Belay Zeleke Dilnesa - Eawag-Empa Library

CHAPTER 3 SYNTHETIC FE-CONTAINING HYDRATES
Using the compositions determined from XRD, the solubility products of both
hydroandradite phases were calculated according to the following equations:
KS0 (Ca3Fe2(SiO4)0.95(2)(OH)8.20(2)) = {Ca 2+ } 3 . {FeO2 - } 2 {HSiO3 - } 0.95 . {OH - } 3.05 . {H2O} 2.1
KS0 (Ca3Fe2(SiO4)1.52(2)(OH)5.92(4)) = {Ca 2+ } 3 . {FeO2 - } 2 {HSiO3 - } 1.52 . {OH - } 2.48 . {H2O} 0.96
The calculated values from the dissolution experiments are given in Table 31. The
resulting solubility products are log Ks0= -32.34 and –34.50 at 20 °C and log Ks0 = -33.68
and -35.76 at 50 °C for C3FS0.95H4.1 and C3FS1.52H2.26, respectively. The solubility
products of Fe-Si-hydrogarnet synthesized at 20 °C and equilibrated under oversaturation
are very similar (log Ks0 = -32.98 and –34.63 at 20 °C for C3FS0.95H4.1 and C3FS1.52H2.96).
The values are 7 log units lower than those of the Al-containing siliceous hydrogarnet
determined in this study. Hence, hydroandradites are significantly more stable than the Al
containing hydrogrossulars.
Table 31 Measured ion concentrations of solids synthesized at 110 °C (re dissolved and
equilibrated for 4 months at 20 °C and 50 °C) and at 20 °C(equilibrated for 3 years
under oversaturated condition).
Phases
Temp.
[°C]
Ca
[mmol/l]
K
[mmol/l]
Si
[mmol/l]
Fe
[mmol/l]
+ pH log Ks0
Ca3Fe2 (SiO4)0.95(OH)8.2 20 0.10 118 0.05 0.0061 13.04 -32.34±0.20
*Ca3Fe2 (SiO4) 0.95(OH) 8.2 20 0.20 85 0.26 0.0008 13.10 -32.98±0.20
Ca3Fe2 (SiO4)1.52(OH)5.92 20 0.10 118 0.05 0.0061 13.04 -34.50±0.20
*Ca3Fe2 (SiO4)1.52(OH)5.92 20 0.20 85 0.26 0.0008 13.10 -34.63±0.20
Ca3Fe2 (SiO4)0.95(OH)8.2 50 0.06 125 0.06 0.0044 13.02 -33.68±0.20
Ca3Fe2 (SiO4) 1.52(OH) 5.92 50 0.06 125 0.06 0.0044 13.02 -35.76±0.20
Detection limits (mmol/l): Al = 0.001 Ca = 0.0004, Fe = 0.0000006, K = 0.0024, S=0.0014, Si=0.0002 measurement uncertainty ±10%,
+
pH measured at 20°C, *solubility of Fe-siliceous hydrogarnet equilibrated for 3 years under oversaturated condition.
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