DEVELOPMENTS IN EVAI,UATING ENVIRONMENTAL IMPACT … · DEVELOPMENTS IN EVAI,UATING ENVIRONMENTAL IMPACT FROM UTILIZATION OF BULK INERT WASTES ... Netherlands Energy Research

Pergamon

PII: S0956-4)53X(96)00028-1

Waste Management, Vol. 16, Nos I-3, pp. 65-81, 1996 Copyright © 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0956-053X/96 $15.00 + 0.00

KEYNOTE LECTURE

DEVELOPMENTS IN EVAI,UATING ENVIRONMENTAL IMPACT FROM UTILIZATION OF BULK INERT WASTES USING LABORATORY LEACHING TESTS AND FIELD VERIFICATION

H. A. van der S loo t Soil & Waste Research, Netherlands Energy Research Foundation (ECN), P.O. Box 1 1755 ZG Petten, The Netherlands

ABSTRACT. In recent years leaching tests for construction materials and wastes have been developed with the emphasis on using them as prediction tools for release in the long term rather than as arbitrary pass/fail tests. In the first stages of development, the mechanisms of release and release-controlling parameters have been assessed. For each of these aspects leaching tests have been selected, developed and in part standardized. Not all relevant aspects of leaching are yet cover- ered. The reducing properties of materials, for example, are still not taken into account properly. The need for these more elaborate tests is increasing as the policy of re-using waste materials in construction is expanding. As a consequence, the desire to improve material quality is increasing, For this purpose more detailed knowledge on the chemical speciation of contaminants is needed, as a treatment of waste carried out to reduce a few contaminants with too high leach rates may lead to an undesired increase in the release of other contaminants, which were previously not a problem. This requires a more integral approach and a good understanding of the consequences of changes in material properties. Tests focused on two main aspects of leaching can be identified: (1) release as a function of time; (2) release as a function of main leaching controlling parameters, such as pH, redox and complexation. The relation between these tests and the data interpretation associated with them is discussed. Another distinction in the use of tests is related to the level of understanding needed. In CEN TC 292 tests have been distinguished as: characterization tests, compliance tests, and on-site verification tests. An important aspect of the new development of tests with release prediction capabilities is the verification of such predictions in the field. An example of the relation between predictions of release from laboratory test data and field observations is presented: release from MSWI bottom ash monofills. Finally, recent developments in the EC DGXII Standard, Measurements & Testing Programme and CEN TC 292 Characterization of Wastes are addressed. Copyright © 1996 Elsevier Science Ltd

INTRODUCTION

In recent years the discussion about test methods to assess the leaching behaviour of waste materials has entered a new phase as the urge to recycle and re-use waste materials is increasing. This policy of recycling and re-use of waste materials requires a better control over the undesired release of contaminants into the environment. The present regulatory test methods I 5 are insufficient to evaluate the variety of waste materials in the wide range of possible application. 6~9 The release of contaminants from waste is influenced by a large number of physical (e.g. particle size, temperature, mode of contact with water, porosity) and chemical (e.g. pH, redox, sorption properties, complexing agents, reaction kinetics) parameters.

65

A variety of test procedures is available to charac- terize materials with respect to their leaching behaviour.10 14 Each addresses certain aspects of leaching. The question arises, which test is adequate for what purpose? In this respect it is important to distinguish between regulatory requirements, impact assessment, scientific evaluation of leaching and management tools for daily practice.

For regulatory purposes, the protection of the environment (quality of air, water, soil) and of human health is the prime concern. This requires a judgement of both short- and long-term environmental effects.

For environmental impact assessment site-specific conditions, waste/soil interaction, transport and long-term changes in the utilization/disposal conditions need to be addressed. A more rigorous

66 H.A. VAN DER SLOOT

approach is needed to cope with a wide range of technical, physical, chemical and economic aspects. Such an assessment should be the basis for the development of regulation.

In scientific studies, a detailed knowledge of phenomena and modelling of processes under controlled conditions is aimed at. This may involve testing under conditions that are not likely to occur under environmental conditions. This type of testing is solely to understand how the process of leaching is affected by specific controlling parameters.

For daily practice in waste management a relatively simple, fast screening test or compliance procedure is needed, allowing reliable judgement of treatability, re-use, acceptance at landfill, etc.

It has been stated before 8'~5,t6 that assessing a wide variety of materials in an even broader range of applications and disposal situations cannot be ade- quately addressed by one single extraction test. Con- sequently, the aim of a test has to be clearly identified before a decision can be made as to which test is most appropriate in a given situation. The situation represented by a test result should reflect as closely as possible the utilization or disposal conditions to be judged.

In deciding which test or combination of tests is appropriate for regulatory purposes, a choice between measuring a maximum leachate concentration in the short- or the long-term, or measuring a maximum release has to be made. At present, tests addressing each of these different aspects are used rather indiscriminately. Tests are very often used in an arbitrary way irrespective of the ultimate fate of a material.

Several waste materials from large-scale industrial processes possess technical properties that would allow their use in certain construction applications, e.g. coal fly ash, slags from large-scale industrial melting and ore processing, and incinerator residues. The disposal of such materials requires space and controlled landfills to minimize long-term environmental risks. The beneficial use of such bulk materials is an attractive alternative, if it can be shown that such applications are environmentally acceptable. For this new management of wastes and the decision to either dispose or use, more information on the environmental properties of materials is needed. New concise leaching tests are being developed to assess such properties, in which maximum benefit can be derived from knowledge of the systematic behaviour of materials. Once the characteristics for a given material class are known, this information can be used to relate information obtained in relatively short tests to the more exten- sive data set for consistency.

Modelling the release of contaminants taking into account factors influencing release, the finite dimen-

sions and leachable mass of the source will be an important aspect for environmental assessment. This route leads to a more fundamental understanding of the factors controlling undesired emissions from waste and from products containing waste materials. This ultimately allows improvement of the quality of waste materials and as such produces better management tools to prevent future soil pollution.

The following discussion is limited to aspects of leaching from granular materials. The issue of leaching from stabilized waste and construction materials containing secondary materials is not addressed here.

LEACHING TEST M E T H O D S

Worldwide, many different leaching tests have been developed to assess the release from waste materials under a variety of conditions, t6-~9 The same type of judgement (e.g. disposal conditions) is passed on the same type of materials using widely different leaching tests. In recent studies, ~5 the leaching data of different tests carried out on the same material have been compared. In all cases, a re-evaluation of the approaches chosen in the 1960s and 1970s is important in the light of present needs. The philosophy behind the development of a specific test has been lost in the application of tests with regulatory status. Applicable or not in a given situation, the lack of an appropriate test leads local authorities to choose an inappropriate test, such as applying the EP-TOX test that uses acetic acid (municipal landfill scenario) for assessing a marine application of stabilized waste as an artificial reef. 2°

Classification of Leaching Tests From a technical point of view, tests can be classified into tests aimed at attaining equilibrium conditions at the end of the leaching experiment and tests aimed at dynamic aspects of leaching (e.g. time- dependence)/6 The first type is generally based on batch-type leaching tests. Tests with controlled pH also fall in this category. Dynamic tests are typically tests in which time is an important variable. Examples of dynamic tests are diffusion tests for monolithic materials and column leaching tests for granular materials.

