Methods, materials and mechanisms for laboratory leaching techniques
In the environmental sector, the term leachate is used in a variety of contexts. A leachate is defined as any liquid that, by passing through or making contact with matter, extracts soluble and suspended material from the matter.
Landfill leachate is a widely used phrase that relates to pore water from a landfill site. This type of leachate is quite different from laboratory prepared leachates both in terms of how they are leached and the complexity of the leachate matrix.
There are a number of laboratory leaching methods designed to mimic a variety of environmental leaching events, however, they all have a common aim – to obtain data relating to analyte leachability, which corresponds to determination of an associated risk in a conservative manner. In the contaminated land industry, this leachability may be used to simulate natural attenuation, evaluate the risk of contaminant leaching and reaching the natural ground water, or even determine the appropriate waste disposal route if the risk is found to be too high.
All leachate tests are essentially empirical and the quality of the analytical data obtained may be questionable unless the method protocol is followed exactly. Considering the many and complex interactions that can occur between the solute and solvent, the importance of consistent application of any method is key.
There are a number of primary factors that may control leaching: • Leaching mechanism – dynamic or static, liquid/solid (L/S) ratio • Chemistry of the contaminants of interest • pH, conductivity and redox potential of the solute • Solvent components and chemistry • Physical, chemical or biological changes in the material over time • Temperature of extraction • Duration of leaching • Agitation method
The release of contaminants from the solid to the liquid phase results from the solvent passing over the solid and contaminants being dissolved or mobilised into the liquid. As a consequence, the rate at which the two phases come into contact is an important influence in the leaching behaviour. Static tests exist typically for monolithic waste materials, where the test material is immersed in a tank of liquid, and it is only the surface interaction of the monolith with the solvent that leads to the leaching. For granular materials, dynamic leaching methods are more appropriate. Most established methods employ end over end rotation at ambient temperature, with a rate of 30 rpm for a period of time defined in the method.
The chemistry of the contaminants of interest is perhaps the most difficult to evaluate, but it may be the most influential. Some methods require the testing of the pH of the test material so that the leaching solvent can be chosen appropriately; however, there is no consideration given to the speciation of the contaminants of interest. This may be of particular interest when considering redox active contaminants such as hexavalent chromium. Another important factor is the primary component of the sample which influences adsorption or desorption rates. A high organic matter content, for example, may affect the leachability of organic components, whereas a factor for inorganics and heavy metals may be the clay content – zeolites are known to be effective ‘contaminant sponges’ in balancing pond reed bed systems.
Sample basic characterisation and choice of solvent
It is well known that the basic characteristics such as sample pH, conductivity and redox have a key influence over the leachability of some parameters. An acidic soil, for example, is more likely to leach heavy metals when mixed with the leaching solute. The electrochemical properties are more influential with regards to analyte stability than leaching behaviour, but as such do have an influence on the sorption and desorption of analytes from solid material.
For most European test methods, distilled or deionised water is the solvent of choice. The theory being that distilled water is clean of contaminants and therefore acts as an effective solvent for leachable analytes. Some methods, however, prescribe the use of biocides such as sodium nitride to prevent biological activity affecting the leaching properties. In the USA, acetic acid is added as an organic matrix modifier to enhance the solubility of organic constituents.
Change in material
There are few methods of preserving the samples prior to leachate testing. Generally only cooling the sample and preventing exposure to light is appropriate, both serving to limit biological and chemical degradation over time. Unlike their European counterparts, the US Environmental Protection Agency (USEPA) has defined holding times for the period between sampling and leachate preparation for a selection of key analytes.
These are also considered to be best practice guidelines for other methods.
So, having considered factors that may influence leaching behaviour, it is important to understand exactly how the testing methods are carried out.
Essentially there are four key stages in preparing and analysing a leachate:
• Sample preparation – which may comprise proprietary analysis of the sample, particle size reduction, etc
• Sample leaching – the leaching matrix used, the mode of contact with the solid material, agitation method, etc
• Preparation of the leachate – the type of separation used to obtain the eluate for analysis (centrifugation, decantation or filtration) and any sample preservation
• Leachate analysis – final chemical analysis of the prepared eluate
Some methods clearly define the critical parameters in each stage whereas others provide only general instructions. A good way to grasp the potential differences in leaching methods is to compare and contrast the European Standard methods BS EN 12457 with the USEPA Toxicity Characteristic Leaching Procedure (TCLP) method.
