ZEOLITE-P IN CEMENTS - ITS POTENTIAL FOR IMMOBILIZING TOXIC AND RADIOACTIVE-WASTE SPECIES

Zeolite P, approximately (Na2O,CaO).Al2O3.2SiO(2).4H(2)O, has been shown to develop spontaneously in appropriate cement formulations at >40 degrees C, and to be a stable phase. Suitable composites can be made from mixtures containing Ca(OH)(2) or Portland cement, with high proportions of the pozzolans, metakaolin or class F fly ash. Alternatively, zeolite P is easily prepared in phase-pure form using laboratory chemicals. The latter method was used to obtain zeolite P of composition 0.9CaO.0.1Na(2)O.Al2O3.2.66SiO(2).4H(2)O, on which characterization studies were performed for its sorption potential in cement-analogue environments. R(D) values are reported for the 25 and 85 degrees C isotherms, for a range of initial sorbate concentrations (10-10,000 mu mol/l). The sorbates investigated were: Cs, Sr, Pa, Pb and U(VI). In water media, zeolite P shows good selectivity for Cs, Sr, Ba and Pb, at. 25 and 85 degrees C. The highest R(D) recorded was for Pb2+, at >800,000 ml/g (1000 mu mol/l initial concentration). In NaOH media, Pa and Sr sorption values remained high. Cs and Pb show a marked decrease in sorption, although R(D)s are still reasonable, at similar to 750 and similar to 400 ml/g, respectively. On account of its large ion size, UO22+ uptake into zeolite P is negligible, remaining in solution or precipitating as soddyite or Na uranate. Cements conditioned to form stable zeolites offer great potential in the treatment of hazardous waste streams.