4.3. Results and discussion
4.3.2. Selectivity of inorganic anions
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reveal the coordination of anions species [5]. Thus, the FTIR spectra of solid residues after immobilization of various anionic species with ettringite were exhibited in Fig. 4.4. After incorporation of subjected anions, the FTIR spectra of ettringite were not changed significantly, especially for the OH stretching region (Fig. 4.4(a)). According to the reference compounds (Fig. 4.5) and previously reported data [5, 7, 14, 20, 21], the OH stretching region was also analyzed by peak separation. As shown in Fig. 4.4(b), after incorporating various anions, the OH stretching mode related to Ca-OH2 was not significantly changed, indicating the incorporated anionic species were immobilized in the "solution-like" environment via hydrogen bonds and electrostatic force in ettringite.
The immobilization efficiency ((C0-Ce/C0)100%) of various anionic species which were co-precipitated with ettringite was shown in Fig. 4.6(a). The ettringite exhibits a significant different preference for various anions with different ionic radius and valance states. The ettringite showed special preference to immobilize B(OH)4–, IO3–, and AsO43–. Although B(OH)4– and IO3– are monovalent, the immobilization efficiency of these anions are much higher than some divalent anions, such as SeO42– and CrO42–. However, some monovalent anions, such as I– and ClO4–
, exhibit quiet low immobilization efficiency in ettringite, indicating that the valance state of anions is not the main factor to control the anionic species immobilization efficiency by ettringite. Zhang and Reardon hypothesized that the size of the substituting anion is more important than anion geometry for controlling anion removal
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because the anions are generally immobilized in the intercolumn spaces through electrostatic force in ettringite [4]. The anions having smaller ionic radius than SO42– (ettringite structural formation anion) is easier to be incorporated in ettringite [4]. Thus, correlations for the changes of immobilization efficiency of anionic species as a function of anionic radius are shown in Fig. 4.6(b), where the anionic radius was averaged on the x-axis where different values were reported for the same anion [22-26].
Considering the main anionic species in ettringite is SO42– to form solid solution, the anionic species were clearly separated into two groups based on the SO42– ionic radius. In general, the smaller ionic radius of subjected anions exhibits higher immobilization preference by ettringite [4]. Although I– exhibits smaller ionic radius than SO42–, the immobilization efficiency is still low by coprecipitation with ettringite. For the anions exhibits larger ionic radius than SO42–, there is no special relationship between anionic radii and immobilization efficiency. This observation could not well obey the assumption which is proposed by Zhang and Reardon (2003), indicating ionic radius is not the main factor to control the subjected anionic species selective immobilization by ettringite.
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0 20 40 60 80 100
AsO 3-CrO2- 4
WO2-4 4
SeO 2-I- 4
IO -ClO- 4
B(OH)-4 4
Immobilization efficiency (%)
(a)
2.1 2.2 2.3 2.4 2.5 2.6 2.7
0 20 40 60 80 100
(b)
SO2-4
CrO2-4
WO2-4
SeO2-4 AsO3-4
ClO-4 B(OH)-4
I -IO-3
Immobilization efficiency (%)
r (Å) (2.32 Å)
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0
20 40 60 80 100
(c)
ionic radius larger than SO2-4 ionic radius smaller than SO2-4 weakly
hydrated hydration ability
I- ClO-4
CrO
2-4
SeO2-4 WO2-4
AsO3-4 B(OH)-4
IO-3
Immobilization efficiency (%)
viscosity B coefficients
y=10.32+145.98x R2=0.867
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 0
20 40 60 80 100
(d)
structural making structural
breaking
R2=0.961 y=54.97+53.68x
ClO -I- 4
ionic radius larger than SO2-4 ionic radius smaller than SO2-4 CrO
2-4
SeO2-4 WO2-4
AsO3-4 IO-3
B(OH)
-4
Immobilization efficiency (%)
GHB
Fig. 4.6 The immobilization efficiency of (a) various 1mM anions coprecipitation with ettringite, Correlationship between immobilization efficiency and (b) ionic radius, (c) Jones–Dole coefficient B coefficient, and (d) geometrical factor (∆GHB).
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Based on the previously reported structure model of ettringite [27, 28], the unit cell of ettringite consists of columnar {Ca6[Al(OH)6]2･24H2O]}6+. Each Ca atom is eight coordinated by 4 H2O and 4 OH– ions and the columnar parts are surrounded by H2O molecules. The guest anions could occupy in the intercolumn space and hold the columns together through electrostatic force and hydrogen bonds. Therefore, the hydration ability of anions might be a crucial factor to affect the selective immobilization efficiency by ettringite.
Some researchers have already observed the difference in hydration of I– and IO3– ions [29].
