The pH dependence of the isotopic composition of boron adsorbed on amorphous silica

Giuseppe D. Saldi (1) , Pascale Louvat (2) , Jacques Schott (1) , Jérôme Gaillardet (2)

1 GET - Géosciences Environnement Toulouse
2 IPGP (UMR_7154) - Institut de Physique du Globe de Paris

The use of the boron content and isotopic composition of secondary silicate minerals and siliceous organisms to trace weathering reactions and past ocean pH requires characterizing the fundamental reactions that govern the incorporation and subsequent isotope fractionation of this element in these materials. Toward this goal we have investigated boron adsorption on the surface of amorphous silica (SiO₂·0.32H₂O) and quantified its isotopic fractionation. Boron adsorption envelopes and corresponding isotope fractionation factors were measured in dilute aqueous solutions (0.01 M) of NaCl and CaCl₂. B maximum adsorbed fraction was found to be about 2 times higher in CaCl₂ solutions than in NaCl. The modelling of chemical and isotopic data in NaCl solutions allowed to identify the formation of two main B surface species at the SiO_2(am)/water interface: a neutral trigonal (B3) inner-sphere complex, >SiOB(OH)₂⁰, characterized by a fractionation factor of ~-16‰ relative to aqueous boric acid, and a negatively charged tetrahedral (B4) inner-sphere complex, >SiOB(OH)₃^-, with fractionation factors of -5‰ with respect to aqueous borate. In CaCl₂ solutions the data modelling indicates the presence of B4 inner-sphere complexes with a fractionation factor of -6.5‰ relative to aqueous borate, but excludes the presence of trigonal surface species and suggests instead the formation of a Ca-B(OH)₄^- complex that could partly account for the observed increase of boron adsorption in these solutions. These observations indicate that changes in the surface charge density and interfacial water structure due to different background electrolytes can induce changes in concentration and both chemical and isotopic composition of B adsorbed on silica surfaces.

Although this study suggests that the B adsorption reaction on SiO_2(am) plays a minor role in the B incorporation and isotopic signature of clay minerals and siliceous cements forming during weathering reactions, the acquired data should allow for an improved knowledge of the biomineralization reactions. The observed B isotopic fractionations between adsorbed and aqueous B species should help determining the relative contribution of various processes, such as cellular transport, biological mediation and silicification reactions, on the amount of B incorporated in siliceous microorganisms and its isotopic signatures. In addition, the presence in NaCl solutions of both trigonal and tetrahedral boron complexes at the silica surface could explain the observed weak pH dependence of the boron isotopic composition of marine diatoms.

Mots clés: Boron adsorption; Boron isotope fractionation; Amorphous silica; Triple-layer model; B pH-proxy

Domaines: Planète et Univers [physics]

Giuseppe D. Saldi, Pascale Louvat, Jacques Schott, Jérôme Gaillardet. The pH dependence of the isotopic composition of boron adsorbed on amorphous silica. Geochimica et Cosmochimica Acta, 2021, 308, pp.1-20. ⟨10.1016/j.gca.2021.05.052⟩. ⟨insu-03590016⟩