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Synthesis of lead vanadate iodoapatite utilizing dry mechanochemical process
Journal of Nuclear Materials 454 (2014) 223–229
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Synthesis of lead vanadate iodoapatite utilizing dry mechanochemical process Yasushi Suetsugu ⇑ Biomaterials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba-shi, Ibaraki 305-0044, Japan
h i g h l i g h t s Pb10(VO4)6I2 was successfully synthesized at low temperature in an open system. Leaching tests for Pb10(VO4)6I2 were performed using high-density sintered bodies. Substitution of VO4 by PO4 in Pb10(VO4)6I2 affected the iodine retention properties.
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Article history: Received 3 March 2014 Accepted 30 July 2014 Available online 7 August 2014
a b s t r a c t For consolidation and subsequent deep geological repository of radioactive 129I, a novel synthesis method of iodine-bearing lead vanadate apatite Pb10(VO4)6I2 was developed. In order to reduce the reaction temperature, a dry mechanochemical process was applied and iodoapatite was successfully synthesized without heating. Calcining the as-synthesized low crystalline specimen at 300 °C gave a single-phase with high crystallinity. The obtained powder was consolidated by hot-press sintering at 400 °C under uniaxial pressure of 90 MPa and submitted to leach tests by immersion into pure water at room temperature. The normalized leach rate of iodine on the first day was about 2.6 101 g m2 d1. After 56 days of immersion, the iodine leach rate was reduced to less than 1/30 of the initial rate in newly changed water. 3 Iodoapatite specimens in which part of VO3 4 was substituted with PO4 , Pb10(VO4)4.8(PO4)1.2I2, were also synthesized by the same method. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Radioactive 129I is generated as a fission product in the consumption process of nuclear fuel. Currently, some of it is discharged into the sea and some is captured by silver absorbent during the reprocessing of spent nuclear fuel. 129I has quite a long half-life of 15.7 million years and is considered to be one of the radioactive isotopes with the most impact on the biosphere. Eventually, depositing captured 129I in a geological repository becomes unavoidable. However, 129I has very low absorbability in ordinary artificial and natural barrier materials, making it difficult to isolate from the biosphere. Therefore, novel matrix materials that can immobilize 129I for a long period of time and control its release into the environment are required. For this purpose, synthetic lead vanadate iodoapatite, Pb10(VO4)6I2 [1], which contains iodine in its apatitic crystal structure, or its derivative, lead vanadate/phos-
⇑ Tel.: +81 298 860 4504. E-mail address: [email protected] http://dx.doi.org/10.1016/j.jnucmat.2014.07.073 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
phate iodoapatite Pb10(VO4)6x(PO4)xI2 [2], have been proposed as potential waste forms. A mineral with a composition of Pb10(VO4)6Cl2 that belongs to the apatite group is named vanadinite; therefore, it may be suitable to refer to Pb10(VO4)6I2 as ‘‘iodovanadinite’’, but the better known name apatite is used in this paper. For the synthesis of Pb10(VO4)6I2, two kinds of methods have already been proposed. In one, Pb10(VO4)6I2 is synthesized directly by the reaction of PbO, V2O5 and PbI2 [3], and in the other, Pb3(VO4)2 is first synthesized from PbO and V2O5, and then reacted with PbI2 to obtain Pb10(VO4)6I2 [1,4]. Both methods require high temperature for the reaction, 500–700 °C, in a closed system that requires special equipment to avoid the volatilization of iodine. Le Gallet et al. [5] reported the successful direct reaction-sintering of Pb10(VO4)4.8(PO4)1.2I2 utilizing a spark plasma sintering method. Stennett et al. [6] adopted a microwave heating technique to synthesize Pb10(VO4)6I2. Both of these took place in an open system. The author applied a dry mechanochemical activation technique to lower the reaction temperature for efficient synthesis of lead vanadate/(phosphate) iodoapatite. This paper reports the results of this synthesis along with sintering and leach tests using the obtained sintered samples.
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2. Experimental PbO, V2O5 and PbI2 powders (Wako Pure Chemical Industries) were used as starting materials. For the synthesis of PO3 4 containing Pb10(VO4)6x(PO4)xI2 samples, Pb3(PO4)2 (Kojundo Chemical Laboratory) was added. Expected reactions are described as follows.
9PbO þ 3V2 O5 þ PbI2 ! Pb10 ðVO4 Þ6 I2 7:2PbO þ 2:4V2 O5 þ 0:6Pb3 ðPO4 Þ2 þ PbI2 ! Pb10 ðVO4 Þ4:8 ðPO4 Þ1:2 I2 3 Here, the ratio of VO3 4 and PO4 in Pb10(VO4)6x(PO4)xI2 was set at 4:1 according to the previous reports [2,5,7]. Each starting powder was weighed in a stoichiometric ratio and mixed roughly in a mortar. The mixture was subjected to a mechanochemical activator (Mechano-MicrosÒ, Nara Machinary) [8] that had a drum-shaped vessel whose inner wall was lined with zirconia ceramics. In the vessel, four steel shafts (termed ‘‘sub-shafts’’ in the report of Nakayama et al. [8]) supported stacks of small toroidal disks made of zirconia. These orbited at high speed around the axis of the vessel such that the disks were centrifugally pressed against and rolling along the inner wall of the vessel. The reactant powder was ground between the disks and the wall and underwent compressive and shear stresses. For the present work, the sub-shafts revolved at 600 min1 in the vessel which was 105 mm diameter and 67 mm depth and also rotated slowly at 80 min1 coaxially and reversely to the sub-shafts’ revolution. At every 150 min, the sample powder was raked out and re-subjected to the activator to avoid non-uniformity of the reaction. The total activation time was set to 600 min. In order to avoid OH contamination, dry N2 gas was flowed into the vessel at a rate of 5 L h1 and the outflowing gas was introduced into ethanol. The ethanol remained colorless throughout the process indicating no I2 emission from the sample. The vessel was cooled by flowing water from outside to keep the temperature of the sample under 40 °C throughout the process. To complete the reaction and crystallization, the samples after mechanochemical grinding were placed in a platinum crucible and calcined at 300 °C for 1 h in vacuum or in air. Vacuum calcination was performed utilizing a hot press chamber (Nems NP-12-G, the same machine used for sintering of the obtained iodoapatites as mentioned later) without using the pressing function. Calcination in air was carried out using a Yamato Scientific FP31 muffle furnace. The temperatures of the samples were checked by thermocouples contacting the crucibles. At certain stages of the synthetic process, small portions of the samples were subjected to powder X-ray diffraction (XRD) studies, Fourier-transfer infrared (FT-IR) absorption analyses, thermogravimetric and differential thermal analyses (TG–DTA), temperatureprogrammed evolved gas analyses (EGA), scanning electron microscope (SEM) observations, energy dispersive X-ray spectroscopy (EDS) analyses, and specific surface area measurements. XRD measurements were carried out at room temperature using a Rigaku RINT-Ultima III, by h–2h scanning (in 0.02° steps) using Cu Ka radiation. FT-IR spectra were measured with a PerkinElmer Spectrum 2000 system. The sample was diluted ten-fold with KBr powder, and data were collected with the diffuse-reflection method. TG–DTA was performed by a Rigaku Thermo Plus 2, with a heating rate of 20 K min1, sample quantity of 100 mg, and atmosphere gas flow rate of 300 mL min1. EGA was carried out by a Rigaku TPD type R Photo and a Rigaku Thermo Mass Photo. The heating rate, sample quantity and gas flow rate were set identical to the TG–DTA measurements. SEM images were observed by a JEOL JSM-5600 with an electron acceleration voltage of 20 kV. EDS analyses were performed with a JEOL JED-2200 system (X-ray takeoff angle 30°). Pressed compact pellets of each starting material reagent were used as standard samples for quantitative analysis and acquired raw X-ray intensity data were compensated
by methods of Springer [9], Bishop [10] and Philibert [11]. Specific surface areas were measured by N2 absorption using the BET method with a BEL Japan BELSORP-mini II. The synthesized iodoapatite powder was packed into a graphite cylindrical die with an inner diameter of 20 mm plugged by two graphite stubs at both ends. This assembly was placed in the hot press machine (the above mentioned Nems NP-12-G). The instrument was equipped with an electric resistance graphite heater surrounding the sample assembly which was uniaxially pressed by a hydraulically operated graphite piston rod. The compressive load and moving distance of the piston rod were measured by a load cell and plunger gauge, respectively, and the temperature of the sample was monitored by a thermocouple inserted into a small hole drilled in the sample die. The sample was pressed at 90 MPa and heated to 400 °C at 20 K min1 with a 10 min plateau at 400 °C. The load was released slowly to zero over 10 min without changing the temperature so that the annealing effect would prevent residual stress in the sample. The obtained sintered bodies of iodoapatite were immersed into water for the leach tests according to MCC-1P Static Leach Test Methods of Pacific Northwest Laboratory [12]. In a glove box filled with deoxygenated Ar gas simulating an underground reductive environment, each discus-shaped sample 20 mm in diameter and 4 mm in thickness was hung with nylon thread in a polypropylene bottle filled with vacuum-degassed pure water at room temperature. The ratio of sample surface area/liquid volume (S/V) was regulated to be 0.01 cm1. Here, the sample surface areas were calculated from the geometric dimensions and hence the liquid volumes were around 880 mL. The pH of leachants remained near neutral throughout the total testing duration time, 56 days. At specified time intervals, a small portion of the leachant was sampled and the iodine in it was quantified by an inductively coupled plasma-mass spectrometer (ICP-MS, Perkin Elmer ELAN DRC II) and the concentrations of lead and vanadium were measured by an inductively coupled plasma-atomic emission spectrometer (ICPAES, Perkin Elmer OPTIMA 3000 XL).
