Structural characterization and iron(III) binding ability of dimeric and polymeric lignin models

To understand the complexation in solution and the sorption of iron(III) on soluble and solid fractions of lignin, a dimeric model (guaiacyl-beta -guaiacylglycerol ether, called beta -O-4) and a polymeric model (dehydrogenation polymer resulting from polymerization of coniferyl alcohol) of lignin have been synthesized and characterized with chromatographic, solution, and solid state C-13 CP-MAS NMR and XPS spectroscopies. The beta -O-4 dimer is a monoacid (HL). Potentiometric studies in aqueous solution at 25 degreesC and 1 mol L-1 ionic strength (KNO3) indicated formation of two stable complexes, FeL2+ and probably FeL(OH)(+) which shows that the soluble fraction of lignin binds metals, indicating that they are transported by water through the soils. The binding of iron(III) on the DHP polymer was then investigated. The sorption experiments have shown a great affinity of iron for the solid with a maximum of adsorption since pH 5. A pulsed-ESR study has revealed surface oxidation by the iron(III) cation, which leads to iron(II) and semiquinonic radicals on the polymer surface, with a radical concentration of about 5 x 10(17) spin/g. (C) 2001 Academic Press.