『Abstract
The sequestration of silicon in soil clay-sized iron oxides may
affect the terrestrial cycle of Si. Iron oxides indeed specifically
adsorb aqueous monosilicic acid (H4SiO4o), thereby influencing Si concentration
in soil solution. Here we study the impact of H4SiO4o adsorption on the fractionation
of Si isotopes in basaltic ash soils differing in weathering degree
(from two weathering sequences, Cameroon), hence in clay and Fe-oxide
contents, and evaluate the potential isotopic impact on dissolved
Si in surrounding Cameroon rivers. Adsorption was measured in
batch experiment series designed as function of time (0-72 h)
and initial concentration (ic) of Si in solution (0.61-1.18 mM)
at 20℃, constant pH (5.5) and ionic strength (1 mM). After various
soil-solution contact times, the δ30Si vs. NBS28 compositions
were determined in selected solutions by MC-ICP-MS (Nu Plasma)
in medium resolution, operating in dry plasma with Mg doping with
an average precision of ±0.15‰ (±2σSEM).
The quantitative adsorption of H4SiO4o by soil Fe-oxides left a solution
depleted in light Si isotopes, which confirms previous study on
synthetic Fe-oxides. Measured against its initial composition
(δ30Si = +0.02±0.07‰ (±2σSD),
the solutions were systematically enriched in 30Si
reaching maximum δ30Si values ranging between +0.16‰
and +0.95‰ after 72 h contact time. The enrichment of the solution
in heavy isotopes increased with increasing values of three parameters:
soil weathering degree, iron oxide content, and proportion of
short-range ordered Fe-oxide. The Si-isotopic signature of the
solution was partly influenced by Si release, possibly through
mineral dissolution and Si desorption from oxide surfaces, depending
on soil type, highlighting the complex pattern of natural soils.
Surrounding Cameroon rivers displayed a mean Si-isotopic signature
of +1.19‰. Our data imply that in natural environments, H4SiO4o adsorption
by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic
signature of the soil solution exported to water streams.』
1. Introduction
2. Materials and methods
2.1. Environmental setting
2.2. Sampling
2.3. Sample characterization
2.3.1. Soil and clay properties
2.3.2. Silicon in river waters
2.4. Adsorption experiments
2.5. Silicon isotopes analyses
3. Results
3.1. Soil weathering degree and implications for soil properties
3.2. Silicon adsorption in soils
3.3. Si-isotopic signatures induced by adsorption of monosilicic
acid in soils
3.4. Dissolved and particulate Si in river waters
4. Discussion
4.1. Quantitative Si adsorption and soil components
4.2. Si-isotopic variations during Si adsorption as influenced
by soil weathering degree
4.3. Potential impact of desorption and mineral dissolution
4.4. Comparison with Si-isotopic fractionation induced by synthetic
Fe-oxides
4.5. Implications on soil solution and surrounding rivers
5. Conclusions
Acknowledgments
References