3. Results and discussion 3.1. Synthesis of Pb10(VO4)6I2 Fig. 1 shows the changes in XRD patterns for a mixture of PbO, V2O5 and PbI2 measured at different duration times of
Fig. 1. Changes of XRD patterns for mixture of PbO, V2O5 and PbI2 powders with increase of mechanochemical grinding time. X-ray source: Cu Ka. Grinding time: (a) 0, (b) 150, (c) 300, (d) 450, and (e) 600 (min). Open inverted triangles: a-PbO; filled inverted triangles: b-PbO; filled circles: V2O5; filled squares: PbI2; open diamonds: Pb10(VO4)6I2.
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mechanochemical grinding. The pattern (a), for the mixture without grinding, indicates a superposition of diffraction patterns for each pure starting material. As the grinding time increases, the height of each diffraction peak decreases. The peak for PbI2 reduces remarkably due to the inherent vulnerability of its layered crystal structure against shear stress in accordance with the report of Clark and Kern [13]. PbO transformed from a yellow colored orthorhombic b-phase to a red colored tetragonal a-phase, whose diffraction peaks also decreased with additional grinding time. Commercially available PbO was mostly in the b-phase though this was not the stable phase at room temperature, and mechanochemical grinding converted it to the stable a-PbO. The peaks ascribed to a-PbO are somewhat broad. This is consistent with the observation by Clark and Rowan [14] that a-PbO synthesized from the b-phase by grinding was structurally distorted and therefore had high chemical activity. The most important aspect illustrated in Fig. 1 is that diffraction peaks indicating the existence of iodoapatite phase appear. The peaks intensified as the grinding time became longer. The peaks are very broad, which is indicative of low crystallinity. It is plausible that high reactivity of mechanochemically ground starting materials contributed to the formation of iodoapatite at room temperature. After 600-min grinding [Fig. 1(e)], almost all the peaks observed were those ascribed to the apatite phase except for trace peaks at 20.3° derived from 0 0 1 diffraction of V2O5 (ICDD 00-0411426), at 30.3° for 0 2 0 diffraction of b-PbO (ICDD 00-038-1477) and at 31.9° for 1 1 0 diffraction of a-PbO (ICDD 01-085-1288). The existence of Pb3(VO4)2, while strongly suspected, was not detected by the present XRD measurements. The color of the unground starting mixture was orange–brown and unchanged significantly by mechanochemical grinding. The specific surface areas before and after the grinding were 1.2 m2 g1 and 2.6 m2 g1, respectively. After calcination at 300 °C in vacuum for 1 h, the color of the powder changed to grayish-green yellow and the specific surface area turned to 2.1 m2 g1. Fig. 2(a) shows an XRD pattern for the sample calcined at 300 °C in vacuum after 600-min grinding. The diffraction pattern is very similar to the data of Pb9.85(VO4)6I1.70 reported by Audubert et al. [15] (ICDD 00-051-0119) and the peaks for residual unreacted starting materials such as a- and b-PbO or V2O5 completely disappeared. The unit cell parameters calculated from the 2h locations of diffraction peaks were a = 1.045(1) nm and c = 0.748(1) nm, which were closer to the values for Pb10 (VO4)6I2 by Stennett et al. [6], a = 1.04429(3) nm and c = 0.74865(2) nm, rather than those for Pb9.85(VO4)6I1.70 by Audubert et al. [15], a = 1.0422(5) nm and c = 0.7467(3) nm. Stennett et al. [6] synthesized stoichiometric Pb10(VO4)6I2 as a mixture
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with Pb3(VO4)2 from starting materials including excess amount of PbO and V2O5 as a buffer against iodine substoichiometry reported by Audubert et al. [1,15]. The present results indicate that the mechanochemical method could yield Pb10(VO4)6I2 as a single phase from stoichiometric mixture of the starting materials. Fig. 3(a) and (b) show TG–DTA curves and EGA of I2 (mass-tocharge ratio, m/z 254) for the unground mixture of starting powders and the 600-min mechanochemically ground sample, respectively. The measurements were performed under He gas flow. In the temperature range from room-temperature to 300 °C, about 1% weight loss was observed for both samples. EGA measurement for m/z 18 and 44 (not shown in the figure) revealed that this loss was attributed chiefly to desorption of H2O (100–200 °C) and CO2 (200–300 °C). At temperatures higher than 300 °C, the behaviors of the two samples differed significantly. In the TG curve for the unground sample [Fig. 3(a)], weight loss occurs at wide temperature range of 300–800 °C. The loss at 300–700 °C is attributed mainly to I2 emission as indicated in the EGA curve with prominent two peaks. Main reactions expected to occur at this temperature range are, a reaction between PbO and PbI2 to form lead oxyiodides [16,17], and another reaction between PbO and V2O5 to form lead vanadates, and a subsequent reaction of lead oxyiodides and lead vanadates which yields an apatite phase. In the course of this sequence of reactions, part of iodine volatilizes in the form of I2 by some side reactions without incorporation into the apatite phase. Formation of an apatite phase is considered to take place at least above 450 °C, but Le Gallet et al. [5] observed apatite formation from Pb3(VO4)1.6(PO4)0.4 and PbI2 at 300 °C. This comparatively low synthesis temperature might be achieved perhaps by a mechanochemical effect due to mechanical milling they applied on the starting materials before the heating process. The present EGA measurements did not detect any signals large enough to explain the weight loss at 700–800 °C. This loss was presumably caused by PbI2 emitted accompanying decomposition of the apatite phase. The reason for undetectability of PbI2 by the present EGA measurements was probably because PbI2 vapor pressure was so low that it condensed and deposited somewhere along the path to the detector. Supposing that the total weight loss excluding desorption of H2O and CO2, namely 11.0%, was comprised mostly of I2 and PbI2 emission, the ratio of iodine lost as I2 to the total iodine content is calculated to be about 63%. On the other hand, for the sample that underwent mechanochemical grinding [Fig. 3(b)], the TG curve shows simpler profile similar to that of Pb10(VO4)6I2 reported by Uno et al. [3] or Stennett et al. [6]. Since no corresponding I2 emission is observed in the EGA curve, the remarkable weight losses observed in two stages at 500–650 °C and 650–780 °C are considered to be PbI2 emission like the one that appears at 700–800 °C in Fig. 3(a), during which a decomposition reaction Pb10(VO4)6I2 ? 3Pb3(VO4)2 + PbI2" is quite likely to take place. In this case the total amount of weight loss excluding desorption of H2O and CO2 is 15.0%, which corresponds well with the theoretical weight ratio of PbI2/Pb10(VO4)6I2 (15.3%). In the DTA curve, a large endothermic peak can be observed at 960 °C, which is the melting point of Pb3(VO4)2. Heat emission attributed to formation reaction or improvement in crystallinity of Pb10(VO4)6I2 is observable at around 300 °C, though this is not very clear. 3.2. Iodine evacuation by heating in air atmosphere
Fig. 2. XRD patterns for iodoapatites synthesized by mechanochemical grinding and subsequent calcination at 300 °C. (a) Calcined in vacuum, Pb10(VO4)6I2. (b) Calcined in air, Pb10(VO4)6I1.3(OH)0.7. (c) Calcined in vacuum, Pb10(VO4)4.8(PO4)1.2I2.
The color of the iodoapatite sample powder calcined in air instead of vacuum at 300 °C for 1 h after 600-min mechanochemical grinding was light yellow, which was different from the grayish-green yellow color of the sample calcined in vacuum. This suggested a difference in chemical composition between the two samples.
