There are many leaching tests worldwide. Most of them address more or less the same issue when placed in perspective to end pH in the test (see Relation between leaching tests – Relationship between single batch tests and pH dependence test). Below key leaching tests necessary to provide basic information to address long term release behaviour are highlighted.
Batch leaching tests are relatively simple to perform, as only a granular solid is mixed with a leachant such as demineralized water (EN 12457 series, EPA 1316) or a leachant with a specific composition (TCLP – acetic acid , CaCl2, NaNO3). The batch tests consist of one or a number of parallel extractions of a material during 24 hours at pre-set L/S values (EN 12457 series – L/S=2 and L/S=10; EPA 1316 – L/S=10, 5, 2, 1 and 0.5 L/kg). After 24 hours of equilibration (or longer in case of coarser particles) by roller table or end over end rotation in PE, PP or glass containers depending on the substances to be assessed, the suspensions are filtered (0.45 µm) and analysed for the desired substances. The pH, conductivity, dissolved organic matter (recommended) and redox state (pE) of the eluate are determined.
For better understanding, the results can best be placed in context with pH dependence test data as a function of pH and/or percolation test data as a function of L/S of the same or a similar material.
More information on relations between leaching tests.
The pH dependence test provides information on the pH sensitivity on the leaching behaviour of the material (EN 14429:2005, EN 14997:2005, ISO/TS 21268-4:2007 and EPA method 1313:2009). The listed methods lead to very comparable results (Van der Sloot et al. 2010). In EN 14429 and EPA 1313 the different pH values are obtained through initial addition of acid or base and a subsequent check of pH and adjustment after 6 hours of equilibration. In CEN/TS 14997 the different pH values are pre-set and maintained by feedback control and continuous addition of acid or base. The tests consist of a number of parallel extractions of a material at L/S = 10 during 48 hours at a series of pre-set pH values. The pH is adjusted with either HNO3 or NaOH (or KOH). In a separate titration experiment the amounts of acid or base required can be assessed or derived from prior studies on similar material. After 48 hours of equilibration by rotating on a roller table or end over end rotation in PE, PP or glass containers depending on the substances of primary interest, the suspensions are filtered (0.45 µm) and analysed for the desired substances. The pH, conductivity, dissolved organic matter and redox state (pE) of the eluate are determined. The test provides an acid-base neutralisation curve and liquid-solid partitioning (LSP) curve as function of eluate pH. This curve shows the response of a material to imposed pH changes and an acid-base titration to understand the response of the material to acid or base reactions under environmental scenarios (e.g. carbonation, infiltration, sulphur oxidation, changes at material-soil interfaces).
Advantages and applicability of pH dependence leaching test
– Provides leaching behaviour of substances as a function of pH
– Provides acid/base neutralization capacity information
– Basis for geochemical speciation modelling
– Identification of sensitivity of leaching to small pH changes
– Provides information on pH conditions imposed by external influences (e.g. carbonation, acidification)
– Basis for comparison of international leaching tests
– Mutual comparison of widely different materials to assess similarities in leaching behaviour
– Recognition of factors controlling release
– For non-interacting species possible to assess sub-sampling error
Applicable to almost any material
The monolith leach test provides information on the release per unit surface as a function of time and it is performed on regular shaped product samples according to standardized procedures (EN 15863:2009, EPA method 1315:2009 and CEN/TS 16637-2:2009). The listed methods lead to very comparable results (Van der Sloot et al. 2010). The specimen is subjected to leaching in a closed tank. Demineralised water is used as the leaching solution at a leachant to surface area (L/A) of 80 l/m2 (CEN methods) or leachant-to-product volume ratio (L/V) of approx. 5 (EPA method). The leaching solution is renewed at defined time intervals. The pH, electrical conductivity (EC) and, if needed Eh, are measured in all eluates before filtration (0.45 µm) followed by chemical analysis of desired substances. The test provides flux and cumulative release as a function of leaching time. From this method the predominant release mechanism can be derived. From non-interacting substances (e.g. Cl, Na, K) tortuosity can be estimated.
Advantages and applicability of dynamic surface leaching test
– Provides release as a function of time largely controlled by diffusion
– Estimation of material tortuosity based on release behaviour of highly soluble substances (often soluble salts)
– Isolation of surface wash-off effects
– Relevant for materials with monolithic character (durable materials) or materials behaving as monolith (low permeability soil and sediments)
– Identification of solubility control versus dynamic leaching possible
– Basis for verification of reactive/transport modelling
– Quantification of intrinsic release parameters
– Projection towards long term behaviour possible
Applicable to a wide variety of monolithic construction materials (e.g. concrete, masonry, asphalt) and (cement) stabilised products.
The Compacted Granular Leach Test (CGLT) is applicable to sediments and clayey soils (low permeability) with only one side covered with a layer of inert glass beads exposed to the leachant.
