A CONCISE SCHEME FOR TESTING OF GRANULAR ... - ScienceDirect

Environmental Aspects of Construction with Waste Materials JJJ.M. Goumans, H A . van der Slmt and Th.G.Aalbers (Editors) 01994 Elsevier Science B. K AN rights resewed.

453

APPROACH TOWARDS INTERNATIONAL STANDARDIZATION:A CONCISE

SCHEME FOR TESTING OF GRANULAR WASTE LEACHABILITY. H.A. van der SIoot ', D.S Kossonb, T.T Eighmy', R.N.J. Comama, 0. HjelmaP.

'ECN, P.O. Box 1, 1755 ZG Petten, The Netherlands. bRutgers University, Dept. of Chem. and Biochem. Engineering, P.O. Box 909, Piscataway, NJ 08550909, USA

' University of New Hampshire, Environmental Research Group, Durham, NH 03824, USA Water Quality Institute, 1 1 Agern Alle, DK-2970 Horsholm, Denmark

Abstract A series of short and relatively simple leaching tests is proposed for rapid compliance testing of granular (waste) materials. The test conditions selected are based on results obtained from extensive testing programs that have identified several critical factors influencing leachability of a particular granular (waste) material. These factors include specific element solubility and availability or release potential. Solubility can be influenced by pH, complexation by inorganic species or dissolved organic matter, and reducing properties of the waste. The sum of all of these factors reflect the chemical speciation of constituents in the material. Most current regulatory protocols do not explicitly consider these hndamental waste properties during evaluation. The proposed testing protocol includes two serial batch extractions with deionized water, first at liquid to solid ratio of 2.1 followed by L/S 10; and two static pH extractions at L/S 50, first at pH = 8 and then at pH = 4.The entire procedure can be completed within 32 hours and is designed to be simple, concise and reliable. Some typical examples of test results are presented in comparison with more extensive test data.

1. INTRODUCTION

The leaching behaviour of wastes is important for assessing the environmental impact of waste utilization, requirements for treatment, recycling or disposal options for waste materials. Many regulations, both existing and under development, use leaching test results to evaluate the potential for environmental and health risks associated with a given material. However,

454 leaching is a complex phenomenon where many factors may influence the release of specific constituents from a waste over prolonged time intervals. These factors include major element chemistry, pH, redox, complexation, liquid to solid ratio (LS), contact time and biological activity. A distinction between leaching tests for waste characterization, (detailed assessment of leaching behaviour), compliance and on-site verification has been made in the framework of CEN standardization [ 13. This recognizes that the properties of wastes that govern release of potentially harmful substances is too complex to be evaluated by a single leach test. Classification of current leach tests leads [I-41 to a limited number of generic tests that can be used to quantify the majority of factors controlling leaching. Prior classification has been based on specific disposal scenarios rather than from an integrated waste management perspective. The waste management perspective indicates that initially detailed information is required to evaluate the properties of a material and potential management options. Subsequently, simpler testing protocols (compliance tests) can be selected based on knowledge gained from detailed characterization for quality control purposes and comparison with regulatory thresholds. The maximum acceptable time interval for completion of testing increases from less than one hour for on-site verification to 48 hours for compliance testing, and up to several weeks for detailed characterization. An alternative classification of leach tests is from the perspective of controlling physical and chemical release mechanisms. This approach differentiates between equilibrium (batch extraction and controlled pH tests) and dynamic tests (column tests, tank leaching or diffusion controlled tests). The following selection of characterization tests would cover almost all aspects of leaching based on the identification of the properties assessed by the different leaching tests in use. This selection is given below: Material Granular (waste) materials

Stabilized waste, construction materials and other monolithic materials

Test method pH controlled test Availability test Column test Reducing capacity test Availability test Tank leach test Reducing capacity test

Conditions pH 4 - 13, LS*=5, d, < 4mm pH =7, 3 hrs, LS=50, LS=4, 3 hrs, LS=50, d,< 125 pm LS = 0.1 - 10, d, < 4mm

1

pH =7, 3 hrs, LS=50, LS=4, 3 hrs, LS=50, d,< 125 pm Leachant renewal (8 cycles: 6 hrs up to 64 days), LV=5 , dmin > 40 mm I