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Fig. 3. Curves of DTA, TG and EGA of I2 (m/z 254) for: (a) mixture of PbO, V2O5 and PbI2 without grinding, measured in He atmosphere; (b) mechanochemically synthesized Pb10(VO4)6I2, measured in He; (c) mechanochemically synthesized Pb10(VO4)6I2, measured in simulated air (80% He + 20% O2); and (d) mechanochemically synthesized Pb10(VO4)4.8(PO4)1.2I2, measured in simulated air. (a and b): measured by Rigaku TPD type R Photo (c and d): measured by Rigaku Thermo Mass Photo.
Fig. 4 shows FT-IR spectra for the mechanochemically ground mixtures calcined at 300 °C in vacuum (a) and in air (b), respectively. In spectrum (a), almost no peaks are observed. On the other hand, in spectrum (b), an absorption peak at 3535 cm1 is found that is attributed to the stretching vibration of OH [18], which replaced some of the I sites. A shoulder peak at 3555 cm1 is also presumed to be from OH, but no literature mentioning this peak has been found yet. Fig. 2(b) shows the XRD pattern for the mechanochemically ground sample calcined in air at 300 °C for 1 h [the same sample as in Fig. 4(b)]. Compared to the XRD pattern for the sample
Fig. 4. FT-IR spectra for iodoapatites synthesized by mechanochemical grinding and subsequent calcination at 300 °C. (a) Calcined in vacuum, Pb10(VO4)6I2. (b) Calcined in air, Pb10(VO4)6I1.3(OH)0.7.
calcined in vacuum [Fig. 2(a)], it is noted that many peaks are shifted to higher 2h angles, except, for example, 0 0 2 and 0 0 4 peaks, which show almost no change at 23.8 and 48.7°, respectively. These results indicate that Fig. 2(b) shows an intermediate pattern between Pb10(VO4)6I2 {a = 1.04429(3) nm and c = 0.74865(2) nm [6]} and Pb10(VO4)6(OH)2 {hydroxyvanadinite, a = 1.02242(3) nm and c = 0.74537(2) nm [19]}. Therefore, given the existence of a solid-solution between these two end-members, namely Pb10(VO4)6I2y(OH)y, Fig. 2(b) corresponds well with the FT-IR result [Fig. 4(b)]. The diffraction data of Fig. 2(b) give cell parameters a = 1.036(1) nm and c = 0.749(1) nm, which leads to a substitutional parameter y = 0.7 ± 0.1, providing Vegard’s Law is applicable in this solid-solution system. The resultant composition Pb10(VO4)6I1.3(OH)0.7 was supported by relative elemental concentration data acquired by an EDS analysis of the same sample (Pb:V:I = 10.09 ± 0.24:6.00:1.33 ± 0.27 in atomic ratio). In Fig. 2(b), small peaks at 28.2° and at 31.1° which are not ascribed to apatite phase are also observed. Identifications of these peaks are to be mentioned at the last part of this subsection. Fig. 3(c) shows results of TG, DTA and EGA for the mechanochemically ground sample measured under a flow of simulated air (mixture of 80% He and 20% O2). Compared to the results measured in pure He gas flow [Fig. 3(b)], the major weight loss occurs at lower temperatures ranging from 330 °C to 570 °C, and the corresponding I2 emission is apparent in the EGA curve. Redfern et al. [20] reported that Pb10(VO4)6I2 broke down into Pb3(VO4)2 emitting iodine upon heating in air in the temperature range of 267– 407 °C. The difference in the temperatures from those in Fig. 3(c) is probably caused by the difference in experimental conditions. Since the weight loss shown in Fig. 3(c) is 8.0% whereas that in Fig. 3(b) is 15.0%, substitution of I2 M O likely occurred in this case considering the formula weight ratio (I2–O)/Pb10(VO4)6I2 = 7.9%.
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DTA behavior at 800–960 °C with endothermic peaks at 812 and 954 °C agrees well with the incongruent melting of 10PbO3V2O5 composition [corresponding to Pb10(VO4)6I2 – I2 + O] decomposing into Pb3(VO4)2 + liquid in the phase diagram made by Shimohira et al. [21]. From these results, it was considered plausible that some iodine was replaced by oxygen when the mechanochemically ground mixture was calcined in air at 300 °C. In order to confirm whether OH could also directly replace I, simulated air humidified with H2O vapor of partial pressure 2 kPa at room temperature was used as atmosphere gas for TG, DTA and EGA measurements. The results showed no meaningful difference in comparison with the results obtained using dry simulated air without H2O. This implies that the exchange of I M OH by heating in air did not directly take place through hypothetical reactions such as Pb10(VO4)6I2 + yH2O ? Pb10(VO4)6I2y(OH)y + yHI or Pb10(VO4)6I2 + y/4O2 + y/2H2O ? Pb10(VO4)6I2y(OH)y + y/2I2, but partial exchange of I2 M O occurred initially and H2O molecules were subsequently induced at lower temperature to form Pb10(VO4)6I2y(OH)y. In the case of calcium phosphate apatite, Ca10(PO4)6O (oxyapatite) can actually exist at least as a solid-solution with Ca10(PO4)6 (OH)2 (hydroxyapatite) at high temperature. Under ordinary wet atmosphere at room temperature, H2O molecules are rapidly introduced into O2 sites to form complete Ca10(PO4)6(OH)2 [22,23]. We can imagine that a similar mechanism turns Pb10(VO4)6O (‘‘oxyvanadinite’’) into Pb10(VO4)6(OH)2 by absorbing H2O from air, but the existence of neither Pb10(VO4)6O nor even solid-solution Pb10(VO4)6(OH)22zOz has been confirmed in any reports yet. However, according to Redfern et al. [20], upon heating in air, Pb10(VO4)6I2 decreases its unit cell parameter a in two stages associated with the decomposition; the first one begins prior to the onset of breakdown and the other is in almost the same temperature range as the breakdown occurs. This a-axis contraction in either or both of these temperature stages might indicate the possibility of the existence of solid-solution Pb10(VO4)6(I2yOy/2) with limited y. This type of composition was mentioned by Audubert et al. [1]. According to Shimohira et al. [21], a binary system with the composition corresponding to ‘‘oxyvanadinite’’ (10PbO3V2O5) actually adopts the biphase of 2Pb3(VO4)2 + Pb4O(VO4)2 below the incongruent melting point (807 °C). Provided that the replacement of I2 by O in Pb10(VO4)6I2 beyond the acceptable limit of the apatitic frame structure leads to breakdown into 2Pb3(VO4)2 + Pb4O(VO4)2, the small peaks observed at 28.2° and at 31.1° in Fig. 2(b) can be identified as 6 2 0 and 0 4 0 diffractions, respectively, of Pb4O(VO4)2 (ICDD01-073-7788). Alternatively, attributions to 0 1 5 and 1 1 0 diffractions of c-Pb3(VO4)2 (ICDD 01-080-1957) are also possible, which phase is stable above 104 °C in phase diagrams but could be stabilized at room temperature. These results indicate that in order to prevent emission of iodine, the heating process, calcination and sintering should be carried out in an atmosphere excluding oxygen.
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sintering process. The relative density calculated from the dimensions of the sintered body was 97.8 ± 0.2% given the theoretical density of 7.082 g cm3 calculated from the crystallographic data reported by Stennett et al. [6]. Fig. 7 shows an SEM image of the fracture surface of the sintered Pb10(VO4)6I2. Fractures took place along grain boundaries. The diameter measured for circles with the same area of the grain images in Fig. 7 was 200 ± 40 nm. 3.4. Synthesis and sintering of Pb10(VO4)4.8(PO4)1.2I2 Synthesis of Pb10(VO4)4.8(PO4)1.2I2 was carried out by the same process except that Pb3(PO4)2 was added to the starting materials. The XRD pattern of the mixed powder calcinated at 300 °C in vacuum after 600-min mechanochemical grinding is shown in Fig. 2(c). Peak broadening is observed, likely due to low crystallinity or compositional fluctuation. Specific surface areas measured before and after the mechanochemical grinding, and after the subsequent calcination were 2.0, 2.6, and 2.3 m2 g1, respectively. The results of TG, DTA and EGA measurements under simulated air atmosphere are shown in Fig. 3(d). The profile of each curve is similar to that of Pb10(VO4)6I2 [Fig. 3(c)] except for some thermal shifts. In the DTA curve, shifts of the endothermic peaks to lower temperatures (812 ? 768 °C, 954 ? 937 °C) are ascribed to temperature depression of incongruent melting due to impurity effects. In the EGA curve, the higher one of two iodine emission peaks shifts its position by 40 K (520 ? 560 °C) though iodine emission starts at almost the same temperature (330 °C). Two shrinkage curves of Pb10(VO4)4.8(PO4)1.2I2 observed in the process of sintering are shown as curves (c) and (d) in Fig. 5, together with the cases of Pb10(VO4)6I2 [(a), (b)]. The shrinkage starts at 280 °C and finishes at 370 °C. Both temperatures are lower than those in the cases of Pb10(VO4)6I2. This difference seems to be intrinsic considering that the specific surface area measured for each type of iodoapatite did not differ significantly (2.1 m2 g1 for Pb10(VO4)6I2 and 2.3 m2 g1 for Pb10(VO4)4.8(PO4)1.2I2). 3.5. Leach test Fig. 8 shows temporal concentration changes of elements dissolved from sintered samples of Pb10(VO4)6I2 [Fig. 8(a)] and of Pb10(VO4)4.8(PO4)1.2I2 [Fig. 8(b)] separately immersed in pure water. Each graph presents results of two equal tests. Vanadium was not detected above the detection limit, 5.9 102 lmol L1 throughout the 56-day test. Lead appeared above the detection
3.3. Sintering of Pb10(VO4)6I2 Shrinkage curves of iodoapatites observed in the process of sintering under the uniaxial pressure of 90 MPa are shown in Fig. 5. The figure presents two results of Pb10(VO4)6I2 sintering as curves (a) and (b) for reproducibility test. The shrinkage starts at 320 °C and finishes at 390 °C for both (a) and (b). Fig. 6 shows the appearance of one of the sintered bodies. The color of the surface was metallic dark gray, distinctly different from the former grayishgreen yellow color before sintering. However the streak color of the sintered body was the same grayish-green yellow as the raw powder. By XRD and EDS measurements, no significant differences from the raw powder were detected indicating that the creation of impurities and/or changes in composition did not occur during the
Fig. 5. Shrinkage curves for iodoapatites in the process of hot press sintering under the uniaxial pressure of 90 MPa. Initial thickness (shrinkage = 0) was about 5.3 mm. (a), (b): Pb10(VO4)6I2. (c), (d): Pb10(VO4)4.8(PO4)1.2I2.
