Adsorption Behavior of Metasilicate on N‑Methyl d‑Glucamine Functional Groups and Associated Silicon Isotope Fractionation

Significant 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 k_a of 1.25 × 10^–4 s^–1 and the desorption rate constant k_d of 4.00 × 10^–6 s^–1 are obtained at 293 K. Both k_a and k_d 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., α_{solid–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.