Practical routine testing protocols must be able to be completed in less than two days if they are to serve as the basis for immediate decision making. The approach advocated in this paper is that management decisions are made based on a combination of simple and rapid tests for which results are evaluated in conjunction with background information provided by initial detailed characterization of typical waste samples. This approach statistically establishes

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"characteristic" waste leaching behaviour for categories of similar waste streams and compares quality control and on-site verification results to the defined characteristic behaviour. Thus the question being asked is "is this waste sample the same as the defined waste stream?" This approach permits the development of a range of management options for different waste characteristics and relatively easy site-to-site transference of prior knowledge. Application of this approach calls for a good understanding of the underlying mechanisms and parameters controlling constituent release. This allows the selection of an optimized combination of simple tests which allow more profound conclusions with respect to environmental consequences than just a pass or fail criterion on an arbitrary test. The aim of this work is to provide a first approach towards a concise, but at the same time comprehensive test covering as many relevant aspects as possible with a minimum of effort. 2. EXPERIMENTAL

2.1 Characterizationtests Based on the summary of test methods by Environment Canada [2],Wallis et al [3] and Van der Sloot et a1 [4] the following test procedures are considered most appropriate for characterization: 2.1.1 Capacity oriented Availability test r5l. This test assesses the fraction of the total concentration that under extreme environmental conditions could become available for leaching. Solubility contraints are minimized in this test by using a high dilution (Liquid to solid ratio (LS) = 100) and a maximum particle size of 125 him(). The data obtained with the pH static test at pH=4 may come close to the quantity determined with this test. Redox capacity test 161. A proper test for this property is not yet avialable. COD measurements [7] lead to too high values due to the fact that carbon and chloride are reactive as well. The test result is needed to evaluate how long a material may remain reducing upon exposure to atmospheric conditions to decide whether a material should be tested under reduced conditions or whether prior oxidation of the material leads to a better estimate of the possible release.

2.1.2 Equilibrium oriented pH static test and Acid Neutralization Capacitv testL8.91. These tests cover the leachability of a material over a large pH range, which may not be relevant entirely for the environmental assessment but will give information on the chemical speciation of the constituents considered. Many regulatory and other tests produce data that are consistent with the results of the pH stat test [4,10]. The pH static test or ANC test have succeshlly been used for geochemical modelling of the leaching process. Dissolved Organic Carbon (DOC) and Total Organic Matter (TOC). DOC and TOC are important parameters to assess the possible complexation of constituents - inorganic and organic- with dissolved organic substances like humic and hlvic acids. DOC is the actual property as measured in a leachate, whereas TOC is the potential property that may lead to the formation of DOC.

456 2.1.3 Dynamics oriented:

Column testsrl 11. In a column test the dynamic aspects of leaching that may occur due to slow transformation processes, sequential release and depletion of species may be reflected. The release is usually expressed in m a g leached against the LS ratio. This latter property is related to a time scale through the cumulative infiltration rate. Serial batch test[ 1,2,3]. This procedure provides almost the same information that a column test provides but in a shorter time span and with less resolution with respect to the low LS ratios. Tank leach test112.131. This test is relevant for monolithic specimen, where the release is mostly diffision dominated. Provided the monoliths are sufficiently durable size reduction of such materials would lead to a significant overestimation of the actual release to the environment. The aspect of leaching from stabilized materials and construction materials is not hrther addressed here, but covered in other studies [ 14,15]. ComDacted granular leach test [16J This test focusses on release by diffision from granular materials. Some fine grained materials in a surrounding of coarser material or those compacted to a low permeability during placement show a release which is governed by diffision rather than by percolation. A test has been developed to assess similar release controlling properties (physical retardation and chemical retention) as in the tank leaching test. 2.2 Concise tesr The relevant aspects to be addressed in assessing the environmental properties of waste materials are the changes that take place at the longer term, which can be derived from testing at different LS values, when it is realized that in most applications of granular materials slow percolation is the prevalent transport process. The actual release is controlled by sorption and solubility of specific phases, which is very much controlled by the chemistry of major elements in the wastes. Important leaching controlling factors are the pH, the redox situation, the presence of complexants- inorganic such as chloride and organic substances, such as DOC, and the presence of active sorption sites. Apart from the actual pH it is important to know how long a material is likely to maintain a given pH. As the pH is known to be an important parameter controlling release, changes in pH with time are important for the long term leaching behaviour of wastes. At present the role of reducing properties of wastes or applications under reducing conditions are not addressed in the regulatory framework. Such changes may lead to order of magnitude differences between the laboratory test and the actual situation in the field [ 171. From all this it is clear that a single extraction giving just one number to compare with a regulatory threshold is quite inadequate and likely t3 lead to many erroneous decisions- both as false positives (e.g. reducing materials in an oxidizing environment) and false negatives (e.g. materials with metal leachability due to the high pH showing a much lower pH under actual field conditions). The proposed concise protocol consists of a optimized serial batch extraction with as wide an LS range as practical in a batch procedure and two pH-controlled conditions in accordance with the unified approach of leaching for characterization purposes [10,18]. In the leachates some additional measurements are carried out to fill in some other crucial properties (Redox potential - EH, Dissolved Organic Carbon, Total Dissolved Solids). The duration of the serial batch extraction has been designed such that the whole procedure can be carried out within two days. In this paper we will focus on granular waste materials. A

