THERMODYNAMICS AND KINETICS OF THE ADSORPTION OF CELLULASE ON BLEACHED AND UNBLEACHED PULP

摘要

The enzymatic hydrolysis of cellulose is a complex heterogeneous process including the cellulase adsorption at the cellulose surface as an initial step. The kinetic characteristics of the process are determined by the adsorption step regularities. The aim of this work is to study the adsorption of cellulase at the surface of bleached and unbleached kraft pulp and to establish the thermodynamic and kinetic parameters of the process. The effect of various experimental parameters such as the lignin content, the initial and the equilibrium enzyme concentrations, the contact time and different temperatures was investigated. The enzyme complex cellulase NS50013 (product of Novozymes AS) of an activity of 700 (EGU/g) was added in different quantities referred to the pulp mass. The temperature varied up to 40°C, the pH was kept at its optimal value, while the reaction time is varied up to 30 min. The enzyme adsorption was studied by measuring the residual protein contents with the application of the colorimetric Bradford method. The Langmuir, Freundlich and other adsorption isotherm models were applied to describe the equilibrium stage. The model of the unhomogeneous surface was applied to describe the adsorption equilibrium in the bleached and unbleached pulp - cellulase system. The lignin and the hemicellulase deteriorate the cellulase adsorbability. It was established that the values of the equilibrium constant increase with the temperature increase. The thermodynamic parameters such as the change of the Gibbs energy △G0, of the enthalpy △H0 and the entropy △S0 of the process were calculated. The process investigated was found proceeding spontaneously although it was endothermic and the entropy change was positive. Thus it was concluded that the entropy factor was determining. The kinetics of the process was studied and the temperature-time dependences were established. The adsorption kinetics of celluase on the bleached and unbleached kraft pulp was described by the exponential kinetic equation. That equation was found providing good interpretation of cellulase behavior at the pulp surface. The values of the activation energy and the preexponential factor were estimated.