Both methods are used similarly – both for waste characterisation and site risk assessment purposes.
BS EN 12457:2002 is a multi-part standard with each of the four parts detailing different leachate methodology; however, they all follow the same general protocol.
The series is broken down into a string of single stage batch test leachates at either the 2:1 or 10:1 L/S ratio, in addition to BS EN 12457-3, which is a cumulative two-stage leachate that has been used traditionally for waste classification purposes.
The BS EN 12457 series provides a great amount of detail with regard to the sample types that are fit for analysis and how they are prepared. For example the method deals with liquid waste, sludges, monolithic wastes and granular wastes. The method also prescribes how to calculate the volume of liquid to add to a wet sample and how to back calculate all the contaminants to their equivalent mg/kg content in the original sample. The procedure is also not without quality control samples and the use of blanks and their data is discussed in each of the standards.
This use of these standard methods has now evolved, with UK Environment Agency Guidelines recommending the use of the single stage leachate at 10:1 (BS EN 12457-2) instead of the cumulative two batch test (BS EN 12457-3). Their “Waste Sampling and Testing for Disposal to Landfill” published in March 2013 updates and supersedes previous guidance and the justification for the change in leachate methodology and ration is derived from the 2010 Environmental Permitting regulations. This states that the 10:1 L/S preparation should be used for all waste types, unless short term (rapid) leaching of contaminants is expected, and in this case the 2:1 L/S (BS EN 12457 – 1) is suggested as appropriate.
The BS EN 12457 series does not detail sample holding times, analytical preparation methods or chemical testing techniques.
The US has an established approach with the TCLP that covers sample preparation, leaching procedure, preparation of the leachate for analysis and final chemical analysis techniques, whereas other methods define only the sample preparation and leaching procedure.
The TCLP approach is somewhat different from European approaches and is typically used to establish leaching characteristics for waste material. Unlike other methods, the TCLP protocol details how wastes with varying solid content are dealt with, for example those which are predominately liquid rather than solid comprising less than 0.5% solid material are filtered first to obtain the solid content. There are two extraction fluids which may be used depending on the properties of the material, both have an acetic acid component that mimics disposal with organic waste but the pH of each is different (4.93 and 2.88) and the selection of which to use is prescribed in the method. The soil sample is mixed with the extraction fluid in a ratio of 20:1, liquid:solid, and the leaching procedure begins using an end over end shaker at 30 RPM for a period of 18 hours. The mixture is then filtered and the liquid portion retained for analysis.
The TCLP is unique in that it has defined holding times from sampling to extraction, and also offers a zero headspace extraction for leaching volatiles. It also defines the analytical methods to be employed when determining the concentration of parameters in the leachate.
The TCLP method is used for a wide range of purposes. It is used for site investigation purposes, waste classification, or even assessing the effectiveness of a remediation programme. In terms of waste, the USEPA has an established list of contaminants and trigger levels for waste (List D), which unlike its UK counterpart includes parameters such as VOCs, SVOCs and pesticides in addition to the more traditional and common heavy metals.
Selection of methods
Given the range of differences between these two methods essentially designed for the same purpose, it is clear that unless the method is specifically designed for the task in hand then additional guidance should be sought.
Fortunately this additional guidance exists and a good source of information is BS ISO 18772:2008, in addition to guidance provided by national agencies such as the Environment Agency.
The first question to be asked is whether the leaching information is for an environmental impact assessment, or whether it is for assessment of compliance or comparison with defined criteria. If it is the latter, then often the defined criteria will indicate the methodology. This is the case with UK Waste Acceptance Criteria, which was formerly performed using BS EN 12457-3 and now is recommended to be performed using BS EN 12457-2. In this case the analytical method to be followed is a standard method and is clearly defined in the Environment Agency specifications.
