Sorption of iodide, cesium and strontium on organophilic bentonite

Modification of bentonite by substituting natural interlayer cations by quaternary alkylammonium ions, like hexadecylpyridinium (HDPy+), greatly influences the sorption behaviour of many sorbates. In batch experiments, the sorption and desorption of iodide was investigated using equilibrium solutions with different ionic strengths (0.037, 0.259 and 0.346 mol . L-1, respectively). Additionally, the competing sorption of cesium and strontium on original and HDPy-bentonite was examined. 125I, 134Cs and 85Sr were used as radiotracers. Compared with bidistilled water, reduced sorption (Ra-values) of iodide was observed when electrolytes with higher ionic strengths were used. Sorption was found to be almost completely reversible in the latter cases, whereas with bidistilled water partial reversibility was observed. Sorption and desorption were linear over a wide range of iodide concentrations (up to ∝ 10-1 mmol . g-1) suggesting ion exchange as the principal sorption mechanism. Cs+ was preferentially sorbed in competition with Sr2+ in both original and HDPy-bentonites. According to thermogravimetric (TG), calorimetric (DTA) and in situ powder X-ray diffraction (XRD) measurements, the thermal decomposition of the HDPy-bentonite complexes seems to be a gradual process depending on temperature: It starts at about 200 C and proceeds further with higher temperatures reaching complete decomposition at 600 C. These investigations are performed to optimize the engineering of clay barriers in nuclear waste repositories. (orig.)