Radu Barna

THE EFFECTS OF CARBONATION AND DRYING DURING INTERMITTENT LEACHING ON THE RELEASE OF INORGANIC CONSTITUENTS FROM A CEMENT-BASED MATRIX

A Portland cement mortar was submitted to cycles of intermittent wetting (IW) in which tank leaching was interspersed with periods of storage in either an inert or a reactive atmosphere. Relative humidity (RH) (23%, 48% and 98%) was maintained during storage to control the drying process. The effects of IW were qualified by comparing flux and cumulative release of matrix constituents (Ca, OH, Na, K and Cl) to that of continuous water saturation. The carbonation process was associated with the degree of drying occurring due to storage. Cumulative release of most major constituents was suppressed in samples storage under 100% CO2 in comparison to the inert atmosphere (100% N2). Results suggest that accurate long-term performance assessment must account for the potential impact of phenomena associated with IW.

Leaching behavior of pollutants in stabilized/solidified wastes

The objective of the study is to characterize pollutant retention in wastes solidified with hydraulic binders in order to predict the long term leaching behavior. The stabilized/solidified wastes are APC MSW (air pollution control residues from municipal solid waste incineration), lead secondary smelting slag and different salts. The binders used are ordinary Portland cement (OPC 55) or blast furnace slag cement (CLK 45) and additives, such as metakaolin. Leaching tests are used in the characterization procedure. Modelling of the observed leaching behavior is conducted to improve the description of the physical and chemical phenomena involved in the release of soluble species. The main results of the study allow a distinction to be made between the species whose solubility is not sensitive to the chemical context of the porewater, and the other species, such as amphoteric metals, whose solubility is sensitive to the context (especially the pH). In the first case, a diffusion or shrinking front model is appropriate. In the second case, a coupled solubilization/diffusion model must be developed in order to describe the leaching behavior of heavy metals (lead, etc.) contained in the stabilized/solidified wastes.

Long-term prediction of the leaching behavior of pollutants from solidified wastes

Abstract A methodology based on three experimental tests and a behavioral model was developed for the assessment of inorganic contaminant release from cement-based waste materials placed in an ‘immersed scenario’. The experimental ‘toolbox’ consists of the Pore water test (PWT), the Acid neutralization capacity test (ANC), and the Monolithic leaching test (ML). The experimental data supplied by the tests (leaching–available quantity, pore water composition, solubility vs. pH, released mass at the end of each leaching sequence) are the necessary input parameters for the behavioral model. The leaching model takes into account the transport phenomena coupled with main physico-chemical reactions in the saturated pore system of the material as well as in the eluate. The model, adapted to different scenarios, provides the concentration and the cumulative quantities released for each species. The methodology was applied on the wastes containing lead and stabilized/solidified with different hydraulic binders: two cements and a coal fly ash with lime. The stabilization of lead was better in the case of cement-based materials. The 10 years simulation of Pb release in the case of the particular scenario example, i.e. a water-storage reservoir built with the waste materials show that the lead concentration in stored water is higher than the accepted limit. Pb precipitation in the eluate can occur.

Pollutants leaching behaviour from solidified wastes : A selection of adapted various models.

Leaching tests are essential in the environmental assessment of stabilized wastes. Research programmes were conducted on their interpretation in order to develop tools for the evaluation of long term release of pollutants contained in solidified wastes. Models for the leaching of porous materials are discussed in this paper according to the specificity of the chemical species (i.e. transport model with total dissolution of species-diffusional model; transport model with progressive dissolution of species due to limitation of solubility-shrinking core model; and the model coupling transport and chemical phenomena). The leaching behaviour of pollutants (i.e. lead) solidified in a cement matrix was studied under different chemical conditions. Results have shown that the release of species whose solubilities depend on the physico-chemical conditions, and especially the pH (e.g. amphoteric metals), is governed by the solubility of the species in the pore water at local conditions and by the pH evolution within the matrix. A coupled dissolution/diffusion model was developed to describe the release of chemically complex species contained in a porous medium in contact with water. Leaching tests of cement matrices and artificial porous matrices containing calcium hydroxide and pollutants were conducted in order to validate the coupled dissolution/diffusion model. A good assessment of the retention of some pollutants contained in cement matrices could then be obtained by the association of two tests: solubilization of the pollutants related to the chemical context (pH) under steady state conditions and monolithic long term dynamic leaching tests in order to characterize the evolution of the chemical context (pH) and consequently the release of pollutants. The objective is to integrate this approach in the standardization process (CEN TC 292- WG 6, in progress).

Environmental behaviour of a construction made of a mixture of hydraulic binders and air pollution control residues from municipal solid waste incineration Part 1. Physico-chemical characterisation and modelling of the source term

Abstract The reuse of waste materials requires the development of assessment methods for the long-term release of pollutants (source term) from wastes (or materials containing wastes) in contact with water. These methods depend on the scenario conditions: characteristics of the materials (especially physical structure and composition), contact with water. The scenario studied here is a water storage reservoir for fire fighting. The reservoir construction is made of a mixture of hydraulic binders and air pollution control (APC) residues from a municipal solid waste incinerator (MSWI). The modelling of the source term is performed in 5 steps ranging from the physico-chemical characterisation of the material to the validation of the proposed model by means of field simulation devices. This article presents the first steps of the methodology: physico-chemical characterisation of the source term, identification of the main transfer mechanisms and laboratory scale modelling of the source term. During the physico-chemical characterisation, it has been shown that the solidified waste shows a high basic capacity and that a relative decrease in pH during leaching favours retention of the main pollutants. During the first leaching sequences, the dynamic leaching tests show that the release of pollutants such as cadmium, arsenic, zinc and lead is extremely low but that the release of alkaline species (sodium and potassium) and chloride is very high from the beginning, whereas the release of calcium remains very high even after 3600 h of leaching. Identification of the main transfer mechanisms concludes that the release of soluble pollutants is the combined result of diffusional transfer of pollutants in the solution and the physico-chemical specificity of the species. The modelling based on these features enables a good simulation of the release but reveals a deviation from the experimental results after 500 h for alkaline species and 1000 h for Ca and Cl leaching. However, this deviation only appears after release of the major part of these elements.

