Leaching behaviour and leaching tests
Jutta Laine-Ylijoki, Markku Juvankoski, Tommi Kaartinen, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen, Henna Punkkinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.
No single technique or test alone suffices to identify minerals and compounds or to characterize a material or waste, therefore a combination of chemical analysis, leaching studies as well as mineralogical studies is a necessity for complete characterization of elemental composition, mobility and stability of a material. Evaluation of leaching behaviour of wastes from the extractive industry using appropriate leaching tests is essential, even if there were no sulphide or iron-sulphide minerals.
Leaching tests are in principle applicable to any type of residue from the extractive industry, whether appropriate or not will depend on whether they present an efficient way to produce the information sought in a specific case. Standard chemical or leaching methods are not sufficient to assess element release related to, or driven by, sulphide oxidation. In case of sulphide oxidation leaching test gives important input for assessment together with mineralogical data and modelling. The percentage of the content of a specific constituent that can be leached from a waste at a relevant pH is a measure of the potential leachability. The leachability varies strongly for each material and each element. It may range from close to 100% to 0.001% demonstrating that total composition is a poor measure to predict the potential environmental impact of an element. There are a large number of standardized leaching tests available. The characterization leaching tests comprise methods for measuring (i) solid-aqueous partitioning as a function of pH, (ii) solid-aqueous partitioning as a function of liquid to solid ratio (L/S), and (iii) mass transfer rates for monolithic or compacted granular materials. The preferred and most commonly applied leaching tests for waste characterization are the CEN/TS 14405 column leaching test and the pH dependence test as complement. (Technical Committee CEN/TC 292 2012)
Methods for determination
Leaching tests should be carefully chosen depending on which data is needed to satisfy the defined objectives of the characterization exercise. As stated above the preferred test in this context is the CEN/TS 14405 column leaching test.
Percolation test / column test
The column leaching test (CEN/TS 14405) is an up flow percolation test designed to resemble common percolation scenarios. The liquid to solid ratio is related to a time scale through the infiltration rate, density and height of the material. Through modelling, the effect of preferential flow can be quantified. The first eluate from a column test reflects the pore water conditions of the material considered. Control measures can be taken for testing of materials that are sensitive to oxidation to avoid changes in initial conditions. US-EPA has adopted the method as EPA method 1314 (US EPA 2013a) for inclusion in SW 846 (US EPA 2013b).
In column test seven eluate fractions are collected within the range of L/S = 0.1 l/kg to 10 l/kg. Typical total test duration ranges from two weeks to a month depending on material density. The leachant is demineralised water (DMW). The test material is applied as received or size reduced and up-flow (13 ml/h) is applied through a cylindrical column. The low L/S condition gives an indication of pore water concentrations, which are relevant in field conditions with relatively low infiltration. The percolation test CEN/TS 14405 is performed in water saturated conditions and it should be noted that, in the case of oxidising waste material, this test does not capture the change in leachability over time.
The pH dependence leaching tests (CEN/TS 14429, CEN/TS 14997) provide information on the pH sensitivity of leaching behaviour of the material. Also the acid neutralisation capacity (ANC) can be determined using this test. The test consists of a number of parallel extractions of a material at a liquid/solid ratio (L/S) of 10 (l/kg) for 48 h at a series of pre-set pH values. Since pH is one of the main leaching controlling parameters, the information can be used to evaluate the repeatability in testing (resulting from measurement at steep concentration – pH slopes) and to provide information on the sensitivity to pH in specific field scenarios. The changes in mineral solubility and sorption resulting from acidification are captured by this method but it does not give information on the weathering process leading to changes in pH and solubility. For this type of information, repeated testing is needed on the same material exposed to weathering for different duration. In combination with geochemical speciation modelling (van der Sloot & van Zomeren, 2010) test provides insight in minerals controlling acid neutralisation behaviour other than carbonates. For material characterization this has been proven to be a very useful method (e.g. van der Sloot et al. 2010). The method is standardised in two experimental modes by CEN/TC 292/WG 6 (CEN/TS 14429, CEN/TS 14997). US-EPA has adopted the method as EPA method 1313 for inclusion in “Test Methods for Evaluating Solid Waste, Physical/Chemical Methods”, also known as SW-846 (USEPA 2013b).
Compliance test or short term tests
Short tests or compliance tests are mostly single step tests that by themselves give a very limited answer to the question of release from waste. In a tiered approach (Kosson et al. 2002, EN 12920) consisting of a hierarchy of tests, these type of tests are very functional as they provide in conjunction with prior characterization testing a cost effective and comprehensive data management system.
EN 12457 is a standard compliance test for granular and powdered waste materials pointed out by Council Decision establishing criteria and procedures for the acceptance of waste at landfills (2003/33/EC). EN 12457-1, -2 and -4 are single step procedures. Part 3 consists of two extraction steps, first at L/S = 2 and subsequently at L/S 8 to yield cumulative L/S of 10. In EN 12457-3 the sample is agitated with demineralised water for 6 hours first at a liquid-to-solid-ratio (L/S) of 2. The eluate and the solid are separated by filtration (0.45 µm membrane). The solid is subsequently mixed for 18 hours with demineralised water at L/S 8 and the eluate separated by filtration. The concentrations of leached components in both collected eluates are then measured. The final test results are reported in mg/kg DM at cumulative liquid to solid ratio 10 l/kg. Other parts of EN 12457 follow the same principles but are only performed as 1-stage procedures.
Alternative compliance procedures are single steps derived from the full characterization tests, e.g. first fraction of percolation test. The advantage of such approach is that when the full method is validated, the compliance procedure is implicitly validated. In addition there will be no debate on the comparability of the methods, as they are essentially the same.
Table 1. Leaching tests (Technical Committee CEN/TC 292 2012).
*The method is used for quick verification against quality objectives. A single number without context can be very misleading.
EN 12920 2004. Characterization of waste – Methodology for the determination of the leaching behaviour of waste under specified conditions.
Kosson, D.S., van der Sloot, H.A., Sanchez, F. & Garrabrants, A.C. 2002. “An integrated framework for evaluating leaching in waste management and utilization of secondary materials”, Environmental Engineering Science, 19(3), 159-204.
Technical Committee CEN/TC 292 2012. Characterization of waste – Overall guidance document for characterization of wastes from extractive industries. CEN/TR 16376:2012.
Van der Sloot, H.A. & van Zomeren, A. 2010. Geochemical speciation modelling of pH dependence test data as alternative to sequential chemical extraction. In: Third international symposium on energy from biomass and waste, Venice 2010.
Van der Sloot, H.A., Hjelmar, O. & Kosson.D.S. 2010. Recent Developments in Testing, Modelling and Environmental Impact Assessment for Soil, Waste and Construction Products. Depotech 2010, 3 – 5 November, Leoben, Austria.
US EPA 2012. Method 1313. Liquid-solid partitioning as a function of extract pH using parallel batch extraction procedure. http://www.epa.gov/wastes/hazard/testmethods/sw846/pdfs/1313.pdf Accessed 16th June 2015
US EPA 2013a. Draft method 1314. Liquid-solid partitioning as a function of liquid-solid ratio for constituents in solid material using an up-flow percolation column procedure. http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/1314.pdf Accessed 12th June 2015
US EPA 2013b. SW-846 On-line. http://www.epa.gov/osw/hazard/testmethods/sw846/online/index.htm Accessed 12th June 2015