Retention of radionuclides by organophilic bentonite

摘要

Distribution ratios (Rd-values), and isotherms orsorption of iodide (125I), technetium (95mTc), cesium (134Cs) andstrontium (85Sr) were measured on MX-80-bentonite, which hadbeen modified with hexadecylapridinium (HDPy+). Syntheticground water was used as the equilibrium solution (ionicstrength: 0.037 mol l-1). Additionally, the mineralogicalcharacteristics, like regular an din situ powder X-ray diffraction(XRD), thermogravimetric (TG), calorimetric (DTA)measurements, IR spectral analysis of the organo-bentonitesamples, and the exchange behaviour of HDPy+ (chemicalanalysis) were investigated. With HDPy+ -modified MX-80-bentonite, iodide andpertechnetate ions exhibited increasing adsorption (Rd), whilecesium and strontium showed decreasing adsorption withincreasing organophilicity. Sorption of technetium was found tobe almost completely reversible with samples of relatively high(>100% of the cation exchange capacity, CEC) HDPy+ loading.Generally, sorption and desorption were linear over a wideconcentration range of the elements investigated (up to ~10-1mmol g-1 organo-bentonite) suggesting ion exchange as theprincipal sorption mechanism. Sorption capacities for the anionsinvestigated were estimated to be ~5×10-1 mol kg-1. Concerningthe cationic radionuclides, higher distribution coefficients werefound for the Cs+ compared to the Sr2+ ions in the untreated andmodified samples. On the basis of mineralogical and chemical analysis it wasconcluded that the alkylammonium ions are adsorbed as: 1,HDPy+ cations; 2, HDPyCl molecules; and 3, micelles withdecreasing binding intensities in this order. The observed uptakeof the organic cations in excess of the CEC leaks to theformation of modified microstructures different from sampleswith relatively low HDPy+ saturation. As the organo-bentonitesshowed a fairly sufficient thermal stability, their use in nuclearwaste isolation should be taken into consideration.