Evolution of C02 during the fixation of chromium containing wood preservatives on wood

Abstract

A titration procedure was used to confirm carbon dioxide evolution from wood treated with solutions containing chromic acid and to quantify the effects of species (red pine vs soft maple), solution concentration and reaction temperature on the rate and amount of CO2 evolved. Small blocks or chips were vacuum treated with either chromated copper arsenate (CCA) wood preservative or chromic acid solutions and the release of CO2 monitored until the reaction was complete. Significant volumes of CO2 were measured. This is attributed to the oxidation and subsequent decarboxylation of primary hydroxyl groups on wood constituents. The ratio of moles of CO2 produced to moles of chromium added to the wood ranged from about 0.07 to 0.24 depending on the wood species, solution properties and fixation conditions. This accounted for from 9 to 32% of the total oxidation potential of the hexavalent chromium applied. The relative amounts of CO2 produced were higher for maple than for red pine. The rate of CO2 evolution was also higher in the maple samples, consistent with the higher rate of chromium reduction in soft maple compared to red pine. The amount of C02 produced was approximately proportional to the amount of hexavalent chromium in the CCA treating solution although the ratio of CO2 produced to chromium added to the wood increased slightly with increasing solution concentration. The relative amounts of CO2 produced increased with increasing fixation temperature over the 50–90 °C range in both species. The rate of CO2 evolution was accelerated as the fixation temperature was increased. The rates and amounts of CO2 produced were similar for CCA and Cr03 treatments containing the same concentration of chromic acid. Copper and arsenic components of the CCA solution did not appear to have any effect on the decarboxylation reaction.