Several aspects of testing materials by leaching tests are addressed, such as efficiency in testing, typical issues in testing of specific materials, use of leaching apparatus and role of particular test conditions.
Evaluation scheme using characterization tests, factors affecting release, modelling, simplified testing, regulatory aspects and field data illustrating changes in release behaviour with time are given.
In the PDF an evaluation for leaching of Pb is shown.
The underlying goal of environmental assessments and regulatory criteria is to provide a judgement as to the acceptability of using, storing or disposing of a material in the context of a specific scenario (e.g., construction use, landfill disposal, mine reclamation). The potential for great detail in testing, chemical analysis and simulation is limitless – economic resources and time are not! Thus, balance must be struck between evaluation requirements, inherent uncertainty, prior information from the same or related cases, and the value of new information. Tiered approaches to environmental assessment and regulatory criteria are essential to striking the necessary balance. These approaches typically combine prior detailed understanding of related cases with step-wise assessment of the specific case under consideration.
When conclusions are drawn on environmental acceptability, one must ensure that it is based on a balanced evaluation of possible critical components and not on one or two perceived critical constituents. Drawing too far reaching conclusions on limited testing should be avoided. Referring for acceptance on a waste treatment option or use of a waste in construction to regulation-defined parameters may not be sufficient for a scientific paper, as other parameters may surface that were not before considered relevant or were not sufficiently known at the time the regulation was drafted. Particularly, in case of treatment options the conditions may change so drastically that an entire new spectrum of constituents can be of relevance for judging the new material.
In case of leaching test methods, there are many single step leaching tests, that each provide information on one specific experimental condition, and more sophisticated tests, that allow release mechanisms to be identified and a quantification of intrinsic leaching parameters. Modelling of leaching can also be performed at different levels of detail (hyperlink to modelling impact scenarios).
An example of a tiered assessment involves a first evaluation at total content level. A second tier can be the available content as defined by the maximum quantity leached over the pH range 1.5 – 13 using the pH dependence test. A next level narrows down on the release over the applicable pH domain relevant for the material considered in the application scenario to be judged. A next level takes the release mechanism – percolation of diffusion dominated – into consideration over a time frame of relevance. At each level substances of potential concern can be dropped as they prove not to be critical for the evaluation.
Proper waste management, satisfying environmental and health related criteria, requires a fair amount of information on a wide range of materials. This information need covers a.o. a proper material description, composition data, physical properties of a material, data on leaching of inorganic and organic constituents and, whenever relevant, biological properties. Release of constituents through leaching is a key aspect for judging environmental impact of management options focussed on beneficial applications as well as on disposal, because most pathways of transport go in one way or another through a dissolution phase.
For any country or industry facing waste management problems, generating all this information alone is costly. As all countries and industries are faced with the similar problems on very much the same types of waste, pooling information across boundaries is therefore a sheer necessity. However, without proper guidance on how to collate such information and identify the gaps in the knowledge, such an action is futile.
In principle, a sufficient number of particles is needed for a solid sample to be representative. However, this is not the only factor at play as a distinction must be made between content analysis and leaching of solids.
In content analysis sometimes rather small samples are processed, which puts additional requirements on sample representativity. This is generally solved by extensive size reduction. There are limits as metallic parts or plastics can not be size reduced that easily and may lead to large fluctuations in content when a subsample does or does not contain a small solid metal piece.
In leaching, additional factors play a role that limit the effect of particle size as solubility limitations have a large effect on leachability with pH as an important control variable. In leaching also the presence of organic matter and as a consequence dissolved organic matter plays a role.
In the graphs below the leaching of two MSWI bottom ash size fractions is presented using pH dependence leaching test (EN 14429). The fractions tested are the 0 mm – 4 mm fraction normally required in the standard and the 2 mm – 10 mm fraction separated from the original bottom ash laboratory sample. According to both number of particles and considerations relating to surface area, the results should not be as close as they appear to be. This is linked to the solubility control that governs leaching. If there is no solubility control the results obtained in the pH dependence test would result in a flat line, like the one observed for K. In case of Cu and Fe, the increased mobility in the pH range 6 – 11 is related to the different level of DOC in the 0 mm – 4 mm vs the 2 mm – 10 mm fraction. None of the other elements shown feature this type of release behaviour, which implies that for all other elements one or the other form of solubility control is active (mineral precipitation/dissolution, sorption to iron oxides or particulate and dissolved organic matter).
Leaching of MSWI bottom ash size fractions (0 – 4 mm; nominal median around 0.05 mm and 2 – 10 mm; nominal median around 3 mm) illustrating role of solubility controlled release and hence more significant effect of pH differences than an effect of particle size on release.
In batch leaching, an end over end tumbler or excentric roller table is the preferred agitation device as other types of agitation devices lead to too much size reduction (wrist shaker) of the test material or incomplete exposure (shaker table) to leachant.
Performing a percolation test with alkaline flue gas treatment waste and similar materials with self-cementing properties may lead to disruption of the column apparatus when a peristaltic pump is applied for controlled leachate flow. As the material start to cement the net flow through the column decreases and pressure builds up. A solution to the problem is to pre-wet the material, thus allowing the cementation reaction to complete , then crush the materials and place in a column for percolation.
When in the leaching tests with long contact times (percolation and monolith leaching) eluates are exposed to the surrounding atmosphere there is a substantial chance that the pH in resulting alkaline eluates is affected by uptake of CO2 from the laboratory atmosphere. This effect can be very large when carbonation experiments or other CO2 producing activities in the laboratory environment take place. The collection bottles should be closed off as much as possible and verification of the magnitude of the effect can be tested by placing a similar collection vessel filled with solution adjusted to pH 11 for the same period of the eluate collection cycles and measuring pH at regular intervals.
A few questions arise around this topic:
Overall, it is important in case of heterogeneous materials to keep the purpose of the testing in mind in finding a solution to a heterogeneity issue as it may not be relevant for the main purpose of the test as shown in the case of leaching of MSWI bottom ash for environmental impact evaluation.
In the case of waste treatment for beneficial use, the current often limited regulatory tools mainly consisting of a single step extraction are applied as the tool against which environmental behaviour of treatment of materials is judged. In addition, in most cases a very limited set of parameters is tested. What is insufficiently recognised is that making a change in the material properties may induce changes to the new material that possibly reduces the release of the constituents of primary concern but lead to an increase in the release of other parameters. For instance, high pH materials may exceed Pb limits, but upon neutralisation oxyanions like Sb, Mo, V, As may show an increase in their release. When such constituents are not analysed, such changes go unnoticed and false claims are made that after treatment the material is “environmentally” acceptable.
