Evolution of confining properties of a clay material soaked by a copper solution
Résumé
In the framework of domestic waste disposals, the in-situ compaction of soil is usually used to obtain engineered clay barriers with suitable confining properties. Permeability and pollutant retention are the major properties which should be investigated in this context for barrier evaluation. Considering clay barriers in contact with waste leachates, they may be saturated by solutes of various chemical compositions, including in particular toxic heavy metals. In this context, the expansive Fo-Ca clay, a natural Ca-smectite from the Paris basin of Ypersian (Sparnacian) age, was chosen because of its very low permeability and ability for pollutant retention through cationic exchanges. The clay mineral is associated with kaolinite (up to 20%), and minor quartz, calcite, goethite and gypsum. An experimental work was performed to analyse the Fo-Ca performances when submitted to coupled chemo-hydro-mechanical effects produced by soaking the clay with a polluted solute. For this purpose, copper was chosen as the pollutant and tests were performed using copper solutions of concentrations, varying from 10-3 up to 10-1 mol/l. The evolution of the chemical composition of the leachate was carried out as a function of duration for each test in order to characterise the ability of the clay to copper retention. The results highlight that: 1. cation exchanges occur between copper and compensating cations of the clay, 2. an increase of macroscopic porosity and of the permeability, due to two simultaneous chemical processes: calcite dissolution and cation exchanges, 3. the initial water saturation stage plays an important role on the chemical kinetics and copper diffusion through the Fo-Ca clay, 4. the copper concentration in the porous phase is not significantly influenced by the initial water saturation stage. All these results show that the good exchange properties of Fo-Ca allow to restrain pollutant transport, such as copper cations, which lead to modify the clay microstructure, the porous volume, and also the macroscopic behaviour. These cation exchanges also produce changes of the Fo-Ca rheological behaviour without eliminating swelling ability and without producing an unacceptable increase of permeability.