Zeolite P in cements: Its potential for immobilizing toxic and radioactive waste species

Zeolite P, approximately (Na2O,CaO).Al2O3.2SiO2.4H2O, has been shown to develop spontaneously in appropriate cement formulations at> 40°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.0.1Na2O.Al2O3.2.66SiO2.4H2O, on which characterization studies were performed for its sorption potential in cement-analogue environments. RD values are reported for the 25 and 85°C isotherms, for a range of initial sorbate concentrations (10-10,000 μmol/l). The sorbates investigated were: Cs, Sr, Ba, Pb and U(VI). In water media, zeolite P shows good selectivity for Cs, Sr, Ba and Pb, at 25 and 85°C. The highest RD recorded was for Pb2+, at> 800,000 ml/g (1000 μmol/l initial concentration). In NaOH media, Ba and Sr sorption values remained high. Cs and Pb show a marked decrease in sorption, although RDs are still reasonable, at 750 and 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.