Retention of radionuclides by organophilic bentonite

Abstract

Distribution ratios ( R d-values), and isotherms for sorption of iodide ( 125I), technetium ( 95mTc), cesium ( 134Cs) and strontium ( 85Sr) were measured on MX-80–bentonite, which had been modified with hexadecylpyridinium (HDPy +). Synthetic ground water was used as the equilibrium solution (ionic strength: 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-bentonite samples, and the exchange behaviour of HDPy + (chemical analysis) were investigated. With HDPy +-modified MX-80–bentonite, iodide and pertechnetate ions exhibited increasing adsorption ( R d), while cesium and strontium showed decreasing adsorption with increasing organophilicity. Sorption of technetium was found to be 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 wide concentration range of the elements investigated (up to ~10 −1 mmol g −1 organo-bentonite) suggesting ion exchange as the principal sorption mechanism. Sorption capacities for the anions investigated were estimated to be ~5×10 −1 mol kg −1. Concerning the cationic radionuclides, higher distribution coefficients were found for the Cs + compared to the Sr 2+ ions in the untreated and modified samples. On the basis of mineralogical and chemical analysis 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 CEC leads to the formation of modified microstructures different from samples with relatively low HDPy + saturation. As the organo-bentonites showed a fairly sufficient thermal stability, their use in nuclear waste isolation should be taken into consideration.