If the purpose of the leaching information is for another purpose such as environmental impact assessment, then careful selection of the test method from the available options is critical. If the chemistry of the site in question is quite unusual, then there may even be scope to develop specific leaching methods for a particular site investigation.
An application which is proving slightly contentious in the UK relates to leachate preparation to simulate groundwater risk. Current Environment Agency guidelines recommend using the BS EN 12457-1 method, which is a 2:1 L/S extraction; this replaces previous guidelines which stipulated a 10:1 L/S extraction ratio. The final analysis of the leachate typically comprises a broad range of analytes, from heavy metals, poly aromatic hydrocarbons, and even pesticides or herbicides.
The problem with much of the organic analysis is that a large volume of liquid eluate is required for liquid/liquid or solid phase extraction processes. With the 2:1 L/S ratio, this means preparing a large volume of sample and the inherent problems of filtering a sample with such a high solid ratio may mean that further interactions continue between the solid and liquid phases once the “leaching” process is complete. The potential leachability of the organic analytes is broadly linked to the solubility of the determinand. For example, heavier PAHs and PCBs are strongly adsorbed onto the soil, volatile compounds such as BTEX and trichloroethylene are more soluble, and ethers and alcohols such as MTBE and Phenol are highly soluble.
Despite the analytical problems relating to the 2:1 L/S extraction ratio, it is more appropriate in representing pore or perched water from the potentially contaminated groundwater source. Additionally, it should be noted that despite the BS EN 12457 methods being based on waste classification of solid materials the method is recommended for use in determining leachability from natural, contaminated and agricultural soils and soil materials.
It is obvious that given the number of different methods available there must be a good level of clarity and intelligibility in the way in which the methodology and data are reported. The TCLP method in the US is followed exactly as detailed and reported as prescribed, but the leachate methods within the UK and Europe often have in-house modifications due to lack of flexibility in the standard. To this end, the Environment Agency in the UK have clearly defined what modifications may not be made to BS EN 12457-2 (10:1 L/S) for waste classification purposes:
• The liquid to soil ratio of 10:1 must be used and the volume of liquid to be added to the soil determined using the moisture content calculation given in the standard
• At least 95% of the sample must be of a particle size less than 4mm
• End over end or roller bed shaking may be used – flat bed orbital shakers are not suitable
• The eluate must be filtered through a 0.45 µm filter
They also define that for waste purposes, all data must be reported in mg/kg as well as mg/l (or µg/l). Where the leachate is used for risk assessment purposes, values of the direct concentration of the analyte in the leachate are required (either mg/l or µg/l).
BS EN 12457-1:2002 Characterisation of waste. Leaching. Compliance test for leaching of granular waste materials and sludges. One stage batch test at a liquid to solid ratio of 2l/kg for materials with high solid content and with particle size below 4mm (without or with size reduction).
BS EN 12457-2:2002 Characterisation of waste. Leaching. Compliance test for leaching of granular waste materials and sludges. One stage batch test at a liquid to solid ratio of 10l/kg for materials with particle size below 4mm (without or with size reduction).
BS EN 12457-3:2002 Characterisation of waste. Leaching. Compliance test for leaching of granular waste materials and sludges. Two stage batch test at a liquid to solid ratio of 2l/kg and 8l/kg for materials with a high solid content and with a particle size below 4mm (without or with size reduction).
BS EN 12457-4:2002 Characterisation of waste. Leaching. Compliance test for leaching of granular waste materials and sludges. One stage batch test at a liquid to solid ratio of 10 l/kg for materials with particle size below 10mm (without or with size reduction).
U.S. Environmental Protection Agency (EPA), Washington, DC (2008). “Test Methods for Evaluating Solid Waste, Physical/Chemical Methods.” Document no. SW-846. 3rd Edition.
BS ISO 18772:2008 Soil quality – Guidance on leaching procedures for subsequent chemical and ecotoxicological testing of soils and soil materials.
Environment Agency Report: Waste Sampling and Testing for Disposal to Landfill, EBPRI 11507B, Final March 2013.
Published: 19th Aug 2014 in AWE International