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Fig. 6. Sintered Pb10(VO4)6I2.
Fig. 7. SEM image of fracture surface of sintered Pb10(VO4)6I2.
limit, 1.3 101 lmol L1, at 28 and 56 days [Fig. 8(a)] and at 7– 56 days [Fig. 8(b)] after the immersion. The amount of leached iodine from Pb10(VO4)4.8(PO4)1.2I2 was 4–5 times as high as those from Pb10(VO4)6I2. Fig. 8 indicates that the amount of leached iodine is the largest among the three elements, which implies that iodine was not leached out by a simple congruent dissolution of bulk apatites. This is in agreement with the previous reports [2,7,24,25]. There are several possibilities for what occurred at the surface of iodoapatite immersed in water, e.g., OH replaced I to form a OH rich apatite layer [25], or iodine was leached out with lead and left a lead vanadate (or lead vanado-phosphate) layer on the surface [7]. A diffusion process of iodine through those layers of secondary products is supposed to control the leach rate [7]. For Pb10(VO4)6I2 samples, normalized iodine leach rates on the first day are calculated to be 2.7 101 and 2.6 101 g m2 d1. Those for Pb10(VO4)4.8(PO4)1.2I2 are 9.0 101 and 8.0 101 g m2 d1. Here, the reactive surface areas used for the calculation were values obtained from the geometric dimensions of the samples. In the literature of Maddrell and Abraitis [24], the normalized iodine leach rate for Pb10(VO4)6I2 immersed in pH 11 buffer solution at 90 °C is in the region 2–5 g m2 d1. Also, from the figures in the literatures of Audubert and Lartigue [2] and Guy et al. [7], normalized iodine leach rate at the earliest stage of dissolution of Pb10(VO4)4.8(PO4)1.2I2 immersed in 90 °C pure water is read to be 0.2 g m2 d1. For comparison, these data are converted to the values expected if acquired under the same experimental conditions as those for the present work (25 °C and neutral pH) using apparent activation energy of 37 kJ mol1 and the effect of pH difference presented by Guy et al. [7]. The calculated values are in the region
Fig. 8. Concentration changes of iodine (circles) and lead (triangles) in leachant in which a sintered iodoapatite was immersed. Sample surface area/liquid volume (S/ V) = 0.01 cm1. Each graph, (a) Pb10(VO4)6I2 and (b) Pb10(VO4)4.8(PO4)1.2I2, shows results of two equal tests. The data sets of iodine and lead marked in the same color were obtained from the same test.
1.4 102–3.6 102 g m2 d1, which are 1/10–1/60 of those acquired from Fig. 8. One possible explanation for this discrepancy is as follows. At least at the early stage of leach tests in the previous works [2,7,24], if Pb2+ had dissolved with I to keep charge balance, the leachants would have been instantly saturated with respect to Pb(OH)2, which is found by means of equilibrium calculations taking S/V ratios and OH concentrations at 90 °C into account, hence Pb2+ must have remained in the solid phase. According to the results by Zhang et al. [25], a most likely way for the iodine leaching to take in these cases was substitution by OH rather than coleaching with Pb2+ accompanied by Pb(OH)2 reprecipitation, while in the case of present work, co-leaching with Pb2+ was probably dominant because the amount of leachant was abundant for Pb2+ dissolution. Now suppose that the OH rich apatite layer formed by I M OH exchange served as a stronger protective layer against ongoing leaching of iodine than lead vanadate layers formed by simultaneous dissolution of lead and iodine, the higher leach rate would be observed by the present leach test. The leached samples retrieved after 56 days of immersion were again immersed separately into newly changed pure water. Iodine leach rates of Pb10(VO4)6I2 samples on the first day were decreased to 8.8 103 g m2 d1 and 7.1 103 g m2 d1. Those of Pb10(VO4)4.8(PO4)1.2I2 were 5.0 102 and 3.3 102 g m2 d1. These data are 1/20–1/40 of the values for the previous tests but still much higher than expected values (1.7 104 g m2 d1) calculated from the steady state leach rate at 90 °C (2.4 103 g m2 d1) provided by Guy et al. [7], suggesting that the
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experimental condition applied for the present leach test was not favorable for the formation of efficient protecting layers. 4. Conclusions The present work showed that Pb10(VO4)6I2 could be synthesized in an open system by mechanochemical grinding of a powder mixture of PbO, V2O5 and PbI2. It was confirmed that without the mechanochemical process, a closed experimental system was indispensable to prevent volatilization loss of iodine. The application of the mechanochemical process also enabled incorporation of PO3 partly substituting VO3 in iodoapatite. 4 4 3 PO4 incorporation changed the iodine emission temperature but not far enough to definitely improve thermal durability of iodine immobilization. PO3 4 including iodoapatite also exhibited a lower sintering temperature and higher iodine leaching property in comparison with Pb10(VO4)6I2. One of the noteworthy features of iodoapatite may be an exchangeability of I by OH. It was found likely that, in air, OH replaces I in apatite structure through a temporary incorporation of O2 at high temperature. In water, OH rich apatite layer formed by I M OH exchange was suspected to play a quite important role for iodine immobilization having strong effect on the leaching property of iodoapatite. Acknowledgements The author is deeply grateful to the staff at Rigaku Corporation, especially to Dr. Kazuko Motomura, for generous support of TG, DTA and EGA measurements. The author also expresses his gratitude to Dr. Masanori Kikuch at National Institute for Materials Science for insightful suggestions.
