『Abstract
Although phytoliths, constituted mainly by micrometric opal,
exhibit an important control on silicon cycle in superficial continental
environments, their thermodynamic properties and reactivity in
aqueous solution are still poorly known. In this work, we determined
the solubility and dissolution rates of bamboo phytoliths collected
in the Reunion(eの頭に´) Island and characterized
their surface properties via electrophoretic measurements and
potentiometric titrations in a wide range of pH. The solubility
product of “soil” phytoliths (pK sp0 = 2.74 at 25℃) is equal to that of vitreous
silica and is 17 times higher than that of quartz. Similarly,
the enthalpy of phytoliths dissolution reaction (ΔH r25-80℃ = 10.85
kJ/mol) is close to that of amorphous silica but is significantly
lower than the enthalpy of quartz dissolution. Elecrophoretic
measurements yield isoelectric point pHIEP
= 1.2±0.1 and 2.5±0.2 for “soil” (native) and “heated” (450℃ heating
to remove organic matter) phytoliths, respectively. Surface acid-base
titrations allowed generation of a 2-pK surface complexation
model. Phytoliths dissolution rates, measured in mixed-flow reactors
at far from equilibrium conditions at 2≦pH≦12, were found to be
intermediate between those of quartz and vitreous silica. The
dissolution rate dependence on pH was modeled within the concept
of surface coordination theory using the equation:
R = k 1・{>SiOH2+}n+k
2・{>SiOH0}+k
3・{>SiO-}m,
where {>i } stand for the concentration of the surface
species present at the SiO2-H2O
interface, k 1 are the rate constants
of the three parallel reactions and n and m represent
the order of the proton- and hydroxy-promoted reactions, respectively.
It follows from the results of this study that phytoliths dissolution
rates exhibit a minimum at pH〜3. This can explain their good preservation
in the acidic soil horizons of Reunion(eの頭に´)
Island. In terms of silicon biogeochemical cycle, phytoliths represent
a large buffering reservoir, which can play an important role
in the regulation of silica fluxes in terrestrial aquatic environments.』
1. Introduction
2. Materials and methods
2.1. Phytoliths characterization
2.2. Solubility measurements
2.3. Electrophoretic measurements
2.4. Surface titration
2.5. Dissolution kinetics
3. Results and discussion
3.1. Phytoliths solubility
3.2. Electrophoretic measurements
3.3. Surface titration and surface complexation model
3.4. Dissolution kinetics and behavior of phytoliths in natural
settings
4. Conclusions
Acknowledgments
Appendix A
Appendix B
Appendix C
Appendix D
References