CHEMISTRY ARTICLE

SORPTION AND DISPERSION OF STRONTIUM RADIONUCLIDE IN THE BENTONITEQUARTZ-CLAY AS BACKFILL MATERIAL CANDIDATE ON RADIOACTIVE WASTE

The experiment of sorption and dispersion characteristics of strontium in the mixture of bentonite-quartz, clay quartz, bentonite-clay-quartz as candidate of raw material for backfill material in the radioactive waste repository has been performed. The objective of this research is to know the grain size effect of bentonite, clay, and quartz on the weight percent ratio of bentonite to quartz, clay to quartz, bentonite to clay to-quartz can be gives physical characteristics of best such as bulk density (ρb), effective porosity (ε), permeability (K), best sorption characteristic such as distribution coefficient (Kd), and best dispersion characteristics such as dispersivity (α) and effective dispersion coefficient (De) of strontium in the backfill material candidate. The experiment was carried out in the column filled by the mixture of bentonite-quartz, clay-quartz, bentonite-clay-quartz with the weight percent ratio of bentonite to quartz, clay to quartz, bentonite to clay to quartz of 100/0, 80/20, 60/40, 40/60, 20/80, 0/100 respectively at saturated condition of water, then flowed 0.1 N Sr(NO3)2 as buffer solution with tracer of 0.05 Ci/cm3 90Sr as strontium radionuclide simulation was leached from immobilized radioactive waste in the radioactive waste repository.

 The concentration of 90Sr in the effluents represented as Ct were analyzed by Ortec β counter every 30 min, then by using profile concentration of Co and Ct, values of Kd, α and De of 90Sr in the backfill material was determined. The experiment data showed that the best results were -80+120 mesh grain size of bentonite, clay, quartz respectively on the weight percent ratio of bentonite to clay to quartz of 70/10/20 with physical characteristics of ρb = 0.658 g/cm3 , ε = 0.666 cm3 /cm3 , and K = 1.680x10-2 cm/sec, sorption characteristic of Kd = 46.108 cm3 /g, dispersion characteristics of α = 5.443 cm, and De = 1.808x10-03 cm2 /sec can be proposed as candidate of raw material of backfill material in the radioactive waste repository.

EXPERIMENTAL SECTION Materials The materials used in this experiment were bentonite has chemical composition in the weight % as follows: H2O- (1.80%), pH at 25 °C with 10% solid (8.05%), result of dry sample at 105 °C: SiO2 (64.73%), Al2O3 (13.14%), Fe2O3 (2.92%), CaO (4.13%), MgO (1.40%), TiO2 (0.36%), K2O (1.64%), Na2O (1.97%), H2O+ (6.03%) [7]. Whereas, quartz used in this experiment from accumulating basin of the Sermo Kulon Progo with SiO2 content of 77.89 wt.%, and clay from the Kasongan Bantul. Whereas as simulation of radionuclide in the radioactive waste will be dispersed is tracer of 90Sr. The simulation of radionuclide is 90Sr tracer with concentration of 0.01 Ci/cm3 in the liquid of Sr(NO3)2 0.1 N as buffer. Instrumentation The equipments utilized in this experiment were sieving pans of ASTM ISO 585-R20 standard, oven Sybron, analytic balance Sartorius, β counter Ortec, glass column, glass apparatus. Procedure Preparation of Local Minerals (Bentonite, Quartz, Clay) Powders Gravels of bentonite from Nanggulan Kulon Progo were dried in a oven until its constant weight, then it was crushed to powders. The bentonite powders were poured in the sieving pan of ASTM ISO 585-R20 standard with the sieve from upper to bottom of 16 mesh and 30 mesh, and then were sieved. The bentonite powder grain size of -16+30 mesh was kept on the sieving pan of 30 mesh. The same method was used for the sieve from upper to bottom of 30 and 40 mesh, 40 and 50 mesh, 50 and 60 mesh, 60 and 70 mesh, 80 and 120 mesh respectively until the grains size bentonite of -30+40 mesh, -40+50 mesh, -50+60 mesh, -60+70 mesh, -80+120 mesh respectively were obtained. Preparation of quartz with SiO2 content of 77.89 wt.% from accumulating basin of Sermo Kulon Progo and clay powders from Kasongan Bantul was carried out by the same method as that of bentonite powders. Determination of Bulk Density of Bentonite, Quartz, and Clay Powders by ASTM D1895B A21.A Method The bentonite powders were poured excessively into the bowl of cylindrical glass with known volume (V) and weight (M1). The excess of bentonite powders was scraped horizontal by using thin blade precise at upper segment of the bowl. The bowl was filled with bentonite powders weighed as M2. Bulk density of bentonite (ρb) is determined by equation [8]: 2 b M M V ρ − = 1 (2) Determination of bulk density of quartz and clay powders was carried out by the same method as that of bentonite powders.

