Hans A. van der Sloot

Process identification and model development of contaminant transport in MSWI bottom ash

In this work we investigate to what extent we are able to predict experimental data on column leaching of heavy metals from municipal solid waste incinerator (MSWI) bottom ash, using the current knowledge on processes controlling aqueous heavy metal concentrations in combination with a multicomponent reactive transport computer model. Heavy metal concentrations were modelled with a surface complexation model for metal sorption to (hydr)oxide minerals in the bottom ash matrix. For transport modelling it was necessary to simplify the sorption modelling approach. Therefore, we determined a minimal set of components and species that still provided an adequate description of the pH dependent heavy metal behaviour. The concentration levels of the heavy metals are generally predicted to within one order of magnitude. Discrepancies between the model and the data are caused by uncertainty in modelling parameters and a still insufficient description of the dynamics of macroelement leaching and pH. In general, the simulated leaching curves show much more abrupt changes than the measurements. This observation might be an indication of non-equilibrium. Processes that have to be taken into account for further model development are the influence of non-equilibrium effects and the facilitated transport of heavy metals by dissolved organic matter.

Comparison of the release of constituents from granular materials under batch and column testing

Column leaching testing can be considered a better basis for assessing field impact data than any other available batch test method and thus provides a fundamental basis from which to estimate constituent release under a variety of field conditions. However, column testing is time-intensive compared to the more simplified batch testing, and may not always be a viable option when making decisions for material reuse. Batch tests are used most frequently as a simple tool for compliance or quality control reasons. Therefore, it is important to compare the release that occurs under batch and column testing, and establish conservative interpretation protocols for extrapolation from batch data when column data are not available. Five different materials (concrete, construction debris, aluminum recycling residue, coal fly ash and bottom ash) were evaluated via batch and column testing, including different column flow regimes (continuously saturated and intermittent unsaturated flow). Constituent release data from batch and column tests were compared. Results showed no significant difference between the column flow regimes when constituent release data from batch and column tests were compared. In most cases batch and column testing agreed when presented in the form of cumulative release. For arsenic in carbonated materials, however, batch testing underestimates the column constituent release for most LS ratios and also on a cumulative basis. For cases when As is a constituent of concern, column testing may be required.

pH-dependent leaching of constituents of potential concern from concrete materials containing coal combustion fly ash

Current concerns about the environmental safety of coal combustion fly ash have motivated this evaluation of the impact of fly ash use as a cement replacement in concrete materials on the leaching of constituents of potential concern. The chemical effects of fly ash on leaching were determined through characterization of liquid-solid partitioning using EPA Method 1313 for four fly ash materials as well as concrete and microconcrete materials containing 0% (control materials), 25% and 45% replacement of portland cement with the fly ash source. All source materials, concrete formulations and replacement levels are representative of US concrete industry practices. Eluate concentrations as a function of pH were compared to a broader range of available testing results for international concretes and mortars for which the leaching characteristics of the component fly ashes were unknown. The chemistry of the hydrated cement fraction was found to dominate the liquid-solid partitioning resulting in reduced leaching concentrations of most trace metals compared to concentrations from fly ash materials alone. Compared to controls, eluate concentrations of Sb, As, B, Cr, Mo, Se, Tl and V from concrete products containing fly ash were essentially the same as the eluate concentrations from control materials produced without fly ash replacement indicating little to no significant impact on aqueous partitioning.

Landfill sustainability and aftercare completion criteria

Although many countries are increasing their efforts to recycle and to re-use waste materials, landfilling will still be needed in order to dispose of wastes which cannot be recycled or treated in other ways. Since landfills will most probably be present for a long time, measures will have to be taken to reduce their (long-term) emissions. The European Commission has delegated the competent authorities to decide to end aftercare and several member states have provided regulations for this purpose. However, there is currently no guidance for long-term risk assessment to support an aftercare completion procedure for landfills. The aim of this study is to provide examples of current regulations and to demonstrate an alternative approach for a quantitative risk assessment of landfill leachate. The presented modelling approach clearly demonstrates the added value of site specific risk assessments of the long-term emissions from landfills and might provide a basis for application when the acceptance criteria for landfill will be revisited in the future. In addition, the modelling approach can be used as one of the toolboxes to perform assessments of the long-term emissions from landfill leachates and might help the competent authorities to decide whether the remaining emission potential is acceptable or not. Moreover, the results imply that local environmental conditions contribute to the acceptability of landfill emissions and are important factors in choosing a landfill location.

