Modeling the diffusion of Na+ in compacted water-saturated Na-bentonite as a function of pore water ionic strength

Assessments of bentonite barrier performance in waste management scenarios require an accurate description of the diffusion of water and solutes through the barrier. A two-compartment macropore/nanopore model (on which smectite interlayer nanopores are treated as a distinct compartment of the overall pore space) was applied to describe the diffusion of {sup 22}Na{sup +} in compacted, water-saturated Na-bentonites and then compared with the well-known surface diffusion model. The two-compartment model successfully predicted the observed weak ionic strength dependence of the apparent diffusion coefficient (D{sub a}) of Na{sup +}, whereas the surface diffusion model did not, thus confirming previous research indicating the strong influence of interlayer nanopores on the properties of smectite clay barriers. Since bentonite mechanical properties and pore water chemistry have been described successfully with two-compartment models, the results in the present study represent an important contribution toward the construction of a comprehensive two-compartment model of compacted bentonite barriers

Geochemical filtering of heavy metals by calcareous alluvions in an island of the Seine River

International audienceAquifer sediments are known to have a large potential for filtering heavy metal cations from percolating groundwater. To investigate the role of riverine alluvions from the Seine River in filtering heavy metals, we collected three cakareous alluvion profiles (5 m in depth), one taken from an island and two from adjacent river banks. The major element content (Si, Al and Ca) is independent of the profile and of depth. In the river bank profiles heavy metals and organic C (C(org)) are also rather constant. In the island profile peak concentrations of heavy metals and C(org) are found at a depth of between 1 to 3 m (3.2 ppm Ag, 5 ppm Cd, 47 ppm Cr, 93 ppm, Cu 235 ppm Pb, 260 ppm Zn and 1.8% C(org)). Enrichment in these heavy metals demonstrates the strong potential of island alluvions for natural geochemical filtering

Natural attenuation of inorganic pollutants (copper, sulfate) in the aquifer below an industrial site

The contamination of soils and aquifers by inorganic pollutants is so widespread in industrial sites that it does not seem economically feasible to decontaminate the large areas or soil volumes involved. It is therefore interesting to investigate whether the local environment is capable to attenuate this contamination. Natural attenuation by degradation seems realistic for many organic pollutants. Here we show that it can take place also for inorganic pollutants. The phreatic fill aquifer underlying an industrial plant located on the river banks of the Garonne River is contaminated by acidic water (pH down to 1) and high concentrations of sulfate (up to 50 g/L) and copper (up to 30 g/L). As acid water, rich in Cu and sulfate. migrates away from the contamination source, pH increases due to buffering of aquifer solids, dissolved Cu concentrations decrease by 6 orders of magnitude, while sulfate concentrations decrease little

Adsorptive behavior of nickel with goethite in the presence of EDTA: Kinetics and reversibility

International audienceThe influence of EDTA on the adsorption of nickel by goethite has been studied in batch experiments. The nickel adsorption reaches equilibrium in about 3 hours for free (hydrated) Ni and Ni-EDTA, whereas up to 24 hours may be necessary to attain desorptive equilibrium for adsorbed Ni in the presence of EDTA. Nickel behavior is strongly influenced by the presence of EDTA. Nickel solid/solution partitioning depends on the solution pH, the EDTA/Ni ratio and the sequence of introduction of the adsorbates (Ni and EDTA). Compared to the behavior of free Ni at the solid-solution interface, the occurrence of Ni-EDTA complexes increases Ni adsorption at low pH values and decreases adsorption at high pH. The observed data are explained by the formation of a surface-ligand-metal ternary surface complex. Ni uptake depends on the sequence of introduction of the reactants in the goethite suspension (i.e., whether Ni is added with or prior to the EDTA). When EDTA is added to the suspension after Ni had time to equilibrate with the goethite surface, the behavior of Ni at the water-goethite interface is controlled by reaction kinetics. EDTA adsorbs faster than it can complex the dissolved Ni at low pH, and the adsorbed Ni is only slowly desorbed by the EDTA remaining dissolved at high pH

