A study on zeolite precipitation at alkaline conditions: effect of solution composition on mineralogy and crystallization rate

Document available in extended abstract form only. Designs for engineered barrier systems for the geologic confinement of radioactive wastes involve the extensive use of bentonite, a material known for its capability to retard radionuclide migration into the groundwater system due to its low permeability. Cement and concrete are used in conjunction with bentonite to serve as structural supports and to seal access routes. However, when concrete becomes saturated with groundwater, it produces alkaline fluids that may attack and degrade the bentonite over time. A key concern regarding bentonite alteration by alkaline fluids is the precipitation of secondary minerals, such as zeolites, which may produce changes in the transport properties of the bentonite and compromise the stability of the entire barrier system. More importantly, studies have shown that zeolites form at timescales that are of interest in determining the long-term integrity of the barrier system. It is thus important to understand zeolite precipitation at conditions operative during bentonite alteration. The objective of this study, therefore, is to determine the effects of fluid composition on the crystallization rate and zeolite mineralogy. Zeolites were synthesized in batches from solutions with varying ratios of Si and Al at pH 13.5 by combining appropriate amounts of NaOH, NaAlO2 and Na2SiO3.9H2O. The Si and Al concentrations in the starting solutions both ranged from 0.03 to 0.24 M and the solution Si/Al ratios ranged from 0.125 to 8. The solutions were transferred to polypropylene bottles and stored in an oven set at a constant temperature of 70 deg. C for 1, 4, 7, 14 and 30 days. At appropriate reaction times, samples were extracted from the oven and then centrifuged to separate the solids from the liquid. The supernatant was filtered using 0.2 μm filter and the solids were washed several times before drying in a freeze dryer. The solids were characterized by XRD and SEM-EDX to determine the mineralogy, morphology and composition. The solutions were analyzed for dissolved element concentration using ICP-AES for Na and Al. Si was analyzed colorimetrically using the molybdate blue method. At the earliest stages of the crystallization process, an amorphous precursor phase forms, as shown by broad peaks in the XRD patterns. Over time, crystalline material develops at the expense of these amorphous phases. Samples with Al-rich content precipitated cubic crystals of synthetic Zeolite A. Zeolite A crystals were observed between 4 to 7 days, depending on the proportion of Si and Al, with the solutions having the highest amounts of Al precipitating zeolites earlier. On the other hand, in samples with Si-rich content (starting solution Si/Al > 1), the assemblage that crystallized consists of faujasite as the dominant phase and minor amounts of phillipsite. In these samples, crystalline material formed at a slower rate relative to the Al-rich systems, being observed only after a reaction time of 30 days. This suggests that the composition of the starting solution controls the rate at which crystals develop. This relationship has been attributed to the concentration of Al, which controls the formation of aluminosilicate polymers that build up the zeolite structure. SEM analyses of the solids from both the Al-rich and Si-rich samples show that crystalline material begins to develop on the surface of the initially formed amorphous phase. As the reaction proceeds, crystalline materials increase in size and eventually replace the amorphous phase. The growth of the crystals is accompanied by a decrease in the concentrations of the Al and Si in the fluid phase, which indicates the consumption of these elements during crystal growth. Both amorphous and crystalline products were analyzed by EDX to determine their composition. Solids from the Al-rich systems have Si/Al ratios of close to 1, while those from the Si-rich systems have Si/Al ratios of that ranged from about 1.5 to 1.8. In addition, crystalline products have Si/Al ratios that are almost the same a s their amorphous precursors. These results indicate that the composition of the crystalline material is directly inherited from its amorphous precursor. Thus, the lower Si/Al ratio of the amorphous products from the Al-rich systems favored the precipitation of Zeolite A, which has a Si/Al ratio of 1, whereas the higher Si/Al ratio of the amorphous products from the Si-rich systems favored the precipitation of the relatively more siliceous zeolites faujasite and phillipsite. The data, therefore, suggest that the composition of the starting fluid, particularly its Si/Al ratio, control the Si/Al ratio of the precursor phase, which determines the mineralogy. The observed phases are markedly different from zeolites commonly observed in bentonite alteration experiments. In particular, the absence of analcime, which typically forms in Na dominated systems, is noteworthy. This may be attributed to the relatively high concentrations of Si and Al used in these experiments, in contrast to the low concentrations commonly observed in bentonite alteration studies. This observation further highlights the role of solution composition in determining zeolite mineralogy. The results show that at fixed pH, the composition of the solution, particularly its Si/Al ratio controls the rate of crystallization and the mineralogy of the zeolites that precipitate. For the Al-rich solutions with Si/Al < 1, silica deficient Zeolite A crystallized rapidly, whereas the Si-rich solutions with Si/Al > 1, siliceous zeolites faujasite and phillipsite crystallized at a slower rate. The dependence of rate on the composition is explained by the formation of aluminosilicate polymers that build the zeolite framework. The dependence of mineralogy on composition is due to the solution controlling the composition of the amorphous precursor phase, from which the zeolite inherits its composition. (authors)