Tracing water flow and colloidal particles transfer in an unsaturated soil
Résumé
In recent years, many studies have been carried out on colloidal particle transfer in the unsatu- rated zone because they can be a risk to the environment either directly or as a vector of pollut- ants. A study was conducted on the influence of porous media structure in unsaturated conditions on colloidal particle transport. Three granular materials were set up in columns to replicate a flu- vio-glacial soil from the unsaturated zone in the Lyon area (France). It is a sand, a bimodal mixture in equal proportion by weight of sand and gravel, and a fraction of bimodal mixture. Nanoparticles of silica (SiO2-Au-FluoNPs), having a hydrodynamic diameter between 50 and 60 nm, labeled by organic fluorescent molecules were used to simulate the transport of colloidal particles. A non- reactive tracer, bromide ion (Br−) at a concentration of C0,s = 10−2 M was used to determine the hy- drodispersive properties of porous media. The tests were carried out first, with a solution of nano- particles (C0,p = 0.2 g/L) and secondly, with a solution of nanoparticles and bromine. The transfer model based on fractionation of water into two phases, mobile and immobile, MIM, correctly fits the elution curves. The retention of colloidal particles is greater in the two media of bimodal par- ticle size than that in the sand, which clearly demonstrates the role of textural heterogeneity in the retention mechanism. The increase in ionic strength produced by alimenting the columns with col- loidal particle suspension in the presence of bromide, increases retention up to 25% in the sand. The total concentration profile of nanoparticles collected at the end of the experiment shows that the colloidal particles are retained primarily at the entrance of the columns. Hydrodispersive cal- culated parameters indicate that flow is more heterogeneous in bimodal media compared to sand.