The up-flow percolation test provides information on the leaching behaviour of the material as a function of the liquid to solid ratio (L/S in litre/kg) (EN 14405:2004, ISO/TS 21268-3:2007, EPA method 1314:2009, CEN/TS 16637-3:2020). The listed methods lead to very comparable results (Van der Sloot et al. 2010). Seven eluate fractions are collected over the L/S range 0.1 – 10 litres/kg, with the total test duration ranges from about 21 days to 14 days. The leachant is demineralized water. The test material is applied as received and up-flow is applied through a column with a waste height of about 25 cm and a diameter of 5 cm or 10 cm depending on the particle size of the material. The test provides a liquid-solid partitioning (LSP) curve as function of L/S and an estimate of pore water concentrations. L/S can be related to a time-scale through the infiltration rate, density and height of the application (Hjelmar 1990). The interpretation of the test results is based on the local equilibrium assumption, i.e., that near-equilibrium exists between the solid and liquid phases at all points of the column.
Advantages and applicability of the percolation leaching test
– Provides release as a function of liquid to solid ratio
– Identification of wash out versus solubility control (slope 1 in cumulative release – L/S plot)
– Indication of pore water concentrations relevant to field leachate from low L/S data
– Local equilibrium established quite rapidly
– Basis for verification of geochemical speciation modelling
– Allows comparison with lysimeter and field data after estimating L/S value based on local infiltration data.
– Projection towards long term behaviour possible using information on solubility controlled release of some substances and wash-out of non-interacting species.
Applicable to many materials. Limited or not applicable to clayey soils and sediments (low permeability). Reactive matrices (e.g. MSWI fly ash) may require prewetting prior to placement in a column.
From the amounts of acid and base used to change the pH to the desired end values in the pH dependence leaching test, the acid/ base neutralization capacity of the material/product can be obtained. ANC/BNC plotted as a function of pH provides an indication of the amount of acid or base needed to reach a given end pH, whereby CO2 exposure can be recalculated as acid equivalents. As the ANC/BNC is ultimately linked to the mineral assemblage, it means that a match between measured and calculated ANC/BNC is indicative of the suitability of the selection of mineral and sorption phases in the material studied. Below examples of typical ANC/BNC curves.
The purpose of doing leaching tests is to obtain information on release under real field conditions. Gaining information under realistic field conditions is however not very practical in view of the amount of work and the cost involved. Besides, the number if situations to be evaluated makes this approach impossible. Leaching tests are therefore used to simulate the release as close as technically and practically feasible. A key aspect is the comparability of test results in case of dispute. For this purpose, leaching tests are standardised and the sensitivity of test results to limited variation in the test conditions evaluated through robustness testing. Intercomparison validation is subsequently carried out to determine test performance in terms of repeatability and reproducibility.
There are many leaching tests worldwide. Leaching tests can be described in a few general categories:
Due to the amount of work and cost involved the number of tests carried out deceases rapidly going from compliance testing to characterization testing to field simulation testing. Each level of test has its benefits and limitations. Verification of laboratory test results and predictions at field scale provide valuable insights in factors that cannot be mimicked in the laboratory. To solve a specific problem a tiered approach generally will prove very helpful.
More information on lab to field examples
Depending on the nature of the material inorganic substances or organic substances are of primary interest. Currently, inductively coupled plasma (ICP) methods either in combination with optical emission spectroscopy (OES) or mass spectrometry (MS) are mostly applied due to sensitivity and multi-element analysis capabilities. For organic substances, gas chromatography in combination with mass spectrometry is increasingly applied (GC-MS). In addition to these potentially critical substances, parameters such as pH, conductivity, redox state, presence of particulate and dissolved organic matter and reactive surfaces (hydrated metal oxides, clay) are important.
In CEN (European standardisation organisation), ISO (International Standardisation Organisation) and in US EPA (Environmental Protection Organisation) characterisation leaching test have been developed in response to a need for more understanding of release controlling factors, where the single step procedures fell short in preventing mismanagement in waste disposal. In US EPA the Leaching Environmental Assessment Framework (LEAF) has been set up in which guidance is provided on the use of the different characterisation leaching tests for specific questions. In table 1 below the corresponding test protocols in CEN, ISO and US EPA are indicated. Parallel validation between US EPA and CEN has shown that within the uncertainty of the methods the outcome of the corresponding tests is the same. Due to another choice of particle size distribution in the percolation tests for granular construction products, this is not necessarily true for CEN/TS 16637-3 in comparison with EPA 1314.
Table 1. International characterisation leaching tests across different fields of application.
Leaching tests
Terminology, acronyms and abbreviations
Leaching vs. content
Relation between leaching tests
Test vs. material use
Analysis
Comparison with regulation
Performance characteristics
Guidance on test use
