Chemical interaction between amorphous silica mineral and highly alkaline plume with Ca ion Usui H. 1, Niibori Y. 1, Kadowaki J. 1, Tanaka K. 1, Tochiyama O. 2, Mimura H. 1 1 Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University, Sendai, Japan 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan Abstract In the geological disposal system, a large amount of cement is used for the construction. Such a material alters the groundwater to highly alkaline and high Ca concentration [1-3]. This highly alkaline plume containing Ca induces dissolution and precipitation reaction with surrounding rock matrix, and affects the migration of radionuclides. This study focused on the dynamic change of porous amorphous silica in highly alkaline plume containing Ca in both continuous-flow column and batch conditions. The amorphous silica mineral (75-1 diameter, BET surface area 350 m 2 /g) was contacted with Ca(OH) 2 solution (column experiment: 0.78, 4.37, and 8.48 mm; batch experiment: 0.62, 3.00, and 9.22 mm). The ph value was always adjusted to 12.2-12.5 by the addition of NaOH solution. Then, under each experimental condition, the concentration change of both Ca and Si was monitored by ICP-AES. Further, the distribution of Ca on the cross section of silica mineral was observed by EPMA. The solid-liquid ratio of batch experiment was determined from the ratio of solid amount in the column to the total volume of injected solution in the column experiment. The experimental results showed that in either case Ca deposited not only on the surface of the silica mineral but also in the inner pores of the mineral, although the change in Ca consumption amount was quite different under the column and batch condition. Moreover, at relatively high concentration of Ca (8.48 to 9.22 mm), the dissolution rate of SiO 2 in the column condition was larger than that under the batch condition. While the alteration of silica mineral induced by the alkaline plume with Ca ion is very complicated, these results suggest that the flow condition on the solid surface is a key factor to estimate the spatial area altered by the alkaline plume in the near field of the repository. 1. Introduction For the performance assessment of the geological disposal system, the chemical reaction occurring in the natural barrier is one of the key factors that affect the migration of the released radionuclide. Whether natural barrier is sedimentary or crystalline, it contains the SiO 2 as the major component. On the other hand, for the construction of the geological disposal system, a large amount of concrete is used, and such cementitious
material may alter the condition of groundwater to a highly alkaline and high Ca concentration. Such groundwater is called highly alkaline plume. The silica has very large solubility under alkaline conditions [4-5]. On the other hand, soluble silicic acid reacts with Ca species (Ca 2+, CaOH +, etc), and may precipitate immediately as calcium-silicate-hydrate (CSH gel). That is, the reaction between rocks and highly alkaline plume induces dissolution and/or precipitation, depending on the concentration of Ca species. Usui et al. reported that Ca concentration dramatically affect such chemical reaction [6]. Moreover, this phenomenon resulted from the column flow condition. In this study, the dynamic behaviors of porous amorphous silica induced by the highly alkaline plume were examined in the two conditions of continuous-flow column and batch. 2. Experimental Figure 1 shows the experimental apparatus of batch experiment (a) and column experiment (b). In both experiment, Ca(OH) 2 solution is liquid phase and amorphous slilca particle is solid phase, and the system is isolated from air by N 2 gas. The amorphous silica particles were sieved to 75-1 diameter, and deaerated in the pure water. The chemical composition of amorphous silica is SiO2 0.38H 2 O, measured by TG from 378 K to 1073 K (i.e., the amount of sorbed water is excluded). The specific surface area of amorphous silica particle was 350 m 2 /g estimated by BET method using N 2 gas, and the apparent specific gravity was 2.0 g/cm 3 [7] (a) Batch experiment Sampling hole N 2 gas (b) Column experiment Amorphous silica packed bed column N 2 gas Reaction vessel Stirrer Ca(OH) 2 solution Filter Sample collection Ca(OH) 2 solution Amorphous silica particles Pump Figure 1 experimental apparatus of batch experiment (a) and column experiment (b) The ph value of the solutions was adjusted to 12.2-12.5 by the addition of NaOH solution. In general, the
dissolution of amorphous silica decreases the ph value of the solution. However, this study confirmed in advance that the change was negligible small in the limited experiment periods. The Ca concentration value of the solution was 0.78, 4.37 or 8.48 mm in the column experiment, and 0.62, 3.00 or 9.22 mm in the batch experiment. The sample was taken by 1 ml at 20 min intervals until 180 or 200 min, and the Ca and Si concentration of the sample were measured by ICP-AES. Further, in either experiment, the surface structure of solid particle was observed by SEM micrograph, and the distribution of Ca on the cross section of solid particle was measured by EPMA. In the batch experiment, the volume of solution was 500 ml, and the solution was stirred by magnetic stirrer. The solid-liquid ratio of batch experiment was determined by the ratio of solid amount in the column to the total volume of solution injected into the column. The aliquots were filtrated before the measurement, in order to avoid the contamination of the solid particles. In the column experiment, average flow rate of the feed solution was about 1.1 mm/sec. The feed solution was continuously injected from the bottom side of the packed bed. The eluted solution from the packed bed is collected and measured the Ca and Si concentration. 