6.3. Results and discussion
6.3.3 Fine structure analysis and reduction mechanism
Chapter 6
144
To elaborate further explanations on the calcination processes of SeO42–ettringite, the thermogravimetric analysis (TG), differential thermogravimetric analysis (DTG), and differential thermal analysis (DTA) curves are shown in Fig. 6.4. The SeO42–ettringite dehydrated in two stages (Fig. 6.4), where the first stage occurs between 25 and 130 °C, with a total mass loss of approximately ~33-34% corresponding to a loss of freely associated H2O.
This is consistent with one endothermic peak that appeared at approximately 100 °C in DTG curve. After the first stage, SeO42–ettringite shows a progressive weight loss (~15%) until the 900 °C, this is caused by the dehydroxylation process. The pure SeO42–ettringite exhibited similar characteristics to pure sulfate ettringite until the temperature reached 940 °C, as previously reported [36]. However, when the temperature is higher than 940 °C, selenium began to evaporate; leading to about a 20% mass loss. The exothermic peak was observed at approximately 600 °C corresponding to the crystallization of the amorphous mixture. This result was coincident with the PXRD patterns (Fig. 6.1) in which some small new peaks were detected after calcination at 600 °C.
Chapter 6
145
Se(IV) from Se(VI), showing a shift of ∼3 eV between the dotted lines of the Se(IV) and Se(VI) species in Fig. 6.6. The result suggests that the Se(VI) oxidation states were dominantly preserved under the temperature lower than 600 °C. However, when the temperature increased, the Se(VI) was gradually reduced into Se(IV). When the calcination temperature reached at 800 °C, Se species completely changed into Se(IV). This result is consistent with the FTIR spectra (Fig. 6.2(b)).
12600 12620 12640 12660 12680 12700 200 oC
400 oC 600 oC 800 oC 900 oC
NaSeO
4
Normalized absorption
E (eV) NaSeO3
Fig. 6.6 Se K-edge XANES spectra of calcinated selenate ettringite under various temperatures and the related standards (NaSeO3 and NaSeO4).
EXAFS spectroscopy was carried out to gain a deeper insight into the structure of calcined products, and the results of Se K-edge raw spectra and fitted k3-weighted χ functions for calcined solids are shown in Fig. 6.7. For the Fourier transform in uncalcined and calcined samples at 200 °C, the first shell of Se atom is dominated by the backscattering from O atoms,
Chapter 6
146
highlighted by the vertical dashed line at 1.2 Å (not corrected for phase shift). In addition, there are no further peaks above the noise level in the Fourier transform spectra of pristine and calcined samples at 200 °C. Because these spectra showed no further peaks at a longer distance than 2 Å, the structural information obtained from EXAFS analysis is limited to the first coordination shell. This suggests that SeO42− is bound by outer-sphere complexation in the uncalcined and calcined SeO42–ettringite at 200 °C. This finding is consistent with the previous reports [9, 45].This could be explained by the absence of a direct bonding to neighboring atoms of Se. However, when the calcination temperature increased to 800 °C, a new phase was formed (Fig. 6.1) involving the reduction of Se(VI) to Se(IV) (Fig. 6.6). The k3-weighted Se K-edge EXAFS for calcined product at 800 °C is dominated by the backscattering from an oxygen shell approximately 1.3 Å (not corrected for phase shift).
Specifically, additional peaks at approximately 3 Å were also observed in k3-weighted EXAFS for calcined product at 800 °C, indicating there are some atoms with direct bonding to neighbors of Se to form inner-sphere complexation in the newly formed phase. These peaks were not observed in the calcined product at 200 °C and uncalcined SeO42–ettringite.
Chapter 6
147
0 1 2 3 4 5 6
pure SeO
2-4-ettringite 200 oC 800 oC Se(IV)-Al
Se(IV)-O
(a)
Fourier transform amplitude
R(Å) Se(VI)-O
4 6 8 10 12 14 16
200 oC 800 oC
pure SeO
2-4-ettringite k3 (k)
k (Å-1)
Fig. 6.7 (a) k3-weighted Se K-edge EXAFS spectra of SeO32– and SeO42– doped ettringite and reference compounds. (b) Corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).
