Boron isotopic fractionation related to boron sorption on humic acid and the structure of surface complexes formed

Abstract

Boron isotopic fractionation during adsorption onto Ca-flocculated Aldrich humic acid (HA) has been investigated experimentally as a function of solution pH at 25°C and I = 0.15 M. Boron aqueous concentration and isotopic composition were determined by Cs 2BO 2 + Positive Thermal Ionization Mass Spectrometry analysis, while the structure of B surface complexes on HA was characterized using 11B Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Significant B sorption on HA was observed at 6 < pH < 12 with a maximum value of Kd, the partition coefficient between adsorbed and aqueous boron, equal to 40 at pH = 9.5–10. Combined 11B MAS NMR analysis and FITEQL modeling of B sorption on HA showed that this element forms tetrahedrally coordinated five- or six-membered ring chelates, most likely 1,2-diol and 1,3-diol complexes at alkaline pH (8 < pH < 11) and dicarboxylic complexes at near neutral conditions (6 < pH < 9). Results of this study demonstrate for the first time that boron sorption on HA induces a strong pH-dependent isotope fractionation—with 11B depleted at the surface of HA—that reaches a maximum at 5 < pH < 9 (α = 0.975, Δ = −25‰) and decreases sharply at pH >9. The measured isotope fractionation cannot be modeled assuming that the isotopic composition of the sorbed borate species is identical to that of B(OH) 4 - species in the parent solution. It is shown that the extent of isotopic fractionation depends not only on B aqueous speciation but also on the distribution and structure of the borate surface complexes formed. In agreement with energetic constrains, calculation of the isotope fractionation factors between aqueous boric acid and boron surface complexes suggests that the formation of the strained six-membered ring 1,3-diol complex yields a much higher fractionation (α BL P1 −III = 0.954–0.960, Δ = −41/-47‰) than that of the very stable five-membered ring 1,2-diol (α BL P2 −III = 0.983, Δ = −18‰). The results of this study open new perspectives to understand and model boron biogeochemical cycle. It is predicted that boron sorption onto organic matter can have important consequences for the boron isotopic composition of surface water reservoirs (seawater, groundwater, soil waters) in which either abundant organic surfaces or significant boron concentrations are available. In addition, the large isotope fractionation between aqueous boric acid and surface boron-organic complexes found in the present work makes boron a promising tracer of biologic activity.