Foam Flotation of Clay Particles Using a Bifunctional Amine Surfactant.

Understanding clay flotation mechanisms has become a major concern because of the increasing level of environmental contamination of soil and ground water by heavy metals and radionuclides. Clays are often used as sorbents for extracting metals in indirect flotation processes but can function simultaneously as defoamers. However, how foam generation and stability depend on the molecular interactions between the clays and surfactant is still controversial. In the present study, an amine polyethoxylated surfactant was used as a bifunctional surfactant that collected clay particles and acted as a foaming agent in the flotation process. The pH conditions strongly affected the surfactant physicochemical properties, allowing the clay extraction efficiency to be tuned. The interfacial recovery factor of the clays almost reached 100% under acidic (pH < 6) and neutral (pH 6-10) conditions, whereas it was negative under alkaline conditions (pH > 10), contrary to expectations. To elucidate the mechanisms involved in the particle flotation process for each of the pH conditions, the bulk and foam phases were analyzed. The effects of electrostatic interactions between the solutes and multiscale structure on the clay extraction behavior were investigated by electrophoretic measurements, dynamic light scattering, small-angle neutron scattering, and image analysis. Based on these results, three flotation processes were found depending on pH range: surfactant foam fractionation at pH > 10; clay particle foam flotation at pH 6-10; and particle froth flotation at pH < 6.

Size and structure of hexanuclear plutonium oxo-hydroxo clusters in aqueous solution from synchrotron analysis.

The size and shape of a water-soluble hexanuclear plutonium cluster were probed by combining synchrotron small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS). A specific setup coupling both techniques and dedicated to radioactive samples on the MARS beamline endstation at Synchrotron SOLEIL is described. The plutonium hexanuclear cores are well stabilized by the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid ligands and this allows a good evaluation of the setup to probe the very small plutonium core. The results show that, in spite of the constrained conditions required to avoid any risk of sample dispersion, the flux and the sample environment are optimized to obtain a very good signal-to-noise ratio, allowing the detection of small plutonium aggregates in an aqueous phase. The structure of the well defined hexanuclear cluster has been confirmed by EXAFS measurements in solution and correlated with SAXS data processing and modelling. An iterative comparison of classical fit models (Guinier or sphere form factor) with the experimental results allowed a better interpretation of the SAXS signal that will be relevant for future work under environmentally relevant conditions.

Ultrasonically assisted conversion of uranium trioxide into uranium(vi) intrinsic colloids.

Under oxidizing conditions, the corrosion of spent nuclear fuel may lead to the leaching of radionuclides including soluble uranyl-based species. The speciation of the generated chemical forms is complex and the related potential formation of colloidal species appears surprisingly poorly reported in the literature. Their formation could however contribute significantly to the mobility of radionuclides in the environment. A better knowledge in the speciation and reactivity of these species appears particularly relevant. This study describes the preparation and characterization of intrinsic uranium(vi) colloids from amorphous and crystalline UO3 in pure water assisted by 20 kHz ultrasound. In the presence of carbon monoxide preventing the sonochemical formation of hydrogen peroxide, ultrasonic treatment boosts the conversion of UO3 powder into (meta-)schoepite precipitates and yields very stable and notably concentrated uranium(vi) nanoparticles in the liquid phase. Using HR-TEM, SAXS and XAS techniques, we confirmed that the colloidal suspension is composed of quasi-spherical nanoparticles measuring ca. 3.8 ± 0.3 nm and exhibiting a schoepite-like crystallographic structure. The proposed method demonstrates the possible formation of environmentally relevant U(vi) colloidal nanoparticles appearing particularly interesting for the preparation of reference systems in the absence of added ions and capping agents.

Specific salt and pH effects on foam film of a pH sensitive surfactant.

Steady state foams made of a pH sensitive surfactant, nonaoxyethylene oleylether carboxylic acid, with ion complexing properties was studied using small angle neutron scattering (SANS). The effect of pH variation and salt addition on the foam film thickness was investigated and discussed in terms of the influent parameters stabilizing the foam such as surface properties and electrostatic effects determined by tensiometry and zeta potential measurements. The decrease in the film thickness by adding mono (Na(+)) and divalent (Ca(2+)) salts is classically explained by screening of the double layer in foam films (transverse interactions). On the contrary, addition of acid or complexing ion (Nd(3+)) results in an increase in the film thickness and can be analyzed in terms of cohesive forces between surfactants at the liquid/gas interface (lateral interactions). pH and specific salt effects revealed that foams produced by nonaoxyethylene oleylether carboxylic acid are of interest in the potential use of this surfactant in ion separation process.