(299p) Coupled Chemical-Transport Modelling for Material Leaching Behaviour Assessment in Environmental Conditions

The stabilisation/solidification with cement or pouzzolanic binders is one of the favourite management issues for highly polluted inorganic waste, contributing to the environmental impact reduction by decrease of pollutants mobility and release.

However, the contact with water of such materials, used as secondary raw materials in construction or disposed in landfill, can generate pollutant mobilisation and emission towards natural media (soil, groundwater, surface water) and significantly burdens the host ecosystems. The knowledge of the materials' leaching behaviour is a unavoidable step for the real scale leaching behaviour assessment and then the ecological impact foreseeing.

The most reuse or disposal scenarios for the waste-based materials can be schematised as in figure below.

Figure 1. Typical leaching scenario scheme

The solid porous material constitutes a compartment characterised by a mineralogical composition and a porous system totally or partially filled with water. The leachant compartment is characterised by the transport phenomena (mostly convection) and the interaction with a gaseous phase (atmosphere) and the material compartment. The knowledge of the leachant (the pollution vector) composition for long time scales is the key issue of our researches.

Experimental assays (solid characterisation, solid/liquid equilibrium, dynamic leaching assays using water and carbonated water) and modelling approaches are the investigation tools of such systems.

Experimental assays were performed at laboratory scale using a stabilised/solidified mineral waste and for all experiments the concentration of main elements (Ca, Na, K, Si, Al, Mg) and pollutants (S, Ba, Cd, Cu, Mo, Pb, Zn) have been monitored. The solid speciation of the pollutants is very difficult to determine experimentally by usual methods because of their low concentration in the material. An indirect approach was used, i.e. the solid phases identification starting from their solubility (concentration measurement) in water.

The equilibrium tests were performed on samples of crushed material (<1mm), in contact with demineralised water at different pH values (HNO₃ added) and sheltered of air, in order to identify the main pollutants' immobilisation mechanisms and to develop a geochemical model for the solid/liquid system. The geochemical model is a assemblage of phases responsible for the elements release in water and was determined by inverse modelling. The PHREEQC software and LLNL data base were used for geochemical modelling. Phases like: hydrated calcium-silicates, albite, barite, brucite, calcite, gypsum, magnesite, ettringite, Cd(OH)₂, Cu(OH)₂, Pb(OH)₂, Zn(OH)₂ were considered to be very likely to occur in the system. These phases given the more reliable simulation results with respect to the solubility experimental data.

The evolution of the pore-water concentration is the result of two main dynamic processes, the diffusion and the chemical reactions (dissolution/precipitation and reactions in liquid phase). Dynamic leaching tests performed on bloc samples allowed the identification of the diffusion parameters for the mobile chemical species. The leachate, in a fixed liquid/solid ratio, was renewed sequentially (at 0.25, 3.25, 5.25, 10.25, 17.25, 18.25, 37.25, 65.25 days). The diffusion coefficient was determined by inverse modelling applied to very soluble elements (considered as non reactive) like Na and K. A coupled geochemical?transport model taking into account all mineral phases, chemical reactions and the pore diffusion was developed to simulate in a predictive manner the experimental results.

The following step of the study introduces as environmental parameter the interaction with CO₂ as atmospheric constituent, and different leachant flow rates. The simulation of the tests was performed using coupled geochemical-transport models for the material compartment (geochemistry and diffusion) and for the leachate compartment (geochemistry and convection). A good agreement was obtained between the experimental results and the simulation results. Figure 2 shows some results expressed as leachate concentration in function of time, in the case of the use of carbonated water as leachant.

Figure 2. Leaching with carbonated water. Experimental (exp) and simulation (sim) results (DL=detection limit).

The coupled transport ? geochemical model was then used in order to simulate the pollutant release in schematic scenarios, as for example an immerged block (bridge foot, dyke, etc.) in a water flow (river), figure 3.

Figure 3. Immerged material: scenario scheme

The same geochemical model was used with an adapted transport model in order to take into account the scenario characteristics. Simulations were realised for a leaching period of 30 years; the results for some elements are shown in figure 4.

Figure 4. Simulation of the leachant composition in function of time for the real scenario

The experimental and modelling tools developed provided promising results at a laboratory scale and the possibility to extend the application for a real scenario. However, the lake of real data on pollutant release did not allow the model validation at the scenario scale.