Optimization and immobilization kinetics of Aspergillus nidulans cellulase onto modified chitin by response surface approach

The present study deals with the immobilization of Aspergillus nidulans cellulase onto modified chitin (MC). The effect of contact time, cellulase concentration, MC dosage, temperature and agitation rate for maximum immobilization percentage and immobilization capacity is investigated on the basis of five-level-five-factor central composite design (CCD) obtained by response surface methodology (RSM). The equilibrium nature of immobilization is described by Langmuir and Freundlich isotherms. The kinetic data were tested using pseudo first order and pseudo second order kinetic models. The activation energy of immobilization was evaluated to be 6.735 Jmol. Results of the thermodynamic investigation indicate the spontaneity (∆G<0), slightly endothermic (∆H>0) and irreversible (∆S>0) nature of the sorption process. Entropy and enthalpy were found to be 62.31 Jmolmg and 17.04 KJmol respectively. The Gibbs free energy was found to be -36.04KJ mol. At 120 rpm, 304.6 K, 23.3 min, 1.12 mg of MC, immobilized 18.51 mg g of cellulase from an initial cellulase concentration of 16.34 mg ml with retention of 70% of native cellulase activity upto 10 cycles of batch hydrolysis experiments. The diffusion studies carried out revealed the reaction rate as 52 μmol min. At optimized conditions, immobilized cellulase had a higher Km (0.83 mM) and lower Vmax (38.2 μmol min), compared with the free cellulase (0.62 mM and 48.92 μmol min respectively), indicating the affinity of cellulase for MC matrix.