Are clays relevant materials to have both high sorption and suitable hydraulic properties for the retention of metal pollutants and pharmaceuticals ? Application to waste landfill and waste water treatment
Résumé
Clay materials rich in smectites are usually used as geochemical barrier in waste landfill thanks to their low permeability, their expansion ability and their capacity to retain metallic or organic pollutants. These high sorption properties may also be valued as ternary filtration in waste water treatment plant. Batch experiments coupled with sorption models are often performed to evaluate their efficiency and to access the main sorption mechanisms (cation exchange, surface complexation) by varying different parameters (pH, ionic strength, pollutant concentration). However, the low solid/liquid ratio and the non-renewed solution during batch experiments are far from real hydromechanical conditions in field applications. To simulate waste landfill or waste water treatment conditions at the laboratory scale, percolation experiments were performed by using œdometers coupled with an injection system under controlled pressure. So, clay samples are firstly compacted (e.g. at 0.5 MPa to simulate the over-consolidation of an engineered clay barrier) then percolated by a selected solution (e.g. Pb(NO3)2 at pH 5 to simulate young landfill leachate). This experimental approach gives important information on the evolution of macroscopic swelling and hydraulic conductivity of clays, but also on the evolution of their sorption efficiency. For instance, after one year of percolation with Pb or Zn solution, the Ca-exchanged montmorillonite kept its sealing performance (k = 10-12 m.s-1) as well as a high metal retention. If smectites are well-known to adsorb cationic contaminants (e.g. metals), they appear to be less efficient to retain neutral or anionic compounds, such as some pharmaceuticals. Organoclays, hybrid materials based on clay materials and cationic surfactants, were proposed as realistic alternatives for the sorption of pharmaceuticals. If their efficiency was highlighted in batch experiment conditions, we demonstrated on the contrary that the incorporation of surfactants within the interlayer space of the layered materials increases the hydraulic conductivity of about hundred to thousand times to that of the Na-exchanged montmorillonite. This experimental approach was finally used as an analogue for a better understanding on the removal of pharmaceuticals in waste water treatment plants. The objective was here to evaluate the sorption of a pool of pharmaceuticals onto clay-sand filters using percolation experiments by varying the clay-sand ratio, the solution matrix (pure water or natural effluent) and the injection pressure. The different sorption mechanisms onto clay mineral will be discussed as a function of the charge of pharmaceuticals.