457

similar approach is possible for stabilized waste, contruction materials and other monolithic materials, but will not be addressed here. 3. RESULTS AND DISCUSSION

3. I Examples of clzaracterization test data Data obtained from a column study on a neutral reacting coal fly ash [ 191, as shown in figure 1, illustrates some of the typical release profiles as observed in column leaching studies. AII results are expressed as cumulative release in mgkg. For comparison the total composition (straight line) of the respective constituents in the ash as well as the availability (dotted line) are included as relative assymptotes. From the graphs it is clear that a fraction of the Ca is being washed out rapidly, followed by a more slowly dissolving phase as reflected by the difference in the K values (measure for matrix retention; for hrther description see below). In the case of Cr and Mo all of the fraction available for leaching is released. At the end of the experiment (combination of a column experiment up to LS=lO and a serial batch procedure up to LS=lOO) V, Ba, Pb and Zn start to approach the available fraction indicating a release largely controlled by solubility. Contrary to Ca, Ba release is slow at low LS and increases more strongly at LS > 5 . This is most likely related to Ba - solubility control by sulfate, which is released initially in high quantities. The more leachable Ca fraction may well be the gypsum phase present in fresh coal ash. The element Cu features a very slow release, which even at the end of the experiment has not by far approached the available fraction. This is an example of very strong matrix retention. Similar observations have been made for other coal ashes and for other materials. These examples also illustrate the need to include analysis of major- and release controlling species (e.g. Ca, sulfate) during testing, not only the elements of regulatory significance. 3.2 Exaniples of release ns a function of LS Based on the column data given in figure 1 a few general types of release from column experiments can be identified (figure 2): fast (A, wash-out), intermediate(B) and slow release (C; solubility/dissolution control); a decrease in availability due to slow mineral formation or sorption reactions (D); an increase in release due to depletion of a solubility controlling phase (Barium) or changes in chemical conditions, such as pH (oxyanions) redox potential, with time (F); a decrease in release due to changes in chemical conditions or initial release of a different, more mobile, species (E ;Ca , DOC complexed metals). When solubility is the main controlling factor and changes in major element chemistry are limited, a Continuously Stirred Tank Reactor model (CSTR) can be used for initial evaluation of release. Subsequent model refinement would include evaluating the column as aplug flow reactor with dispersion. Use of a CSTR model leads to a description of release (E in mg/kg) from the column by:

E = AVB * ( 1 - e -LsK) + Co with AVB: the availability in mg/kg, LS: the liquid to solid ratio in Vkg, K: a retention factor for the constituent in the matrix in f i g and CO: a constant.

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Figure 1. Column leaching data for coal fly ash with retention data obtained from modelling release.

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In figure 1 this relation has been used to quantie the matrix retention parameter K. In a support document for the development of a new European leaching test for waste [ I ] a comparison of column studies with batch leaching tests of the same material was carried out on a wide range of materials to demonstrate that in many cases the proposed serial batch procedure matches well with the more eleborate column procedure. In table I a comparison of K values for column and serial batch are given for a few different materials.