Determination of Effective Porosity of Bentonite, Quartz, and Clay Bed The porosity of samples was determined by an apparatus as shown in Fig. 2. The 2 glass column filled with the powder of single local mineral (bentonite or quartz or clay) or the mixture of (bentonite-quartz or clay-quartz or bentonite-clay-quartz) of V1 bed volume was flowed with distilled water from burette 1 through bottom of the 2 glass column. When the distilled water stream is precisely at bed bottom of local mineral powder at C boundary on the 2 glass column, then valve K on burette 1 was closed. Noted volume at A boundary, then distilled water was flowed to the 2 glass column filled with the local mineral powder by opening K valve. Grain Size Effect on Physical Characteristics of Bentonite, Quartz, Clay Bed The grain size affects the physical characteristics of bentonite, quartz, and clay. Therefore by knowing the grain size, the best physical properties could be determined. Physical characteristics e.g. Bulk density (ρb), effective porosity (ε), and permeability (k) were showed in the Fig. 5, 6, and 7 respectively. Fig. 5 shows that the smaller the grain sizes of the samples, the higher its density. This case is caused at constant volume; the smaller the grain size, the greater total particles to fill its volume. With total of particles increases, weight of the samples will increase so that the greater of the bulk density such as determined by using equation (2). Fig. 6 shows that the smaller of grain size of samples, the greater the porosity (ε). This case is caused by grain size smaller in the constant bed volume; hence total of particles will be larger. Each particle has internal pore, and then particles to fill bed volume to form external pore between particles. Powders of the backfill material with smaller grain size in the constant bed volume cause internal and external pores greater until total of pore volumes in the bed or usually expressed by effective porosity greater. Effective porosity result of mineral with variation of grain size according to experiment result expressed by Poernomo [11-12]. Fig. 5−7 showed that bentonite, quartz and clay with grain size of -80+120 mesh gave the bigger bulk density, the bigger effective porosity, and the smallest permeability. According to this result, hence bentonite, quartz, and clay with grain size of -80+120 mesh respectively are used as the mixture of local mineral to determine physical characteristics in the backfill material mixture of bentonite-quartz, clay-quartz, and bentonite-clay-quartz. Fig. 5−7 showed that at grain size of -80+120 mesh, bulk density of quartz > clay > bentonite, effective porosity of quartz > clay > bentonite, and permeability of quartz ≈ clay ≈ bentonite. Weight Ratio Effect of Mineral Local Mixture on Physical Characteristics The effect of bentonite-to-quartz, clay-to-quartz, and bentonite-to-clay-to-quartz weight ratio on physical characteristics was showed on the Fig. 8, 9, 10 respectively. Table 1 shows that mixture of bentonite-quartz with the composition bentonite of 80 wt.% and quartz of 20 wt.% give best of physical characteristics (ρb, ε, K). The mixture of clay-quartz with the composition clay of 60 wt.% and quartz of 40 wt.% gave best of physical characteristics. The mixture of bentonite-clay-quartz with the composition bentonite of 70 wt.%, clay of 10 wt.%, and quartz of 20 wt.% is the best of physical characteristics. The biggest bulk density of mineral local mixture can increase the mechanical strength in holding canister

REFERENCE

Centre for the Accelerator and Material Process Technology, National Nuclear Energy Agency, Jl. Babarsari P.O. Box 6101 Ykbb Yogyakarta 55281 Received January 8, 2010; Accepted August 30, 2010