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References [1] F. Audubert, J. Carpena, J.L. Lacout, F. Tetard, Solid State Ion. 95 (1997) 113– 119. [2] F. Audubert, J.-E. Lartigue, International Conference on the Scientific Research on the Back-End of the Fuel Cycle for the 21st Century, Atalante 2000, Avignon France, 2000, P4.13. [3] M. Uno, M. Shinohara, K. Kurosaki, S. Yamanaka, J. Nucl. Mater. 294 (2001) 119–122. [4] L. Merker, H. Wondratschek, Z. Anorg. Allg. Chem. 300 (1959) 41–50. [5] S. Le Gallet, L. Campayo, E. Courtois, S. Hoffmann, Y. Grin, F. Bernard, F. Bart, J. Nucl. Mater. 400 (2010) 251–256. [6] M.C. Stennett, I.J. Pinnock, N.C. Hyatt, J. Nucl. Mater. 414 (2011) 352–359. [7] C. Guy, F. Audubert, J.E. Lartigue, C. Latrille, T. Advocat, C. Fillet, C. R. Phys. 3 (2002) 827–837. [8] S. Nakayama, S. Nagare, K. Hamada, Y. Abe, M. Senna, J. Eur. Ceram. Soc. 27 (2007) 4413–4416. [9] G. Springer, Fortschr. Mineral. 45 (1967) 103–124. [10] H.E. Bishop, J. Phys. D-Appl. Phys. 1 (1968) 673–685. [11] J. Philibert, Third International Symposium on X-ray Optics and X-ray Microanalysis, Sanford, 1962, pp. 379–392. [12] MCC-1P, Nuclear Waste Materials Handbook, Test Methods, Pacific Northwest Laboratory 1983. [13] G.L. Clark, S.F. Kern, J. Am. Chem. Soc. 64 (1942) 1637–1641. [14] G.L. Clark, R. Rowan, J. Am. Chem. Soc. 63 (1941) 1302–1305. [15] F. Audubert, J.M. Savariault, J.L. Lacout, Acta Crystallogr. Sect. C-Cryst. Struct. Commun. 55 (1999) 271–273. [16] H.S. van Klooster, R.M. Owens, J. Am. Chem. Soc. 57 (1935) 670–671. [17] W. Rolls, E.A. Secco, U.V. Varadaraju, Mater. Sci. Eng. 65 (1984) L5–L8. [18] G. Engel, W.E. Klee, J. Solid State Chem. 5 (1972) 28–34. [19] J.G. Eon, C.B. Boechat, A.M. Rossi, J. Terra, D.E. Ellis, Phys. Chem. Chem. Phys. 8 (2006) 1845–1851. [20] S.A.T. Redfern, S.E. Smith, E.R. Maddrell, Mineral. Mag. 76 (2012) 997–1003. [21] T. Shimohira, S. Iwai, H. Takagi, J. Ceram. Assoc. Jpn. 75 (1967) 352–358. [22] M. Kikuchi, A. Yamazaki, M. Akao, H. Aoki, Mineral. J. 18 (1996) 79–86. [23] C.J. Liao, F.H. Lin, K.S. Chen, J.S. Sun, Biomaterials 20 (1999) 1807–1813. [24] E.R. Maddrell, P.K. Abraitis, Mat. Res. Soc. Symp. Proc. 807 (2004) 261–266. [25] M. Zhang, E.R. Maddrell, P.K. Abraitis, E.K.H. Salje, Mater. Sci. Eng. B-Adv. Funct. Solid-State Mater. 137 (2007) 149–155.
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Synthesis of lead vanadate iodoapatite utilizing dry mechanochemical process Yasushi Suetsugu ⇑ Biomaterials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba-shi, Ibaraki 305-0044, Japan
h i g h l i g h t s Pb10(VO4)6I2 was successfully synthesized at low temperature in an open system. Leaching tests for Pb10(VO4)6I2 were performed using high-density sintered bodies. Substitution of VO4 by PO4 in Pb10(VO4)6I2 affected the iodine retention properties.
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Article history: Received 3 March 2014 Accepted 30 July 2014 Available online 7 August 2014
a b s t r a c t For consolidation and subsequent deep geological repository of radioactive 129I, a novel synthesis method of iodine-bearing lead vanadate apatite Pb10(VO4)6I2 was developed. In order to reduce the reaction temperature, a dry mechanochemical process was applied and iodoapatite was successfully synthesized without heating. Calcining the as-synthesized low crystalline specimen at 300 °C gave a single-phase with high crystallinity. The obtained powder was consolidated by hot-press sintering at 400 °C under uniaxial pressure of 90 MPa and submitted to leach tests by immersion into pure water at room temperature. The normalized leach rate of iodine on the first day was about 2.6 101 g m2 d1. After 56 days of immersion, the iodine leach rate was reduced to less than 1/30 of the initial rate in newly changed water. 3 Iodoapatite specimens in which part of VO3 4 was substituted with PO4 , Pb10(VO4)4.8(PO4)1.2I2, were also synthesized by the same method. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Radioactive 129I is generated as a fission product in the consumption process of nuclear fuel. Currently, some of it is discharged into the sea and some is captured by silver absorbent during the reprocessing of spent nuclear fuel. 129I has quite a long half-life of 15.7 million years and is considered to be one of the radioactive isotopes with the most impact on the biosphere. Eventually, depositing captured 129I in a geological repository becomes unavoidable. However, 129I has very low absorbability in ordinary artificial and natural barrier materials, making it difficult to isolate from the biosphere. Therefore, novel matrix materials that can immobilize 129I for a long period of time and control its release into the environment are required. For this purpose, synthetic lead vanadate iodoapatite, Pb10(VO4)6I2 [1], which contains iodine in its apatitic crystal structure, or its derivative, lead vanadate/phos-
⇑ Tel.: +81 298 860 4504. E-mail address: [email protected] http://dx.doi.org/10.1016/j.jnucmat.2014.07.073 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
phate iodoapatite Pb10(VO4)6x(PO4)xI2 [2], have been proposed as potential waste forms. A mineral with a composition of Pb10(VO4)6Cl2 that belongs to the apatite group is named vanadinite; therefore, it may be suitable to refer to Pb10(VO4)6I2 as ‘‘iodovanadinite’’, but the better known name apatite is used in this paper. For the synthesis of Pb10(VO4)6I2, two kinds of methods have already been proposed. In one, Pb10(VO4)6I2 is synthesized directly by the reaction of PbO, V2O5 and PbI2 [3], and in the other, Pb3(VO4)2 is first synthesized from PbO and V2O5, and then reacted with PbI2 to obtain Pb10(VO4)6I2 [1,4]. Both methods require high temperature for the reaction, 500–700 °C, in a closed system that requires special equipment to avoid the volatilization of iodine. Le Gallet et al. [5] reported the successful direct reaction-sintering of Pb10(VO4)4.8(PO4)1.2I2 utilizing a spark plasma sintering method. Stennett et al. [6] adopted a microwave heating technique to synthesize Pb10(VO4)6I2. Both of these took place in an open system. The author applied a dry mechanochemical activation technique to lower the reaction temperature for efficient synthesis of lead vanadate/(phosphate) iodoapatite. This paper reports the results of this synthesis along with sintering and leach tests using the obtained sintered samples.
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2. Experimental PbO, V2O5 and PbI2 powders (Wako Pure Chemical Industries) were used as starting materials. For the synthesis of PO3 4 containing Pb10(VO4)6x(PO4)xI2 samples, Pb3(PO4)2 (Kojundo Chemical Laboratory) was added. Expected reactions are described as follows.
9PbO þ 3V2 O5 þ PbI2 ! Pb10 ðVO4 Þ6 I2 7:2PbO þ 2:4V2 O5 þ 0:6Pb3 ðPO4 Þ2 þ PbI2 ! Pb10 ðVO4 Þ4:8 ðPO4 Þ1:2 I2 3 Here, the ratio of VO3 4 and PO4 in Pb10(VO4)6x(PO4)xI2 was set at 4:1 according to the previous reports [2,5,7]. Each starting powder was weighed in a stoichiometric ratio and mixed roughly in a mortar. The mixture was subjected to a mechanochemical activator (Mechano-MicrosÒ, Nara Machinary) [8] that had a drum-shaped vessel whose inner wall was lined with zirconia ceramics. In the vessel, four steel shafts (termed ‘‘sub-shafts’’ in the report of Nakayama et al. [8]) supported stacks of small toroidal disks made of zirconia. These orbited at high speed around the axis of the vessel such that the disks were centrifugally pressed against and rolling along the inner wall of the vessel. The reactant powder was ground between the disks and the wall and underwent compressive and shear stresses. For the present work, the sub-shafts revolved at 600 min1 in the vessel which was 105 mm diameter and 67 mm depth and also rotated slowly at 80 min1 coaxially and reversely to the sub-shafts’ revolution. At every 150 min, the sample powder was raked out and re-subjected to the activator to avoid non-uniformity of the reaction. The total activation time was set to 600 min. In order to avoid OH contamination, dry N2 gas was flowed into the vessel at a rate of 5 L h1 and the outflowing gas was introduced into ethanol. The ethanol remained colorless throughout the process indicating no I2 emission from the sample. The vessel was cooled by flowing water from outside to keep the temperature of the sample under 40 °C throughout the process. To complete the reaction and crystallization, the samples after mechanochemical grinding were placed in a platinum crucible and calcined at 300 °C for 1 h in vacuum or in air. Vacuum calcination was performed utilizing a hot press chamber (Nems NP-12-G, the same machine used for sintering of the obtained iodoapatites as mentioned later) without using the pressing function. Calcination in air was carried out using a Yamato Scientific FP31 muffle furnace. The temperatures of the samples were checked by thermocouples contacting the crucibles. At certain stages of the synthetic process, small portions of the samples were subjected to powder X-ray diffraction (XRD) studies, Fourier-transfer infrared (FT-IR) absorption analyses, thermogravimetric and differential thermal analyses (TG–DTA), temperatureprogrammed evolved gas analyses (EGA), scanning electron microscope (SEM) observations, energy dispersive X-ray spectroscopy (EDS) analyses, and specific surface area measurements. XRD measurements were carried out at room temperature using a Rigaku RINT-Ultima III, by h–2h scanning (in 0.02° steps) using Cu Ka radiation. FT-IR spectra were measured with a PerkinElmer Spectrum 2000 system. The sample was diluted ten-fold with KBr powder, and data were collected with the diffuse-reflection method. TG–DTA was performed by a Rigaku Thermo Plus 2, with a heating rate of 20 K min1, sample quantity of 100 mg, and atmosphere gas flow rate of 300 mL min1. EGA was carried out by a Rigaku TPD type R Photo and a Rigaku Thermo Mass Photo. The heating rate, sample quantity and gas flow rate were set identical to the TG–DTA measurements. SEM images were observed by a JEOL JSM-5600 with an electron acceleration voltage of 20 kV. EDS analyses were performed with a JEOL JED-2200 system (X-ray takeoff angle 30°). Pressed compact pellets of each starting material reagent were used as standard samples for quantitative analysis and acquired raw X-ray intensity data were compensated
