Alain C. M. Bourg

Tracer diffusion in compacted, water-saturated bentonite

Compacted Na-bentonite clay barriers, widely used in the isolation of solid-waste landfills and other contaminated sites, have been proposed for a similar use in the disposal of high-level radioactive waste. Molecular diffusion through the pore space in these barriers plays a key role in their performance, thus motivating recent measurements of the apparent diffusion coefficient tensor of water tracers in compacted, water-saturated Na-bentonites. In the present study, we introduce a conceptual model in which the pore space of water-saturated bentonite is divided into ‘macropore’ and ‘interlayer nanopore’ compartments. With this model we determine quantitatively the relative contributions of pore-network geometry (expressed as a geometric factor) and of the diffusive behavior of water molecules near montmorillonite basal surfaces (expressed as a constrictivity factor) to the apparent diffusion coefficient tensor. Our model predicts, in agreement with experiment, that the mean principal value of the apparent diffusion coefficient tensor follows a single relationship when plotted against the partial montmorillonite dry density (mass of montmorillonite per combined volume of montmorillonite and pore space). Using a single fitted parameter, the mean principal geometric factor, our model successfully describes this relationship for a broad range of bentonite-water systems, from dilute gel to highly-compacted bentonite with 80% of its pore water in interlayer nanopores.

A Critical Evaluation of Sample Pretreatment for Storage of Contaminated Sediments to be Investigated for the Potential Mobility of their Heavy Metal Load

The identification of the geochemical forms of heavy metals in contaminated sediments gives information on their availability. This requires the use of a geochemical speciation procedure such as the one developed by Tessier et al. (1979). In addition to the imperfections of these protocols, their results can vary depending on the technique used for the preservation of sediments which must be suited to the materials studied and to particularities of the investigation. This study was carried out on superficial river sediments, seriously polluted by Cu, Cd and Pb. Compared to fresh sediment, none of the drying methods studied (freeze-drying, air-drying and oven-drying at 105 °C) completely preserve the distribution of Cu, Pb, Zn and Cd in the various geochemical fractions of the sediment. The modifications depend directly on the quantities of metals present in the various fractions of the sediment, the effects being more marked when the quantity is smallest. This results in a decrease in metals in the exchangeable fraction and in those bound to carbonates under the action of atmospheric oxygen and a corresponding increase in the other fractions. To minimize this, freeze-drying and air-drying are satisfactory techniques which enable preservation of sediments representative of the environment.

Modeling diffusion and adsorption in compacted bentonite: a critical review

The current way of describing diffusive transport through compacted clays is a simple diffusion model coupled to a linear adsorption coefficient (K(d)). To fit the observed results of cation diffusion, this model is usually extended with an adjustable surface diffusion coefficient. Description of the negative adsorption of anions calls for a further adjustment through the use of an effective porosity. The final model thus includes many fitting parameters. This is inconvenient where predictive modeling is called for (e.g., for waste confinement using compacted clay liners). The diffusion/adsorption models in current use have been derived from the common hydrogeological equation of advection/dispersion/adsorption. However, certain simplifications were also borrowed without questioning their applicability to the case of compacted clays. Among these simplifications, the assumption that the volume of the adsorbed phase is negligible should be discussed. We propose a modified diffusion/adsorption model that accounts for the volume of the adsorbed phase. It suggests that diffusion through highly compacted clay takes place through the interlayers (i.e., in the adsorbed phase). Quantitative prediction of the diffusive flux will necessitate more detailed descriptions of surface reactivity and of the mobility of interlayer species.

Solubilization of lead and cadmium during the percolation of EDTA through a soil polluted by smelting activities

EDTA was percolated in laboratory columns through a soil polluted by heavy metals to investigate the efficiency of and processes involved in soil decontamination by chemical extraction. At high EDTA concentration (10−2 M), elution of Pb and Cd was very efficient for one pore volume, after which it decreased to almost zero due to depletion of available Pb and Cd and to competition with Ca and Fe slowly solubilized during the passage of the EDTA front. Clogging occurred after the end of the EDTA plateau. At lower EDTA concentrations (10−3 and 10−4 M), elution was less efficient, but extraction decreased little with the volume percolated; moreover no Ca above background values was dissolved. The optimum EDTA concentration for heavy metal extraction ranges between 10−2 and 10−3 M. The higher the concentration, the greater the extraction efficiency, but as the EDTA concentration is increased there is an optimum point at which clogging takes place and permeability decreases.

Effect of Complexing Agents (EDTA and ATMP) on the Remobilization of Heavy Metals from a Polluted River Sediment

EDTA and ATMP are synthetic chelatingagents used in many industrial applications. Becauseof their low biodegradability, they are present inmany aquatic systems where their strong complexingcapacities can modify the transfer of toxic heavymetals between solid and liquid phases.The remobilization of Cu, Pb and Cd from pollutedsuperficial river sediments in the presence of twosynthetic organic ligands (a polycarboxylate, EDTA,and a phosphonate, ATMP) is investigated as a functionof reaction time to determine the time necessary toreach dissolutive equilibrium, and as a fonction ofthe complexing agent concentration.The dissolutive equilibrium times are similar (10 to20 hours) and the solubilizing power of ATMP isslightly lower than that of EDTA. The curves ofsolubilization of metals in the presence of thecomplexing agents present a shape of a “chelateconcentration” desorption edge, for a chelate rangebetween 10-5 to 10-3 mol L-1, thesolubilised metals increasing dramatically. Thisremobilization is the result of a competition betweenassociation with the solid phase and dissolvedcomplexation. With EDTA, the desorption order ofmetals seems to be imposed by association with thesolid. For ATMP is it the chelate complexing strengthwhich determines this order.

Acidification and solubilisation of heavy metals from single and dual-component model solids

Abstract The solubilisation of Ni, Cd and Pb from 3 model components of soils and sediments is investigated as a function of pH. In presence of increasing acidity, Ni, Cd and Pb are strongly solubilised from kaolinite and amorphous ferric hydroxide. Metal solubility tends to decrease, on the other hand, when humic acid is present. The quantity of metal solubilised from the dual-component, amorphous ferric hydroxide-humic acid system is smallest in the pH range 3.5 to 5.5. In the system kaolinite-humic acid, the solubility is highest at neutral pH. In these solid mixtures, metal solubility is always minimal (close to nil) in the pH range between the pHzpt of the humic acid and the pHzpt of the mineral solid. The solubilisation of heavy metals from soils and sediments is highly pH-dependent but does not always increase with decreasing pH. The solubility of Cd, Ni and Pb depends not only on pH but also on interactions between soil components.

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

The 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 equilibra...

Modelling the mobility of pollutants away from waste disposal sites to the geosphere, a tool for optimal solid waste management

Abstract New regulations in the European Union regarding waste management will require a strong effort in modelling activities. Existing computer models which predict pollutant behaviour in geological environments are reviewed, and their potential use for the management of waste is discussed. The most promising of these are mechanistic models which account for pollutant behaviour in the context of a solution chemistry controlled by solid–water interactions. A major shortcoming of these computer codes is the lack of conceptual knowledge of the processes involved in interactions between wastes and water. Several needs are identified: better data bases (which must include the proper solubility-controlling variables), phenomenological understanding of pollutant behaviour in waste–leachate interactions, and model validation under realistic conditions.