Kinetic model of CCA fixation on wood. Part I. The initial reaction zone

The fixation process for chromated copper arsenate (CCA-C) preservative tre ated wood has at least two distinctly different zones. One of these is a fa st "Initial Reaction," characterized by a rapid increase in pH and a decrea se in available hexavalent chromium (Cr-VI). In the present study we develo p a mathematical model that describes the initial reaction kinetics for red pine (Pinus resinosa Ait.) treated with 1% CCA-C. The results show that th e initial fixation reactions follow pseudo 10th order kinetics. The activat ion energy and pre-exponential factors were found to be 37.8 kJ.mol(-1) and 8.7 x 10(-19) h(-1) mol(-9) l(9), respectively. At all treatment temperatu res tested, the initial reaction resulted in approximately 47% chromium red uction. At 4 degrees C the time required to complete the initial reaction i s approximately 4.5 h; at room temperature the initial reaction is complete in about 1.7 h. At 50 degrees C the initial reaction is complete in about 25 min. The complete model incorporates the rate equation, Arrhenius temper ature dependence, and the fixation definition into a single equation that e xpresses % chromium fixation as a function of initial chromium concentratio n in the treating solution and time and temperature history of the wood fol lowing treatment. This model can also be used as an integral part of an overall fixation mode l that can be used to predict the percent fixation at a given treatment con dition based on knowledge of the temperature history of the wood during fix ation.