The Jones–Dole viscosity coefficient is generally applied to compare the bonding strength of the hydrated ions [25, 29-31]. Among the Jones–Dole viscosity coefficient, the coefficient B, which is resulted from the degree of water structuring around the ions, is well studied and applied to interpret ions properties in aqueous solution [26, 29-31]. The weakly hydrated ions exhibit a smaller change in viscosity with concentration, having negative B coefficients. In contrast, strongly hydrated ions have positive B coefficients [25, 29-31]. Correlations for the changes of immobilization efficiency of anionic species as a function of anionic radius are shown in Fig. 4.6(c), where the Jones–Dole viscosity B coefficient of each selected anion are averaged from previous reports [46, 48]. The anions (I– and ClO4–
) which have negative B coefficients values exhibit less immobilization preference by ettringite. This is consistent with the previous reports about I– and ClO4– which are poorly hydrated [32, 33]. Thus, less hydrated anions are difficult to interact with the ettringite structural H2O molecules in accord
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with the previous report. However, the anions having positive viscosity B coefficient values exhibit higher immobilization efficiency, proving that only the strongly hydrated anions have potentials to be immobilized in ettringite. Moreover, among these strongly hydrated anions, there is no positive relationship between viscosity B coefficient and ettringite immobilization efficiency. As described above, the subjected anions' ionic radius might contribute the immobilization efficiency of ettringite. Therefore, based on the ionic radius, the subjected anions are also classified into two groups. It is difficult to immobilize anions having smaller ionic radius and less hydrated via ettringite. There is a positive correlation between the larger size anions' Jones–Dole coefficient (B) and immobilization efficiency, indicating that the larger size anions roughly obey the speculation that the hydration ability affects the anions immobilization preference via ettringite.
Furthermore, the "structure making" (kosmotropes) and "structure breaking" (chaotropes) via ions hydration ability to interpret their effects on the water structure has been generally established and accepted to explain the behavior of ions in aqueous system [31, 34-36]. The
"structure making" ions are used to express strong hydration interaction. The interactions of
"structure making" ions with water molecules are stronger than water molecules each other.
Therefore, they are capable of forming hydrogen bonds with water. In contrast, the water molecules will increase the mobility when they attached to the "structure breaking" ions and easily removed from these ions. This is because the "structure breaking" ions exhibit weaker
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interactions with water [9, 38]. Ben-Naim and Marcus observed that different ions affect the averaged number of hydrogen bonds of the water structure [38]. According to the Jones–Dole coefficient (B) and water structural entropy (Sstr), the geometrical factor (∆GHB) was established and applied to classify the "structure making" and "structure breaking" ions. The ion which exhibits smaller ∆GHB value has tendency to be classified as "structure breaking"
ion [25, 37]. The water structural entropy and geometrical factor could be expressed as below [25, 37]:
Sstr =20(z2 + |z|) −605B
ΔGHB= −(0.18±0.08) − (10.22±1.26)⨉10−3(Sstr) where z is the charge value of the subjected ion.
The correlations for the changes of immobilization efficiency of anionic species as a function of ∆GHB are shown in Fig. 4.6(d). As described above, the subjected anions ionic radius might be a factor to control the immobilization efficiency of ettringite. Thus, the subjected anions are also separated in two groups based on the SO42– ionic radius. The ions which have smaller ∆GHB also exhibit smaller immobilization efficiency. It can be observed
that the most of anions show a strong relationship between the ΔGHB and immobilization efficiency (Fig. 5(d)). However, the B(OH)4– and IO3– did not obey this regulation. The ionic radii of B(OH)4– and IO3– is much smaller than SO42–, thus this kind of ions can be easy to intercalate in ettringite. Even I– is smaller than SO42–, the ∆GHB of this ion is quite small,
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indicating I– is assigned to "structure breaking" ion and cannot hydrate. Thus, less hydrated anion could not be immobilized in the ettringite structure although this anion exhibits smaller ionic radius than SO42–.
Based on the above results, anions selective immobilization mechanism by ettringite could be proposed. Because of the positively charged columnar parts {Ca6[Al(OH)6]2･ 24H2O]}6+, anions have a tendency to move and interact with the water molecules which are connected with Ca during the formation of ettringite in aqueous solution. However, different anions exhibit significant differences in hydration ability and could be classified as "structure making" and "structure breaking" anions. The water molecules are distinguishing polar molecules that oxygen atom has partial negative charge and hydrogen atoms exhibit partial positive charge. The oxygen of water molecules is connected with Ca in columnar parts of ettringite and the hydrogen atoms are exposed towards the intercolumn spaces. The "structure making" anions will immediately interact with the hydrogen atom of the adjacent structural water molecules when these anions intercalated in the ettringite. In contrast, "structure breaking" anions exhibit weaker interactions with water molecules and thus difficult to be immobilized in ettringite. It is worth noting that the hydrated anions with smaller ionic radius than sulfate have high affinity to be incorporated in ettringite. The ionic radius is another factor to control the subjected anion immobilization preference.
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