THORIUM AND URANIUM UPTAKE BY NATURAL ZEOLITIC MATERIALS

The thorium and uranium uptake from their aqueous solutions by unpretr eated and NaCl-pretreated zeolite-bearing volcanoclastic rock samples from Metaxades (Thrace, Greece) has been studied using a batch-type me thod. 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 t echniques, reached 12.41 mg/g, whereas the uranium uptake by the raw m aterial was 8.70 mg/g. The uptake distribution coefficients (K-d) indi cated that the relative thorium and uranium uptake is higher for initi al concentrations below 250 mg/l. The zeolitic materials showed except ional resistance to the initial low pH of the solutions used. The pH(i n) was significantly increased due to the simultaneous hydrogen-ion up take. The thorium and uranium uptake is a rather complicated phenomeno n related to the aqueous chemistry of the elements, the nature of the constituent minerals and the properties of the zeoliferous rock specim ens. The various metal species are bound through different uptake proc esses such as ion-exchange, adsorption and surface precipitation. Micr oporous minerals (zeolites, phyllosilicates) are mainly responsible fo r the considerable uptake ability of the rock samples studied.