Table 6.1. Se K-edge EXAFS fitting results of uncalcined, 200 °C calcined, and (c) 800 °C calcined selenate bearing ettringite. Coordination number (CN), interatomic distance (R), Debye-Waller factor (ơ2), and residual factor (Rf).
Sample Shell CN R(Å) ơ2 Rf(%)
Pure sulfate ettringite
Se–O 3.76 1.64 0.002 1.67
200 °C
calcined
Se–O 3.72 1.64 0.002 1.67
800 °C
calcined
Se–O 2.97 1.69 0.002 1.61
Se–Al Se–Ca
0.99 0.99
3.30 3.68
0.006 0.007
–
Rf residual factor indicates the quality of fitting results and is expressed by following formula:
3 3 2
1 exp 3 2 1 exp
100
n i theo
n i
k k
Rf
k
The fitting results of EXFAS for uncalcined and calcined SeO42–ettringite are summarized in Table 6.1. The first shell of pure SeO42–ettringite can be fitted with 4 O atoms at 1.64Å, which is characteristic of Se in tetrahedral coordination. For the calcined product of SeO42–
ettringite at 200 °C, the first shell of Se exhibits similar characteristic to pure SeO42– ettringite.
Chapter 6
148
Because the Fourier transforms spectra showed no further peaks at a longer distance than 2 Å, the structural information obtained from EXAFS analysis is limited to the first coordination shell. In contrast, unlike pure SeO42–ettringite, the calcined product at 800 °C exhibits 3 O atoms of coordination for Se atom, which is the characteristic of Se in pyramidal coordination, indicating the SeO42–was reduced to SeO32– in ettringite under calcination. Furthermore, the second shell of Se(IV) is attributed to the backscattering from Al at 3.4 Å and Ca at 3.7 Å.
0 1 2 3 4 5 6
pure SeO2-4-ettringite 800 oC
200 oC Ca-Al
Fourier transform amplitude
R(Å) Ca-O
2 4 6 8 10
800 oC
200 oC
pure SeO2-4-ettringite
k3 (k)
k (Å-1)
Fig. 6.8 (a) k3-weighted Ca K-edge EXAFS spectra of SeO32– and SeO42– doped ettringite and a reference compound. (b) Corresponding Fourier transforms (not corrected for phase shift) showing both raw (solid lines) and fitted data (dash lines).
In terms of Ca coordination environment, the Ca K-edge EXAFS spectra of the untreated and calcined SeO42–ettringite are shown in Fig. 6.8 and the analytical results of EXFAS fitting are shown in Table 6.2. The Fourier transform is dominated by the first shell of backscattering from oxygen in all. For the pure SeO42–ettringite, each Ca atom is coordinated by 8 O atoms at approximately 2.4 Å. In addition, approximately 2 Al shell was fitted at 3.46 Å. This is consistent with the previously reported ettringite and the atomic distances of Ca–O
Chapter 6
149
and Ca–Al [9, 23]. Furthermore, when the calcination temperature increased to 200 °C, the coordination number of Ca center decreased to 5 and atomic of Ca–O is reduced to 2.4 Å (Fig.
6.8, Table. 6.2).
Table 6.2 Ca K-edge EXAFS fitting results of uncalcined, 200 °C calcined, and (c) 800 °C calcined selenate bearing ettringite. Coordination number (CN), interatomic distance (R), Debye-Waller factor (ơ2), and residual factor (Rf).
Sample Shell CN R(Å) ơ2 Rf(%)
Pure sulfate ettringite
Ca–OH/O 7.72 2.41 0.005 1.11
Ca–Al 1.93 3.46 0.004 –
200 °C
calcined
Ca–OH/O 4.55 2.38 0.006 1.55
Ca–Al 1.82 3.83 0.012 –
800 °C
calcined
Ca–OH/O 4.23 2.37 0.007 1.15
Ca–Al 4.23 3.59 0.011 –
Rf is the same as in Table 6.1.
This change is assigned to the dehydration of coordinated H2O molecules linked to Ca.
However, the second shell of Ca contains Al atoms at the distance of 3.4 Å. For the calcined SeO42–ettringite at 800 °C, the first shell of Ca is proved to be 4 O atoms. However, the second shell of Ca is increased to 4 Al atoms. These give some new information for the structure of calcined SeO42– ettringite at 800 °C calcination.
