4.3. Results and discussion
4.3.1. Sorption mechanism of iodide/iodate in ettringite
After equilibrium of co-precipitation, the water chemistry of solution and XRD patterns of solid residues were analyzed. Based on the XRD patterns, the solid products after precipitations were identified as ettringite. The sorption data can be interpreted by Freundlich adsorption isotherms (Fig. 4.1) and the corresponding water chemistry data and the compositions of IO3– and I– doped ettringite depending on the initial IO3– and I– concentrations are shown in Table 4.1.
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88
Table 4.1 Compositions of Iodine-doped ettringite as a function of initial I– and IO3– concentrations.
I initial concentration
(mM)
I equilibrium concentration
(mM)
Ca mmol/g Al mmol/g S mmol/g
I–
0.116 0.114±0.001 7.23±0.04 2.40±0.05 3.54±0.05
0.624 0.617±0.005 7.23±0.03 2.41±0.01 3.57±0.04
1.051 1.039±0.012 7.23±0.16 2.40±0.04 3.56±0.04
6.127 6.118±0.061 7.23±0.19 2.40±0.12 3.59±0.03
10.123 10.118±0.013 7.23±0.04 2.41±0.18 3.58±0.12
15.221 15.203±0.011 7.23±0.09 2.40±0.03 3.58±0.09
IO3–
0.103 – 7.23±0.03 2.45±0.07 3.56±0.13
0.612 0.003±0.004 7.23±0.11 2.42±0.15 3.54±0.01
1.047 0.005±0.004 7.23±0.07 2.43±0.05 3.59±0.10
6.113 0.005±0.017 7.24±0.09 2.41±0.08 3.46±0.05
10.021 0.014±0.024 7.29±0.02 2.42±0.04 3.05±0.18
14.956 0.108±0.024 7.31±0.12 2.42±0.16 2.80±0.11
Based on the water chemistry, IO3–
and I– bearing ettringite yielded a Ca:Al:(SO4+IO3/I) ratio of 5.8:1.9:2.9, which is in good agreement with the theoretical ratio of 6:2:3 in pure ettringite. However, it is contrasting between IO3– and I– in the immobilization preference of co-precipitation with ettringite. In terms of I– immobilization, negligible trance amount of I– was reduced, even though the initial concentrations of I– increased to ca. 15 mM (Fig. 4.1(a) and Table 4.1). I– sorption results indicate that I– ions exhibited lower affinity to ettringite and were difficult to incorporate in the intercolumn to substitute with SO42– ions. This might be caused by the different properties between I– and SO42– in ionic radius or electrostatics force, because I– is monovalent anion and exhibit less electronegativity. In comparison to I– sorption,
Chapter 4
89 IO3–
exhibits high affinity to ettringite (Fig. 4.1(b) and Table 4.1). Approximately 96% IO3–
was removed from solution at various initial concentrations, even though IO3– is monovalent ions. This suggests that the ionic charge is not the main factor for selectivity of incorporation in ettringite. However, the fitted data to adsorption isotherms do not allow distinction between a pure adsorption and formation of a solid solution or complexation [10]. In order to investigate the I– and IO3– immobilization mechanisms by ettringite, the XRD patterns of solid residues are shown in Fig. 4.2. Only the SO42– ettringite can be identified (ICDD PDF # 041-1451) and no new phase was observed. This suggests that the reduced I– and IO3– ions were mainly incorporated in ettringite.
-1.0 -0.5 0.0 0.5 1.0 1.5
-2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4
r2=0.915 y=0.584x-2.194 log I- Qe mmol/g
log I- Ce (mmol/L) (a)
-3.0 -2.5 -2.0 -1.5 -1.0
-2.0 -1.5 -1.0 -0.5 0.0 0.5 (b)
log IO- 3 Qe mmol/g
log IO
-3 Ce (mmol/L) r2=0.989 y=0.948x+1.184
Fig. 4.1 Sorption isotherms of (a) Iodate and (b) Iodide on ettringite after equilibration. Lines represent linear regression of the data points.
However, the XRD patterns of ettringite exhibit significant difference after sorption of I– and IO3–. For I– sorption, SO42– ettringite was characterized and the XRD patterns, which do not change even though the 15 mM I– coprecipitation with ettringite. In general, the guest anions are able to accommodate in the intercolumn spaces of ettringite by substitution of
Chapter 4
90
SO42– [11-13, 4]. This will cause the change of intercolumn space by intercalating different guest anions. Thus, the lattice parameters can be an alternative way to understand the anions immobilization mechanism [5, 14].