by methods of Springer [9], Bishop [10] and Philibert [11]. Specific surface areas were measured by N2 absorption using the BET method with a BEL Japan BELSORP-mini II. The synthesized iodoapatite powder was packed into a graphite cylindrical die with an inner diameter of 20 mm plugged by two graphite stubs at both ends. This assembly was placed in the hot press machine (the above mentioned Nems NP-12-G). The instrument was equipped with an electric resistance graphite heater surrounding the sample assembly which was uniaxially pressed by a hydraulically operated graphite piston rod. The compressive load and moving distance of the piston rod were measured by a load cell and plunger gauge, respectively, and the temperature of the sample was monitored by a thermocouple inserted into a small hole drilled in the sample die. The sample was pressed at 90 MPa and heated to 400 °C at 20 K min1 with a 10 min plateau at 400 °C. The load was released slowly to zero over 10 min without changing the temperature so that the annealing effect would prevent residual stress in the sample. The obtained sintered bodies of iodoapatite were immersed into water for the leach tests according to MCC-1P Static Leach Test Methods of Pacific Northwest Laboratory [12]. In a glove box filled with deoxygenated Ar gas simulating an underground reductive environment, each discus-shaped sample 20 mm in diameter and 4 mm in thickness was hung with nylon thread in a polypropylene bottle filled with vacuum-degassed pure water at room temperature. The ratio of sample surface area/liquid volume (S/V) was regulated to be 0.01 cm1. Here, the sample surface areas were calculated from the geometric dimensions and hence the liquid volumes were around 880 mL. The pH of leachants remained near neutral throughout the total testing duration time, 56 days. At specified time intervals, a small portion of the leachant was sampled and the iodine in it was quantified by an inductively coupled plasma-mass spectrometer (ICP-MS, Perkin Elmer ELAN DRC II) and the concentrations of lead and vanadium were measured by an inductively coupled plasma-atomic emission spectrometer (ICPAES, Perkin Elmer OPTIMA 3000 XL).
3. Results and discussion 3.1. Synthesis of Pb10(VO4)6I2 Fig. 1 shows the changes in XRD patterns for a mixture of PbO, V2O5 and PbI2 measured at different duration times of
Fig. 1. Changes of XRD patterns for mixture of PbO, V2O5 and PbI2 powders with increase of mechanochemical grinding time. X-ray source: Cu Ka. Grinding time: (a) 0, (b) 150, (c) 300, (d) 450, and (e) 600 (min). Open inverted triangles: a-PbO; filled inverted triangles: b-PbO; filled circles: V2O5; filled squares: PbI2; open diamonds: Pb10(VO4)6I2.
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mechanochemical grinding. The pattern (a), for the mixture without grinding, indicates a superposition of diffraction patterns for each pure starting material. As the grinding time increases, the height of each diffraction peak decreases. The peak for PbI2 reduces remarkably due to the inherent vulnerability of its layered crystal structure against shear stress in accordance with the report of Clark and Kern [13]. PbO transformed from a yellow colored orthorhombic b-phase to a red colored tetragonal a-phase, whose diffraction peaks also decreased with additional grinding time. Commercially available PbO was mostly in the b-phase though this was not the stable phase at room temperature, and mechanochemical grinding converted it to the stable a-PbO. The peaks ascribed to a-PbO are somewhat broad. This is consistent with the observation by Clark and Rowan [14] that a-PbO synthesized from the b-phase by grinding was structurally distorted and therefore had high chemical activity. The most important aspect illustrated in Fig. 1 is that diffraction peaks indicating the existence of iodoapatite phase appear. The peaks intensified as the grinding time became longer. The peaks are very broad, which is indicative of low crystallinity. It is plausible that high reactivity of mechanochemically ground starting materials contributed to the formation of iodoapatite at room temperature. After 600-min grinding [Fig. 1(e)], almost all the peaks observed were those ascribed to the apatite phase except for trace peaks at 20.3° derived from 0 0 1 diffraction of V2O5 (ICDD 00-0411426), at 30.3° for 0 2 0 diffraction of b-PbO (ICDD 00-038-1477) and at 31.9° for 1 1 0 diffraction of a-PbO (ICDD 01-085-1288). The existence of Pb3(VO4)2, while strongly suspected, was not detected by the present XRD measurements. The color of the unground starting mixture was orange–brown and unchanged significantly by mechanochemical grinding. The specific surface areas before and after the grinding were 1.2 m2 g1 and 2.6 m2 g1, respectively. After calcination at 300 °C in vacuum for 1 h, the color of the powder changed to grayish-green yellow and the specific surface area turned to 2.1 m2 g1. Fig. 2(a) shows an XRD pattern for the sample calcined at 300 °C in vacuum after 600-min grinding. The diffraction pattern is very similar to the data of Pb9.85(VO4)6I1.70 reported by Audubert et al. [15] (ICDD 00-051-0119) and the peaks for residual unreacted starting materials such as a- and b-PbO or V2O5 completely disappeared. The unit cell parameters calculated from the 2h locations of diffraction peaks were a = 1.045(1) nm and c = 0.748(1) nm, which were closer to the values for Pb10 (VO4)6I2 by Stennett et al. [6], a = 1.04429(3) nm and c = 0.74865(2) nm, rather than those for Pb9.85(VO4)6I1.70 by Audubert et al. [15], a = 1.0422(5) nm and c = 0.7467(3) nm. Stennett et al. [6] synthesized stoichiometric Pb10(VO4)6I2 as a mixture
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with Pb3(VO4)2 from starting materials including excess amount of PbO and V2O5 as a buffer against iodine substoichiometry reported by Audubert et al. [1,15]. The present results indicate that the mechanochemical method could yield Pb10(VO4)6I2 as a single phase from stoichiometric mixture of the starting materials. Fig. 3(a) and (b) show TG–DTA curves and EGA of I2 (mass-tocharge ratio, m/z 254) for the unground mixture of starting powders and the 600-min mechanochemically ground sample, respectively. The measurements were performed under He gas flow. In the temperature range from room-temperature to 300 °C, about 1% weight loss was observed for both samples. EGA measurement for m/z 18 and 44 (not shown in the figure) revealed that this loss was attributed chiefly to desorption of H2O (100–200 °C) and CO2 (200–300 °C). At temperatures higher than 300 °C, the behaviors of the two samples differed significantly. In the TG curve for the unground sample [Fig. 3(a)], weight loss occurs at wide temperature range of 300–800 °C. The loss at 300–700 °C is attributed mainly to I2 emission as indicated in the EGA curve with prominent two peaks. Main reactions expected to occur at this temperature range are, a reaction between PbO and PbI2 to form lead oxyiodides [16,17], and another reaction between PbO and V2O5 to form lead vanadates, and a subsequent reaction of lead oxyiodides and lead vanadates which yields an apatite phase. In the course of this sequence of reactions, part of iodine volatilizes in the form of I2 by some side reactions without incorporation into the apatite phase. Formation of an apatite phase is considered to take place at least above 450 °C, but Le Gallet et al. [5] observed apatite formation from Pb3(VO4)1.6(PO4)0.4 and PbI2 at 300 °C. This comparatively low synthesis temperature might be achieved perhaps by a mechanochemical effect due to mechanical milling they applied on the starting materials before the heating process. The present EGA measurements did not detect any signals large enough to explain the weight loss at 700–800 °C. This loss was presumably caused by PbI2 emitted accompanying decomposition of the apatite phase. The reason for undetectability of PbI2 by the present EGA measurements was probably because PbI2 vapor pressure was so low that it condensed and deposited somewhere along the path to the detector. Supposing that the total weight loss excluding desorption of H2O and CO2, namely 11.0%, was comprised mostly of I2 and PbI2 emission, the ratio of iodine lost as I2 to the total iodine content is calculated to be about 63%. On the other hand, for the sample that underwent mechanochemical grinding [Fig. 3(b)], the TG curve shows simpler profile similar to that of Pb10(VO4)6I2 reported by Uno et al. [3] or Stennett et al. [6]. Since no corresponding I2 emission is observed in the EGA curve, the remarkable weight losses observed in two stages at 500–650 °C and 650–780 °C are considered to be PbI2 emission like the one that appears at 700–800 °C in Fig. 3(a), during which a decomposition reaction Pb10(VO4)6I2 ? 3Pb3(VO4)2 + PbI2" is quite likely to take place. In this case the total amount of weight loss excluding desorption of H2O and CO2 is 15.0%, which corresponds well with the theoretical weight ratio of PbI2/Pb10(VO4)6I2 (15.3%). In the DTA curve, a large endothermic peak can be observed at 960 °C, which is the melting point of Pb3(VO4)2. Heat emission attributed to formation reaction or improvement in crystallinity of Pb10(VO4)6I2 is observable at around 300 °C, though this is not very clear. 3.2. Iodine evacuation by heating in air atmosphere
Fig. 2. XRD patterns for iodoapatites synthesized by mechanochemical grinding and subsequent calcination at 300 °C. (a) Calcined in vacuum, Pb10(VO4)6I2. (b) Calcined in air, Pb10(VO4)6I1.3(OH)0.7. (c) Calcined in vacuum, Pb10(VO4)4.8(PO4)1.2I2.