Chapter 6
150
160 140 120 100 80 60 40 20 0 -20
(c) 800 oC
(b) 600 oC
[4]Al
[4]Al
[4]Al
[4]Al
[5]Al
Abundance/ a.u.
Chemical shift/ ppm
[6]Al
(a) 200 oC
Fig. 6.9 27Al NMR spectra for the calcined solid residues of selenate bearing ettringite at (a) 200 °C, (b) 600 °C calcined, and (c) 800 °C.
As discussed above, the immobilized SeO42–in ettringite was reduced to SeO32– with the ettringite phase transformation after calcination temperature higher than 800 °C. Therefore, the reduction mechanism is discussed by combination with phase transformation via calcination. Based on the structural data of ettringite, 26 H2O molecules and 12 OH groups exist in one crystal unit [23]. Until 800 °C, only the water molecules were removed from ettringite structure. This might be the main factor for reduction of SeO42–. Furthermore, according to the XANES and FTIR results, the SeO42– reduction started at 600 °C and also a new phase was observed in PXRD pattern under this stage coincidence with the dehydroxylation process.
Chapter 6
151
In order to confirm these assumptions and to understand the Al coordination in calcined products, the 27Al-NMR was effectuated and the resulting spectra are presented in Fig. 6.9.
The principal chemical shift in calcined product (200°C) of SeO42–ettringite is at 10.05
ppm assigned to hexa-coordinated Al-O6 to OH groups. Moreover, the penta-coordinated Al-O5 and tetra-coordinated Al-O4 signals also appeared in 200 °C calcined product at 41.96 and 74.75 ppm, respectively. This can be interpreted by the dehydroxylation of Al(OH)63–resulting to the production of some Al-O4 and Al-O5. When the temperature increased to 600 °C, the Al-O6 signal became negligibly small and Al-O5 signal completely disappeared. It worth noting that only the tetrahedral Al-O4 was observed in 800 °C calcined products, implying the octahedral Al-O6 gradually convert to tetrahedral Al-O4 via dehydration process. Specifically, the peak which is assigned to Al-O4 was separated into two peaks at 80.31 and 63.03 ppm, respectively. The splitting might be due to the effect of the second coordination sphere (Ca and Se) or to the effect of an electric field gradient at the quadrupolarAl atom [50].
Chapter 6
152
Fig. 6.10 Schematic illustrations for the reductive phase transformation mechanism of selenate-bearing ettringite via calcination at higher than 800 °C.
Chapter 6
153
The 27Al-NMR and Ca K-edge EXAFS results proved that the coordination number of Al and Ca in ettringite gradually decreased with increasing the calcination temperature. The dehydration process is consistent with the reduction process of SeO42–.
It is reported that some metal hydroxides could form electron donor sites by calcination [46-49]. During the dehydration process, the removal of OH will cause the defection of the metal ion coordination. Defected coordination site of metal ions exhibits high activity and will cause two or more oxide ions gathered around the metal ions and occupied the defected sites, which could form potential electron donor sites [46]. Ettringite is a prototypical compound of the hydroxy-sulfates and the main structure is constituted with Ca-OH-Al frameworks. In addition, the columnar structure of ettringite ({Ca6[Al(OH)6]2･ 24H2O]}6+) exhibits less stability than pure metal hydroxide and easy to be dehydroxylation.Thus, mixed metal hydroxide frameworks of this mineral are easier toform the electron donor sites by calcination.It is already reported that dehydrated ettringite exhibits similar structure to despujolsite that the intercolumnar ions attached to columnar structure [32]. Therefore, when the electron donor is formed during calcination, reduction of Se(VI) is possible. In addition, the numbers of electron donor sites could increase with lower calcination temperatures, since the amount of reduced SeO42– is more important at higher temperature (Fig. 6.6).This is also consistent with the decrease in coordination number of Ca and Al which is due to dehydration process of ettringite (Fig. 6.9 and Table 6.2). Based on the above descriptions, the reduction
Chapter 6
154
mechanism of SeO42– in ettringite by calcination was proposed and schematically illustrated in Fig. 6.10.