Field site leaching from recycled concrete aggregates applied as sub-base material in road construction

The release of major and trace elements from recycled concrete aggregates used in an asphalt covered road sub-base has been monitored for more than 4 years. A similar test field without an asphalt cover, directly exposed to air and rain, and an asphalt covered reference field with natural aggregates in the sub-base were also included in the study. It was found that the pH of the infiltration water from the road sub-base with asphalt covered concrete aggregates decreased from 12.6 to below pH 10 after 2.5 years of exposure, whereas this pH was reached within only one year for the uncovered field. Vertical temperature profiles established for the sub-base, could explain the measured infiltration during parts of the winter season. When the release of major and trace elements as function of field pH was compared with pH dependent release data measured in the laboratory, some similar pH trends were found. The field concentrations of Cd, Ni, Pb and Zn were found to be low throughout the monitoring period. During two of the winter seasons, a concentration increase of Cr and Mo was observed, possibly due to the use of de-icing salt. The concentrations of the trace constituents did not exceed Norwegian acceptance criteria for ground water and surface water Class II.

Effect of coal combustion fly ash use in concrete on the mass transport release of constituents of potential concern.

Concerns about the environmental safety of coal combustion fly ash use as a supplemental cementitious material have necessitated comprehensive evaluation of the potential for leaching concrete materials containing fly ash used as a cement replacement. Using concrete formulations representative of US residential and commercial applications, test monoliths were made without fly ash replacement (i.e., controls) and with 20% or 45% of the portland cement fraction replaced by fly ash from four coal combustion sources. In addition, microconcrete materials were created with 45% fly ash replacement based on the commercial concrete formulation but with no coarse aggregate and an increased fine aggregate fraction to maintain aggregate-paste interfacial area. All materials were cured for 3 months prior to mass transport-based leach testing of constituents of potential concern (i.e., Sb, As, B, Ba, Cd, Cr, Mo, Pb, Se, Tl and V) according to EPA Method 1315. The cumulative release results were consistent with previously tested samples of concretes and mortars from international sources. Of the 11 constituents tested, only Sb, Ba, B, Cr and V were measured in quantifiable amounts. Microconcretes without coarse aggregate were determined to be conservative surrogates for concrete in leaching assessment since cumulative release from microconcretes were only slightly greater than the associated concrete materials. Relative to control materials without fly ash, concretes and microconcretes with fly ash replacement of cement had increased 28-d and 63-d cumulative release for a limited number 10 comparison cases: 2 cases for Sb, 7 cases for Ba and 1 case for Cr. The overall results suggest minimal leaching impact from fly ash use as a replacement for up to 45% of the cement fraction in typical US concrete formulations; however, scenario-specific assessment based on this leaching evaluation should be used to determine if potential environmental impacts exist.

Integration of testing protocols for evaluation of contaminant release from monolithic and granular wastes

Abstract Testing for evaluation of contaminant release from soils and wastes is necessary to compare management options and project potential environmental impacts. Frequently, the available management options include disposal as either an untreated or treated granular or monolithic waste form with varying extents of water contact. In addition, some wastes may be suitable for utilization as fill material or aggregate replacements in construction applications. Selection amongst these choices requires substantially more intensive leaching testing and evaluation then is provided in most regulatory frameworks. Ideal testing and evaluation protocols would permit the necessary comparisons, provide realistic estimates of contaminant release under the scenarios evaluated, and be accomplished through use of a minimum number of analyses carried out over a very short time interval. A series of testing protocols has been under development for granular and monolithic wastes. The goal has been a series of tests that can be carried out in less than approximately one week with approximately six samples for chemical analysis that provide measurement of fundamental leaching parameters. These fundamental parameters then are used to estimate contaminant release under different management scenarios. Substantial reductions in testing requirements can be achieved through integration of the individual test methods. This paper will present the testing evaluation framework, using data obtained from several wastes to illustrate the benefits of the framework.

Release of heavy metals during long-term land application of sewage sludge compost: Percolation leaching tests with repeated additions of compost.

Leaching assessment procedures have been used to determine the leachability of heavy metals as input for evaluating the risk from sewage sludge compost land application. However, relatively little attention has been paid to understanding leaching from soils with repeated application of sewage sludge compost with elevated levels of heavy metals. In this paper, leaching assessment is extended to evaluate the potential leaching of heavy metals during repetitive application of composted sewage sludge to soils. Four cycling of compost additions and percolation leaching were conducted to investigate how leaching behavior of heavy metals changed with repeated additions of compost. Results showed that repetitive additions of compost to soil significantly increased the content of organic matter, which favored the formation of reducing condition due to improved microbial activities and oxygen consumption. Establishment of reducing conditions can enhance the leaching concentrations of As by approximately 1 order of magnitude, especially for the soil rich in organic matter. For Cd, Cr, Cu, and Pb, repeated additions of compost will cause accumulation in total contents but not enhancement in leaching concentrations. The infiltration following compost additions will leach out the mobile fraction and the residual fraction might not release in the next cycling of compost addition and infiltration. The cumulative release of Cd, Cr, Cu, and Pb accounted for less than 5% of the total contents during four times of compost applications.

