Michel Buès

PAH oxidation in aged and spiked soils investigated by column experiments

Soils of former steel-making or coking plants have been contaminated for decades by PAHs. These soils could be cleaned up by In situ chemical oxidation (ISCO) but the low PAH availability may be a drawback. The objective of the present contribution was to study the efficiency of PAH oxidation in two aged soils compared to a spiked soil in dynamic conditions. Column experiments were performed with two oxidants: hydrogen peroxide used in modified Fentons reaction and activated persulfate. The oxidant doses were moderate to ensure the feasibility of process upscaling. Besides, the availability of PAHs in these soils was measured by extraction with a cyclodextrin. Our results showed that oxidation was limited: the higher PAH degradation rate was 30% with the aged soils and 55% with the spiked one. PAH availability was a parameter explaining these results but no direct correlation was found between PAH extractability by the cyclodextrin and oxidation efficiency. Other parameters were also involved, such as the organic carbon content, the calcite content and the pH. This study was a first achievement before studying the influence of a number of parameters on the efficiency of PAH oxidation in aged soils.

Macroscale model and viscous-inertia effects for Navier-Stokes flow in a radial fracture with corrugated walls

The radial Navier-Stokes flow in a fracture bounded by impermeable corrugated rock surfaces is significantly different from the commonly used creeping flow model between two parallel surfaces, described by Darcys law on the macroscale. Continuous variations in the Reynolds number along the radial coordinate determine the important role of the nonlinear inertial effects, which are reinforced by local oscillations of the velocity field caused by wall corrugation. The system behaviour is studied both analytically and numerically. A solution for the full system of Navier-Stokes equations in a thin cylindrical domain with oscillating walls is developed as a biparametric and two-scale asymptotic expansion with respect to fracture aperture and corrugation period. The numerical solution is derived based on the finite volume method. A new macroscale flow equation is obtained, which explicitly displays the relative roles of viscous dissipation caused by corrugation, local and global inertia, and cross inertia-viscous effects. The effective flow parameters are defined using analytical relationships.

Numerical Efficiency Assessment of IB–LB Method for 3D Pore-Scale Modeling of Flow and Transport

In many earth science and environmental problems, the fluid–structure interactions can affect the hydrodynamics properties of the porous medium via the spatial evolution of its solid matrix. A significant insight into these properties can be obtained from pore-scale simulations. Using a 3D pore-scale domain with moving walls, we proposed in this paper a comparison of the numerical accuracy between different approaches in regard to flow and reactive transport. Two direct-forcing immersed boundary (IB) models coupled with lattice Boltzmann method (LBM) are evaluated for the flow calculation against an analytical solution. The IB–LBM showed improvement compared to the classical and interpolated bounce-back lattice Boltzmann model. Concerning the reactive transport, the most accurate IB–LB method was coupled with two non-boundary conforming finite volume methods (volume of fluid and reconstruction). The comparative study performed with different mesh sizes and Damköhler numbers demonstrates better results for the reconstruction method.

Gravity-driven fingers in fractures: experimental study and dispersion analysis by moment method for a point-source injection.

In this study, we investigate the behavior of a dense contaminant injected from a point-source in a fracture. Our experimental model is a transparent Hele-Shaw cell, 0.5 mm of aperture. A saline solution is injected locally representing the point-source pollution. A Laser Induced Fluorescence (LIF) method provides concentration measurement of the pollution plume. Two propagation patterns have been observed: one and two-finger plumes. If the upper part of the plume is stable over time regardless of the second configuration, the moment when the plume separates into two fingers is highly dependent on both injection flow-rate and contaminant concentration. To further investigate the dispersion process inside the fracture, experimental results are interpreted by the spatial and time moment methods. Resulting dispersivities and plume propagation mean velocity are compared to theoretical values derived from a modified Taylor-Aris dispersion tensor. The longitudinal macro-dispersion obtained suggests an asymptotical behavior of the plume spread regardless of the studied configurations. Experimental local dispersivities derived from time and space moments proved to be close at large times to theoretical values predicted by the density-dependent dispersion tensor (Oltéan et al., 2004). Based on those observations the mechanism behind the separation of the plume into two fingers is believed to be significantly impacted by the pre-asymptotic behavior of the dispersion tensor.

