TIGHT METAL-BINDING BY HUMIC ACIDS AND ITS ROLE IN BIOMINERALIZATION

Analytical and thermodynamic data, EPR, FTIR, solution H-1 and solid-s tate C-13 cross polarization magic angle spinning NMR and solid-state extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra have been recorded for purified hu mic acids (HAs) isolated from a German peat (GHA), an Irish peat (IHA) , an unpolluted New Hampshire bog soil (NHA) and their tightly bound c opper(II), iron(III) and manganese(II) forms. Brief water washing of p artly or fully metal-loaded HAs leaves 'tightly' bound metal in the is olated freeze-dried solids. Mo:rt of this metal is removed by washing with 0.1 M HCl, indicating acidic HA functional groups as principal me tal binding sites. The number of nearest-neighbour atoms coordinated t o tightly bound Cu-II (four), Fe-III (six, probably with distorted geo metry) and Mn-II (six, undistorted) in solid GHA, IHA and NHA were det ermined by XANES and EXAFS spectroscopy with reference standards. Isot herms measured at 20.0 degrees C and pH 2.4-3.2 with [M](total) = 0.18 -25.8 mM for tight, reversible Cu2+(aq), Fe3+(aq), and Mn2+(aq) bindin g by solid IHA and NHA fit the Langmuir model and give the pH-independ ent stoichiometric site capacities nu(1) and equilibrium constants K-i for metal binding at specific HA sites i = A, B and C. Tight binding sites A, B and C of IHA are occupied by Cu-II, sites A and B by Fe-III and site A by Mn-II, while only identical metal binding site A in NHA is tight enough to resist metal removal by brief water washing. A new helical HA molecular model based on the empirical formula C36H30N2O15 . xH(2)O visualizes metal binding and the likely roles of HAs in biomi neralization. Site A is suggested to be carboxylate, mixed ligands pro bably constitute site B, and site C is tentatively assigned as the int erior of the HA helix. Binding free energies and EPR evidence suggest that Cu2+(aq), Fe3+(aq) and Mn2+(aq) rapidly transfer between specific HA binding sites. This affects rates of metal release and transfer to minerals.