The chemistry and kinetic behavior of Cu‐Cr‐As/B wood preservatives. I. Fixation of chromium on wood

Abstract

Abstract The kinetic behavior of CrO 3 in its reaction with wood has been elucidated. Various reactions take place between CrO 3 and the lignin and cellulose in wood. CrO 3 reacts with cellulose in a two‐step reaction: the first step is an adsorption of Cr VI onto the cellulose to form Cr VI /cellulose activated complexes. The second step is a Cr VI → Cr III reduction taking place on the cellulose surface. The Cr III formed is only physically adsorbed to the cellulose or very weakly bound as small amounts of Cr III can be released into the reaction medium. The Cr VI adsorbed by cellulose appears mainly to be reduced to Cr III . The reaction of Cr VI with lignin has been shown to be the composition of the three successive reaction of Cr 2 O 7 2− , HCrO 4 − , and CrO 4 2− with the guaiacyl units of lignin. Insoluble and stable Cr VI /lignin complexes in which chromium maintains its hexavalent oxidation state are formed. Rate constants and energies of activation for all the reactions have been determined. The fixation of CrO 3 ‐derived compounds on wood has been explained as the combination of the various reactions investigated. The results indicate that 60% of Cr is fixed irreversibly to the lignin of wood as Cr VI and 40% is weakly bound, probably just precipitated, on the cellulose surface as Cr III of which small amounts can be released in a water medium. The complex Cr VI and Cr III species forming complexes with the guaiacyl units have been identified.