3. Result and discussion Figure 2 shows the Ca consumption amount (a) and Si fraction dissolved (b) with time, where Si fraction dissolved is the ratio of dissolved SiO 2 to the initial SiO 2 amount in the reaction vessel (batch experiment) and packed bed (column experiment). Figure 2-a shows apparent difference in the Ca consumption with time. In the column condition, Ca consumption increases gradually with time. On the other hand, in the batch condition, Ca consumption rapidly increased until 20 min, and gradually increased. As shown in Figure 2-b, when the Ca concentration is relatively high, the dissolution of Si was very limited because of the products of CSH. (a) (b) Ca consumption Ca Á ï amount Ê[mmol] [mmol] column-0.78 column-0.78 mm mmcolumn-4.37 mm mm column-8.48 mm mm batch-0.62 batch-0.62 m M m M batch-3.80 mmm M batch-10.3 mmm M 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 50 100 150 200 250 Time time [min] Si fraction Si n ð dissolved ª column-0.78 column-0.78 mm mm column-4.37 column-4.37 mm mm column-8.48 column-8.48 mm mm batch-0.62 batch-0.62 m M m Mbatch-3.80 mmm batch-10.3 m Mm M 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0 50 100 150 200 250 Time time [min] Figure 2 shows the amount of Ca consumption (a) and Si fraction dissolved (b) with time However, the dissolution rate (calculated by the Si fraction dissolved after 20min) was about 3.6 times higher
than that of batch condition, although the Ca concentration injected into the colume (0.78 mm) was a little higher than the initial Ca-concentration of the batch experiment (0.62 mm). This suggests that the chemical reaction of Ca species to the surfaces of solid phase including the inner pores of particle is not always uniform spatially in the column flow system, particularly when the Ca concentration is relatively small. Figure 3 shows the SEM and EPMA micrograph of the cross section of the packed particle reacted in batch condition and in the column condition. (In this figure, the value such as 0.33 mm is the initial Ca concentration in the batch experiments or the feed concentration of Ca in the column experiments. Further, the time such as 1 min or 180 min means the observation time after the start of each experiment.) Batch experiment, 1 min Batch experiment, 180 min a) SEM Ca d) SEM Ca b) e) c) f) a) 0.33 mm b) 2.87 mm c) 8.35 mm d) 0.62 mm e) 3.80 mm f) 10.3 mm Column experiment SEM Ca g) h) g) 0.78 mm : 170 min h) 4.37 mm : 20 min i ) 4.37 mm : 180 min i) Figure 3 SEM and EPMA micrograph of packed particle after the reaction in batch condition (a ~ f) and column condition (g ~ i) These results indicate the precipitation of CSH gel is not only on the outer surface of the particle but also on the surface of the inner pore of the particle. Moreover, as shown in the micrograph after the 1min, Ca species
immediately spread over the inner regions of the particle through the inner pores, and react with the dissolved silicic acid. Figure 4 shows the surface structure of the amorphous silica particle of the batch and column experiments. At the relatively high Ca concentration, the deposit with fiber form appeared on the outer surface. Such behaviors are confirmed either in the batch and column experiment. Batch experiment (A) (B) (C) (D) Before Reaction [Ca] total = 0.62 mm [Ca] total = 3.80 mm [Ca] total = 10.30 mm Column experiment (a) (b) (c) Before Reaction [Ca] total = 0.78 mm [Ca] total = 4.37 mm Figure 4 SEM micrograph of the amorphous silica s surface after the reaction; in batch condition (A ~ D) and column condition (a ~ c).
4. Conclusions In this study, the chemical reaction between Ca(OH) 2 solution and amorphous silica mineral in the batch condition and column condition was compared. The experimental result showed that Ca species to the silica surfaces including the inner pores of particle is not always uniform spatially in the column flow system. While, in the batch condition, Ca species in the solution were quickly uptaken, and the dissolution of Si was limited compared to that of column condition. These suggest that the flow condition on the solid surface is a key factor to estimate the spatial area altered by the alkaline plume in the near field of the repository, although the alteration of silica mineral induced by the alkaline plume with Ca ion is very complicated. 5. References [1] Savage, D., Noy, D., Mihara, M., Modelling the interaction of bentonite with hyperalkaline fluids, Applied Geochemistry, 17, 207 (2002). [2] Usui, H., Niibori, Y., Tanaka, K., Tochiyama, O., Mimura, H., Fundamental Experiments on Permeability Change of Flow-path by Highly alkaline Plume, Scientific Basis for Nuclear Waste Management XXVIII, edited by Hanchar J. M. and Stroes-Gascoyne S. and Browning L., Mat. Res. Soc. Symp. Proc., 824, 449 [3] (2004). Harris A.W., Manning M.C., Tearle W.M., Tweed C.J., Testing of models of the dissolution of cements leaching of synthetic CSH gels, Cement and Concrete Research, 32, 731 (2002). [4] Stumm, W., Morgan, J. J., Aquatic Chemistry, 3rd ed. (John Wiley & Sons, New York, 1996), p. 368. [5] Chida, T., Niibori, Y., Tochiyama, O., and Tanaka, K., Dynamic Behavior of Colloidal Silica in the Presence of Solid Phase Scientific Basis for Nuclear Waste Management XXVI, edited by Finch R. J. and Bullen D. B., Mat. Res. Soc. Symp. Proc., 757, 497 (2003). [6] Usui H., Niibori Y., Tanaka K., Tochiyama O., Mimura H., Effects of Calcium Ion in Highly Alkaline Plume on Permeability Change of Flow-Path, Mat. Res. Soc. Symp. Proc., (2005) -in press- [7] Niibori, Y., Kunita, M., Tochiyama, O., and Chida, T., Dissolution Rates of Amorphous Silica in Highly Alkaline Solution, J. Nucl. Sci. Technol., 37, 349 (2000).