5 10 15 20 25 30 35 40 45 50 55 60 65 70
(102) (110)
15 mM
10 mM 5 mM
1 mM
0.5 mM
Intensity/ a.u.
Diffraction angle,2[Cu K]/ degree 0.1 mM (a) KI
(100)
5 10 15 20 25 30 35 40 45 50 55 60 65 70
(102)
KIO3
15 mM
10 mM
5 mM
1 mM
0.5 mM
0.1 mM
Intensity/ a.u.
Diffraction angle,2[Cu K]/ degree (b)
(100) (110)
1E-3 0.01 0.1 1
11.22 11.24 11.26 11.28 11.30 11.32 11.34 11.36
(c)
I IO
-3
Lattice parametera (Å))
Incorporated I (mmol/g)
1E-3 0.01 0.1 1
21.30 21.32 21.34 21.36 21.38 21.40 21.42 21.44 21.46 21.48 21.50
I IO
-3
Lattice parameterc (Å)
Incorporated I (mmol/g)
Fig. 4.2 XRD patterns of solid residues after coprecipitation of (a) I– and (b) IO3– with ettringite at various initial concentrations. Changes of lattice parameters (c) a and (d) c of ettringite after incorporating different amount of I– and IO3–.
Based on the results of XRD patterns, the lattice parameters of ettringite were plotted against the incorporated I species as shown in Fig. 4.2(c), (d). Incorporation of I– of in ettringite keeps the lattice parameter a and c as a constant (Fig. 4.2(c), (d)). This suggests that
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91
the reduced negligible trance amount of I– might be physically bonded on the ettringite surface rather than the crystal structure because ettringite surface can be positively charged [15]. On the contrary, XRD patterns of the IO3–-doped ettringite exhibited different trend from I– incorporation case.
As shown in Fig. 4.2(b), the solid residues also can be identified as ettringite, with preferred orientation at Miller index of (100) and (110). However, with increasing the incorporated IO3– amount, the diffraction peak intensity at (100) plane, which is related to the intercolumn distance, gradually decreased. This ascribed to the IO3– ions were substituted with the intercolumn located SO42– ions. In order to balance the charge, two IO3– ions should be substituted for one SO42– ions in ettringite structure, result in disorder at the (100) plane. In contrast, the relative intensity of diffraction peaks at 12.16° (102), corresponding to the cation-cation plane, increased slightly with the increasing the incorporated IO3– amounts. This result implies that the columnar structure ({Ca6[Al(OH)6]2･24H2O]}6+) of ettringite was maintained. The above results clearly indicate that the ettringite structure will be deterioration after incorporating high concentrations of IO3–. In addition, by increasing the incorporated IO3–
amount in ettringite, the lattice parameter a of ettringite increased and the lattice parameter c decreased (Fig. 4.2(c), (d)). This is consistent with the above description that intercolumn spaces were enlarged and incorporated IO3– ions disordered the SO42– sites.
Based on the above results, I– and IO3–
show contrasting immobilization preference via
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92
co-precipitation. Immobilization efficiency of anionic contaminants generally depends on the coordination behavior of intercalated anions in ettringite. It is reported that some anionic species which form an inner-sphere complex with Ca atoms in ettringite exhibits high affinity [5]. However, the anionic species which form outer-sphere complex exhibit relativity lower affinity. This is based on the fact that SO42– ions exhibit more negative charge and smaller ionic size to monovalent ions [16, 17].
0 1 2 3 4 5 6 7 8 9 10 11 12
H2O
IO-3
(ettringite columnar structureAl )
Ca
IO-3
(R=3.84Å 2=0.015 CN=7.3) (R=2.71Å 2=0.008 CN=2.7)
K
KIO3 IO-3-doped ettringite
IO
IK IO
IO
Fouri er t rans for m am pl it ude
R(Å)
(R=1.81Å 2=0.002 CN=2.7) (R=1.81Å 2=0.002 CN=2.7)
(Outer-sphere complexation)
Fig. 4.3 k3-weighted I L3-edge EXAFS Fourier transforms of IO3– doped ettringite and a reference compound (not corrected for phase shift) with both raw (solid lines) and fitted data (dash lines). Schematic illustration of I coordination environment is also exhibited.
