Organophilic bentonites as adsorbents for radionuclides: II. Chemical and mineralogical properties of HDPy-montmorillonite

MX-80 bentonites, which had been modified with hexadecylpyridinium (HDPy+), exhibit the capability of anion as well as cation adsorption and allow its use as effective sorbents for anionic as well as cationic pollutants in engineered barriers of waste repositories and in other geotechnical applications. Chemical and mineralogical properties of MX-80 bentonite undergo marked changes with increasing organophilicity. Mineralogical characteristics of the organo-bentonite samples were investigated by powder X-ray diffraction (XRD), by thermogravimetric (TG) and by calorimetric (DTA) measurements and by IR spectral analysis. Additionally, the exchange behaviour of HDPy+ and the dependence of surface charge on the degree of organophilicity were determined. The surface charge was measured by titration with charge compensating polyelectrolytes; the endpoint was at zero potential. On the basis of chemical and mineralogical analyses, it was concluded that the alkylammonium ions are adsorbed as (1) HDPy+ cations, (2) HDPyCl molecules and (3) micelles with decreasing binding intensities in this order. The observed uptake of the organic cations in excess of the cation exchange capacity (CEC) is due to hydrophobic interactions. HDPy+-uptake leads to the formation of a modified microstructure different from samples with relatively low HDPy+ saturation. As the organo-bentonites possess a fairly sufficient thermal stability up to 200°C, their use in engineered barriers for heat-generating pollutants such as nuclear waste, should be taken into consideration.