlled in order to maintain specific values);
• the granular or the monolithic form of the material;
• the crushing of the material to a certain particle size;
• the porosity and the permeability of compacted granular materials implemented into columns; and
• temperature (which generally is ambient temperature or controlled at 20°C).
Table 7.1 presents some examples of typical leach test methods that can be found in the literature. Also listed, for completeness, are speciation tests that aim to separate out different leaching species.
