PHREEQC Modelling of Leaching of Major Elements and Heavy Metals From Cementitious Waste Forms
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PHREEQC Modelling of Leaching of Major Elements and Heavy Metals From Cementitious Waste Forms Evelien Martens1, Diederik Jacques1, Tom Van Gerven2, Lian Wang1 and Dirk Mallants1 1 Belgian Nuclear Research Centre (SCK•CEN), Institute for Environment, Health and Safety, Boeretang 200, B-2400 Mol, Belgium 2 Katholieke Universiteit Leuven, Faculty of Engineering, Department Of Chemical Engineering, de Croylaan 46, B-3001 Leuven, Belgium ABSTRACT In this study, Ca, Mg, Al, and Pb concentrations leached from uncarbonated and carbonated ordinary Portland cement – dried waste incinerator bottom ash samples during single extraction tests (EN12457 test) at a pH from 1 to 12, were modelled using the geochemical code PHREEQC. A good agreement was found between modelling results and experiments in terms of leached concentrations for Ca, Mg, and Al by defining a single set of pure mineralogical phases for both the uncarbonated and carbonated (three levels) samples. The model also predicted well the observed decrease in Ca leaching with increasing carbonation. Modelling results further revealed that leaching of Pb is not controlled by dissolution/precipitation of pure Pb containing minerals only (carbonates and (hydr)oxides). The addition of solid solutions (calcite-cerrusite and gibbsite-ferrihydrite-litharge solid solutions) and adsorption reactions on amorphous Fe- and Al-oxides improved the model representation of the experimentally observed amphoteric leaching profile of Pb from the cementitious material. INTRODUCTION Solidification/stabilization is a technique for immobilizing hazardous wastes in binding materials, mostly cement-based, to delay dissolution and release of toxic components to the environment. Large amounts of low-level radioactive wastes (LLW) are conditioned in this way to guarantee safe disposal and negligible radiological impact on humans and the environment for long time scales. Besides radioactive elements in LLW, a number of chemically toxic elements, such as Pb, are also present in those wastes. In this study, Ca, Mg, Al, and Pb concentrations leached from uncarbonated and carbonated ordinary Portland cement (OPC) – dried bottom ash from municipal solid waste incinerator (MSWI) samples during single extraction tests at different pH were modelled using the geochemical code PHREEQC. MSWI bottom ash is enriched in Pb with contents varying between 98 to 13700 mg/kg [1]. The objectives were (i) to define mineralogical phases that can be used in modelling to represent the leachate concentrations measured at different pH-values, (ii) to model the effect of carbonation on leaching behaviour in a single general model, and (iii) to assess the inclusion of mechanisms such as a formation of solid solutions and surface complexation on modelling of Pb leaching.
EXPERIMENTAL DATA Mortars with a water-to-cement ratio of 0.5 were prepared by mixing 548 kg/m³ of OPC, 1096 kg/m³ dried MSWI bottom ash and 281 kg/m³ distilled water. The mixtures were poured in moulds of 150 x 150 x 150 mm and vibrated. After a 24 hour se
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