In order to reveal the mechanism of IO3– immobilization and its selectivity in ettringite, EXAFS spectroscopy was applied to gain a deeper insight into the structure of IO3-doped
Chapter 4
93
ettringite. The I L3-edge raw spectra and fitted k3-weighted χ functions for KIO3 (reference compound) and IO3-doped ettringite are shown in Fig. 4.3. The I L3-edge EXAFS data for KIO3 are dominated by the backscattering resulted from an oxygen shell at approximately 1.3 Å in the Fourier transform with additional peaks at approximately 2.2 Å and 3.2 Å. The first shell of KIO3 can be fitted with 3 O atoms at 1.79 Å and other 3 atoms also can be fitted approximately 2.72 Å. This is consistent with the crystal structure that O atoms and I atom form as IO3– ion, because the bond length between I and O in IO3– is approximately 1.81 Å [18, 19]. The other three O atoms bonded with longer distance at 2.72 Å could ascribe to the three neighboring IO3– ion. Furthermore, approximately 8 K atoms were fitted at 3.8 Å. This is also consistent with the previously reported value for KIO3 crystal structure [19]. The I L3-edge EXAFS data for IO3–-doped ettringite is dominated by the backscattering resulted from an oxygen shell at approximately 1.8 Å in the Fourier transform. The first shell of IO3–-doped ettringite can be fitted with 3 O atoms at 1.81 Å, similar to that of KIO3, which is characteristic of I in IO3– ions. However, unlike KIO3, no further peaks above the noise level were found longer than 2 Å in the Fourier transform of IO3–-doped ettringite (Fig. 4.3).
Because the Fourier transforms spectra showed no further peaks at a longer distance, the structural information obtained from EXAFS data analysis is limited to the first coordination shell. This suggests that IO3– ions are immobilized in ettringite via "solution-like"
coordination environment without formation of inner-sphere complex. Because the
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94
incorporated I– amount is quite low in ettringite, I– EXAFS spectra could not be collected in BL 06 at the SAGA-LS.
In addition, it was reported by using FTIR, the crystal water molecules bonded with Ca will be removed when some ions form an inner-sphere complex in ettringite [5, 7]. The FTIR spectra of the ettringite reacted with 15 mM of I– and IO3– and pure sulfate ettringite are shown in Fig. 4.4. For the pure ettringite, the overlapping bands from 3000 to 3650 cm-1 were attributed to the stretching vibration mode of OH groups from channel water and crystal water in ettringite. The O–H bending vibration mode of the OH groups is also observed at 1620 cm-1. The peaks at 872 and 574 cm-1 are ascribed to a bending vibration mode of Al-OH [7, 20]. When the 15 mM of I– reacted with ettringite, the solid residues exhibit the same FTIR spectra of pure ettringite. However, the bands at 678 cm–1 in the FTIR spectra of IO3–
doped ettringite is assigned as the stretching modes of I–O in IO3– [21]. The other peaks are in good agreement with the pure ettringite. This indicates that IO3– ions were incorporated into the ettringite without affecting to the structural water. Incorporation of IO3– ions did not reduce the peak intensity at 3000 to 3650 cm-1. Finally, the FTIR results also indicate that IO3–
ions are immobilized via out-sphere complex in ettringite.
Chapter 4
95 4000 3500 3000 2500 2000 1500 1000 500
I-O stretching Al-OH bending OH stretching
OH bending S-O stretching
15 mM I -15 mM IO
-3
1 mM WO2-4
1 mM CrO
2-4
1 mM ClO
-4
1 mM SeO
2-4
1 mM B(OH)-4 1 mM AsO
3-4
Pure SO
2-4 ettringite
Absorbance/ a.u.
Wavenumber/ cm-1 (a)
4000 3800 3600 3400 3200 3000 2800
(b)
R2=0.9983 R2=0.9981
R2=0.9987 R2=0.9985 R2=0.9985 R2=0.9987 R2=0.9984 R2=0.9981
R2=0.9986 1 mM AsO
4
3-1 mM SeO
4
2-1 mM B(OH)
4
-1 mM ClO
4
-1 mM WO
4
2-1 mM CrO
4
2-15 mM IO
3
-15 mM I
-Pure SO4 ettringite Ca
channel water forms H-bonds with anion
CaAl
Absorbance/ a.u.
Wavenumber/ cm-1
Fig. 4.4 FTIR vibration spectra of (a) ettringite reacted with various anionic species with 15 mM (I– and IO3–) and 1 mM (B(OH)4–, ClO4–, CrO42–, SeO42–, WO42–, and AsO43–), (b) curve fitted FTIR spectra of the 2800~4000 cm-1 region for each compounds.
Chapter 4
96