Another type of classification is the distinction of tests in relation to practice. In that context, tests can be classified as characterization tests aimed at understanding the leaching behaviour of materials, compliance tests, which are generally of much shorter duration that are aimed at a direct comparison with regulatory thresholds, and finally on-site verification tests, which are aimed at verifying a previous evaluation of a charge or batch arriving at a processing plant. The latter distinction has been adopted in the

DEVELOPMENTS IN EVALUATION 67

draft Landfill Directive of the EC 21 as proposed by CEN, the European Standardization Organization, as the basis for leach test development. 22'23

Examples of Most Common Regulatory Leaching Tests

EP-toxicity test~Toxicity Characteristic Leaching Procedure (TCLP USA). The EP-TOX test was issued by the Environmental Protection Agency as a test to classify materials in hazardous and non-hazardous materialsJ The acetic acid applied in the test was meant to simulate conditions in a municipal landfill (co-disposal scenario). The test has been widely used as a regulatory tool, which has led to serious concerns with respect to the applicability of the test far beyond its originally intended use.

The Toxicity Characteristic Leaching Procedure (TCLP) officially replaced the EP-TOX from 1990. 2 The test is designed to determine the mobility of organic and inorganic contaminants in liquid, solid and multiphased wastes. If volatile species are present, a zero-headspace extractor is used. TCLP is very similar to the EP. The most significant change is the option to choose a different extraction fluid in relation to the alkalinity of the material determined in a separate test.

The final pH in both tests is in most cases around 5. However, very alkaline materials may result in final pH values anywhere between 5 and 12. This may result in highly variable data due to the sharp changes in contaminant solubility as a function of pH. 6,v

German leach test procedure (DIN 38414 $4). The German DIN 38414 $4 procedure was developed to assess leaching of sludge and sediments from water and wastewater treatment? The method is considered applicable to solids, pastes and sludges. It is not considered to represent conditions in a disposal site. Additional factors need to be considered in assessing actual conditions in field situations. A distinction is made between readily soluble constituents and spar- ingly soluble components, which therefore may require a second or third extraction. The pH is not controlled in this test.

French leach test (X 31-210). The French leach test X 31-210 is restricted to solid residues? The sampling and sample preparation are described in detail. The pH is not controlled in the French procedure. The method is very similar to the DIN 38414 $4 procedure.

Swiss leach test TVA. The Swiss extraction test described in the Technische Verordnung fiber Abfgille is based on leaching under CO2 saturated conditions? The leach test is not considered to be the only criterion to decide about disposal of waste

materials. Information on behaviour of the waste in the short term and on longer time-scales is sought. Continuous CO2 injection is considered to represent a time-scale reduction.

Availability test (NEN 7341). The availability test described in NEN 73411° is aimed at assessing the fraction of the total mass potentially available for leaching under environmental conditions. This implies that the silicate matrix and very poorly soluble mineral phases are not dissolved. This is achieved by performing the test on finely ground (<125 ~m) material at a high dilution (LS = 100) to minimize solubility limitations and controlling the pH at 7 and 4 as the lowest pH that, apart from rather exceptional conditions in sulphidic ore bodies, can be found in the environment. 24 The extraction at pH 7 is used to obtain a more representative extraction for oxyanionic species? 5

Column test (NEN 7343). The column test described in NEN 734311 is considered to simulate the leaching behaviour of a waste material in the short, medium and long term by relating the release expressed as mg/kg leached to the liquid to solid (L/S) ratio. The relation with the time-scale is obtained from the height of the application and the infiltration rate. Very slow changes in mineral composition can not be addressed with this test. The pH is not controlled, so the waste is allowed to dictate the chemical conditions in the pore-solution.

The relation between this lab test and field conditions is not 1:1, since such factors as temperature, channeling, degree and duration of water contact, ageing effects (carbonation) and others need to be considered.

pH static test. The influence of pH on the leaching of contaminants from waste is assessed by extracting waste at L/S = 5-10 for 24 h under pH-controlled conditions using automated pH control equipment (eight positions) with NaOH or HNO 3 addition. A common pH range covered ranges from pH 4 to 12. j326 This procedure is not standardized. A procedure resulting in comparable results is the Acid Neutralization Capacity (ANC) procedure. 14

Compacted granular leach test. A recently developed test is the compacted granular leach test, which allows the measurement of the release from granular materials by diffusion. This method has been developed in relation to the judgement of stabilization processes, where the original waste needs to be evaluated in a similar manner as the solidified/stabilized waste. 27 In practice, the limited infiltration aimed for in utilization scenarios (e.g. roadbase)justifies the use of this type of release mechanism.


TABLE 1 Characterization Tests for Granular Materials

Method Conditions

Static pH test Availability test

Column test Reducing capacity test

4 <pH <13; LS = 5; d o <4 mm p H 7 , 3 h , L S = 5 0 pH 4, 3 h, LS =- 50 d o <125/zm 0.1 <LS <10; d o <4 mm (under development)

The EP-TOX, TCLP, DIN 38414 $4, X31-210 and TVA are typically compliance tests. Characterization tests for leaching are summarized in Table 1, where the test conditions are specified. For further details, the standards and related d o c u m e n t s 1°']1,z3'28 provide more background information. The tests specified cover release as a function of liquid-to-solid ratio (LS in 1/kg), the leaching potential and the influence of pH. Results obtained by many existing tests are covered by this selection. ~5 An important property not covered is the effect of redox conditions. This aspect is not addressed in any of the existing regulatory leaching tests. In several cases, redox conditions have a strong influence on the release r a t e 24'29 requiring testing under redox controlled conditions. Studies are in progress to assess the role of reducing properties of materials. 3°

On-site verification tests have not been proposed yet. These may be very simple tests, such as pH

measurement, soluble salt content (conductivity) and loss on ignition.

