Boron isotope fractionation during adsorption on aragonite and calcite in artificial seawater and NaCl aqueous solutions

Adsorption of dissolved species on mineral surfaces is a key elementary process, which controls crystal growth and incorporation of trace elements into mineral structures, affecting the isotopic composition of geological archives. Changes of chemical composition of aqueous solution can induce significant variations in the type, concentration and isotopic composition of boron surface species incorporated by CaCO 3 . To better understand and quantify these effects, the boron isotope fractionation associated with B adsorption on calcite and aragonite was investigated in artificial seawater and equimolar solutions of NaCl (0.5 M) at room temperature and 7.5 ≤ pH NBS ≤ 11.9. Boron adsorption on aragonite was 2–4 times stronger than on calcite, in agreement with the higher affinity of aragonite for borate ions reported by previous studies. In seawater solutions at pH > 8.5, B removal from the solution increased due to increasing adsorption on both CaCO 3 polymorphs and coprecipitation with brucite, which was observed to form at more alkaline conditions. Boron sorption reactions on calcite and aragonite surfaces were described using the calcite three-plane model (TPM) assuming the presence of a borate inner-sphere complex ( > B O 3 H 2 + 0.5 ), formed by the replacement of B(OH) 4 − for surface carbonate groups, and the adsorption of B(OH) 4 − at calcium protonated sites (> [ C a OH 2 + 0.5 ⋯ B O H 4 - 1 ] - 0.5 ). The relative distribution of the two species and the associated isotope fractionation factors were different for calcite and aragonite and changed between artificial seawater and NaCl solutions. On calcite the two surface complexes were heavier than aqueous borate, showing an overall fractionation of +5.5 to +6.4 ‰ in NaCl solutions (7.8 ≤ pH NBS ≤ 10.3), the inner-sphere complex being ∼3 ‰ heavier than the outer-sphere complex and 7.7 ‰ heavier than B(OH) 4 − . The isotope fractionation decreased to +4.7 ‰ in seawater (7.9 ≤ pH NBS ≤ 8.9), where the inner-sphere complex accounted for >99 % of adsorbed boron. On aragonite the inner-sphere complex formed in NaCl 0.5 M resulted heavier than aqueous borate by 4.2 ‰, whereas the outer-sphere complex, accounting for 9 to 52 % of adsorbed boron between pH 8 and 11, exhibited the same isotopic composition as the aqueous anion. The overall isotopic fractionation varied from +4.1 ‰ (pH NBS = 8.2) to +1.2 ‰ (pH NBS = 11.3). In seawater both borate surface species formed on aragonite were heavier than in NaCl solutions, resulting in overall fractionations of about +4.05 ± 0.05 ‰ relative to aqueous B(OH) 4 − . The present analysis of boron adsorption and isotope fractionation suggests that the isotopic composition of borate ions adsorbed on CaCO 3 in seawater is affected by the specific interaction of the carbonate surface with different aqueous ions, such as SO 4 2− , which modifies the local surface structure favoring the adsorption of B(OH) 4 − ions that are either lighter (calcite) or heavier (aragonite) relative to NaCl solutions. These findings illustrate the complex behavior of boron upon adsorption on CaCO 3 and support the need for studies that provide a more comprehensive description of the processes by which boron isotopes are incorporated and fractionated during biogenic and abiogenic CaCO 3 formation in seawater.