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Surface properties, solubility and dissolution kinetics of phytoliths, from bamboos of Réunion Island
F Fraysse, O Pokrovsky, J Schott, J Meunier
To cite this version:
F Fraysse, O Pokrovsky, J Schott, J Meunier. Surface properties, solubility and dissolution kinetics of phytoliths, from bamboos of Réunion Island. Goldschmidt Conference, Jun 2004, Copenhague, Denmark. �hal-03170275�
Surface properties, solubility and dissolution kinetics of phytoliths, from bamboos of Reunion Island
Background
Phytoliths, constituted mainly by micrometric opal, exhibit an important control on silicon cycle in superficial continental environments, however, their physico- chemical properties and their reactivity in solution are still poorly known.
The aim of this work is to determine the solubility and dissolution kinetics of bamboo phytoliths from Réunion Island and to characterize their surface properties via electrokinetics measurements and potentiometric titration.
Potentiometric titration
Dissolved silicic acid in soil solutions, trapped by plant roots, precipitate in aerial tissues cells like micrometric opal particles called phytoliths (Fig.1).
Then, this phytoliths are released by organic matter degradation in litters and then can be transferred to soils or evacuated by aerial or hydrographic ways.
The phytoliths studied have been sampled at the northwest of Reunion Island in the M horizon at a depth of 20cm (Fig.2). Their origin is bamboos.
F. Fraysse
a, O.S. Pokrovsky
a, J. Schott
a, J.D. Meunier
b
(a) Laboratoire des Mécanismes de Transfert en Géologie (LMTG), UMR 5563 UR 154 CNRS Université Paul-Sabatier IRD, OMP, 14, Av. Edouard Belin, 31400 Toulouse, France ( fraysse@lmtg.obs-mip.fr )
(b) CEREGE, Europôle méditerranéen de l’Arbois, BP80, 13545 Aix-en-Provence, France
Zêta potential Measurements Solubility
Dissolution kinetics
Conclusion
25°C
0 ,0 E+0 0 5,0 E- 0 4 1,0 E- 0 3 1,5E- 0 3 2 ,0 E- 0 3 2 ,5E- 0 3
0 10 0 2 0 0 3 0 0 4 0 0 50 0
time (days)
[Si], M
p H = 4 p H = 6 p H = 8
0, 01 M NaCl
Log Ksp = f (1/T)
-4 -3,5 -3 -2,5 -2
2,8E-03 3,0E-03 3,2E-03 3,4E-03 1/T
log Ksp
Quartz [1]
amorphous silica [1]
Phytoliths Linéaire (Phytoliths)
For phytoliths, Hr
25-80= 10.85 kJ/mole
For amorphous silica, Hr
25-80= 14.44 kJ/mole
For quartz, Hr
25-80= 25.63 kJ/mole
While the solubility of phytoliths is ~ 20 times higher than that of quartz, their reactivity at
25°C are similar. It follows from the results of this study that phytoliths dissolution rate exhibits a minimum at pH ~ 3. This can explain their good preservation in the acidic soil horizons of
Réunion Island. Phytoliths can represent a real reservoir of biogenic silica in acid soils like
those in Reunion island and consequently must be taken into account to study the global cycle of silicon.
Dissolution kinetics of phytoliths as a function of pH in mixed-flow reactor, at 25°C
1,E-17 1,E-16 1,E-15 1,E-14 1,E-13 1,E-12
0 2 4 6 8 10 12 14
pH R, mol/cm2 /s
Quartz [2]
Vitreous silica [3]
native phytoliths phytoliths burned fresh phytoliths
At 4 ≤ pH ≤ 10, R = k
p* a
OH0.33R ~ k
Si[>SiO
-]
2.0Dissolution kinetics have been made in
« mixed flow » reactors.
Rate of dissolution R, mole/cm
2/s:
R = (( C
out– C
in) * Q ) / ( M * S)
Fig. 1: Phytoliths view by S.E.M. ( Meunier J.D., 1999 ) Fig.2: Study area ( Meunier J.D., 1999 )
T (°C)
C
SiO2mmol/L
log K
spphytoliths
25 1,83 -2,74
50 2,38 -2,62
80 3,6 -2,44
At pH (25°C) = 8, [SiO
2 (aq)]
mesured= [ H
4SiO
4°
(aq)]
[Si] as a function of time, at 25°C, 0,4 mL/min and pH = 7,5 ± 0,1
0,00E+00 4,00E-05 8,00E-05 1,20E-04 1,60E-04
5 10 15 20 25 30 35
Time (days)
[Si], M
0.01 M for pH >2
pH
IEP= 1.15 ± 0.1
At pH
IEP, [>SiO
-] = [>SiOH
2+]
Phytoliths zêta potential as a function of pH, at 25°C
-45 -40 -35 -30 -25 -20 -15 -10 -5 0
0 1 2 3 4 5 6 7 8 9 10 11 pH
Zêta potential, mV
0.001M
0.05M
0.01M
Zêta potentials of bamboo phytoliths as a function of pH, at 0,01M and 25°C
-50 -40 -30 -20 -10 0
0 1 2 3 4 5 6 7 8 9 10 11 pH
Zêta potential, mV
Phytoliths
fresh phytoliths phytoliths burned
Steady state
[1]: Rimstidt and Barnes (1980) [2]: Dove and Elston (1992) [3]: Wirth and Gieskes (1979)
Acid-base properties of phytoliths have been determined by potentiometric titration at 25°C in 0.01M NaCl:
Surface charge density of phytoliths as a function of pH, 20 g/L, 6,51 m2/g
-4 -3 -2 -1 0 1
3 4 5 6 7 8 9 pH
Surface charge, µmol/m2
Experimental FiteQL Model