Presentation of Leaching Test Data Leaching test results can be expressed either as leachate concentration (mg/1) or as constituent release (mg/kg of dry residue). The basis selected for expressing leaching results should depend on the type of data comparison desired. Regulatory test results are most often expressed as leachate concentrations for comparison to limit values, but do not consider the underlying basis for the release phenomena that are observed. Results expressed as leachate concentrations permit a comparison of contaminant solubility which reflects the chemical speciation of the elements and leaching solution conditions (e.g. pH). Transformation of measured concentrations into release is necessary for comparison of the data obtained at different liquid-to-solid (L/S) ratios and for determination of availability. Release is defined as the mass of a contaminant dissolved divided by the mass of dry residue leached. In Fig. 1 the leaching of C1 represents leachability of an avail-ability-controlled species. Data from tests at different L/S expressed in mg/l lead to apparent differences, while data presented in mg/kg show that in all cases the fraction available for leaching is released. The element Si represents a solubility- controlled constituent. Here presentation in mg/kg leads to differences, whereas data represented in mg/l show the solubility control in the pH region 3-8.

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FIGURE 1. Types of release identified from column tests or sequential batch tests. Leach results presented as concentration (mg/l) to visualize solubility control and as release (mg/kg) to visualize availability control.


Mechanisms Controlling Release Mechanisms controlling release can be distinguished in control by:

• the release potential, which is the fraction ultimately available for leaching

• solubility, which may be affected by changes in leaching controlling conditions (e.g. pH, redox)

• mass transfer limitations (e.g. diffusion out of a solid matrix into porewater)

• chemical speciation in the solid and in solution.

In the release patterns observed in column experiments such mechanisms can be observed. In Fig. 2 examples are given of the leaching behaviour of Ca, Ba, Cr, Pb, Mo, Cu, V and Zn from neutral coal fly ash. 31 In the graph the total concentration in the ash is indicated by the horizontal solid line and the availability by the dotted line.

Cr and Mo are clearly availability-controlled, as the cumulative amount leached is reached within L/S -- 1. The leachability of Ca reflects the influence of chemical speciation. The most readily soluble phase at high pH is portlandite, which is depleted in a few column volumes (L/S ~ 2). At later stages gyp- sum and other soluble Ca phases are relevant. The

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FIGURE 2. Column data on Ca, Ba, Cr, Cu, Mo, V, Pb and Zn from coal fly ash showing actual release as a function of LS, the total concentration (solid line) and the availability for leaching (dotted line). The retention values (K) were derived from a simple CSTR modelJ 6

availability level as defined by the availability test is approached for several elements, which reflects the relevance of this test as an asymptote to which leaching may approach in the long term.

Factors Influencing Release Several factors influence the release of contaminants from granular materials. An important factor gov- erning release is the major element chemistry because the major elements dictate the pore water composition, which in turn controls the trace element leach-ability to a large extent. The pH of the solution has been shown to be a crucial parameter in determining the solubility of contaminants. 6'7 In addition, the redox status of the system and the presence of complexants, either inorganic, such as CI-, or organic, humic substances, or other dissolved organic compounds capable of complexing metals are of importance, t6'13 The liquid-to-solid ratio is relevant because it relates to the time-scale through the rate of infiltration/°.32 The role of biological activity should not be neglected as it has an impact on the generation of dissolved organic substances, generation of COz and it can turn a system anoxic. The changes brought about by any of these factors may lead to order of magnitude changes in leachability of specific elements. Some specific underestimated factors are discussed below.

Leaching of different species. In several cases it is possible to identify from the release as a function of time that leaching of an element from one waste matrix may proceed in a different chemical form than in another case. An example is the leaching of Cu from MSWI residues (Fig. 3). The release of Cu from MSWI fly ash in a column test points at a significant retention relative to mobile constituents such as sodium (>5000), whereas in the bottom ash a fraction of dissolved Cu is observed with a low retention (<1). This is apparently caused by a different chemical form (organic complexl6). From a single measurement these sorts of phenomena go unnoticed, which is a serious drawback of single batch extractions. For an assessment of long-term behaviour the difference in chemical form is crucial, since the ultimate release and behaviour of the different chemical species in the underlying soil may be entirely different.

Role of carbon dioxide. In the Swiss test, saturation with CO2 is applied. The question arises whether this extreme condition reached in 24 h is relevant in all long-term disposal scenarios. For disposal of inorganic wastes and stabilized wastes the condition will lead to an overestimation, as only the surface of the material will be in contact with CO 2 from the air and subsequently feature the leaching behaviour as


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FIGURE 3. Unified approach of leaching for Cu from Municipal Solid Waste Incinerator bottom ash and Municipal Solid Waste Incin- erator fly ash. The release as a function of LS and release as a function of pH are given. A comparison with current regulatory tests is included.

depicted by the Swiss test. However, the uptake of CO2 in deeper layers in a site may be very slow, particularly when the material is highly alkaline (sealing effects) and fine-grained. Therefore, a distinction between surface layer exposed to the air and the bulk of the material in a site is essential with respect to the predominant leaching mechanisms. For disposal of organic wastes the implied conditions will be more appropriate due to the CO2 generated by biological degradation, although the reducing properties often associated with this process are not addressed in this test.

Availability for leaching.The time-scale at which the leachable quantity can be leached under environmental conditions is largely dictated by the neutralization through acid rain infiltration and uptake of CO2 from the air or through biological degradation. For very alkaline materials, this time-scale can be in the order of hundreds or thousands of years depend- ing on the conditions of application or disposal. For mutual comparison of materials, it is essential to use

uniform and well defined conditions. When a long- term chemical change is brought about by incorpo- ration in poorly soluble mineral phases resulting in a lower leachable fraction, it is important to be able to quantify such a change. For that purpose, the availability test is qualified.

Repeatability of Tests The repeatability of the different test methods depends on the combination of sampling error, variation in the test performance, the final pH of the test and the analytical error in assessing leachate composition. At very low concentration levels the variation due to the analytical methods is most prominent. Therefore, to isolate the different contributions to the total error, it is important to compare results on elements with low analytical error to isolate test performance and sampling errors from analytical errors.

If an element can be assessed with sufficient sensitivity, repeatability of single extraction tests can be within 5% relative standard deviation for such tests. In the case where the final pH of the test is in a region


where the sensitivity of an element to pH is critical, much larger standard deviations may be expected. The results obtained in this work show these effects as well. For MSWI bottom ash, for example, the final pH values of EP-TOX and TCLP tests were respectively 5.3 and 4.86. This resulted in higher leached quantities for Mg, Cu, Zn, Pb, Ni, Mn and Fe in the TCLP test. ~5 A difference of a few tenths of a pH unit in the final pH of the same test on the same material results in a bias that is in accordance with the relation between pH and leachability.

Inhomogeneity of a material with respect to a particular component can be identified by comparing the standard deviation of a particular component with that of another component in the same sample in relation to the respective limits of determination for these elements. This can be done for the total concentration as well as for the leaching results of a specific test. When the standard deviation is much higher than expected on the basis of the concentration level measured, inhomogeneity of the sample for a particular component may be a likely cause.