Upscaling of transport processes in porous media with biofilms in equilibrium and non-equilibrium conditions

Transport of biologically reactive dissolved solutes in a saturated porous medium including a biofilm-phase occurs in various technological applications such as in biochemical or environmental engineering. It is a complex process involving a wide variety of scales (from the bacteria-scale to the aquifer-heterogeneities-scale in the case of groundwater remediation) and processes (hydrodynamic, physicochemical and biochemical). This work is devoted to the upscaling of the pore-scale description of such processes. Firstly, one-equation macroscopic models for bio-reactive transport at the Darcy-scale have been developed by using the volume averaging method; they will be presented below. These one-equation models are valid for different limit cases of transport; their validity domains in terms of hydrodynamic and biochemical conditions will also be discussed. Finally, in order to illustrate such a theoretical development, an example of application to the operation of a packed bed reactor will be studied.

Impact of biofilm‐induced heterogeneities on solute transport in porous media

In subsurface systems, biofilm may degrade organic or organometallic pollutants contributing to natural attenuation and soil bioremediation techniques. This increase of microbial activity leads to change the hydrodynamic properties of aquifers. The purpose of this work was to investigate the influence of biofilm-induced heterogeneities on solute transport in porous media and more specifically on dispersivity. We pursued this goal by (i) monitoring both spatial concentration fields and solute breakthrough curves from conservative tracer experiments in a biofilm-supporting porous medium, (ii) characterizing in situ the changes in biovolume and visualizing the dynamics of the biological material at the mesoscale. A series of experiments was carried out in a flow cell system (60 cm(3)) with a silica sand (Phi = 50-70 mesh) as solid carrier and Shewanella oneidensis MR-1 as bacterial strain. Biofilm growth was monitored by image acquisition with a digital camera. The biofilm volume fraction was estimated through tracer experiments with the Blue Dextran macromolecule as in size-exclusion chromatography, leading to a fair picture of the biocolonization within the flow cell. Biofilm growth was achieved in the whole flow cell in 29 days and up to 50% of void space volume was plugged. The influence of biofilm maturation on porous medium transport properties was evaluated from tracer experiments using Brilliant Blue FCF. An experimental correlation was found between effective (i.e., nonbiocolonized) porosity and biofilm-affected dispersivity. Comparison with values given by the theoretical model of Taylor and Jaffe (1990b) yields a fair agreement.

A Comparative Study of Three Classes of Boundary Treatment Schemes for Coupled LBM/DEM Simulations

A comparative study of three classes of fluid-solid boundary treatment schemes in the lattice Boltzmann method (LBM) is conducted to evaluate their accuracy and computational efficiency. The three classes are the bounce-back (BB) based schemes, the partially saturated computational (PSC) method and the immersed boundary method (IBM). The flow past a fixed circular cylinder and the sedimentation of a circular particle in a confined cavity are considered as the reference test cases. It is found that all three classes of schemes can yield reasonable results in general, with interpolated BB (IBB) showing better accuracy than others for every case studied. In addition, some force fluctuations are observed with the IBB and BB when the solid is moving. This study may help researchers to select an applicable boundary treatment scheme for their coupled LBM-DEM.

The permeability contrast influence on the plume propagation with variable physical properties

ABSTRACT This paper deals with the modelling of variable-density flow and solute transport in saturated porous media. The mathematical formulation for such problems is solved by a combination of the mixed hybrid finite element method and discontinuous finite element method. The specific point of interest in this study is the investigation of dense plume behaviour in presence of heterogeneity. The model under consideration called FRIPE is used: (i) to study the behaviour of a dense solution locally injected into a stratified porous medium without groundwater flow field and (ii) to simulate the movement of radioactive salt solutions in Lake Karachai (Russia).