Sorption of iodide, cesium and strontium on organophilic bentonite

Modification of bentonite by substituting natural interlayer cations by qua ternary alkylammonium ions, like hexadecylpyridinium (HDPy+), greatly influ ences the sorption behaviour of many sorbates. In batch experiments, the so rption and desorption of iodide was investigated using equilibrium solution s with different ionic strengths (0.037, 0.259 and 0.346 mol.L-1, respectiv ely). Additionally, the competing sorption of cesium and strontium on origi nal and HDPy-bentonite was examined. I-125, Cs-134 and Sr-85 were used as r adiotracers. Compared with bidistilled water, reduced sorption (R-d- values) of iodide w as observed when electrolytes with higher ionic strengths were used. Sorpti on was found to be almost completely reversible in the latter cases, wherea s with bidistilled water partial reversibility was observed. Sorption and d esorption were linear over a wide range of iodide concentrations (up to sim ilar to 10(-1) mmol.g(-1)) suggesting ion exchange as the principal sorptio n mechanism. Cs+ was preferentially sorbed in competition with Sr2+ in both original and HDPy-bentonites. According to thermogravimetric (TG), calorim etric (DTA) and in situ powder X-ray diffraction (XRD) measurements, the th ermal decomposition of the HDPy-bentonite complexes seems to be a gradual p rocess depending on temperature: It starts at about 200 degrees C and proce eds further with higher temperatures reaching complete decomposition at 600 degrees C. These investigations are performed to optimize the engineering of clay barriers in nuclear waste repositories.