Examples are Cd and Cu in MSW incinerator bottom ash. In total composition, inhomogeneity has been demonstrated for these elements. ~6 At the leached amounts of, respectively, 3.8 and 110 mg/kg in the TCLP test, the reproducibility should be much better than 5%. Measured standard deviations of 15 and 8.5% are above this value, indicating sample inhomogeneity. Testing with another test, such as the DIN 38414 $4, does not lead to the same conclusion. This is related to the fact that TCLP is more closely related to the total composition, which was identified to be inhomogeneous for MSWI bottom ash, whereas the DIN test is in the domain of solubility control and therefore not subject to the influence of inhomogeneity in composition. This is an important observation as the TCLP is not relevant for many utilization or disposal scenarios, whereas the solubility-controlled conditions as obtained in the neutral to mildly alkaline pH domain are relevant for field conditions.

Comparison of Leaching Test Data Obtained by Different Test Methods The wide variety of leaching test methods may lead to confusion as to the meaning of the results of the different tests. Placing the various leaching tests in perspective to one another is therefore very useful, in particular in relation to interactions between countries with different regulatory requirements. 15

In Fig. 3 the results of different leaching tests applied to the same materials are shown in relation to the more elaborate testing methods such as column and pH stat test. The materials studied here are Municipal Solid Waste Incinerator fly ash and bottom ash. This type of comparison has been made for

several elements. Here results for Cu are shown. The agreement between the results of single extraction test and the pH stat data is good. This type of comparison forms the basis for the Unified Approach of Leaching as advocated by the International Ash Working group (IAWG). 16"33

The existing tests each tell a small part of the leaching story, but the tests in combination really provide the insight into how factors influence release and can be used to control release.

Concise Tests The requirements on tests for regulatory purposes, testing for scientific understanding of leaching phenomena and tests for management of wastes at a production site are quite different. A management tool has to be quick and simple, whereas a test for scientific understanding may take longer and can be far more complicated in its performance. A test for regulatory purposes has to be a good predictor of environmental impact.

To obtain good predictability with the least effort, it is recommended to require a full characterization of waste material that has not been tested before. This characterization should at least consist of the following steps:

• total concentration of constituents (variability between batches)

• potential leachability of contaminants, acid neutralization capacity and reducing potential

• pH sensitivity of leaching (pH stat test) • column test for granular materials LS = 0.1-10

(simulation of long-term leaching).

Based on the knowledge gained and keeping in mind that there is a certain level of background information, concise tests have been derived covering important factors controlling release in such a way that the test can be performed within 2 days with the analysis of four extracts. In Fig. 4 the specifications are given. 34 The two-extraction step is part of the recently prepared CEN TC 292 compliance test for granular wastes. 23

From this combination of four extractions the following conclusions can be drawn.

Extractions at two L/S values:

• distinction between solubility control and availability control

• measure of retention in the matrix relative to very mobile species (e.g. soluble salts)

• EH measurement (relative to pH) gives indication of possible reducing properties

• DOC (Dissolved Organic Carbon) measurement gives potential mobilization of metals

• TDS (Total Dissolved Solids) gives the fraction of soluble salts.


Extractions at two pH values:

• leachability changes in crucial pH domains • availability for leaching • acid neutralization capacity derived from acid

consumption • DOC measurement gives an indication of the nature

of the mobilized type of DOC.

The combination of leachability and pH data from

the four extractions shows the potential sensitivity of leaching to pH in the pH domain relevant for practice.

SYSTEMATIC LEACHING OF DIFFERENT WASTES

Based on a proper characterization of the leaching behaviour of wastes, systematic trends within a waste class and between waste categories can be established.

T I M E D E P E N D E N T R E L E A S E A N D R E T E N T I O N

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S E R I A L B A T C H T E S T

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L S = 2 - 1 0 ; 18 hrs; c l o s e d v e s s e l

p H C O N T R O L L E D T E S T

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L S = 1 0 - 5 0 ; p H - - 4 c o n t r o l ; 3 hrs

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a n d C o n d u c t i v i t ' ¢ .

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FIGURE 4. Concise testing protocol for granular wastes.


Consistency of Leaching Behaviour Between Different Productions of the Same Waste An important question in many cases is to what extent leaching properties of a given residue stream vary within one installation and between different installations producing the same residue. An important secondary question is: which factors influence the observed variability between charges?

A comparison of the release (mg/kg) of Pb from MSWI bottom ash from different installations in different countries is presented in Fig. 5. It is clear that the data are more consistent than might be expected on the basis of the highly variable chemical composition. ~6 Apparently, solubility control for Pb in a bottom ash matrix leads to consistent results, which is influenced by the pH of the bottom ash as the main controlling parameter. In Fig. 6 the relation between column data for different samples and the pH sensitivity of leaching is presented, which illustrates that the main difference in released amount can be attributed to pH. This in turn raises the question of what is controlling the pH of ash. Upon ageing the pH drops rapidly due to CO2 uptake from the air and CO2 derived from biological degradation. Size reduction for testing tends to lead to a higher leachate pH. Consequently, one might query the relevance of the test result for practice.

Comparison of Different Waste Materials Since release of contaminants is largely controlled by chemical reactions, to what extent do similarities exist between the leaching behaviour of different wastes that would enable a transfer of knowledge on leaching behaviour from a waste that has been char- acterized in great detail to other comparable wastes

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v .~-~)~ +~- / / - '~" size-reduced Field , , , ' / ~

/ / ~ , / " Fresh

. . . . . ,~ i - ' " non-size reduced

0.001 . . . . . . . . . . , . . . . . . . . . ~ . . . . . . . . . ~ . . . . . . . . .

9 lO II 12 13

pH

FIGURE 6. Significance of leaching data obtained in the laboratory on fresh size-reduced laboratory samples in relation to untreated material direct from the installation and material aged and weath- ered in the field.

for which less information is available. An aspect of importance in this context is which factors (limited number) control the differences in leaching behaviour between different wastes. Figures 7 and 8 provide a comparison of release as a function of pH for Cd and Zn in 12 different waste materials of widely different origin. 6.W6'3°.35 The wastes compared are refuse- derived fuel ash (RDF ash), coal fly ash, MSWI fly ash, MSWI bottom ash, cement-stabilized MSWI fly ash, vitrified MSWI fly ash, soil amended with

1 oooo Total

lOOO

Availability 1 0 0

j , ~ 0.1

0.01 ~ 0 .001

0 . 0 0 0 1 , ' . . . . . . . : . . . . . . . . ',

0.1 1 10

L$ (I /kg)

Pb

1 0 0

10

E

c 8

0.1

ID J

0 . 0 1 • o

o

0 . 0 0 1 I , , , e : ~ , , , i . . . . i

1 0 11 1 2 p H 1 3

Pb

:' at o • 0/

. , , --°o r=o o ° ° ° Column data, LS=I

' - ' - - - d

14

FIGURE 5. Leaching behaviour of Pb from MWSI bottom ash as a function of the liquid-to-solid (L/S) ratio in relation to total composition and availability for leaching. Relation between column test data and pH-dependent leaching for Pb as obtained from pH stat experiments show that release from column tests is largely determined by pH.

