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.

Introduction

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.

pH-dependence test

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).

Parameter / Test Standard number and name Applicability Comment
Upflow percolation CEN/TS 14405: Characterization of waste – Leaching behaviour tests – Upflow percolation test Applicable to wastes from extractive industries for each type (i.e. tailings, waste rock, overburden and topsoil) Provides the basis for impact assessment by percolation. Very low liquid to solid ratio (L/S) matches porewater composition. No kinetic information.
Influence of pH CEN/TS 14429: Characterization of waste – Leaching behaviour tests – Influence of pH on leaching with initial acid/base addition Applicable to wastes from extractive industries for each type (i.e. tailings, waste rock, overburden and topsoil) pH dependence test results in combination with speciation modelling provide an understanding of the release controlling phases (mineral or sorptive phases). Identification and quantification of potential acid neutralizing minerals. No kinetic information.
Continuous pH control CEN/TS 14997: Characterization of waste – Leaching behaviour tests – pH dependence test with continuous pH control Applicable to wastes from extractive industries for each type (i.e. tailings, waste rock, overburden and topsoil) Same as CEN/TS 14429, except for computer controlled acid/base addition.
Compacted granular leach test FprCEN/TS 16637-2 : Construction products – Assessment of release of dangerous substances – Horizontal dynamic surface leaching test  (see Annex C:  Method for compacted granular construction products (CGLT-model) Applicable to wastes     from extractive industries for each type (i.e. tailings, waste rock, overburden and topsoil) Relevant in special cases with very fine grained tailings.
Acid and base neutralization capacity CEN/TS 15364: Characterization of waste – Leaching behaviour testes – Acid and base neutralization capacity test Applicable to wastes from extractive industries for each type (i.e. tailings, waste rock, overburden and topsoil)  
One stage batch test: liquid/solid ratio 2 l/kg < 4 mm EN 12457-1*: Characterization of waste – Leaching – Compliance test for leaching of granular waste materials and sludges – Part 1: One stage batch test at a liquid to solid ratio of 2 l/kg with particle size below 4 mm (without or with size reduction Applicable to wastes from extractive industries for each type, but several restrictions have to be considered or methods are not yet validated. The four parts of EN 12457 are also applicable to wastes from extractive industries.
One stage batch test: liquid/solid ratio 10 l/kg < 4 mm EN 12457-2*: Characterization of waste – Leaching – Compliance test for leaching of granular waste materials and sludges – Part 2: One stage batch test at a liquid to solid ratio of 10 l/kg with particle size below 4 mm (without or with size reduction) Applicable to wastes from extractive industries for each type, but several restrictions have to be considered or methods are not yet validated. See EN 12457-1
Two stage batch test: liquid/solid ratio 2 l/kg and 8 l/kg < 4 mm EN 12457-3 : Characterization of waste – Leaching; Compliance test for leaching of granular waste materials and sludges – Part 3: Two stage batch test at a liquid to solid ratio of 2 l/kg and 8 l/kg for materials with high solid content with particle size below 4 mm (without or with size reduction) Applicable to wastes from extractive industries for each type, but several restrictions have to be considered or methods are not yet validated. See EN 12457-1
One stage batch test: liquid/solid ratio 10 l/kg < 10 mm EN 12457-4*: Characterization of waste – Leaching – Compliance test for leaching of granular waste materials and sludges – Part 4: One stage batch test at a liquid to solid ratio of 10 l/kg with particle size below 10 mm (without or with limited size reduction) Applicable to wastes from extractive industries for each type, but several restrictions have to be considered or methods are not yet validated. See EN 12457-1
One stage basket test < 32 mm EN 1744-3: Test for chemicals properties of aggregates – part 3: preparation of eluate by leaching of aggregates Relevant only for assessment of waste rock. Results of this test will be different to the ones from other methods listed above.

*The method is used for quick verification against quality objectives. A single number without context can be very misleading.

References

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