IMPROVED SORPTION ABILITY FOR RADIONUCLIDES
BY CEMENTITIOUS MATERIALS
Kenji Noshita, Takashi Nishi, Masami Matsuda
Hitachi Ltd.
ABSTRACT
Radionuclides sorption by cementitious materials should be clarified to ensure the long term safety of radioactive waste repositories. The sorption behaviors of C-14, Co-60 and Am-241 onto calcium silicate hydrogel(C-S-H gel), which is the major component of hydrated cement, were studied using batch sorption experiments and zeta potential measurements. A high level of C-14(CO32-) sorption onto C-S-H gel occurred when the Ca/Si ratio was more than 0.1(mol/mol). This meant the C-S-H gel became positively charged above this Ca/Si ratio so that negative C-14 ions were adsorbed. Co and Am were also highly adsorbed onto C-S-H gel in the same way as C-14, meaning that Co and Am were also negatively charged in cement. To raise the distribution coefficient(Kd) of cement, blast furnace slag(BFS) was added to Ordinary Portland cement(OPC) to produce enough C-S-H gel in the cement. Each Kd value was increased about 10 times by adding 50wt% BFS to OPC.
INTRODUCTION
Cementitious materials have been widely used as a means of containing radioactive waste and as a man-made barrier surrounding repository sites. To ensure isolation of radionuclides, it is important to clarify both sorption abilities and sorption mechanisms of cementitious materials. Various cements have been found to have high sorption abilities for many radionuclides[1-4]. However, the second item has not been investigated in detail since the cement hydrates are so complex.
In this study, one of the single cement hydrates, C-S-H gel, was chosen for investigation as it is major hydrate in cement[5,6]. First, the inorganic C-14 sorption mechanism onto C-S-H gel was studied from the viewpoint of electrostatic adsorption. Next, Co-60 and Am-241 sorption behaviors were compared with that of C-14. Finally, these results were used for cement blending to enhance the sorption ability of each radionuclide.
EXPERIMENTAL
Hardened Cement Paste
Ordinary Portland cement(OPC) and blended cement(OPC/blast furnace slag;BFS) were used to prepare the test specimens with 31wt% water content. Test specimens were cured for 28 days and then ground into particles (<0.15mm) for batch sorption experiments.
Calcium Silicate Hydrogel(C-S-H gel)
C-S-H gel was synthesized using the reagents of Ca(OH)2 and amorphous SiO2. The reagents were mixed at the desired ratio and cured in deionized water(solid/liquid ratio=1g/10mL) at 20° C for 7 days.
Batch Sorption Experiments
A specimen(5.00g) of a ground cement or a C-S-H gel was added to deionized water(50.0mL) in a polypropylene jar. Then an aliquot of the radioisotope(C-14,Co-60 or Am-241) tracer was added to the solution. The contents of the jars were mixed once a day for 7 days prior to sampling for analysis by gamma spectrometry or liquid scintillation counting. The samples for analyses were prepared by centrifuging to separate liquid from solid. Each experiment was carried out at 20° C in air. The results of the analyses allowed calculation of the distribution coefficient(Kd) as follows:
|
Kd(mL/g) = Csolid/Cliquid |
(1) |
where Csolid denotes radioisotope concentration on solid(Bq/g); Cliquid, radioisotope concentration in liquid(Bq/mL).
Zeta Potential Measurements
Particle electrophoresis experiments were carried out to clarify the effects of electrostatic adsorption. The velocities of C-S-H gel particles were measured when a voltage was applied between the electrodes in a flat cell. Zeta potential was calculated using the velocities as follows:
|
· (mV) = 4· · · /· · E |
(2) |
where · denotes zeta potential(mV); · · ,viscosity(poise); · · ,velocity (cm/s); · ,permittivity(F/m); E,applied field strength(V/cm).
XPS Analyses
X-ray photoelectron spectroscopy analyses were carried out using an X-ray beam(Mg-K· ,12kV,20mA). These were performed to distinguish SiO- (101.7eV) from SiO2(103.2eV) in C-S-H gel based on the binding energy of the Si-2p orbital.
RESULTS
C-14 Sorption onto C-S-H Gel
Hydrated cement consists of amorphous C-S-H gel and some crystalline hydrates. For example, fully hydrated OPC is estimated to consist of about 30% C-S-H gel, 50% Ca(OH)2 and 20% monosulphate[7]. The C-S-H gel was synthesized by itself to evaluate the sorption ability.