Leaching of Cd and Pb from a polluted soil during the percolation of EDTA: Laboratory column experiments modeled with a non-equilibrium solubilization step

International audienceChelating agents such as EDTA, when present in water percolating through polluted soils, are capable of solubilizing heavy metals, increasing their downward mobility to groundwater as they form water-soluble and negatively charged complexes. The mobilization and movement of heavy metals in an EDTA flux is investigated with both laboratory experiments and mathematical modeling. The solubilization of Cd and Pb is monitored during the percolation of EDTA, in both pulse and step modes, through columns filled with a contaminated soil. The experimental breakthrough curves (BTCs) are used to validate a numerical model that links solute transport of EDTA and EDTA - metal chelates to the metal solubilization process. An implicit difference scheme is used to solve advection - dispersion equations for free and complexed EDTA with, for both, inclusion of a second-order kinetic law to express the extraction reaction. The hydrodispersive parameters of the column are calculated using the bromide BTCs. The ability of the model to simulate various EDTA injection modes (pulse or step) at different chelate concentrations is tested by fitting the kinetic rate constant K of solubilization with the observed Cd and Pb BTCs. Pulse and step experiments give the same K values, 2.4 x 10-6 s-1 for Cd and 2.1 x 10-5 s-1 for Pb. The model accounts for the diminishing metal extraction efficiency as the metal in the solid is depleted or as the available EDTA concentration decreases. Methodological suggestions for necessary investigations prior to remediation operations are described.Chelating agents such as EDTA, when present in water percolating through polluted soils, are capable of solubilizing heavy metals, increasing their downward mobility to groundwater as they form water-soluble and negatively charged complexes. The mobilization and movement of heavy metals in an EDTA flux is investigated with both laboratory experiments and mathematical modeling. The solubilization of Cd and Pb is monitored during the percolation of EDTA, in both pulse and step modes, through columns filled with a contaminated soil. The experimental breakthrough curves (BTCs) are used to validate a numerical model that links solute transport of EDTA and EDTA-metal chelatestothe metal solubilization process. An implicit difference scheme is used to solve advection-dispersion equations for free and complexed EDTA with, for both, inclusion of a second-order kinetic law to express the extraction reaction. The hydrodispersive parameters of the column are calculated using the bromide BTCs. The ability of the model to simulate various EDTA injection modes (pulse or step) at different chelate concentrations is tested by fitting the kinetic rate constant K of solubilization with the observed Cd and Pb BTCs. Pulse and step experiments give the same K values, 2.4 × 10-6 s-1 for Cd and 2.1 × 10-5 s-1 for Pb. The model accounts for the diminishing metal extraction efficiency as the metal in the solid is depleted or as the available EDTA concentration decreases. Methodological suggestions for necessary investigations prior to remediation operations are described

Characterization of the chromium retention potential of non polluted aquifer solids in an industrial site

A hydrogeochemical study of an industrial site where sulfuric acid and copper sulfate (“bouillie bordelaise") are manufacture showed that the phreatic aquifer is contaminated by copper, sulfate, chromium, arsenic and has an acid pH. Field observations and laboratory experiments, both necessary if we are to understand the processes controlling tranfers at the solid-liquid interface, were used to investigate the behaviour of chromium. In the field, monitoring the mixing of polluted and unpolluted water with a conservative tracer, we shows that chromium disappears from solution. In the laboratory, the potential for retendon of dissolved chromium by an unpolluted aquifer solid was studied as a function of pH, with and without a complexing agent. Adding high concentrations of a complexing agent, EDTA, mobilizes the previously fixed chromium at basic pH (90%). Addition of Cr(III) with EDTA gathered, sorption is greatest (50%) for neutral and basic pH values. Unpolluted aquifer solids have a high sorption capacity for Cr(III). However, adding a complexing agent significantly mobilizes the chromium. Sorption of the complexed chromium is also considerably decreased. There should, therefore, be little risk of immediate chromium pollution in the absence of dissolved complexing agents, as long as the pH of the system is neutral