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72
This is consistent with the above results that SO4 ettringite forms and has a positive charge. In contrast, in the absence of SO42−, the surface charge of the precipitates shows significant fluctuation until 60 min. After 2 min, the zeta potential is around 5 mV at pH 10.5.
It rapidly decreases to 0 mV after 15 min, and then gradually increases and remains constant at around 4 mV. As a reference, the zeta potentials of SeO4 ettringite and Ca(OH)2 were measured against the pH. Figure 3.7 shows that the pHzpc values of SeO4 ettringite and Ca(OH)2 are around 13.2 and 13.7, respectively. This indicates that during the SeO42−removal process, the intermediate products significantly affect the surface charge in the absence of SO42−.
0 20 40 60 80 100 120
-2 0 2 4 6 8 10 12 14
11.5 11.4
11.4 11.1 10.8 10.5 10.2
With SO
4
2-Without SO4
2-11.5 11.4 11.3
10.6 11.1 10.5 10.4
Zeta potential (mV)
Time (min)
Fig. 3.6 Time course of zeta potentials for the solid residues after co-precipitation of SeO42– with and without SO42– in solution. Numbers are pH values of the solution where the solid residues were suspended.
Chapter 3
73
it is expected that Ca2+ and OH− ions are rapidly released from Ca(OH)2:
Ca(OH)2⇄Ca2++2OH− (2)
The OH− ions increase the pH value, which could facilitate formation of ettringite. At the same time, there is a large amount of released Ca2+ in the solution, which would immediately act as a mineralizer for formation of ettringite (Fig. 3.2). Based on the water chemistry, the (S+Se)/Al molar ratios in the solid residues were plotted against time (Fig. 3.8). It has been reported that the (S+Se)/Al molar ratios of AFt–SeO4 and AFt–SO4 are around 1.5 [2]. When SO42−coexists with SeO42−, the (S+Se)/Al molar ratio in the solid phase saturates in the first 2 min (Fig. 3.8). Furthermore, only AFt–SO4 is observed at this time (Fig. 3.2), indicating that coprecipitation of SeO42− and SO42− would directly form ettringite and some SO42− would substitute for SeO42−:
2Al(OH)4− +4OH− +6Ca2+ + (3−x)SO42−+ xSeO42−+(24+n)H2O
⇄ Ca6Al2(SO4)3−x(SeO4)x(OH)12·(24+n)H2O (3) The remaining ion concentration gradually decreases after 2 min and reaches equilibrium at 30 min (Fig. 3.1(a)). The surface charge (Fig. 3.7) and (S+Se)/Al molar ratio (Fig. 3.8) are relatively stable during the whole process of ettringite formation. These results show that SO42− and SeO42− immediately incorporate into the channel of columns and are stabilized as ettringite.
Chapter 3
74
11.0 11.5 12.0 12.5 13.0 13.5 14.0
-5 0 5 10 15 20 25 30 35
Zeta potential (mV)
pH
SeO42--substituted ettringite Ca(OH)2
Fig. 3.7 Zeta potentials for SeO42–-substituted ettringite and Ca(OH)2 against pH.
The ionic radius of SO42− is much smaller than that of SeO42− because the Se–O bond length of SeO42− (161 pm) is longer than the S–O bond length of SO42− (149 pm) [11]. In addition, the Pauling electronegativities of S and Se are 2.58 and 2.55, respectively [12].Therefore, SO42− is more acidic than SeO42−. This may also affect the anion preference of ettringite. These factors are the reasons why a small amount of SeO42− is immobilized in AFt–SO4.
Without SO42−, combining the XRD and SEM results (Fig. 3.3 and Fig. 3.4(a)), an unknown XRD peak at 2θ = 31.1 ° and some flower-like structure crystals are observed.
These flower-like crystals may correspond to AFm-phase, and a similar morphology has been reported for AFm-phase. Furthermore, it has been reported that SeO42− hydrocalumite forms within corporation of SeO42− in the structure of hydrocalumite [13]. The characteristic (003) and (006) diffraction peaks of hydrocalumite gradually weaken and almost completely
Chapter 3
75
disappear, although both the position and intensity of the (110) diffraction peak at 2θ = 31.1°
related to the a lattice parameter did not change. It can be supposed that the coprecipitation system contains not only SeO42– but also Cl– and OH–. Cl– and OH– have possibility to be intercalated in the interlayer spaces of selenate hydrocalumite. As the ionic radii and electronegativity of SeO42–, Cl–, and OH– are different, the layer will be bending after intercalating different ions. Thus, the intensity of characteristic (003) and (006) diffraction peaks of hydrocalumite decreased. In the present work, the XRD patterns are assigned to SeO42− hydrocalumite, which is confirmed by the Ca: Al: Se molar ratio. The Se/Al molar ratio in the solid residue is around 0.5 after 2 min (Fig. 3.8 and Fig. 3.5(a)), corresponding to the theoretical molar ratio of Se/Al in SeO42− hydrocalumite. In terms of the surface charge, highly positively charged Ca(OH)2 did not completely dissolve after 2 min (Fig. 3.3), so the zeta potential of the mixture in solution shows a positive charge on average (Fig. 3.6). The SeO42− hydrocalumite was observed as an intermediate during formation of SeO4 ettringite.
