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

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

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

Interaction of selenite with MX-80 bentonite: Effect of minor phases, pH, selenite loading, solution composition and compaction

We propose a simple model describing the retention of selenite, Se(IV), by the MX-80 bentonite in a synthetic groundwater (SGW) in dispersed and compacted states. The model was calibrated on a pure montmorillonite from data obtained for 4 < pH < 10 and total selenite concentrations between 10−7 and 5 × 10−3 mol/L. Furthermore, the matrix solution covers a wide range of conditions regarding the ionic strength and the nature of the background electrolyte. Three selenite surface species had to be considered. Below pH 5, sorption is governed by a ligand exchange reaction with H2SeO3. Between pH 5 and 7, the experimental data are well described considering the formation of a surface complex implying HSeO3−. Finally, at pH above 7, we propose ternary surface complexes involving Ca2+ and Mg2+. The model, which we consider as operational, appears in agreement with spectroscopic data available in the literature and predicts surprisingly well selenite sorption on MX-80 bentonite even in the compacted state at a dry density of 1100 kg/m3. Based on the model, the solid phase montmorillonite is responsible for selenite retention. Minor solid phases containing iron (pyrite, hematite) had not to be considered in the modelling. Interestingly, calcite (an important phase in MX-80 bentonite) has an indirect effect via the release of calcium into solution and its subsequent contribution to Se(IV) sorption through a ternary surface complex