460 Table I. Comparison of modelling parameters for evaluating matrix retention

* MSWI FA = MSW incinerator fly ash, FGD = flue gas desulhrization residue. 3.3 Reducing conditions When materials contain reducing substances, either due to the fact that reducing substances were introduced in the materials (e.g. stabilization with sulfides) or the materials were produced in oxygen starved conditions (industrial slags), the leaching behaviour will be significantly different from normal oxygenated conditions[20]. In addition, disposal or utilization in reducing environments will have consequences for the evaluation of material behaviour. Materials containing organic degradable matter can turn reducing due to biological Demineralized water

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46 1 degradation. transformations of sulfate to sulfide and reduction of FeIII to Fe I1 are important in this respect. Metals will be retained as their sulfides, Ba mobility will increase due to the decrease in sulfate concentration. The leachability of Fe and Mn will increase as well, since the reduced forms - FeII and MnII- are more mobile than the oxidized forms. This can mobilize trace elements associated with Fe and Mn sorptive sites. To be able to address the issue of reducing materials and materials in reducing environments, it is important to know which materials exhibit reducing properties and what circumstances in the field are relevant. The reducing capacity is an important property, in this connection, which has to be known in order to define how long a material will maintain its reducing properties. Figure 3 shows the redox potential of a range of materials measured in batch extractions in a closed vessel relative to the stability lines of water. For comparison the redox potential measured in demineralized water used in the extractions at different pH values is given as well. Coal fly ash is obviously oxidized, but all slags exhibit reducing properties. Upon exposure to the air both oxidation and neutralization (COz) will take place simultaneously. 3.4 p H dependence of Ienclring

Many studies have pointed at the relevance of pH as one of the most important controlling factors of leaching [ l - 4,10, 21, 22, 231. It is important to realize that the major element chemistry largely dictates the leachate composition and should therefore not be neglected [21,22]. The leaching behaviour of an individual element as a function of pH has been shown to feature generic and systematic characteristics to the extent that a baseline leaching characteristic can be identified for an element, which will deviate as a function of elementspecific interactions. In figure 4 the leaching behaviour as a function of pH is given for Cu from four different wastes. The coal ash reflects the Cu leachability curve governed mainly by inorganic Cu species. In the case of MSWI bottom ash the Cu is strongly complexed with DOC [24]. The modelling by MINTEQA2 [25] indicates the differences between inorganic solubility control and DOC complexation [10,22]. In car shredder waste the large organic fraction leads to a qualitatively predictable high leachability of Cu. Similar generic pH dependent leach curves have been established for other element as well [I, 101. It leads to the conclusion that for each element a limited number of parameters can be identified as the most crucial ones, which tends to make the task to characterize waste more manageable.

3.5 Description of the concise test procedure and its interpretation The proposed test consists of a two step serial batch extraction and a two step pH controlled batch extraction at pH = 8 and pH = 4, respectively. It complies with the unified approach to leaching as developed in the framework of the IAWG work[ 181. The serial batch test is carried out in a closed bottle using degassed water. Agitation in the serial batch test is achieved by test placing the bottles on a rollertable at about 10 rpm. The pH static extractions can be carried out in an open vessel and agitation by strirring is adequate. The four extracts obtained are analysed separately. The material to be tested should be size - reduced to 95% less than 4 mm for the serial batch and to 95% less than 300 pm for the pH controlled test. The serial batch and the pH controlled test can be run in parallel, which implies that the entire procedure can be completed within two days. In the extracts generated by the batch tests relevant major- and trace elements, pH, EH,TDS, conductivity and DOC are measured. In the pH controlled test

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the acid or base consumption is recorded. This combination of extractions allows several relevant conclusions on the properties of a material, which go well beyond a statement as to pass fail in relation to a regulatory limit value. When a material has been characterized extensively and shown to behave quite sytematically, part of the protocol may prove sufficient for quality control purposes (compliance testing). The following aspects can be addressed on the basis of the test results: - For each constituent an indication is obtained on the retention in the waste matrix and consequently the potential risk for short term release. Wash-out (no retention) and depletion of a mobile species can be distinguished from solubility-controlled release. - The release under low infiltration conditions can be derived from the LS=2 data as well as an estimate of the pore water concentrations for constituents that are not retained in the matrix.