Prediction of inorganic pollutant release from various cement based materials in disposa/utilisation scenario based on the application of a multi-parameter leaching tool box

The aim of this study is to assess the long term release of contaminants from cement based stabilised wastes. According to the methodology proposed by the European standard ENV 12 920, the impact of waste in disposal/utilisation scenarios has to be evaluated before being exposed to the environment. For this objective we propose a combined experimental and modelling procedure. Firstly a “tool box” consisting of some tests as the Pore Water simulation test (PW), the Maximum Leachable Fraction test (MLF), the Acid Neutralisation Capacity test (ANC), and the Monolithic Leaching Test (MLT) is used to characterise the waste containing material. Secondly the results of the tests are the necessary input parameters for the coupled physico-chemical leaching model that provides the long-term leaching behaviour. For applying the tool box and the model validation, Ordinary Portland Cement with additives was used to prepare the samples containing 25% of binder, 1% of Pb, and sand. The release mechanisms are described. The model provides the long-term release amounts of pollutants as well as the expected concentrations of pollutants in surrounding natural water over several centuries in the considered scenario conditions.

Valorization of lead-zinc primary smelters slags

Lead and zinc primary smelters in Europe produce and dump every year about 1 Mt of slag. The industrial objectives of the Brite Euram project that is reported in this chapter are to valorize the primary lead and zinc slags, and to avoid their dumping through their use in safe construction and civil engineering materials representing high tonnages. To maintain its competitiveness regarding overseas competitors, the European non-ferrous metallurgy industry has to avoid the burden of dumping cost, and must hence develop beneficial uses for its slags. While in recent years, significant improvements in both “conventional” (lead blast furnace or ISF) or “new” (Kivcet - QSL) processes have been achieved, the solid slags generated by these processes are still largely dumped. The main goal is to work toward a common approach and understanding of the leaching properties of “metallurgical slag”. This implies a better understanding of the phenomena involved in the leaching process to be able to adapt, if necessary, the metallurgical process to produce a slag whose chemical composition, particle size and morphology, will result in reduced leachability. And moreover to develop cost effective processes and techniques to use the non-ferrous slags in environmentally compatible construction and civil engineering materials, as a partial or total substitute to quarry raw materials. Slags from other industrial processes such as blast furnace slag, steelslag, and phosphate slag are used in coastal protection and in roadbase applications. The environmental consequences of these applications have been shown to meet strict utilization criteria.

Models for Leaching of Porous Materials

Abstract The release of soluble species contained in solidified/stabilized wastes are assessed by leaching tests. Interpretation of experimental results must be supported by precise modeling of the different phenomena involved hydrodynamics, dissolution, chemical interaction, diffusive transport. The models are presented, which can apply according to the leaching scenario (with or without advection). - solubilization shrinking core model, - diffusionnal model, - coupled dissolution/diffusion model.

French qualification procedure for solidification processes

Abstract In the framework of the new French technical and regulatory requirements of industrial waste landfilling, solidification/stabilization processes are expected to be one of the main tools to render wastes physically and chemically compatible with the Environment on the long term. Consequently, it appears necessary to propose a comprehensive qualification procedure to assess the real performances of the different industrial processes in front of the wide range of industrial wastes to stabilize. This paper presents the main steps of the procedure, proposed by ADEME (French Agency for the Environment and Energy Management) and designed by POLDEN and L.C.P.A.E.(INSA de Lyon).

Environmental behaviour of a construction made of a mixture of hydraulic binders and air pollution control residues from municipal solid waste incineration Part 2. Simulation tests and validation of the source term modelling

Abstract The reuse of waste materials requires the development of assessment methods for the long-term release of pollutants (source term) from wastes (or materials containing wastes) in contact with water. These methods depend on the scenario conditions: characteristics of the materials (especially physical structure and composition), contact with water… The scenario studied here is a water storage reservoir for fire extinguishing. The reservoir construction is made of a mixture of hydraulic binders and air pollution control (APC) residues from municipal solid waste incinerator (MSWI). The modelling of the source term is performed in five steps ranging from the physico-chemical characterisation of the material to the validation of the proposed model by means of field simulation devices. This paper follows a first publication on source term modelling using laboratory tests which therefore concerns the comparison of the results obtained with the previously established model. The first laboratory scale simulation test aims at taking into account the role of the leachate carbonation in the leaching behaviour of the studied material. The results obtained show that air carbonation of the leachate does not fundamentally change mass transfer mechanisms of easily soluble species (especially for alkaline metals). For these species, the use of the apparent diffusional model (model proposed in the previous paper) is, therefore, at first, a satisfactory solution for the prediction of long term leaching behaviour. The field scale test enables us to validate and calibrate the release model determined on a laboratory scale basis.