Environmental behaviour of recycled waste using molecular approach and modelling
Résumé
Waste is usually valorised as natural materials substitutes or as energy source. To predict the release of the pollutants present in the resulting products, we study the evolution of the pollutants speciation (i.e. oxidation state and retention sites) during alteration and directly through the solid product. Different non-destructive and efficient investigation tools are used (XRD, SEM, µ-XRF, XAFS). Data are then introduced in a reactive transport modelling code (CHESS-HYTEC) to validate the experimental results and then to predict pollutants release with time. Waste, which may contain pollutants, is usually valorised as natural materials substitutes or as energy source. The products are in many cases in permanent contact with water and are likely to be altered and therefore to release pollutants in environment. To assess environmental behaviour of the products, i.e. long-term evolution of the solids, our methodology combines the determination of the structural and crystal-chemistry evolution of the solid matrix down to the molecular level to a modelling approach of leaching experiments at macroscopic scale (laboratory and field tests). The evolution of the pollutants (i.e. oxidation state and retention sites) together with the evolution of the matrix mineralogy are studied using non-destructive and efficient investigation tools (XRD, SEM, µ-XRF, XAFS). Results are translated into thermodynamic data, which are introduced in a reactive transport modelling code (CHESS-HYTEC) to test the proposed speciation model and then to predict pollutants long-term behaviour. This work introduces the behaviour of a Cr(VI)-doped Portland Cement (2000 mg/kg) during leaching. In the altered layer of the cement, the ettringite mineral retains Cr(VI), probably as sulphate substitute. When ettringite is dissolved at the leachate-cement interface, Cr(VI) would be retained by another mineral: hydrotalcite. The model reproduces very well Cr(VI) retention by ettringite and also, the presence of another bearing-phase at the leachate interface, as ettringite dissolves. The methodology applied here is efficient and necessary in term of long-term behaviour prediction problem, especially when complex matrices are considered and when the pollutants concentration is below their oxide or hydroxide solubility limit.