463 MOBLLIZATION A N D WASH-OUT EFFECTS

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SERIAL BATCH TEST 1LS=2; 6 hrs; closed vessel LS=2-10; 18 hrs; closed vessel pH CONTROLLED TEST LS=50; pH=8 control; 4 hrs LS=50-100; pH=4 control; 3 hrs Record pH, E,, DOC, TDS and Conductivity.

r

REDOX PROPERTIES OF WASTES

Acid consumption in pH=8 and pH=4 test + alkalinity of L S = 2 and L S = 2 - 1 0 +Acid Neutralization Capacity

Fig 5. Concise testing protocol for the leachability of granular (waste) materials.

I

464

- Based on the deviation from CSTR behaviour a distinction can be made between changes leading to decreased release on the long term (remineralization, slow sorption kinetics) and increased release at the longer term, which depending on the level at which this occurs, requires further action to identify the causes for this behaviour and the potential risk at the long term. - In combination with the data from the serial batch extraction procedure the pH controlled test will allow a comparison with existing pH dependent leaching behaviour of the same waste or the same category of wastes. From this a conclusion can be drawn whether the material fits the pH pattern or deviates significantly from it. A deviation can often be related to element specific chemical speciation aspects, e.g. increased leachability of Cd in the pH range 4 to 9 due to a higher CI concentration or increased Cu leaching in the pH range 6 to 12 due to complexation with DOC. Abnormal deviations require further evaluation of its cause to minimize unexpected leaching behaviour in the long term. - The pH controlled test will give a measure for the fraction available for leaching and the acid neutralization capacity, which is a useful property to assess how long a material may be expected to maintain its own equilibrium pH when exposed to acid rain or carbon dioxide from the atmosphere or from biological activity. - By measuring the total dissolved solids (TDS) and conductivity in the leachate of the serial batch extractions an indication of the quantity of soluble salts can be obtained, which will have consequences for leachate analysis and may trigger more detailed analysis of soluble salts, which are generally not considered hazardous but may pose a serious threat to groundwater quality. - The Eh measurement will allow an identification of materials exhibiting reducing properties, when the redox potential in the leachate is compared with a &-pH relation for oxygenated demineralized water. A value of more than 50 mV lower than the value for oxygenated water at the corresponding pH value is considered to indicate reducing properties. This deviation has been chosen because of the uncertainty associated with this type of measurement and can only be regarded as indicative. If the material is considered to be reducing a reducing capacity needs to be determined (to be developed). Based on the destination of the material the material may have to be oxidized and tested again to represent the conditions to which it will be exposed. - By performing a DOC measurement in the leachate generated in the serial batch test an indication of possible mobilization of metals and organic contaminants can be obtained. The information generated with this concise protocol will help focus on key issues to improve environmental quality of (waste) materials. In the process of recycling, treatment of wastes, use as secondary materials and in disposal, this information is essential for proper management. 4. CONCLUSIONS

The proposed concise protocol for testing of (waste) materials has been developed on the basis of a number of integrated activities in the field of waste leaching and as such forms a synthesis of many existing leaching tests around the world . With a test duration of less than two days it meets the requirement of fast turn around needed for control purposes in industrial processes. The integrated approach that has been chosen to cover many aspects of leaching allows one to deal with time-dependence of leaching, initial leaching behaviour of materials, pH dependence, occurrence of reducing properties, aspects of chemical speciation and retention in the (waste)

465 matrix. Observations derived from the test may require hrther action to allow a better judgement of potential environmental effects. Parts of the test may be used in compliance testing, when the most crucial factors governing the leaching of constituents from a given waste material are known. This implies that optimization in terms of test use for specific applications is feasible. An optimization of the analytical effort is also possible based on previous knowledge of constituent behaviour.

Acknowledgement

This approach has been developed following discussions in the framework of the activities of the Int. Ash Working Group (IAWG-IEA), discussions with colleagues from Working Goup 2 of CEN TC 292 “Characterization of Waste” in relation to the development of leaching test methods for waste characterization and compliance testing, activities in the Dutch standardization committee on leaching of Contruction Materials and Waste Materials and through collaboration under US-EPA Cooperative Agreement CR 8 18178-01. The financial support by the Netherlands Agency for Energy and the Environment under contract 222.3.6600.10 is gratefully acknowledged.

