Boron isotopic fractionation during incorporation of boron into Mg(OH)_2

Experiments of boron incorporated into Mg(OH)2 from magnesium-free synthetic seawater were carried out at various pH values, in order to investigate the adsorption species and the variation of isotopic fractionation of boron on Mg(OH)2. The results showed that the incorporation of boron into Mg(OH)2 was very rapid and reached the equilibrium after 4 h. The [B]s and the partition coefficient Kd between Mg(OH)2 and final solution decreased with the increasing pH. The maximum values of [B]s and Kd were much higher than that of boron adsorbed on metal oxide or clay minerals, indicating that the incorporation capability of boron into Mg(OH)2 was very strong. When the adsorption reached the equilibrium, the δ 11Bfsw was lower than δ 11Bisw. The boron isotopic fractionation αs-fsw was between 1.0186 and 1.0220 with an average of 1.0203. All these indicated that 11B incorporated into Mg(OH)2 preferentially due to B(OH)3 incorporation into Mg(OH)2 preferentially. The deposition reaction of B(OH)3 with Mg(OH)2 was the direct reason for B(OH)3 incorporation into Mg(OH)2. During the boron incorporation into Mg(OH)2, the isotopic fractionation characteristic of boron was decided by the simultaneous existence of adsorption of boron on Mg(OH)2 and the deposition reaction of H3BO3 with Mg(OH)2. Different from the fact that only B(OH)4-species incorporated into bio-carbonate, B(OH)3 and B(OH)4 incorporated into Mg(OH)2 simultaneously, and B(OH)3 incorporated into it preferentially. The lower pH is, the more incorporated fraction of B(OH)3 will be. Mg(OH)2 exists widely in madrepore, which influences the quantitative correspondence of the boron isotopic composition δ 11Bcarb of corals on the pH of the seawater badly, and brings serious uncertainty to the δ 11Bcarb as the indicator of the ancient seawater pH.