The color of the iodoapatite sample powder calcined in air instead of vacuum at 300 °C for 1 h after 600-min mechanochemical grinding was light yellow, which was different from the grayish-green yellow color of the sample calcined in vacuum. This suggested a difference in chemical composition between the two samples.
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Fig. 3. Curves of DTA, TG and EGA of I2 (m/z 254) for: (a) mixture of PbO, V2O5 and PbI2 without grinding, measured in He atmosphere; (b) mechanochemically synthesized Pb10(VO4)6I2, measured in He; (c) mechanochemically synthesized Pb10(VO4)6I2, measured in simulated air (80% He + 20% O2); and (d) mechanochemically synthesized Pb10(VO4)4.8(PO4)1.2I2, measured in simulated air. (a and b): measured by Rigaku TPD type R Photo (c and d): measured by Rigaku Thermo Mass Photo.
Fig. 4 shows FT-IR spectra for the mechanochemically ground mixtures calcined at 300 °C in vacuum (a) and in air (b), respectively. In spectrum (a), almost no peaks are observed. On the other hand, in spectrum (b), an absorption peak at 3535 cm1 is found that is attributed to the stretching vibration of OH [18], which replaced some of the I sites. A shoulder peak at 3555 cm1 is also presumed to be from OH, but no literature mentioning this peak has been found yet. Fig. 2(b) shows the XRD pattern for the mechanochemically ground sample calcined in air at 300 °C for 1 h [the same sample as in Fig. 4(b)]. Compared to the XRD pattern for the sample
Fig. 4. FT-IR spectra for iodoapatites synthesized by mechanochemical grinding and subsequent calcination at 300 °C. (a) Calcined in vacuum, Pb10(VO4)6I2. (b) Calcined in air, Pb10(VO4)6I1.3(OH)0.7.
calcined in vacuum [Fig. 2(a)], it is noted that many peaks are shifted to higher 2h angles, except, for example, 0 0 2 and 0 0 4 peaks, which show almost no change at 23.8 and 48.7°, respectively. These results indicate that Fig. 2(b) shows an intermediate pattern between Pb10(VO4)6I2 {a = 1.04429(3) nm and c = 0.74865(2) nm [6]} and Pb10(VO4)6(OH)2 {hydroxyvanadinite, a = 1.02242(3) nm and c = 0.74537(2) nm [19]}. Therefore, given the existence of a solid-solution between these two end-members, namely Pb10(VO4)6I2y(OH)y, Fig. 2(b) corresponds well with the FT-IR result [Fig. 4(b)]. The diffraction data of Fig. 2(b) give cell parameters a = 1.036(1) nm and c = 0.749(1) nm, which leads to a substitutional parameter y = 0.7 ± 0.1, providing Vegard’s Law is applicable in this solid-solution system. The resultant composition Pb10(VO4)6I1.3(OH)0.7 was supported by relative elemental concentration data acquired by an EDS analysis of the same sample (Pb:V:I = 10.09 ± 0.24:6.00:1.33 ± 0.27 in atomic ratio). In Fig. 2(b), small peaks at 28.2° and at 31.1° which are not ascribed to apatite phase are also observed. Identifications of these peaks are to be mentioned at the last part of this subsection. Fig. 3(c) shows results of TG, DTA and EGA for the mechanochemically ground sample measured under a flow of simulated air (mixture of 80% He and 20% O2). Compared to the results measured in pure He gas flow [Fig. 3(b)], the major weight loss occurs at lower temperatures ranging from 330 °C to 570 °C, and the corresponding I2 emission is apparent in the EGA curve. Redfern et al. [20] reported that Pb10(VO4)6I2 broke down into Pb3(VO4)2 emitting iodine upon heating in air in the temperature range of 267– 407 °C. The difference in the temperatures from those in Fig. 3(c) is probably caused by the difference in experimental conditions. Since the weight loss shown in Fig. 3(c) is 8.0% whereas that in Fig. 3(b) is 15.0%, substitution of I2 M O likely occurred in this case considering the formula weight ratio (I2–O)/Pb10(VO4)6I2 = 7.9%.
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DTA behavior at 800–960 °C with endothermic peaks at 812 and 954 °C agrees well with the incongruent melting of 10PbO3V2O5 composition [corresponding to Pb10(VO4)6I2 – I2 + O] decomposing into Pb3(VO4)2 + liquid in the phase diagram made by Shimohira et al. [21]. From these results, it was considered plausible that some iodine was replaced by oxygen when the mechanochemically ground mixture was calcined in air at 300 °C. In order to confirm whether OH could also directly replace I, simulated air humidified with H2O vapor of partial pressure 2 kPa at room temperature was used as atmosphere gas for TG, DTA and EGA measurements. The results showed no meaningful difference in comparison with the results obtained using dry simulated air without H2O. This implies that the exchange of I M OH by heating in air did not directly take place through hypothetical reactions such as Pb10(VO4)6I2 + yH2O ? Pb10(VO4)6I2y(OH)y + yHI or Pb10(VO4)6I2 + y/4O2 + y/2H2O ? Pb10(VO4)6I2y(OH)y + y/2I2, but partial exchange of I2 M O occurred initially and H2O molecules were subsequently induced at lower temperature to form Pb10(VO4)6I2y(OH)y. In the case of calcium phosphate apatite, Ca10(PO4)6O (oxyapatite) can actually exist at least as a solid-solution with Ca10(PO4)6 (OH)2 (hydroxyapatite) at high temperature. Under ordinary wet atmosphere at room temperature, H2O molecules are rapidly introduced into O2 sites to form complete Ca10(PO4)6(OH)2 [22,23]. We can imagine that a similar mechanism turns Pb10(VO4)6O (‘‘oxyvanadinite’’) into Pb10(VO4)6(OH)2 by absorbing H2O from air, but the existence of neither Pb10(VO4)6O nor even solid-solution Pb10(VO4)6(OH)22zOz has been confirmed in any reports yet. However, according to Redfern et al. [20], upon heating in air, Pb10(VO4)6I2 decreases its unit cell parameter a in two stages associated with the decomposition; the first one begins prior to the onset of breakdown and the other is in almost the same temperature range as the breakdown occurs. This a-axis contraction in either or both of these temperature stages might indicate the possibility of the existence of solid-solution Pb10(VO4)6(I2yOy/2) with limited y. This type of composition was mentioned by Audubert et al. [1]. According to Shimohira et al. [21], a binary system with the composition corresponding to ‘‘oxyvanadinite’’ (10PbO3V2O5) actually adopts the biphase of 2Pb3(VO4)2 + Pb4O(VO4)2 below the incongruent melting point (807 °C). Provided that the replacement of I2 by O in Pb10(VO4)6I2 beyond the acceptable limit of the apatitic frame structure leads to breakdown into 2Pb3(VO4)2 + Pb4O(VO4)2, the small peaks observed at 28.2° and at 31.1° in Fig. 2(b) can be identified as 6 2 0 and 0 4 0 diffractions, respectively, of Pb4O(VO4)2 (ICDD01-073-7788). Alternatively, attributions to 0 1 5 and 1 1 0 diffractions of c-Pb3(VO4)2 (ICDD 01-080-1957) are also possible, which phase is stable above 104 °C in phase diagrams but could be stabilized at room temperature. These results indicate that in order to prevent emission of iodine, the heating process, calcination and sintering should be carried out in an atmosphere excluding oxygen.