5. REFERENCES

H.A van der Sloot, 0. Hjelmar, Th.G. Aalbers, M. Wahlstrom and A.-M. Fallman. CEN TC 292 WG2 document: Proposed leaching test for granular solid waste. February, 1993. 2. Compendium of waste leaching tests. Environment Canada. Environmental Protection series. Report EPS 3/HA/7. May 1990. S.M. Wallis, P.E Scott and S. Waring. Review of leaching test protocols with a view to 3. developing an accelerated anearobic leaching test. AEA-EE-0392. Environment Safety Centre. 1992. 4. H.A van der Sloot, D. Hoede and P. Bonouvrie. Comparison of different regulatory leach test procedures for waste materials and construction materials. ECN-C-91-082, 1991. NEN 7341, Determination of the leaching behaviour of granular materials. Availability 5. for leaching. “I, 1993. H.A van der Sloot, D. Hoede en P. Bonouvrie. Invloed van redox condities op het 6. uitlooggedrag van reststoffen. ECN-C-93-037, 1993. NEN 323 5.3 Determination of the Chemical Oxygen Demand. ”I, Delft, 1976. 7. J.V. DiPietro, M.R. Collins, M. Guay, and T.T. Eighmy, Proc. International Conference 8. on Municipal Waste Combustion, Hollywood, 1989. Test methods for solidified waste characterization, Acid Neutralization Capacity test, 9. method #7, Environment Canada and Alberta Environmental Center, 1986. 10. H.A van der Sloot, Leaching aspects of MSWI residues. Special session on MSWI residues properties. This conference. 11. NEN 7343. Determination of leaching behaviour of granular materials. Column leaching I1993. test, “ 12. A N S 16.1 American Nuclear Society, Lagrange Park, 11, 1986. 1.

466 13. NEN 7345. Detemination of the leaching behaviour of construction materials and I1993. monolithic materials. Diffusion test. " 14. Proc. Sec. Int. Symp. Stabilizatiodsolidification of Hazardous, Radioactive and Mixed wastes. Williamsburg, Virginia, May, 29 to June 1, 1990. 15. H.A van der Sloot, G.L van der Wegen, G.J. de Groot and D. Hoede. BCR intercomparison of leaching tests for stabilized waste. This conference. 16. D.S.Kosson, T.T.Kosson, H.A. van der Sloot.,"USEPA Program for Evaluation of Treatment and Utilization of Municipal Waste Combustor Residues", Cooperative agreement CR 8 18178-01-O.USEPA/RREL,Cincinnatti, September 1993. 17. Several papers at this conference. 18. T.T. Eighmy and H.A. van der Sloot. A unified approach to leaching behaviour of waste materials. These proceedings. 19. H.A van der Sloot, G.J de Groot and 0.Hjelmar, EC contract EN3F-0032 NL . ECN-R91-008, 1991. 20. H.A van der Sloot, D. Hoede, R.N.J. Comans. The influence of reducing properties on leachingof elements from waste materials and construction materials.These proceedings. 21 T.T.Eighmy, D. Domingo, J.R.Krzanowski, D. Stampfli and D. Eusden. 1993. Proc Municipal Waste Combustion. VIP 32. Air & Waste Management Association Pittsburg, Pennsylvania. 1993. 457 -478. 22. R.N.J.Comans, H.A.van der Sloot, P.Bonouvrie. Proc. Municipal Waste Combustion. VIP 32. Air & Waste Management Association, Pittsburg, Pennsylvania. 1993. 667 -679. 23. C.S. Kirby and DXmstedt. 1993, Proc Municipal Waste Combustion. VIP 32. Air & Waste Management Association Pittsburg, Pennsylvania. 1993. 479 - 51 1. 24. H.A. van der Sloot, R.N.JComans,T.T.Eighmy, D.S.Kosson., Ruckstande aus der Mullverbrennung, Ed. Martin Faulstich, EF-Verlag fiir Energie und Umwelttechnik, GmbH, Berlin, 1992. 331-346. 25. A.R. Felmy, D.C. Girvin, and E.A. Jenne, MINTEQ--A2, EPA-600/3-84-032, U.S. Environmental Protection Agency, Athens, 1984.