Silicon isotopic fractionation during adsorption of aqueous monosilicic acid onto iron oxide

The quantification of silicon isotopic fractionation by biotic and abiotic processes contributes to the understanding of the Si continental cycle. In soils, light Si isotopes are selectively taken up by plants, and concentrate in secondary clay-sized minerals. Si can readily be retrieved from soil solution through the specific adsorption of monosilicic acid ( H 4 Si O 4 0 ) by iron oxides. Here, we report on the Si-isotopic fractionation during H 4 Si O 4 0 adsorption on synthesized ferrihydrite and goethite in batch experiment series designed as function of time (0–504 h) and initial concentration (ic) of Si in solution (0.21–1.80 mM), at 20 °C, constant pH (5.5) and ionic strength (1 mM). At various contact times, the δ 29Si vs. NBS28 compositions were determined in selected solutions (ic = 0.64 and 1.06 mM Si) by MC–ICP–MS in dry plasma mode with external Mg doping with an average precision of ±0.08‰ (±2 σ SEM). Per oxide mass, ferrihydrite (74–86% of initial Si loading) adsorbed more Si than goethite (37–69%) after 504 h of contact over the range of initial Si concentration 0.42–1.80 mM. Measured against its initial composition (δ 29Si = +0.01 ± 0.04‰ (±2 σ SD)), the remaining solution was systematically enriched in 29Si, reaching maximum δ 29Si values of +0.70 ± 0.07‰ for ferrihydrite and +0.50 ± 0.08‰ for goethite for ic 1.06 mM. The progressive 29Si enrichment of the solution fitted better a Rayleigh distillation path than a steady state model. The fractionation factor 29ε (±1 σ SD) was estimated at −0.54 ± 0.03‰ for ferrihydrite and −0.81 ± 0.12‰ for goethite. Our data imply that the sorption of H 4 Si O 4 0 onto synthetic iron oxides produced a distinct Si-isotopic fractionation for the two types of oxide but in the same order than that generated by Si uptake by plants and diatoms. They further suggest that the concentration of light Si isotopes in the clay fraction of soils is partly due to H 4 Si O 4 0 sorption onto secondary clay-sized iron oxides.