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sintering process. The relative density calculated from the dimensions of the sintered body was 97.8 ± 0.2% given the theoretical density of 7.082 g cm3 calculated from the crystallographic data reported by Stennett et al. [6]. Fig. 7 shows an SEM image of the fracture surface of the sintered Pb10(VO4)6I2. Fractures took place along grain boundaries. The diameter measured for circles with the same area of the grain images in Fig. 7 was 200 ± 40 nm. 3.4. Synthesis and sintering of Pb10(VO4)4.8(PO4)1.2I2 Synthesis of Pb10(VO4)4.8(PO4)1.2I2 was carried out by the same process except that Pb3(PO4)2 was added to the starting materials. The XRD pattern of the mixed powder calcinated at 300 °C in vacuum after 600-min mechanochemical grinding is shown in Fig. 2(c). Peak broadening is observed, likely due to low crystallinity or compositional fluctuation. Specific surface areas measured before and after the mechanochemical grinding, and after the subsequent calcination were 2.0, 2.6, and 2.3 m2 g1, respectively. The results of TG, DTA and EGA measurements under simulated air atmosphere are shown in Fig. 3(d). The profile of each curve is similar to that of Pb10(VO4)6I2 [Fig. 3(c)] except for some thermal shifts. In the DTA curve, shifts of the endothermic peaks to lower temperatures (812 ? 768 °C, 954 ? 937 °C) are ascribed to temperature depression of incongruent melting due to impurity effects. In the EGA curve, the higher one of two iodine emission peaks shifts its position by 40 K (520 ? 560 °C) though iodine emission starts at almost the same temperature (330 °C). Two shrinkage curves of Pb10(VO4)4.8(PO4)1.2I2 observed in the process of sintering are shown as curves (c) and (d) in Fig. 5, together with the cases of Pb10(VO4)6I2 [(a), (b)]. The shrinkage starts at 280 °C and finishes at 370 °C. Both temperatures are lower than those in the cases of Pb10(VO4)6I2. This difference seems to be intrinsic considering that the specific surface area measured for each type of iodoapatite did not differ significantly (2.1 m2 g1 for Pb10(VO4)6I2 and 2.3 m2 g1 for Pb10(VO4)4.8(PO4)1.2I2). 3.5. Leach test Fig. 8 shows temporal concentration changes of elements dissolved from sintered samples of Pb10(VO4)6I2 [Fig. 8(a)] and of Pb10(VO4)4.8(PO4)1.2I2 [Fig. 8(b)] separately immersed in pure water. Each graph presents results of two equal tests. Vanadium was not detected above the detection limit, 5.9 102 lmol L1 throughout the 56-day test. Lead appeared above the detection
3.3. Sintering of Pb10(VO4)6I2 Shrinkage curves of iodoapatites observed in the process of sintering under the uniaxial pressure of 90 MPa are shown in Fig. 5. The figure presents two results of Pb10(VO4)6I2 sintering as curves (a) and (b) for reproducibility test. The shrinkage starts at 320 °C and finishes at 390 °C for both (a) and (b). Fig. 6 shows the appearance of one of the sintered bodies. The color of the surface was metallic dark gray, distinctly different from the former grayishgreen yellow color before sintering. However the streak color of the sintered body was the same grayish-green yellow as the raw powder. By XRD and EDS measurements, no significant differences from the raw powder were detected indicating that the creation of impurities and/or changes in composition did not occur during the
Fig. 5. Shrinkage curves for iodoapatites in the process of hot press sintering under the uniaxial pressure of 90 MPa. Initial thickness (shrinkage = 0) was about 5.3 mm. (a), (b): Pb10(VO4)6I2. (c), (d): Pb10(VO4)4.8(PO4)1.2I2.
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Fig. 6. Sintered Pb10(VO4)6I2.
Fig. 7. SEM image of fracture surface of sintered Pb10(VO4)6I2.
limit, 1.3 101 lmol L1, at 28 and 56 days [Fig. 8(a)] and at 7– 56 days [Fig. 8(b)] after the immersion. The amount of leached iodine from Pb10(VO4)4.8(PO4)1.2I2 was 4–5 times as high as those from Pb10(VO4)6I2. Fig. 8 indicates that the amount of leached iodine is the largest among the three elements, which implies that iodine was not leached out by a simple congruent dissolution of bulk apatites. This is in agreement with the previous reports [2,7,24,25]. There are several possibilities for what occurred at the surface of iodoapatite immersed in water, e.g., OH replaced I to form a OH rich apatite layer [25], or iodine was leached out with lead and left a lead vanadate (or lead vanado-phosphate) layer on the surface [7]. A diffusion process of iodine through those layers of secondary products is supposed to control the leach rate [7]. For Pb10(VO4)6I2 samples, normalized iodine leach rates on the first day are calculated to be 2.7 101 and 2.6 101 g m2 d1. Those for Pb10(VO4)4.8(PO4)1.2I2 are 9.0 101 and 8.0 101 g m2 d1. Here, the reactive surface areas used for the calculation were values obtained from the geometric dimensions of the samples. In the literature of Maddrell and Abraitis [24], the normalized iodine leach rate for Pb10(VO4)6I2 immersed in pH 11 buffer solution at 90 °C is in the region 2–5 g m2 d1. Also, from the figures in the literatures of Audubert and Lartigue [2] and Guy et al. [7], normalized iodine leach rate at the earliest stage of dissolution of Pb10(VO4)4.8(PO4)1.2I2 immersed in 90 °C pure water is read to be 0.2 g m2 d1. For comparison, these data are converted to the values expected if acquired under the same experimental conditions as those for the present work (25 °C and neutral pH) using apparent activation energy of 37 kJ mol1 and the effect of pH difference presented by Guy et al. [7]. The calculated values are in the region
Fig. 8. Concentration changes of iodine (circles) and lead (triangles) in leachant in which a sintered iodoapatite was immersed. Sample surface area/liquid volume (S/ V) = 0.01 cm1. Each graph, (a) Pb10(VO4)6I2 and (b) Pb10(VO4)4.8(PO4)1.2I2, shows results of two equal tests. The data sets of iodine and lead marked in the same color were obtained from the same test.
1.4 102–3.6 102 g m2 d1, which are 1/10–1/60 of those acquired from Fig. 8. One possible explanation for this discrepancy is as follows. At least at the early stage of leach tests in the previous works [2,7,24], if Pb2+ had dissolved with I to keep charge balance, the leachants would have been instantly saturated with respect to Pb(OH)2, which is found by means of equilibrium calculations taking S/V ratios and OH concentrations at 90 °C into account, hence Pb2+ must have remained in the solid phase. According to the results by Zhang et al. [25], a most likely way for the iodine leaching to take in these cases was substitution by OH rather than coleaching with Pb2+ accompanied by Pb(OH)2 reprecipitation, while in the case of present work, co-leaching with Pb2+ was probably dominant because the amount of leachant was abundant for Pb2+ dissolution. Now suppose that the OH rich apatite layer formed by I M OH exchange served as a stronger protective layer against ongoing leaching of iodine than lead vanadate layers formed by simultaneous dissolution of lead and iodine, the higher leach rate would be observed by the present leach test. The leached samples retrieved after 56 days of immersion were again immersed separately into newly changed pure water. Iodine leach rates of Pb10(VO4)6I2 samples on the first day were decreased to 8.8 103 g m2 d1 and 7.1 103 g m2 d1. Those of Pb10(VO4)4.8(PO4)1.2I2 were 5.0 102 and 3.3 102 g m2 d1. These data are 1/20–1/40 of the values for the previous tests but still much higher than expected values (1.7 104 g m2 d1) calculated from the steady state leach rate at 90 °C (2.4 103 g m2 d1) provided by Guy et al. [7], suggesting that the
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experimental condition applied for the present leach test was not favorable for the formation of efficient protecting layers. 4. Conclusions The present work showed that Pb10(VO4)6I2 could be synthesized in an open system by mechanochemical grinding of a powder mixture of PbO, V2O5 and PbI2. It was confirmed that without the mechanochemical process, a closed experimental system was indispensable to prevent volatilization loss of iodine. The application of the mechanochemical process also enabled incorporation of PO3 partly substituting VO3 in iodoapatite. 4 4 3 PO4 incorporation changed the iodine emission temperature but not far enough to definitely improve thermal durability of iodine immobilization. PO3 4 including iodoapatite also exhibited a lower sintering temperature and higher iodine leaching property in comparison with Pb10(VO4)6I2. One of the noteworthy features of iodoapatite may be an exchangeability of I by OH. It was found likely that, in air, OH replaces I in apatite structure through a temporary incorporation of O2 at high temperature. In water, OH rich apatite layer formed by I M OH exchange was suspected to play a quite important role for iodine immobilization having strong effect on the leaching property of iodoapatite. Acknowledgements The author is deeply grateful to the staff at Rigaku Corporation, especially to Dr. Kazuko Motomura, for generous support of TG, DTA and EGA measurements. The author also expresses his gratitude to Dr. Masanori Kikuch at National Institute for Materials Science for insightful suggestions.
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