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Improvement of the recovery of heavy chlorinated organic compounds in saturated porous media by thermal and chemical enhancements : experimental and two-phase flow modeling approaches

Thesis

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Abstract

Groundwater pollution by chlorinated organic compounds is a major problem. Actually, these particularly toxic pollutants, permanently degrade soil and groundwater quality. Their dispersion (by solubilization and volatilization) from the pollution source zone can generate large contaminants plumes.Chlorinated organic compounds are recovered as pure product (Dense Non-Aqueous Phase Liquids-DNAPL) mainly using pump/treat technologies. However, these technologies are time-consuming and do not recover the pure product in an efficient way. A significant amount of DNAPL remains trapped in soil as residual saturation (Srn). The objective of this PhD project was to enhance DNAPL recovery rate and yield using chemical and thermal enhancements during the pumping process. Temperature increases aimed to reduce the viscosity of DNAPL (and therefore to increase its mobility) while the addition of surfactant aimed to reduce the capillary forces that trap the DNAPL. Experiments at the laboratory scale (based on monitoring of permittivities, electrical resistivities and optical densities) and two-phase flow modeling were performed to quantify the effects of these enhancements. Heating the DNAPL up to 50 °C (to avoid volatilization) decreases the viscosity by a factor of two. The addition of surfactant, Sodium Dodecyl Benzene Sulfonate-SDBS, at its Critical Micelle Concentration (to prevent DNAPL solubilization) decreases interfacial tensions by a factor of 12. Drainage-imbibition experiments were carried out in 1D cells to obtain the retention curves of the two-phase system (capillary pressure as a function of water saturation). The decreases of Srn obtained with SDBS were 28% for 0.5 mm glass beads (GB) diameter and 46% for 0.1 mm GB. We reported no significant improvement in the remediation yield with thermal enhancement. The curves were fitted with the van Genuchten – Mualem model to generate data for modeling.Drainage-imbibition experiments were carried out in 1D columns to characterize two-phase flow (and in particular the displacement of the DNAPL-water interface according to the pressures applied). The two-phase flow model used a pressure-pressure formulation (using COMSOL Multiphysics®). The modeling of recovered volumes and the displacement of the interface agreed with the experimental results. The remediation yields with chemical and thermal enhancements were of the same order of magnitude as those reported in 1D cells. For 2D tank experiments, pumping was performed at different flow rates with 0.5 mm and 0.1 mm GB. The experiments were also performed with and without enhancement. Models were compared with image interpretation (based on the optical density calibration). Comparing experimental and modeled values shows that the model fitted well with the experiments. The VDNAPL, chemical/VDNAPL, reference ratios were for low and high flow rates on average respectively 2.90 and 1.40 for 0.5 mm GB and 1.37 and 1.18 for 0.1 mm GB. Thermal enhancement had no beneficial effect on DNAPL recovery rate or yield.Indirect measurements of water saturations (Sw) for 1D or 2D experiments yielded the following results: i. the measured permittivities were very similar to the values modeled with the CRIM model; ii. modeling of electrical resistivities with Archie's Law was less accurate; iii. optical densities allow accurate Sw estimation. At field scale, the combination of monitoring both electrical resistivities (which provide a global picture) and permittivities (which provide precise but spatially limited data), is expected to provide Srn data

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