Due to its being amorphous, the Ca/Si molar ratio in C-S-H gel can range widely from 0.8 to 2 in hydrated cement[8,9]. Then synthesized C-S-H gels with different Ca/Si ratios were used for inorganic C-14(CO32-) batch sorption experiments.
It was clarified that C-S-H gel had a high sorption ability for C-14 as shown in Fig.1. Kd values increased with Ca/Si ratios less than 0.1. However, Ca/Si ratios over 0.1 led to a decrease of Kd values.
Sorption Mechanism by C-S-H Gel
Inorganic C-14 is present in a negative ion in cement. The effect of electrostatic adsorption onto C-S-H gel was clarified by zeta potential measurements. Fig.2 shows the zeta potential on C-S-H gel. At low Ca/Si ratios, C-S-H gel was negatively charged, but became positive as the ratios were increased. However, Ca/Si ratios over 0.1 led to a decrease of zeta potential. This fact agreed with the decrease of Kd values. It indicated that C-14 was adsorbed onto C-S-H gel by electrostatic force.
To confirm this, C-14 batch sorption experiments were carried out for calcium silicate(CaSiO3), which is neutrally charged, in 0.03M-NaOH solution(pH=12.5). The Kd value of CaSiO3 was less than 1(mL/g). This also meant that electrostatic force contributed to the adsorption ability of C-S-H gel.
Optimum Ca/Si Ratio of C-S-H Gel
Synthesized C-S-H gels were made from Ca(OH)2 and amorphous SiO2. This synthetic process can be divided into two reactions.
|
(-Si-O-Si-) + 2OH- -> (-SiO- + -OSi-) + H2O |
(3) |
|
(-SiO- + -OSi-) + 2Ca2+ -> (-SiOCa+ + +CaOSi-) |
(4) |
where (-Si-O-Si-) denotes the amorphous SiO2 chain structure. These reactions can explain the appearance of a positive charge on C-S-H gel and also suggest the existence of an optimum Ca/Si ratio which is determined by the amount of -SiO-.
XPS analyses were carried out to measure -SiO- using amorphous SiO2 after reaction with 0.03M-NaOH solution(pH=12.5) for 7 days. The results are shown in Table I. A 10 mol% amount of SiO- was produced in pH12.5 solution. This value agreed with that the optimum Ca/Si molar ratio of 0.1 found in the sorption experiments and by zeta potential measurements.
In summary, it was proposed that C-S-H gel was positively charged due to the generation of -SiOCa+ so that C-14 was adsorbed onto C-S-H gel. This explanation also suggests that the optimum Ca/Si ratio varied with the SiO2 phase, especially in the actual cement. The ratio should depend on the binding energy of -Si-O-Si-.
Co-60 and Am-241 Sorption onto C-S-H Gel
In a high pH solution(~13), Co and Am are thought to be present as hydroxides(Co(OH)2, Am(OH)3)[10]. However, these nuclides form carbonic complexes or colloids in cement, so that actual chemical form was not verified in detail.
To clarify the effect of electrostatic adsorption, batch sorption experiments were carried out using two specimens. One was the C-S-H gel which was synthesized with the Ca/Si molar ratio of 0.15, and the other was pure amorphous SiO2 in 0.03M-NaOH solution. The former was positively charged, and the latter was negatively charged. The results were compared with those of C-14 as shown in Table II. High sorption abilities(large Kd) for Co and Am were found in C-S-H gel just as for C-14. On the other hand, the pure SiO2 had very low Kd values. These facts indicated that Co and Am were also negatively charged in cement.
Enhancement of C-S-H Gel in Cement
High sorption abilities were found in C-S-H gel for C-14, Co-60 and Am-241. This indicated that radionuclides sorption abilities of cementitious materials could be enhanced by producing more C-S-H gel. OPC does not contain enough SiO2 agent compared with its CaO content to produce a sufficient amount of C-S-H gel, but it is well known more C-S-H gel can be produced by adding BFS to OPC[7].
Blended cements were prepared to clarify this effect. After curing for 28 days, batch sorption experiments were carried out. The experimental results are shown in Fig.3. C-14 Kd values increased about 10 times by adding 50wt% BFS to OPC. Over 50wt% BFS, Kd values were reduced by the lack of CaO. Co and Am Kd values were also obtained using this optimized cement. Fig.4 summarizes data on the effectiveness of this blended cement. Kd values were increased about 10 times for each radionuclide.
CONCLUSION
The sorption behaviors of C-14, Co-60, Am-241 onto C-S-H gel were studied by batch sorption experiments, zeta potential measurements and XPS analyses. The following results were obtained;
REFERENCES