74 H . A . VAN DER S L O O T

I ]oo Brown coal ash _y

I II 8 s h J O I

I g '

° " °"I O.01 O,OI O,OI

~ O.OOI 0.001 ~ 0.001

° °-r I O 0.0001 0.0001

o o o o o , • ' ? ~ ' ? 7 . " ' 7 7 ~ , ?", . . . . . o o o o o , - ' ' . . . . o o o o o , . . . . . . . . . . . . . . 3 4 5 6 7 8 9 I0 II 12 13 21 4 5 6 7 8 9 10 II 12 13 3 4 5 6 7 8 9 I0 " 12 13

100

10

I

0.1

0.01

0.00!

0.000 I

0.00001

100

I0

I00

10

1

0.1

0.01

0.001

0.0001

0.00031

Cement-stabilized

MSWI fly ash

0.1

0.01

0.001

0.0001

J , , , , , , I t 0.0000l

4 5 6 7 8 9 I 0 I I 1 2 1 3

MSWI Fly ash Shredder Waste l m l I ' I l l ' I l I l I I I l l • l l , i , l . i l l , i l i , I l i l

4 3 6 7 8 9 I0 II 12 13 3 3 4 5 6 7 8 9 I0 II 12 13

1 0 0

10

I

0.1

0.01

0.001

0.0001

0.00001

I00

io

I

0.1

0.0!

0.001

0 . 0 0 0 1 ,

0.00001

3

M S W Incinerator Boltom Ash

i ° . ° . t . , . , r , . , • i , . .

3 4 5 6 7 8 9 I0 II 12 13

Vitrilied MSW1 fly ash

| i , , , | , i |

4 5 6 7 8 9 tO 11 12 13

100

10

1

0.1

0.01

0.001

0.000 I

0.00001

100

l0

I

0.1

0.01

0.001

0.0001

0 . 0 0 0 0 1

Jarosile • , . , . , . . . , .

3 5 7 9 II 13

Phosplmte slag

O x

. . . . . " 3 4 5 6 7 8 9 I0 11 12 13

I00

I0

I

0.1

0.01

0.00 I

0 . 0 0 0 1

I00

IO

I

OA

0.01

0.001

0.000 I

LO0001

Sewage amended soil

I I I I I I I I I

4 5 6 7 8 9 I0 I1 12 13

Sewage sludge

4 5 6 7 8 9 I0 I I 12 13

p H

FIGURE 7. Cd leachability of 12 different bulk materials as a function of pH.

sewage treatment sludge, sewage sludge, a natural soil (terra rossa), waste from car shredders (shredder waste), phosphate slag, brown coal ash and jarosite. General trends in release as a function of pH are similar but the absolute release for each element varies by

several orders of magnitude. It is clear that the major elements that dictate the leachate composition for the most part control the leachability of trace contaminants. The differences between the behaviour of indi- vidual elements in different wastes can be attributed


10000

1000

-.~ 100

E lo 1 =

. ~ O.1

¢~ 0.01 =

0.001 O ~ 0.0001

0

~.) 10000

Ct:S 1000 Z)

100 0

,-2 t o

1

0.1

0.01

0.(,01

0.0001

10000

1000

100

|0

0

0.(q

I,.001

0.0001

R D F ash 1 a [ t ] * I 1 I a I t ] i I i I l

3 4 5 6 7 8 9 10 11 12 13

MSWI fly ash

i i I i

3 5 7 9 I | 13

MSWI bottom ash

1 0 0 0 0

1000

100

I0

1

0.1

0.01

0.001

, , , , , i , t I

4 5 6 7 S 9 10 11 12 13

Vitrified

MSWI fly ash

0.0001 . . . . . . . . . . . . . . . . ' ' '

3 4 5 6 7 8 9 10 I I 12 13

10000

1000

100

10

1

0.1

0.01

0.001

0.0001

10000

1000

100

10

1

0.1

0.01

0.001

0.0001

10000

1000

100

10

1

0.1

0.01

0.001

0.0001

10000

1000

100

10

1

0.1

0.01

0.001

0.0001

Brown coal ash

3 4 5 6 7 g 9 10 11 12 13

Cement-stabilized MSWI fly ash

I

t i i l i i i i i

3 4 5 6 7 g 9 10 11 12 13

Jarosite i t i i i i i i , J i

1 2 3 4 5 6 7 g 9 10 11 12 13

Phosphate slag

t i J L , , i i ,

3 4 5 6 7 8 9 10 11 12 13

10000

100010010 I Coal fly ash

o:F' o.o,I o=f . . . . 7 ".,

3 4 5 6 7 8 9 10 11 12 13

1oooo ~

100~ f Shredder waste

0.1 r E

o,oi ~

0.001

0 . 0 0 0 1 ' ' ' ~ ' ' ' '

3 4 5 6 7 S 9 10 11 12 13

10000

1000

100

,o 0.1 I

0.01

0.001

0.0001

Sewage amended soil

Terra rossa i i i i i i i i i

3 4 5 6 7 S 9 10 11 12 13

10000

1000

100

10

1

Sewage sludge

0.1

0.01

0.001

0.0001 . . . . . . . . .

3 4 5 6 7 8 9 10 11 12 13

pH F I G U R E 8. Z n l e a c h a b i l i t y o f 12 d i f f e ren t b u l k m a t e r i a l s as a f u n c t i o n o f p H .

to specific factors, such as the presence of high concentrations of dissolved organic matter (sewage sludge, shredder, soil), strong sorptive properties by high Fe oxide contents (brown coal ash, soil) and the occurrence of reducing conditions (industrial slags).

For Cd the influence of CI on release is substantial. The release of Cd from a variety of materials (RDF ash, MSW bottom ash, MSWI fly ash, shredder waste, coal fly ash) with different CI levels shows the sensitivity to the CI level. The increase in Cd release


at higher pH with increasing C1 concentration in the leachate is very pronounced and can be modelled with geochemical speciation models./3"37'3s The rela- tionship between vitrified MSWI fly ash, MSWI bottom ash, MSWI ESP ash and RDF ash forms an interesting sequence in this respect. The total C1 contents (equals the availability) are, respectively, < 100, 1500, 18,000 and 50,000 mg/kg.

