Analcime reactions at 25–90°C in hyperalkaline fluids

Abstract

Extensive use of cement and concrete is envisaged in the construction of geological disposal facilities for radioactive wastes. The hyperalkaline porefluids typical of groundwaters that have reacted with these materials have the potential to react chemically with other engineered barrier components such as bentonite, potentially degrading their performance. Analcime, NaAlSi2O6.H2O, has been identified from previous modelling and experimental studies as a potential alteration product of bentonite.Laboratory experiments to investigate the stability of analcime under hyperalkaline porefluid conditions have been performed. Experiments used both batch and fluidized bed equipment at 25, 50, 70 and 90°C in K-based pH buffer solutions, both under- and over-saturated with respect to analcime. Results from dissolution experiments demonstrate that release of Na was greatly enhanced (by up to a factor of thirty) over that for Si and Al, particularly at pH 10 and 11. However, enhanced release of both Na and Al occurred in the batch experiments at pH 12–13. Near stoichiometric dissolution was observed in fluidized bed experiments under steady-state conditions at 70°C. Sodium was removed from the analcime structure by ion exchange for K, without involving dissolution and re-precipitation of the analcime framework. Scanning electron microscopy of reacted analcime grains showed that some grains had pronounced cracks parallel to original cleavage traces. These cracks were a result of volume decrease due to the substitution of K for Na ions and water molecules in the analcime structure to form leucite, KAlSi2O6.Synthesis of the dissolution data shows that the rate of dissolution increased with increasing temperature in the range 25–70°C and with pH at each temperature. Absolute rates of dissolution ranged from 10−10 mol m−2 s−1 at pH 9.5 at 25°C to 10−7 mol m−2 s−1 a pH 12 at 70 and 90°C. The rate of dissolution at any temperature was pH-dependent, such that the rate could be described by k (aH+)n, where k is the rate constant and n is −0.3 at 25°C, −0.4 at 50°C, −0.6 at 70°C and −0.7 at 90°C. Attempts to measure the growth rate of analcime in supersaturated solutions at 70 and 90°C were unsuccessful, although a limiting rate at 70°C, pH 10 was calculated to be 4 × 10−11 mol m−2 s−1, roughly 100× less than the rate of dissolution under the same conditions.These results imply that any trace amounts of analcime in bentonite will be converted to leucite by reaction with cement fluids with a high K/Na ratio. In some instances, leucite may thus incorporate K+ in preference to other phases (e.g. illite, K-feldspar) during alteration of bentonite by cement porefluids.

Fossilized Endolithic Microorganisms in Pillow Lavas from the Troodos Ophiolite, Cyprus

“…The high clay content in all the samples indicates pH higher than 5 [64]. Mordenite is usually found in environments with a pH between 7-9 [65], analcime is found in environments with pH higher than 8 [66], while Ca carbonates are found in environments with pH higher than 8-9 [67]. Living fungi in aerobic environments have been cultivated and show a preference for pH around 4.5-8.3 [68], and studies of living fungi from subterranean environments shows that they can survive in extreme environments with pH up to 10 [69].…”

Coupling between mineral reactions, chemical changes in groundwater, and earthquakes in Iceland

“…The lack of a corresponding change in δ 18 O implies that analcime had already acquired its δ 18 O value from groundwater. In a laboratory study of analcime dissolution, which was run at 70°C and pH 10–11, i.e., similar conditions to groundwater at Hafralækur, Savage et al . [] showed that release of Na was enhanced by up to a factor of 30 compared to Si and Al.…”

“…3. In a laboratory study of analcime dissolution, which was run at 70°C and pH 10-11, i.e., similar conditions to groundwater at Hafralaekur, Savage et al [2001] showed that release of Na was enhanced by up to a factor of 30 compared to Si and Al.…”

Dissolution Kinetics of Stilbite at Various Temperatures under Alkaline Conditions

“…Increase in temperature increased the n values slightly (0.33, 0.35, 0.38, and 0.38), but the values were the same within statistical range and, therefore, showed little temperature dependence of the reaction order within the temperature range. The dissolution rate increased with temperature and pH, however, Data from other studies are: analcime (Savage, 2001), heulandite (Ragnarsdottir,1993), anorthite (Brady et al,1989), quartz (Brady and Walther,1990) and illite (Kohler et al, 2003).…”