BORIC ACID ADSORPTION ON HUMIC ACIDS: AB INITIO CALCULATION OF STRUCTURES, STABILITIES, 11B NMR AND 11B,10B ISOTOPIC FRACTIONATIONS OF SURFACE COMPLEXES

Abstract

Boric acid, B(OH)3, forms complexes in aqueous solution with a number of bidentate O-containing ligands, HL-, where H2L is C2O4H2 (oxalic acid), C3O4H4 (malonic acid), C2H6O2 (ethylene glycol), C6H6O2 (catechol), C10H8O2 (dioxynaphthalene) and C2O3H4 (glycolic acid). McElligott and Byrne [McElligott, S., Byrne, R.H., 1998. Interaction of B(OH)30 and HCO3- in seawater: Formation of B(OH)2 CO3-. Aquat. Geochem. 3, 345-356.] have also found B(OH)3 to form an aqueous complex with HCO3- 1. Recently Lemarchand et al. [Lemarchand, E., Schott, J., Gaillardeet, J., 2005. Boron isotopic fractionation related to boron sorption on humic acid and the structure of surface complexes formed. Geochim. Cosmochim. Acta 69, 3519-3533] have studied the formation of surface complexes of B(OH)3 on humic acid, determining 11B NMR shifts and fitted values of formation constants, and 11B, 10B isotope fractionations for a number of surface complexation models. Their work helps to clarify both the nature of the interaction of boric acid with the functional groups in humic acid and the nature of some of these coordinating sites on the humic acid. The determination of isotope fractionations may be seen as a form of vibrational spectroscopy, using the fractionating element as a local probe of the vibrational spectrum. We have calculated quantum mechanically the structures, stabilities, vibrational spectra, 11B NMR spectra and 11B,10B isotope fractionations of a number of complexes B(OH)2L- formed by reactions of the type:B (OH)3 + HL- ? B (OH)2 L- + H2 Ousing a 6-311G(d,p) basis set and the B3LYP method for determination of structures, vibrational frequencies and isotopic fractionations, the highly accurate Complete Basis Set-QB3 method for calculating the free energies and the GIAO HF method with a 6-311+G(2d,p) basis for the NMR shieldings. The calculations indicate that oxalic acid, malonic acid, catechol and glycolic acid all form stable complexes (?G < 0 for Reaction (1)),which are deshielded (less negative ?) vs. the (C2H5)2OBF3 reference by 3.6, 1.5, 6.5 and 5.4 ppm, respectively, and which are isotopically lighter than B(OH)4- (more negative ? 11B) by 3?, 2?, 5? and 2?, respectively. The calculated 11B NMR shifts match well literature values and with the results of Lemarchand et al. (2005), while the calculated isotopic fractionations are also consistent with their results, but show much smaller deviations from B(OH)4- t than indicated by these authors. This is a consequence of the use by Lemarchand et al. (2005) of a value of 1.0194 [Kakihana, H., Kotake, M., Satoh, S., Nomura, M., Okamoto, M., 1977. Fundamental studies on the ion-exchange separation of boron isotopes. Bull. Chem. Soc. Jpn. 50, 158-163.] for the 11B,10B isotopic exchange equilibrium constant of the B(OH)3, B(OH)4- pair which is obsolete and should be replaced by the new purely experimentally determined value of 1.0285 for 0.63 molar ionic strength [Byrne, R.H., Yao, W., Klochko, K., Kaufman, A.J., Tossell, J.A., 2006. Experimental evaluation of the isotopic exchange equilibrium 10 B(OH)3 + 11 B(OH)4- = 11 B(OH)3 + 10 B(OH)4-in aqueous solution. Deep-Sea Res. 1 (53), 684-688.] or 1.0308 for pure water [Klochko, K., Kaufman, A.J., Yao, W., Byrne, R.H., Tossell, J.A., 2006. Experimental measurement of boron isotope fractionation in seawater. Earth Planet Sci. Lett. 248, 276-285]. Given this correction the B(OH)2L- complexes are observed to be isotopically lighter than B(OH)4- by only a few ?. Changes in 11B NMR shift and 11B,10B isotope fractionations for the B(OH)2L- complexes, compared to B(OH)4-, are found to be correlated to some extent with distortions of the O-B-O angles from tetrahedral values and/or with B-O bond strength sums. Similar free energies for the corner-sharing and 4-ring isomers of B(OH)2 CO3- suggest a mechanism for creation of both BIII and BIV environments when B is incorporated into calcite.