The Zn leachability curves for different wastes are very consistent. A slight shift in solubility with pH occurs for RDF ash as a result of the high C1 content in this waste. Mobilization of Zn through DOC interaction is noted for shredder waste, sewage sludge and soil in the neutral to alkaline pH range. In Fig. 9 the generic pH-dependent leaching curve for Zn is given with modifying influences specific to a material or the surrroundings.

These observations hold promise for a further evaluation and characterization of waste materials. If the observed leaching patterns, which make perfect sense in relation to the basic chemistry involved, can be proven to be consistent for almost any waste it would be useful to consider the possibility of charac- terizing wastes by constituent (about 20 environmentally relevant elements) rather than by waste (hundreds of different waste types).

MODELLING LONG-TERM RELEASE

The processes occurring in the field are complex. To derive conclusions on a full account of interactions in any given field situation is impossible. How- ever, the arbitrary approach to deciding on disposal based on a single extraction test is another extreme. Through simplified modelling several factors affect- ing release can be accounted for to lead to better decisions on acceptable practices. As a first approach

100 % E ,-, 10 e-, o ~ 1

= 0.1 o O

0.01 e-, 0

0.001

0.0001

Cd

tom exa on

- - _ d / ~ b ~ l i C t i ~ n

vitrification ~ , , ~

I I [ I I I I I I

4 5 6 7 8 9 10 11 12 13

a continuous stirred tank reactor (CSTR) model is used, which is based on the fact that changes in leaching behaviour are not so much related to the process of percolation as to significant changes in chemical conditions at time-scales that are long relative to the rate of percolation. With the exception of soluble salts the release of most constituents is solubility controlled. If elements are present in different dissolved chemical forms, only those that feature limited or slow chemical exchange will generally be observed as separate species in leaching test data (e.g. Cr III and Cr VI, Cu 2+ and DOC-bound Cu). In such cases the more mobile form may be depleted before any significant release of the other species occurs.

The release ELSr~e,d at a given exposure time of a waste material in the field, expressed in mg/kg leached, is given by

ELSneld = A vail (1 - e LSF,o~d/X) (mg/kg) (1)

with Avail as the availability for leaching NEN 7341, ~° which is the asymptote to which leaching approaches in the long term, unless separate chemical species can be identified requiring adjustment of this parameter, K is a factor expressing the retention of the element of concern in the matrix relative to a mobile constituent such as Na, which is obtained by curve fitting from the test results obtained in a column experiment according to NEN 7343. u LSFiel d is the actual LS reached after t years of exposure to field conditions and is obtained from

LSFidd = /Via f X tlh X d (1/kg) (2)

with Ni,r the net infiltration at the given location as obtained from meteorological data (m/m2/year), t is the exposure time in the field (years), h is the height of the application (m) and d is the bulk density of the material concerned (kg/m3).

10000

1000

100

10

1

0.1

0.01

Variation with the [

. ~ [ ~Region of solubility control ~

- g~uc:mg! / ~¢omplcxation] cenditiorts [ _ v ~ ~ 1

. i \ / . / , - . Fe hnd r,~ o,,i,k.s \ I.I

0 2 4 6 8 10 12 14

pH pH

FIGURE 9. Main factors influencing the leachability of Cd and Zn in specific pH domains. The solid lines represent generic leaching behaviour of these elements in common, mainly inorganic, matrices without organic matter, high contents of sorptive phases and complexants.


The emission to the underlying soil is then obtained from

/'max = db X h × ELSFiel d (mg/m 2) (3)

The retention value K is not a constant when the conditions in terms of pH, redox or complexation change. Through the K value adjustments for chemical speciation or externally imposed changes can potentially be made. This aspect needs to be exploited further.

LABORATORY FIELD VERIFICATION

The ultimate aim of testing and modelling is to be able to assess what the long-term release in a given field application or disposal situation will be, in order to decide on the acceptability of the action. However, it is important to realize that it will be difficult to reach a 1:1 relation between laboratory and field data for all constituents of interest. In the translation, a number of factors need to be taken into account:

• temperature • mode of contact with water (permanent, inter-

mittent, different flow regimes) • channeling • redox conditions • waste/soil interactions • remineralization/ageing

Release from fine-grained materials is a function of pH, liquid-to-solid (LS) ratio, redox potential, sorption, complexation and some minor factors with a smaller contribution to the overall release. To arrive at a conclusion on the acceptability of a material in a given situation implies that the magnitude of contribution of the different factors to the overall release is known. Starting from major variables, LS and pH, a release pattern can be identified for indi- vidual contaminants. The other variables can modify this general pattern and the trends should be known for a given material (identification of material characteristics).

Factors in the Translation of Laboratory Test Data to Release under FieM Conditions

Time. The projected life-time of the application is a factor obtained through estimates of the infiltration rate. Based on a model description of the release- controlling mechanism(s) --solubility, sorption, percolation, diffusion-- the release as a function of time can be described.

climates (arctic, temperate or tropical) this factor needs to be taken into account.

pH. The pH conditions in the laboratory and in the field may differ substantially due to carbonation or self-neutralization effects. Since the difference in release as a function of pH may change by orders of magnitude over relatively small pH ranges, 6,13'16 this factor is not to be neglected. Generally, the pH measured in the laboratory is higher than that encountered in the field. Particularly for metals which show a higher leachability at high pH (e.g. Pb, Zn), this may lead to an overestimation of release.

Redox. The material tested in the laboratory may be reducing and be tested as such. If such a material becomes oxidized in a field scenario, the data obtained in the laboratory no longer reflect conditions encountered in the field and should thus be adjusted in testing. A normally oxidized material may become exposed to reducing conditions in the field and therefore not reflect the conditions to be assessed in a standard test. 24

Geometry. The shape and dimensions of a material affect the rate of release. A relatively thin layer of material may become depleted of soluble constituents in a relatively short time-scale. The geometry may have a significant effect on the infiltration rate.

Water-to-solid contact frequency. The degree of contact with water is a factor of importance as the testing is generally carried out under fully saturated conditions. In the field, wet/dry cycles may occur as well as variations in the degree of water saturation. Corrections for the wet/dry cycles may be based on the average annual rainfall rate at a specific location.

Interface reactions. In several cases interface effects have been shown to occur as a result of changes in the chemistry of the product in a thin surface section. Since this is the surface exposed to the leachant, it is likely to dictate release. Even surface sealing effects have been noted 39 as a result of precipitation reactions.

Compar&on of Laboratory Data with FieM Measurements The number of useful correlation studies between laboratory and field conditions is still limited. 4° In many cases, the field verification measurements have been carried out too far away from the waste/soil interface to be useful.