When the SeO42− hydrocalumite formed, the AFt–SeO4 and calcium hydroxide also can be characterized in this stage (Fig. 3). It is quite difficult to quantify the accurate H2O numbers in SeO42−
hydrocalumite, because other phases (SeO4 ettringite) also contain H2O groups in their structures. Thus, the chemical formula of SeO42− hydrocalumite is exhibited as Ca4Al2(SeO4) (OH)12·mH2O. Therefore, with dissolution of Ca(OH)2 (Eq. (2)), the released Ca2+ immediately reacts with SeO42−
and Al3+ to form SeO42−
hydrocalumite, and then SeO4
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76
ettringite forms in the solution as described by Eqs (4) and (5):
4Ca2+ + 2Al(OH)4− + SeO42− +12OH− + mH2O Ca4Al2(SeO4)(OH)12·mH2O (4) 6Ca2+ + 2Al(OH)4− + 3SeO42−+ 4OH− + 24±nH2O Ca6Al2(SeO4)3(OH)12·(24H2O + nH2O)
(5)
In summary, SEM-EDX analysis (Fig. 3.5(a)) and the Se/Al molar ratios in the solid residues (Fig. 3.8) prove that SeO42− is mainly immobilized in SeO42− hydrocalumite in the first 2 min.
0 20 40 60 80 100 120
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
1.8 Throretical ratio of ettringite
Throretical ratio of selenate AFm phase
With SO
4
2-Without SO4
2-Molar ratio of (S+Se)/Al in solids
Time (min)
Selenate AFm phase
Fig. 3.8 Changes in the molar ratio (Se+S)/Al in solid phase with increasing time during co-precipitation in the presence and absence of SO42–.
After 5 min, SeO4 ettringite is already one of the dominant phases, where as the XRD peaks of Ca(OH)2 and SeO42− AFm-phase gradually disappear with time (Fig. 3.3). It is suggested that SeO4 ettringite progressively forms. SeO42−
hydrocalumite is unstable below pH 12 [7]. Based on the dissolution–reprecipitation mechanism [14], hydrocalumite is unstable and the following dissolution reaction occurs [15]:
Ca4Al2 (OH)12SeO4·mH2O4Ca2++2Al(OH)4−
+ SeO42−
+ 4OH−+mH2O (6)
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77
SeO42− hydrocalumite then progressively transforms o SeO4 ettringite, as described by Eqs. (5) and (6). After 30 min, all of the Ca(OH)2 has dissolved (Fig. 3.3), so the zeta potential of the mixture gradually decreases to 0. Because SeO42− ettringite and the Ca(OH)2
surface should be positively charged in a pH range 10.5 to 13 (Fig. 3.7), the decrease in the surface charge might result from gradual dissolution of Ca(OH)2 because pHzpc˂10.6 for SeO42− hydrocalumite. The pHzpc of nitrate AFm-phase has been reported to be 7.32 [16].
SeO42− hydrocalumite may have a similar pHzpc to nitrate hydrocalumite. After 45 min, the reaction reaches equilibrium: SeO42− hydrocalumite completely disappears and the surface charge of the mixed phases increases from 0 to 4 mV (Fig. 3.6). SeO4 ettringite has a positive charge below pH 11.5 (Fig. 3.7). As a result, the surface charge of the solid in solution is positive. This is consistent with the SEM results: the newly formed SeO4 ettringite electrostatically adsorbs on the surface of layered crystals because of their opposite charges below pH 11. In terms of the Se/Al molar ratios in the solid residues, SeO42− hydrocalumite progressively transforms to AFt–SeO4, which gradually becomes the dominant phase, and then the Se/Al molar ratio increases from 0.5 to approximately 1.5. In addition, as shown in Fig. 8, mole ratio of Se/Al is lower than 1.5, indicating the oxyanion sites are not always occupied by selenate. Furthermore, the lattice parameter a is 11.42 Å, which is slightly smaller than in the pure SeO4 ettringite (11.47 Å) [9]. Muhamad et al. (1993) have already reported that the OH– can be substituted with oxoanions in ettringite [17]. Thus, some
Chapter 3
78
oxoanion sites might be occupied by some OH– ions to decrease the mole ratio of Se/Al in solid residues. These results can be used to interpret the AFt–SeO4 formation process. Firstly, SeO42− hydrocalumite forms and SeO42−isimmobilized in this phase. Because SeO42−
hydrocalumite is not stable, it gradually transforms to SeO4 ettringite. SeO42− is eventually immobilized in SeO4 ettringite. Based on the above description, the formation mechanism of SeO4 ettringite is schematically illustrated in Fig. 3.9.
Without SO42−, precipitation of SeO42− as SeO4 ettringite takes longer to reach equilibrium.
In this system, there may be several reasons. One is that Se, Al, O, and Ca are constituents of SeO42− hydrocalumite, which immediately forms with addition of Ca(OH)2. However, this phase is unstable and gradually dissolves. During this period, SeO42− hydrocalumite releases SeO42−, Al3+, and Ca2+ into the solution, which could prevent Ca(OH)2 dissolution and result in a lower supply of Ca2+ ions from Ca(OH)2. Another possible reason is that SeO42−
hydrocalumite already has a negative charge during the SeO42− removal process. In contrast, the surface of Ca(OH)2 has a positive charge during this period, and the immediately formed SeO42− hydrocalumite would cover the surface of Ca(OH)2.
Chapter 3
79
Fig.3.9 Schematic illustrations for the formation mechanism of AFt–SeO4.
Chapter 3
80
Thus, dissolution of Ca(OH)2 would also be inhibited. Furthermore, the Pauling electronegativities of SeO42− and SO42− could also affect the formation kinetics of SeO4
ettringite. Consequently, immobilization of SeO42− without SO42− takes longer to reach equilibrium than with SO42−.
