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Download fileAdsorption Behavior of Metasilicate on N‑Methyl d‑Glucamine Functional Groups and Associated Silicon Isotope Fractionation
journal contribution
posted on 2016-08-07, 00:00 authored by Wei Wang, Hai-Zhen Wei, Shao-Yong Jiang, Christopher
J. Eastoe, Qi Guo, Yi-Bo LinSignificant
isotope fractionation of silicon provides a powerful
geochemical tracer for biological and physicochemical processes in
terrestrial and marine environments. The exact mechanism involved
in silicon uptake as part of the biological process is not well known.
The silicon uptake in biological processes is investigated using silicate
adsorption onto the N-methylglucamine functional
group (sugarlike structure, abbreviated as L) of Amberlite IRA-743
resin as an analogue of the formation of silicate–sugar complexes
in plants. This study provides new evidence that certain sugars can
react readily with basic silicic acid to form sugar–silicate
chelating complexes, and the equilibrium adsorption behavior of silicate
can be well described by the Langmuir isotherm with a Gibbs free energy
(ΔG) of −11.94 ± 0.21 kJ·mol–1 at 293 K. The adsorption kinetics corresponds well
to a first-order kinetic model in which the adsorption rate constant ka of 1.25 × 10–4 s–1 and the desorption rate constant kd of 4.00 × 10–6 s–1 are obtained at 293 K. Both ka and kd increase with increasing temperature. The
bonding configurations of silicate–sugar complexes imply the
principal coordination complex of hexacoordinated silicon (silicon/L
= 1:3) in the liquid phase and the dominant tetracoordinated silicon
in the solid phase. Similar to those of many natural processes, the
biological uptake via the sugar–silicate chelating complexes
favors the preferential enrichment of light Si isotopes into solids,
and the Rayleigh model controls the dynamic isotope fractionation
with an estimated silicon isotope fractionation factor (i.e., αsolid–solution = (S30i/S28i)solid(S30i/S28i)solution) of 0.9971. This study advanced the fundamental
understanding of the dynamic isotope fractionation of silicon during
silicon cycling from the lithosphere to the biosphere and hydrosphere
in surficial processes.