Temperature. The difference between testing in the laboratory and conditions in the field can not be neglected. In particular, for decisions on release in different

Comparison of field data from MSWI ash disposal with laboratory data. In Fig. 10 the data obtained in field measurements ~6 are related to the concentrations


10 1000 11 ~_~ 0.1 -- /

O . O l '

o 0.001 u 0.01

o.ooo, o.oo, "b; 0 . 0 0 o 0 1 . . . . . . . . . . . . : . . . . : . . . . : . . . . . . . . : o.ooo,, , , . . . . = . . . . , , , , , ; . .

0 2 4 6 pH 8 10 12 14

FIGURE 10. Comparison of data from Municipal Solid Waste Incinerator residue lysimeter studies and from disposal sites for Cd and Pb in comparison with laboratory leaching data.

measured in laboratory leaching tests. Data were gathered from the studies shown in Table 2. The pH range in field measurements and lysimeter studies was significantly lower (pH 7-10) than the pH range found in laboratory tests on fresh ash (pH 9.5-12). This also suggests that translation of laboratory data to actual field conditions requires adjustment based on pH-solubility curves for evaluation of the long-term behaviour of bottom ash. The pH-solubility curve for the sample to be evaluated can be obtained through use of pH static testing or similar tests. However, pH data generated in both field and lysimeter studies should be regarded with caution, since collection tanks or drainage systems are usu- ally not isolated from contact with air. It is possible that the alkaline pH of collected pore water is neutralized during prolonged contact with air. The concentrations measured in field leachate and lysimeter studies are in equilibrium with the measured (lower) pH, which are not representative of the condition within the ash.

The range of the concentrations measured in the field relates well for Pb and Cd with the range of results obtained in the laboratory. The leachability of Pb deviates in some cases by an order of magnitude from the laboratory data (reducing conditions). The Cd data from the field measurements show a good correlation with the laboratory test data. Particu- larly at the lower pH range, where due to biological

TABLE 2 Sources of Data for Fig. 10

Ash type Location Country Ref.

Bottom ash Rotterdam Netherlands 41

Bottom ash Copenhagen Denmark 42

Combined ash Several monofills USA 43

Combined ash Woodburn, OR USA 44 Bottom ash USA 45 Bottom ash HW 15 Netherlands 46

activity (high level of unburnt material) acidic conditions occur, the Cd data show a good correlation. With just a few exceptions (e.g. some Zn field data) the field data generally fall below the results obtained in the laboratory, which makes the laboratory evaluation a conservative approach, j6

Laboratory-field verification of a roadbase appfication of stabilized coalfly ash. As part of coal ash utilization demonstration projects, a test section of road was constructed in 1983, where a road with a coal ash stabilization layer (thickness 30-50 cm) was covered with an asphalt wearing course. During first two years after placement leachate was collected and analysed. Eleven years after placement field verification measurements 47 have been performed. This work, which is one of the first in which true impact under an application of secondary materials is veri- fied in the field will be presented in a separate paper.

CURRENT EC ACTIVITIES RELEVANT TO LEACHING

In relation to new directives in preparation by the EC, leaching tests are needed to decide on the environmental quality of waste materials and subsequently on the options for utilization or disposal.

European Standardization of Leaching Tests ( CEN) In the framework of CEN TC 292 Characterization of Waste, this aspect has been taken on as an item for European standardization. In Working Group 2 a compliance test for granular material has been prepared, 23 which will be at stage level 40. The test consists of three options: extraction at low LS (L/S = 2) relevant to the new disposal conditions with minimal infiltration, extraction at L/S -- 10 and a combination of L/S = 2+2 and L/S -- 2-10. The test currently in preparation in this working group is a compliance test for monolithic materials.


A new Working Group 6 has been installed to deal with the standardization of characterization tests for long-term leaching behaviour. In addition to this item a guidance document will be prepared to identify relevant steps in scenarios for assessing long- term release in utilization and disposal of materials.

Standards, Measurement & Testing Programme (S ,M&T) In recent years prenormative work on standards development has been carried out with the support of this DG XII Programme. This relates to work on extraction methods for soil 48 and sediments 49 an intercomparison study of leaching tests for stabilized waste 5° and prenormative work on the development of test procedures for coal fly ash in concrete. As a result of the increase in proposed test methods, it was felt necessary to harmonize the development of tests in different fields to allow as much interaction as possible between them, thus avoiding unnecessary development of procedures already available in other related fields.

A pilot project, Harmonization of Leaching/ Extraction tests, started in 1995 with its first round of consultation of experts in a broad spectrum of fields (soil, contaminated soil, sediments, sludges, compost, waste, stabilized waste, construction materials, preserved wood)? ~

CONCLUSIONS

The aim of a leaching test must be described as well as its limitations. It should not stretch outside the scope of its inherent limitations. It is clear that all tests developed so far focus on a specific aspect of leaching, but cannot cover individually all aspects that need to be covered. A rational combination of existing procedures can deal with the complexity caused by the most relevant parameters influencing the release of contaminants through leaching (concise test). Existing regulatory tests are implied in the concise procedure.

Release from waste may be dictated by solubility or by availability. This distinction is crucial for the interpretation of the leaching data obtained from a leaching test. Identification of the release-controlling mechanism allows prediction of release at different time-scales. When changes in pH, redox or degree of complexation by dissolved organic matter occur, the possible changes in release can be quantified once the element-specific relations have been identified for a given matrix.

Leaching behaviour from bulk waste materials is more consistent than one would expect from the results obtained in single extraction tests, as the sensitivity to specific leaching controlling factors cannot be recognized. The variability in leaching behaviour

is in many cases less than judged from single tests, as minor shifts in pH or redox state go unnoted. Such changes can lead to order of magnitude differences in leachability.

The results from pH-dependent leaching can help to identify the crucial factors controlling release from a given waste material through similarities in leaching behaviour with other waste materials.

Modelling the release of contaminants from granular materials is an essential part of the long-term evaluation of the behaviour of waste materials in disposal scenarios and in beneficial applications. A finite dynamic source term has to be described and the consequences of changes in chemical speciation have to be included. It is important to relate intrin- sic leach parameters derived from batch extractions and column leaching with each other. Application of geochemical speciation models will help to improve the basic understanding of processes controlling long-term release.

In the interpretation of leaching test data it is important to know whether the conditions in the test bear any resemblance to the actual conditions under field conditions. Alkaline materials will be neutralized upon contact with atmospheric CO2. Even in testing this phenomenon may lead to ini- tially unexplained variabilities in testing the same material. Reducing materials will be mostly reduced during short-term testing, but upon prolonged contact with atmospheric oxygen may become oxidized, which may lead to totally different release characteristics.

