Thorium and uranium uptake by natural zeolitic materials

The thorium and uranium uptake from their aqueous solutions by unpretreated and NaCl-pretreated zeolite-bearing volcanoclastic rock samples from Metaxades (Thrace, Greece) has been studied using a batch-type method. The concentration of the solutions varied between 50 and 20 000 mg/l. The NaCl pretreatment of the materials improved the thorium but not the uranium uptake. The absolute thorium uptake by the pretreated material, determined using neutron activation and X-ray fluorescence techniques, reached 12.41 mg/g, whereas the uranium uptake by the raw material was 8.70 mg/g. The uptake distribution coefficients (Kd) indicated that the relative thorium and uranium uptake is higher for initial concentrations below 250 mg/l. The zeolitic materials showed exceptional resistance to the initial low pH of the solutions used. The pHin was significantly increased due to the simultaneous hydrogen-ion uptake. The thorium and uranium uptake is a rather complicated phenomenon related to the aqueous chemistry of the elements, the nature of the constituent minerals and the properties of the zeoliferous rock specimens. The various metal species are bound through different uptake processes such as ion-exchange, adsorption and surface precipitation. Microporous minerals (zeolites, phyllosilicates) are mainly responsible for the considerable uptake ability of the rock samples studied.