Retention of radionuclides by organophilic bentonite

Distribution ratios (R-d-values), and isotherms for sorption of iodide (I-1 25), technetium (Tc-95m), cesium (Cs-134) and strontium (Sr-85) were measur ed on MX-80-bentonite, which had been modified with hexadecylpyridinium (HD Py+). Synthetic ground water was used as the equilibrium solution (ionic st rength: 0.037 mol l(-1)). Additionally, the mineralogical characteristics, like regular and in situ powder X-ray diffraction (XRD), thermogravimetric (TG), calorimetric (DTA) measurements, IR spectral analysis of the organo-b entonite samples, and the exchange behaviour of HDPy+ (chemical analysis) w ere investigated. With HDPy+-modified MX-80-bentonite, iodide and pertechnetate ions exhibite d increasing adsorption (R-d), while cesium and strontium showed decreasing adsorption with increasing organophilicity. Sorption of technetium was fou nd to be almost completely reversible with samples of relatively high (> 10 0% of the cation exchange capacity, dCEC) HDPy+ loading. Generally, sorptio n and desorption were linear over a wide concentration range of the element s investigated (up to similar to 10(-1) mmol g(-1) organo-bentonite) sugges ting ion exchange as the principal sorption mechanism. Sorption capacities for the anions investigated were estimated to be similar to 5 x 10(-1) mol kg(-1). Concerning the cationic radionuclides, higher distribution coeffici ents were found for the Cs+ compared to the Sr2+ ions in the untreated and modified samples. On the basis of mineralogical and chemical analysis it was concluded that t he alkylammonium ions are adsorbed as: 1, HDPy+ cations; 2, HDPyCl molecule s; and 3, micelles with decreasing binding intensities in this order. The o bserved uptake of the organic cations in excess of the CEC leads to the for mation of modified microstructures different from samples with relatively l ow HDPy+ saturation. As the ogano-bentonites showed a fairly sufficient the rmal stability, their use in nuclear waste isolation should be taken into c onsideration. (C) 1999 Elsevier Science B.V. All rights reserved.