Field verification measurements have been carried out to test the agreement between model predictions and improve the definition of testing required for a given application through an iterative process of model prediction, verification and identification of underestimated factors. Field verification has shown better agreement between laboratory data and field measurements than generally believed possible. The difficulties in relating laboratory to field should, however, not be underestimated.

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1. EPA Toxicity Test Procedure (EP-TOX). Appendix I1, Fed- eral Register, Vol. 45(98), 1980, 33127-33128. Government Printing Office, Washington DC (1980).

2. Toxicity Characteristic Leaching Procedure (TCLP). Federal Register, Vol. 51, No 114, Friday, 13 June, 1986, 21685- 21693 (proposed rules). Federal Register, Vol. 261, 29 March, 1990 (final version). Government Printing Office, Washington DC (1990).

3. DIN 38414 $4. German Standard Procedure for Water, Wastewater and Sediment testing--Group S (Sludge and Sed- iment); Determination of Leachability ($4). Instittit for Nor- mung, Berlin (1984).

4. D6chets: Essai de Lixiviation X 31-210, 1988. Association Frangaise de Normalisation (AFNOR), Paris (1988).

80 H . A . VAN DER S L O O T

5. Bericht zum Entwurf for eine technische Verordenung fiber Abfalle (TVA). 1988. Drpartement Frdrral de l'Intrrieur. Switzerland (1988).

6. Goumans, J. J. J. M., van der Sloot, H. A. and Aalbers, Th. G. (eds). WASCON 1991. Waste materials in construction. Elsevier, Amsterdam (1991).

7. Goumans, J. J. J. M., van der Sloot, H. A. and Aalbers, Th. G. (eds). WASCON 1994: Environmental Aspects o f Construc- tion with Waste Materials. Elsevier, Amsterdam (1994).

8. Van der Sloot, H. A. Leaching behavior of waste and stabilized waste materials; characterization for environmental assessment purposes. Waste Manage. Res. 8:215-228 (1990).

9. Hjelmar, O. Field studies of leachates from landfilled combustion residues. In: Waste Materials in Construction, Goumans, J. J. J. M., van der Sloot, H. A. and Aalbers, Th. G. (eds). Elsevier, Amsterdam, Supplementary issue NOVEM (1991).

10. NEN 7341. Determination of the Availability for Leaching From Granular and Monolithic Construction Materials and Waste Materials (1993).

11. NEN 7343. Determination o f Leaching from Granular Con- struction Materials and Wastes by Means o f a Column Test (1994).

12. NEN 7345. Determination o f Leaching from Monolithic Con- struction Materials and Waste Materials by Means o f a Diffu- sion Test (1994).

13. Comans, R. N. J., van der Sloot, H. A. and Bonouvrie, P. Geochemical reactions controlling the solubility of major and trace elements during leaching of municipal solid waste incinerator residues. Proc. Municipal Waste Combustion VIP 32, Air & Waste Management Association, Pittsburg, PA, 667- 679 (1993).

14. Test methods for solid waste characterization. Acid Neutral- ization Capacity Test, method 37. Environment Canada and Alberta Environmental Center (1986).

15. van der Sloot, H. A., Hoede, D. and Bonouvrie, P. Com- parison of Different Regulatory Leach Test Procedures for Waste Materials and Construction Materials. ECN-C-91-082 (1991).

16. Chandler, A. J., Eighmy, T. T., Hartlen, J. et al. Interna- tional Ash Working Group: Treatise on Municipal Solid Waste Incinerator Residues, IAWG summary document, ECN, Petten, The Netherlands, p. 77 (1994).

17. Environment Canada. Compendium of Waste Leaching Tests. Environmental Protection series, Report EPS 3/HA/7 (1990).

18. Wallis, S. M., Scott, P. E. and Waring, S. Review of Leaching Test Protocols with a View to Developing an Accelerated Anaerobic Leaching Test. AEA-EE-0392. Environment Safety Centre (1992).

19. Characterization of waste in Europe. State of the art report for CEN TC 292. STB/94/28 (1994).

20. Parker, J. H., Woodhead, P. M. J., Duedall, I. W. and Car- leton, H. R. Coal Waste Artificial Reef Plan. EPRI report CS 2574 (1992).

21. Landfill Directive of Waste EC DG XI, draft (1995). 22. Van der Sloot, H. A., Hjelmar, O., Aalbers, Th. G.,

Wahlstr~m, M. and F~tllman, A.-M. CEN TC 292 Document: Proposed Leaching test for Granular Solid Waste. February, 1993. (Also as ECN-C-93-012) (1993).

23. Compliance test for leaching of granular materials, CEN TC 292 Characterization of Waste, Working Group 2 Draft European Standard (1994).

24. Van der Sloot, H. A., Hoede, D. and Comans, R. N. J. The influence of reducing properties on leaching of elements from waste materials. In: WASCON 1994: Environmental Aspects o f Construction with Waste Materials. Elsevier, Amsterdam, 483-490 (1994).

25. SOSUV-I. Samenvattend rapport afrondende werkzaamhe- den (1989).

26. Di Pietro, J. V., Collins, M. R., Guay, M. and Eighmy, T. T. Leachability of municipal solid waste incinerator residues. Int. Conf. On Municipal Waste Combustion, Hollywood, FL, U.S.A., April (1989).

27. Kosson, D. S., Kosson, T. T. and van der Sloot, H. A. USEPA Program for Evaluation o f Treatment and Utilization o f Municipal Waste Combustor Residues. Cooperative agreement CR 818178-01-0. USEPA/RREL, Cincinnati, Septem- ber (1993).

28. Aalbers, Th. G. et al. Milieuhygienische Kwaliteit van Primaire en Secundaire Bouwmaterialen in Relatie tot Herge- bruik en Bodem- en Oppervlaktewateren Bescherming. RIVM rapport 7771402006/RIZA rapport 93.042 (1993).

29. Comans, R. N. J., van der Sloot, H. A., Hoede, D. and Bonouvrie, P. A. Chemical processes at a redox/pH interface arising from the use of steel slag in the aquatic environment. In: Proc. WASCON 1991: Waste Materials in Construction. Goumans, J. J. J. M., van der Sloot, H. A. and Aalbers, Th. G. (eds). Elsevier, Amsterdam, 243-254 (1991).

30. Van der Sloot, H. A. and Hoede, D. Laboratorium onderzoek naar de invloed van reducerende eigenschappen op het emissie-gedrag van industrie-slakken in oeverbeschermingen. ECN-C-94-094 (1994).

31. Van der Sloot, H. A., de Groot, G. J., Hoede, D. and Wykstra, J. Mobility of trace elements derived from combustion residues and products containing these residues in soil and groundwater. EUR report 14403. EN Commission of the European Communities DG XIII, Luxembourg (1993).

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