Reactions between Cr(VI) and wood and its model compounds - Part 1: A qualitative kinetic study of the reduction of hexavalent chromium

Wood, macromolecular and simple model compounds, were reacted with CrO3 or K2CrO4 aqueous solutions. Extracted lignin, guaiacol, vanillin, vanillyl al cohol and homovanillyl alcohol were chosen as model compounds for lignin, w hilst cellulose, gum Ghatti, xylan, extracted hemicellulose from pine, meth yl-beta-D-glucopyranoside and methyl-beta-cellobioside were used as models for wood polysaccharides. The kinetics of the reduction reactions of Cr(YI) were monitored using UV-Vis spectroscopy and the results obtained for seve ral temperatures are discussed. In general terms, wood, lignin and lignin m odel compounds reduced Cr(VI) faster and to a greater extent than polysacch arides or simple sugar molecules. Moreover, lignin model compounds were red uced even faster than lignin. Simple sugars showed a reduction pattern simi lar to that of cellulose. Extracted hemicellulose revealed to be a poorer r eductant while gum Ghatti was the strongest among the polysaccharides. As e xpected, CrO3 aq. behaved as a more powerful oxidant than K2CrO4 aq. for th ese substances. Even at 100 degrees C, sugars or polysaccharides did not se em to be oxidised by R2CrO4 aq. 0.01 M. These results suggest that, because of the differences in reactivity, lignin reacts preferentially when wood i s treated with Cr(VI)-containing formulations, like those which are applied in wood preservation treatments.