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

Abstract

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 SiO 4 0 ) by iron oxides. Here, we report on the Si-isotopic fractionation during H 4 SiO 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 d 29 Si 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& (±2r 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 (d 29 Si = +0.01 ± 0.04& (±2r SD )), the remaining solution was systematically enriched in 29 Si, reaching maximum d 29 Si values of +0.70 ± 0.07& for ferrihydrite and +0.50 ± 0.08& for goethite for ic 1.06 mM. The progressive 29 Si enrichment of the solution fitted better a Rayleigh distillation path than a steady state model. The fractionation factor 29 e (±1r SD ) was estimated at À0.54 ± 0.03& for ferrihydrite and À0.81 ± 0.12& for goethite. Our data imply that the sorption of H 4 SiO 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 SiO 4 0 sorption onto secondary clay-sized iron oxides.

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…However, the isotope composition of the bSiO 2 seemed to remain constant within the reactive surface layer where most of the dissolution and precipitation occurred, which makes isotopic fractionation during dissolution unlikely. From leaching experiments on older Pleistocene and Pliocene sediments, Tatzel et al (2015) report that the formation of authigenic Al-Si phases should be 2‰ lighter than the pore waters they formed from, in agreement with previous experimental results (Delstanche et al, 2009;Geilert et al, 2014;Oelze et al, 2014Oelze et al, , 2015. However, the authors observed an increase of the Si isotope composition of the preserved SiO 2 during phase transformation from opal-A to opal-CT.…”

“…reviews of Opfergelt and Delmelle, 2012;Frings et al, 2016), Figure 1, Supplementary Table 1 and references therein). Adsorption of silicic acid onto Fe oxy-hydroxides (Delstanche et al, 2009) and Al hydroxides (Oelze et al, 2014) has also been demonstrated to preferentially immobilize the lighter Si isotopes. As discussed in section Rivers, all these processes are particularly complex on seasonal to geological timescales and explain most of the higher δ 30 Si DSi of continental waters.…”

Measuring production—dissolution rates of marine biogenic silica by 30Si‐isotope dilution using a high‐resolution sector field inductively coupled plasma mass spectrometer

“…No chromatographic or MAGIC preconcentration steps are to be expected on such samples, since salinity is low and DSi contents are usually high. This method could also expand the studies of Si adsorption-desorption processes in batch experiments on sediments (e.g., Van Cappelen et al 2002) and soils (Delstanche et al 2009). It should permit more particularly the measurement of low rates on short time scales.…”

Si isotope variability in Proterozoic cherts

“…Alternatively, silica may have been transported to deeper water BIFs via the 'iron shuttle' (Fischer and Knoll, 2009) by adsorption on Fe-oxide/hydroxide particles and accompanying Si isotope fractionation (c.f. Delstanche et al, 2009) leading to the progressive enrichment of heavy Si isotopes in the ambient water.…”