Characterisation Leaching Tests and Associated Geochemical Speciation Modelling to Assess Long Term Release Behaviour from Extractive Wastes

A pH dependent leaching test (CEN/TS 14429) and a percolation leaching test (CEN/TS 14405) developed in CEN/TC 292 have been used for the first time to characterise the release behaviour of different sulphidic mining wastes. Geochemical speciation modeling using LeachXS Orchestra provides another type of partitioning between mineral and sorptive phases than is currently practised in the mining industry. Comparing new leaching test data for seven tailings and two waste rock samples with model predictions gives new insights into release behaviour. In leaching, mineral transformations on the surface of waste rock or tailings particles, rather than bulk mineral composition, dictate release, which implies that mineralogical examinations are not necessarily relevant from a leaching perspective. Kinetic aspects of release from sulphidic waste can be addressed by testing material in different stages of oxidation or exposure to atmospheric conditions.ZusammenfassungZum ersten Mal wurde ein in der CEN/TC 292 entwickelter pH-Wert-abhängiger Auslaugungstest (CEN/TS 14429) und eine Perkolationsprüfung im Aufwärtsstrom (CEN/TS 14405) genutzt, um das Austragsverhalten verschiedener sulfidischer Bergbauabfälle zu charakterisieren. Geochemische Speziesmodellierung mit der LeachXS Orchestra Software ermöglicht eine andere Methode der Unterteilung von Mineral- und Anreicherungsphasen als die derzeit in der Bergbauindustrie praktizierten Methoden. Neue Ergebnisse der Auslaugungstests von sieben Schlämmteich- und zwei Abraumproben ermöglichen einen Vergleich mit Modellvorhersagen und erlauben neue Einblicke in das Austragungsverhalten. Beim Auslaugen wird die Lösung eher durch mineralische Transformationen auf der Oberfläche des Abraums oder der Rückstandspartikel bestimmt als durch die Gesamtgesteinszusammensetzung. Daraus lässt sich schließen, dass mineralogische Untersuchungen aus Sicht des Auslaugungsverhaltens nicht unbedingt relevant sind. Kinetische Aspekte der Austragung aus sulfidischem Abraum können berücksichtigt werden, indem das Material in verschiedenen Oxidationsstadien oder unter atmosphärischen Bedingungen getestet wird.ResumenPara caracterizar el comportamiento de liberación desde diferentes residuos de minería de sulfuros, se han usado por primera vez un test de lixiviación pH dependiente (CEN/TS 14429) y un test de lixiviación en percolación (CEN/TS 14405), desarrollados en CEN/TC 292. Un modelo de especiación geoquímica usando LeachXS Orchestra, proporciona otro tipo de particionamiento entre mineral y las fases de sorción, que es actualmente utilizado en la industria minera. La comparación entre los datos de este nuevo test de lixiviación y las predicciones del modelo, para siete colas y dos muestras de rocas residuales, muestra nuevos aspectos en el comportamiento de estos residuos. En lixiviación, las transformaciones en la superficie de la roca o de las partículas de la cola, más que la composición del seno del mineral, dictaminan la liberación, lo que implica que los exámenes mineralógicos no son necesariamente relevantes desde una perspectiva de lixiviación. Los aspectos cinéticos de la liberación desde residuos sulfurados, pueden ser realizados testeando el material en diferentes estados de oxidación o exposición a condiciones atmosféricas.抽象欧盟PH依赖溶滤试验(CEN/TS 14429)和欧盟管柱渗透浸出实验(CEN/TS 14405)首次用于描述含硫化物矿山废石的溶出特性。利用LeachXS Orchestra专家系统建立地球化学形态分析模型而提出另一种不同的矿物及吸附形态分析方法。七种尾矿和两种废石样品的溶滤试验结果与模型预测结果的对比加深了废石溶滤特征的认识。在溶滤过程中,控制滤出过程的主要因素是废石或尾矿颗粒表面的矿物质转化而不是它们的矿物质总成分,废石或尾矿的矿物学特征不一定与溶滤直接相关。测试含硫化物废石在不同的暴露、氧化阶段的物质成分可以揭示其溶出动力学性质。

Effects of aerobic and anaerobic biological processes on leaching of heavy metals from soil amended with sewage sludge compost.

The risk from leaching of heavy metals is a major factor hindering land application of sewage sludge compost (SSC). Understanding the change in heavy metal leaching resulting from soil biological processes provides important information for assessing long-term behavior of heavy metals in the compost amended soil. In this paper, 180days aerobic incubation and 240days anaerobic incubation were conducted to investigate the effects of the aerobic and anaerobic biological processes on heavy metal leaching from soil amended with SSC, combined with chemical speciation modeling. Results showed that leaching concentrations of heavy metals at natural pH were similar before and after biological process. However, the major processes controlling heavy metals were influenced by the decrease of DOC with organic matter mineralization during biological processes. Mineralization of organic matter lowered the contribution of DOC-complexation to Ni and Zn leaching. Besides, the reducing condition produced by biological processes, particularly by the anaerobic biological process, resulted in the loss of sorption sites for As on Fe hydroxide, which increased the potential risk of As release at alkaline pH.